PRISM COUPLER SURFACE SENSING
by
William David Mason
A thesis submitted for the degree of Doctor of Philosophy in the
University of London.
Department of Electrical Engineering,
Imperial College of Science and Technology,
Exhibition Road,
London SW7 2BT.
August 1988
1 To my parents and their family.
2 ABSTRACT
The mechanism of resonant optical coupling to a surface wave on a thin silver film, with consequent absorption of the optical power, forms the basis of a device for sensing changes in surface characteristics. This "Surface Plasmon
Resonance" (SPR) can be detected as a sharp reduction in the reflected intensity over a narrow range of incident angles in an observation of Total Internal Reflection.
The advantages of this technique for sensing include : a large fraction of the propagating field resides in the dielectric cladding beyond the silver surface; the con finement of the field to a region close to the surface; the coupling resonance occurs over a narrow angular range, typically 0.2 degree with an air cladding and 2.0 degrees with an aqueous cladding.
This thesis considers the design and theoretical per formance of alternative structures, not involving SPR, that migh,t offer improvements in sensitivity or in material characteristics.
The excitation process of SPR relies essentially on a "Prism Coupler" phenomenon, similar to that used routinely for excitation of dielectric waveguides, where a planar waveguide is separated from a high index coupling prism by a low index buffer layer. For such a waveguide excitation there is no inherent loss mechanism, a corollary of which is that the coupling resonance can be made arbitrarily sharp to give improved resolution. Although practical considerations limit the ultimate resolution attainable,
3 angular widths of order 0.1 degree have been obtained with both air and agueous clad polymer guides. In addition to the angular resolution, the fractional power in the sensing cladding medium is a crucial factor in determining the overall device sensitivity. A dielectric waveguiding structure fabricated from Magnesium Fluoride and Zinc Sulphide can have a large fractional power in the sensing cladding (19% with air cladding and 37% with an aqueous cladding). For the polymer structures the power in the sensing cladding is much lower in both cases because the guiding layer is much wider resulting in stronger confinement. The values for the polymer struc ture used are 2% with an air cladding and 4% with an aqueous cladding. Thus the ZnS/MgF2 configuration is sub stantially more attractive for sensing applications. The overall sensitivity to surface layer changes for
ZnS/MgFz dielectric structure with an air cladding is com parable to that for SPR, but with an aqueous cladding, it is shown that this dielectric system would be substan tially more sensitive.
4 ACKNOWLEDGEMENTS
I wish to express sincere thanks to my supervisor Dr John Cozens for his innovation, encouragement and patience.
I would also wish to thank Professor Mino Green and Dr Keith Leaver for their helpful suggestions and discussions and to thank my colleagues in the Optics group for their willingness to advise and explain.
I am also indebted to the S.E.R.C for financial support throughout my course of study.
5 CONTENTS Page
Abstract 3 Acknowledgements 5 Table Of Contents 6 List Of figures 11
Chapter 1 Optical Surface Measurement Techniques 16 1.1 Total Internal Reflection Spectroscopy 16
1.2 Ellipsometry 19
1.3 Thin Film Waveguide Measurement 20 1.4 Surface Plasmon Excitation 23
1.4.1 Applications Of Surface Plasmon
Excitation 26
1.5 The Prism Coupler Dielectric Sensor 28
Chapter 2 Optical Excitation Of Thin Film
Waveguides 32
2.1 Coupled Mode Theory Of Prism Coupler
Operation 32
2.2 Solutions Of Maxwells Equations For
The Prism Coupler Geometry 35
2.3 The Theory Of Prism Coupler Operation
Using Modified Fresnel Coefficients 40 2.4 Representation Of The Amplitude Field
Solutions In The Waveguiding Layer As A Resonance Phenomena 42
2.5 Excitation By A Beam Of Finite Width 44
2.6 Physical Description Of Power Transfer 47
2.7 "Leaky" Mode Field Distribution Matching 50
6 2.8 Prism Coupler Experimental Observations 52 2.9 Device Operation 53
Chapter 3 Silicon Oxide Waveguide Structures 55 3.1 Initial Choice Of Thin Film Structure 55 3.2 Substrate Preparation 56 3.3 Magnesium Fluoride Deposition 59 3.4 Reviewing The Design Of The Structure 62
3.5 Deposition Of Silicon Oxide 64 3.5.1 Summary Of The Reviewed Literature On Silicon Oxide
Deposition 65
3.5.2 Experimental Deposition Results 67
3.6 Excitation Of The Guiding Structure 71
3.6.1 Experimental Arrangement 71
3.6.2 Experimental Observation 7 3
3.7 Expected Observations For Ideally Flat Films 79
3.7.1 Focusing The Beam 80
3.7.2 Expanding The Beam 82 3.8 Quantitative Measurement Of Mode Indices 83
3.9 Suitability Of Silicon Oxide Films 83
Chapter 4 Fabrication Of "Lossless" Polymer
Structures 85
4.1 Choice Of Deposition Technique 85
4.2 Fabrication Details - The Spin Coating
Technique 87
4.2.1 The Proposed Polymer Structure 91
4.2.2 Determination Of Film Refractive
Indices 91
7 4.3 Locating The Resonance Experimentally 93 4.3.1 Detection Of Excitation By Phase Shifts 94
4.4 An Alternative Babinet Compensator 95 4.5 Transmission Through The Output Plane
Polariser 95 4.6 Experimental Demonstration 96 4.6.1 Experimental Agreement With
Theoretical Curves 100 4.7 Predicted Appearance Of The Output Spot 104 4.8 Experimental Observations Of The Output Spot 107
4.9 Explanation Of The Effect Of Non-Uniformity 108
Chapter 5 Optimisation Of The Sensing Structure 112
5.1 Sensitivity Of The Polymer Structure 112
5.2 Choice Of Layer Thickness 112
5.3 Sensitivity To Guide Width Variation 118
5.4 Comparison With A Critical Angle Sensor 120
5.5 Choice Of Spacing Layer Thickness And Index 121
5.6 Proposed Fabricatable Structure 124
5.7 Possible Use Of The TM Fundamental Mode 126
5.8 Theoretical Performance Compared With The
Surface Plasmon Resonance 127
5.8.1 Resonance Width 127
5.8.2 Sensing Field Decay lengths 131
5.8.3 Theoretical Mode Index Shifts 132
5.9 Predicted Resonance Shift Comparison 134
5.10 Use Of Bimodal Structures 138
5.11 Use Of Higher Order Modes 143
8 5.12 Practical Fabrication Of Two Mode Structure 143
Chapter 6 Experimental Fabrication Of The Nominally Optimised Structure 145 6.1 Deposition Of ZnS Films By Evaporation 145
6.2 Properties Of ZnS Films 148
6.2.1 Microstructure 148
6.2.2 Stress Stability 149 6.2.3 Stability Of Zinc Sulphide On Magnesium Fluoride 151 6.2.4 Refractive Index Inhomogeneity 152
6.3 Estimation Of The Error Due To Refractive Index Inhomogeneity 153
6.4 Film Evenness 154 6.5 Buffer Thickness Variation 155
6.6 Measured Thickness Variation 155
6.7 The Source Of Scattering 160
6.8 Deposition Of Zinc Sulphide Onto PMMA 160
6.9 The Fabricated Structure 161
Chapter 7 "Prism Coupler Surface Sensing" Applications 167 7.1 Qualitative Illustration Of Surface Sensing 167
7.2 Quantitative Demonstration Of Thin Film
Surface Sensing 172
7.3 Demonstration Of The Resolution Of The Device 174
7.4 Comparison Of Dielectric Structure And SPR
Sensitivities 179
9 7.5 Suggestion For Future Work 1 8 0 7.6 Conclusions 183
References 186
10 LIST OF FIGURES Page
Fig 1.1(a) Resonant field enhancement in TIR spectroscopy using an optical cavity layer N3 and spacing layer N2 ...... 18 Fig 1.1(b) Typical mode of operation of ellipsometric
thin film measurement ...... 18 Fig 1.1(c) Thin film measurement by coupling into
an optical waveguide layer ...... 22
Fig 1.1(d) Surface plasmon excitation on a silver film with a coating adlayer ...... 22
Fig 1.2 Comparison of field amplitudes for surface plasmon excitation and dielectric waveguide
excitation ...... 29 Fig 1.3 "Prism coupler surface sensor" experimental
arrangement ...... 31
Fig 2.1 Coupled mode explanation of prism coupler
excitation ...... 33
Fig 2.2 Four media planar structure representing a
prism coupler configuration ...... 36
Fig 2.3(a) Convention used for modified Fresnel coefficients ...... 36
Fig 2.3(b) Representation of prism coupler
excitation using modified Fresnel coefficients ...... 36
Fig 2.4 Excitation of waveguide mode resonances for
increasing gap thickness ...... 43
Fig 2.5 Spatial power redistribution of the
reflected spot on waveguide structure
excitation ...... 46
11 F i g 3.1(a) Movement of low intensity band across the output spot ...... 74 Fig 3.1(b) Scattered and reflected output intensity
from a structure using evaporated
inorganic layers ...... 74 Fig 3.2 Reflected output intensity for an
evaporated structure with theoretical
mode index width 10~4 ..... 76 Fig 3.3 Reflected spot profile on excitation of a scattering waveguide structure ...... 78
Fig 3.4 Appearance of the reflected laser spot at resonance for focused and unfocused beams at various azimuths ...... 81
Fig 4.1(a) TE and TM polarisation resonances for an
air clad polymer structure ...... 98
Fig 4.1(b) Experimental reflectance for the
structure of Fig 4.1(a) ...... 99
Fig 4.2(a) TE and TM polarisation resonances for an
aqueous clad polymer structure ...... 101
Fig 4.2(b) Experimental reflectance for the
structure of Fig 4.2(a) ...... 102
Fig 4.3 Appearance of the laser spot on reflection
from a polymer structure for different
polariser arrangements ...... 106
Fig 4.4 Invariance of the output spot for azimuthal
rotation ...... 106
Fig 4.5 Surface contour of a polystyrene guide ..... 109
Fig 5.1 Schematical power distributions for surface plasmon and dielectric
waveguiding structures ...... 113
12 F i g 5.2(a) Cladding power percentage as a function of guide thickness ...... 115
Fig 5.2(b) Cladding power percentage as a function of. waveguide mode index ...... 115 Fig 5.3 Combinations of guide thickness and index which give a chosen value of mode index .... 117 Fig 5.4 Cladding power percentage as a function of guide index with mode index fixed ...... 117 Fig 5.5(a) Mode index sensitivity as a function of
guide width ...... 119 Fig 5.5(b) Mode index sensitivity as a function of
mode index ...... 119
Fig 5.6(a) Increase in cladding power percentage by
lowering the buffer index ...... 123
Fig 5.6(b) The effect of guide assymetry on
cladding power percentage ...... 123
Fig 5.7 Estimation of the improvement in field distribution for surface sensing using
available dielectric materials ...... 125
Fig 5.8 Optical reflectivity of a 3 phase system
with a middle layer of complex
refractive index ...... 128
Fig 5.9 Direct comparison of surface plasmon
and dielectric waveguide minima
(i)-(ii) ..... 129 (iii)-(iv) ...... 130
Fig 5.10 Illustration of the breakdown of the
dielectric perturbation calculation ...... 133
1 3 Fig 5.11 Comparison of surface plasmon shift
with a dielectric structure using MgF2 and ZnS and an air cladding ...... 135
Fig 5.12 Experimental mode index shift of a strongly perturbed surface plasmon
and corresponding curve for indicated dielectric structure ...... 135 Fig 5.13 Comparison of silver surface plasmon shift
with a dielectric structure using MgF2
and ZnS and aqueous cladding ...... 137 Fig 5.14 Illustration of the use of two different
mode resonances to determine two
adlayer parameters ...... 139
Fig 5.15 A waveguiding structure with a thin
guiding layer and large index
differences ...... 141
Fig 5.16(a) Error in adlayer thickness determination
due to 10-4 error in TE and TM resonance
location ...... 142
Fig 5.16(b) Error in adlayer index determination
due to 10-4 error in TE and TM
resonance location ...... 142
Fig 6.1 Evaporated ZnS thickness distribution ...... 156 Fig 6.2(a) Surface profile of evaporated ZnSfilm .. 157
Fig 6.2(b) Contour relief of ZnS f i l m ...... 157
Fig 6.3 Build up of ZnS film onto PMMA coated
substrate ...... 162
Fig 6.4 Broadening of the resonance curve due to
film unevenness ...... 164
1 4 Fig 7.1 Condensation and evaporation of a layer of
water vapour ...... 168 Fig 7.2 Restoration to initial intensity level as residual water evaporates ...... 169
Fig 7.3 Deposition of acetone onto the sensor surface with subsequent evaporation ...... 171 Fig 7.4 Demonstration of the difference in porosity between evaporated films of ZnS and SiO .... 173
Fig 7.5 Shift of waveguiding structure resonance
caused by a 40 A polystyrene adlayer ...... 175 Fig 7.6 Detection of increasing adlayer thicknesses
of P M M A ...... 17 6
Fig 7.7 Calculated film thickness versus speed of rotation in spin deposition ...... 178
15 1. OPTICAL SURFACE MEASUREMENT TECHNIQUES The use of optical techniques to determine the properties of thin surface layers is well established in the fields of reflection spectroscopy, ellipsometry, total internal reflection spectroscopy, optical waveguide devices and the varied applications of the surface plas- mon. In the following a brief review is made of each of these techniques and an outline is given of the device to be fabricated during this thesis. During the course of
the investigation it has become apparent how aspects of the proposed device are closely related to the last three
techniques mentioned above. A recent report on "Optical Biosensors" (Norris 1986) concludes that "techniques using total internal reflection are promising areas for poten tial optical sensors".
1.1 Total Internal Reflection Spectroscopy
(Harrick 1967) A light beam undergoing total internal reflection has
a non zero evanescent field in the medium of lower index, whose penetration increases as the beam direction is moved towards the critical angle - just beyond the critical angle the field becomes radiative, indicative of transmis
sion into the second medium. The reflectivity of the in
terface in the vicinity of the critical angle is very sen
sitive to changes in absorption or refractive index, either for a bulk sample or a thin layer at the interface. This allows measurement (by means of an optical
monochromating system) of the attenuated total reflection
spectrum of a sample placed on the interface which will
16 be characteristic of the material. The spectrum is not a unique "fingerprint" for the sample because of complications arising from variation in the "effective thickness" of the sample with wavelength - for example the volume of interaction of the beam with the sample increases monotonically with increasing wavelength as a result of increasing depth of penetration of the evanescent field. Moreover the spectrum obtained is not a direct measure of either the absorption coefficient or the refractive index of the material, but a combination of both, determined by the evanescent field shape , sample thickness, and the complex index of refraction. In order to enhance the effect of the sample on the reflectivity it is usual to have an Internal Reflection Element (IRE) in the system which produces multiple reflections - the simplest being an internal reflection plate several millimetres thick in which a light beam is multiply reflected between the upper and lower sampling surfaces. When the sample being studied is only available
in small quantities a more feasible alternative is to use
a Frustrated Total Reflection cavity as in Fig. 1.1(a), in which a high index film has a resonant field enhancement for light incident at a particular angle. The high index
layer (N3) is separated from the TIR prism by a low index
spacing film (N2) and the absorbing sample (N4) is placed
on the high index cavity layer (N3) - unless the sample
itself forms the cavity (Fig. 1.1 (a)). The structure can give rise to enhancements of spectra by a factor of ten but can cause spectral distor
tion if the absorption bandwidth is of the same order as
17 Fig 1.1(a) Resonant Field Enhancement In TIR Spectroscopy Using An Optical Cavity Layer N3 And Spacing Layer N2
ELLIPTICALLY POLARISED SAMPLE
LASER
Of Ellipsometric Thin Film Measurement
18 the cavity bandwith. It should be remarked that in all this discussion of TIR the assumption of infinite plane waves to represent the incident light beam has been made and that each spectrum is made at fixed angle.
1.2 Ellipsometrv (Azzam 1977, Zaininaer 1964) An infinite plane wave incident from air as an am bient medium on an absorbing substrate coated with a thin film is reflected with a reduced amplitude and a phase shift relative to the incident beam. The plane wave is considered to be partially reflected at the surfaces of the film and phase delayed on traversing the film with subsequent summation producing the final output. In equality of reflection coefficients at the interfaces for parallel and perpendicular polarisations means that the phase and amplitude changes are different for these two components. A linearly polarised input wave will in general become elliptically polarised on reflection (Fig
1.1(b)), with the direction of the axes of the ellipse and its ellipticity providing independent parameters which can be used to determine two properties of the film (usually thickness and refractive index) if the substrate index is known. Although the above implies transparent films, the technique can be used for absorbing films also. Because the phase delay for propagation through the film is cyclic, there is a corresponding ambiguity in the calculated value of thickness so it is necessary to know the "order" of the thickness - i.e to have an independent estimate of this quantity. This cyclic ambiguity also manifests itself in the need of a judicious choice of film
19 thickness to avoid a measurement very close to an order where accuracy is very low. Commercial instruments can measure indices to 5*10-3 and thickness with errors of or der tens of angstroms. As well as its standard use in thin film measurement of evaporated, sputtered, or grown inorganic layers and organic polymer layers, the ellip- sometric technique has also been patented by Battelle (Norris 1986) for applications on bioassays (organic films deposited from aqueous solution). Although used almost exclusively in external reflec
tion (with air as ambient medium), it would be possible to
use ellipsometry in total internal reflection except that there will be no amplitude change unless the film is ab
sorbing. Azzam (1976) has proposed such a scheme for analysing biological cells in vitro - basically using el-
lipsometric modelling and measurement for angles of in
cidence in the vicinity of the critical angle.
1.3 Thin Film Waveguide Measurement Determination of the optical parameters of a thin
film by fabricating a layer as a planar waveguide on a
substrate of lower refractive index has been one of many useful applications of integrated optics - the accuracies
claimed for index and thickness are generally superior to
any other technique - (Kersten 1975 , Paul 1982, Swalen et al 1977, Tien et al 1970, Westwood et al 1979). A prism
placed above the film with an air spacing layer of a few
tenths of a micron is used to couple a laser beam into the
guide via the evanescent field of the gap, after which
coupling is terminated abruptly by the edge of the prism.
20 A second prism couples out the radiation after propagation along the guide Fig. 1.1(c). If the guiding film has a strong loss mechanism it is possible to locate the angles of waveguide excitation (0j_) without the need of a ter minated prism by reflection from the base of a single prism. Measurement of the angle for excitation of the mode yields the mode index of the structure and if the waveguiding layer is thick enough to support more than one mode, providing several measurements, it is possible to determine both the thickness and refractive index of the film. The guiding layer is chosen thick enough so that most of the power is confined in this, the region of measurement (rather than in cladding) but not so thick that the different modes become completely degenerate.
Typical sensitivities would be of order 2*10-4 in index and a few per cent for thickness of the film - i.e one order of magnitude more accurate in index measurement than ellipsometry (Kersten 1975 , Adams et al 1979). An alternative waveguide technique involves deposit
ing the film directly onto a prism of higher index with
the air layer above. If the layer is sufficiently thick
(several optical wavelengths) "quasimodes" of a film guid
ing on one interface (film\air) and radiating on the other (prism\film) are observed, (Ding et al 1983, Leclerc et al
1984). The situation corresponds to modified total inter
nal reflection, where the finite thickness of the second
medium causes the waveguiding mode effect - at angles just
above the critical angle, reflection at the leaky inter
face is very strong (provided the prism index is suffi
ciently high) so that the loss can be quite low. These
21 a ir g a p FILM. SUDSTRATt
Fig 1.1 (c) Thin Flm Measirement By Coupling Into An Optical Waveguide Layer
Fig 1.1 (d) Sirface Plasmon Excitation On A Silver Film With A Coating Adlayer - The Inset Indicates The Reflected Intensity Variation
22 "quasimodes" are not true eigemnodes of the structure and since they are close in mode index, conversion between them is quite strong even in the absence of scattering (Ding et al 1983). This technique yields an accurate value for the film index (to order 10_4), so that
birefringence in films a few microns thick can be measured (Leclerc et al 1984) but because the resonance positions are very insensitive to thickness the accuracy of measure ment of this parameter is very poor. It is possible to
offset this insensitivity to film thickness by measuring the mode indices of all the excited resonances and by fit ting these data numerically to obtain a better estimate.
1.4 Surface Plasmon Excitation
The excitation of a Surface Electromagnetic Wave (or
Surface Plasmon) at the interface of a dielectric and a metallic film has been thoroughly reviewed (Raether 1977,
Abeles 1976 , Nylander et al 1982). Application of
Maxwells equations to a boundary between a semi-infinite dielectric and a semi-infinite ideal metal indicates the
existence (for the TM polarisation) of an electric field
distribution which decays into both the metal and also into the dielectric if the propagation vector of the inci
dent radiation is sufficiently large.
For a thin metal film bounded by two different
dielectrics there are two different mode index values
which give large boundary field amplitudes one for each of the two interfaces. It is not possible to excite the
structure with high field amplitude at the high index
dielectric boundary because its propagation constant is
2 3 too large to be accessed. The resonant excitation with large field amplitude on the low index boundary can be excited through the metal film by a radiation field inci dent from the higher index dielectric medium. The surface plasmon excitation on this interface has a high percentage of power in the lower index medium (99.6% for silver) and consequently its propagation constant is : 1/ Very nearly identical to that of a plane wave propagating completely in the dielectric (the ratio of this propagation constant to that for vacuum propagation is only slightly greater than the index of the dielectric). 2/ Very sensitive to changes in dielectric constant in this region.
3/ Much less lossy than would be expected from the assumption of propagation in the lossy metal because only a small fraction of the power is in this medium (which has a complex permit'fcivi.fcy) .
In a common practical arrangement the silver film is deposited onto a substrate prism with the dielectric medium being the ambient air. Any changes to the surface of the silver film will have a drastic effect on the sur face plasmon mode on this boundary Fig 1.1(d). This is the basis of the many applications of surface plasmon sensing, relying on the high percentage of the power of the excited mode in the dielectric medium and its surface confinement which gives it great sensitivity to interface perturbation in this region. It should be noted however that although the field decay into the metal is very rapid, the decay into air need be no more drastic than it would be for any
24 other evanescent field decay. The visible detection of the surface plasmon relies on the electromagnetic damping of the metal (i.e the finite imaginary part of its dielectric permitivity) which causes resonant absorption of the incident power when the mode is excited. Indeed the surface plasmon resonance can be considered as a special type of internal reflection spectroscopy as noted by Norris (1986) or alternatively can be viewed as a lossy waveguide excitation. The same plane wave theory can be used for both (Hansen 1968).
For excitation in the Kretschman configuration in which coupling to a surface plasmon on the far side of the film from the incidence prism occurs through the metal film there is an optimum value of thickness of the silver film which will produce zero reflectance of a plane wave at the resonance angle (other plane wave components in the beam may not be completely cancelled). For a finite inci dent beam with angular divergence less than that of the resonant width (the usual arrangement for surface plasmon sensing) this optimum thickness value corresponds to equating the loss due to radiation from the Surface Plas mon back into the prism with that due to absorption in the excited mode. This represents a matching of the power
coupling from the prism radiation field into the absorbing
surface plasmon. With this chosen thickness of metal the
angular width of the reflected resonance minima is deter mined by the material constants of the film.
Deposition of a thin adlayer on top of the metal film
produces a shift in the angular resonance position, which
allows determination of one adlayer parameter (dielectric
2 5 permittivity or thickness). This method is especially suited to measurement of very thin films <100 A, which complements the region of satisfactory operation of ellip- sometric measurement (Abeles 1976). The characteristics of the Surface Plasmon resonance dip are (to first order) determined as follows 1/ Angular position of the minimum - fixed by the dielectric constant of the medium above the metal surface, and the real part of the metals permittivity, (and the choice of prism refractive index). 2/ The angular width of the resonance - fixed by the thickness of the metal film and the imaginary part of the metals permittivity, (and the prism refractive index).
3/ The depth of the minimum - fixed by the ratio of
Surface Plasmon absorption damping to Surface Plasmon radiation damping relative to unity - the latter case causing a minimum of zero reflected intensity.
1.4.1 Applications Of Surface Plasmon
Nylander et al (1982) have demonstrated the use of
surface plasmon to detect anaethestic gas absorbed by an oil layer of a few hundred angstroms on the surface of the
silver film at concentrations of 80 ppm.
Pockrand (1978) has demonstrated the shift in the
surface plasmon minimum by building up inorganic layers above the film and also investigated the effect on the resonance caused by absorbing dye monolayers (ibid 1977) in order to . determine the dispersion curves for these materials to compare with results from previous methods.
2 6 Flannagan et al (1984) have demonstrated the build up of antibody layer from aqueous solution onto a silver film pre-coated with the appropriate antigen. A device with a similar application but using a waveguide evanescent field is also being developed by Battelle, Geneva (Kress-Rogers 1986). PA technology hope to have available in the next year (1988) or so an "Immunoassay" kit based on the Surface Plasmon.
Coupling between a planar dielectric waveguide and a Surface Plasmon mode is also sensitively dependent on the cladding of the metal film. Broadband excitation of such
a structure produces an absorption dip whose minimum
ocurrs at a wavelength determined by the external cladding
(Kreuwel 1986). There have also been several applications of surface
plasmon to surface nonlinear optical effects (Sarid 1981,
Ariyasu et al 1985) with particular interest in Langmuir
Blodgett deposition of organic films with high second or
der non linear coefficients (Cross 1987). It is claimed
that localisation of the evanescent field at the interface
of the metal and dielectric improves the efficiency of the
non-linear processes. All of these applications utilise the localised
evanescent field on the surface of the metal film. The decay of the field in the metal film is much faster (approximately 15 times for silver) than the decay in the
dielectric, but since it is the field in the dielectric (air usually) which is responsible for the interaction
the surface plasmon sensing field is no better confined
than any other air evanescent field. The angular resolu
27 tion of resonance position measurement is also limited by the absorption in the metal (the width is of order 0.2° for air clad silver, and 2.0° for aqueous clad silver,
(Flanagan et al 1984, Yeatman 1987).
1.5 The Prism Coupler Dielectric Sensor The surface plasmon resonance as a detection mechanism has found wide application and generated much interest, but the mode of operation involving a perturba tion of the evanescent field of a guided (albeit damped wave) is essentially that used in any dielectric direc tional coupler surface sensor. In the Kretschman con figuration the guided wave is launched on the lower side of a silver film by excitation through the film itself by an incident radiation field in a high index prism. The most direct dielectric analogy would be a four medium structure equivalent to a non-terminated prism coupler.
Fig. 1.2 illustrates the comparison of field shapes in these two arrangements.
In both cases the resonant excitation of a guiding structure and the shift of the position of this resonance which is caused by perturbation of the evanescent field in the lower cladding forms the basis of the measurement. In the conventional prism coupler the lower dielectric labelled "sensing dielectric" in Fig. 1.2 is actually the substrate on which the waveguiding film is deposited so access to this region is not available. If however a dielectric film is used as a spacing layer as opposed to an air gap the waveguiding structure can be deposited directly onto the prism itself with an accessible region
28 SURFACE PLASMON EXCITATION DELECTRIC WAVEGUIDE EXCITATION
PRISM 7/ / I I I I I 111 II I 111 //// m e t a l f il m DELECTRIC SPACING LAYER DIELECTRIC CLADDING DiEi7ff ir fljf e / ffqJDiQfc' La y e r
DIELECTRIC CLADDING
Fig 1.2 COMPARISON OF FIELD AMPLITUDES FOR SURFACE PLASMON EXCITATION AND DIELECTRIC WAVEGUIDE EXCITATION
29 adjacent to the waveguiding layer. This "Prism Coupler Surface Sensor" is indicated in Fig. 1.3. The coupling of the laser beam into this waveguide structure will be sensitively dependent on per turbations to the top of the waveguiding film because this effectively alters the planar waveguide which is to be ex cited by the incident radation field from the prism. The geometry of the device and its mechanism of operation most closely resemble those of the Surface PlaSLmon sensor dis cussed above. The regime of thin film thickness measure ment of the device complements that of conventional planar waveguide measurement as the latter requires a film suffi ciently thick to support several guided resonances whereas the former is suited to measurement of adlayers which may well be much too thin to support any guided modes.
Similarly ellipsometry and reflectance spectroscopy are not well suited to very thin film measurement. In the former a 60 A film would be barely detectable and in the
latter an absorbing film of 60 A would produce a change of order 1% in reflectivity (internal and external reflection
McIntyre 1971). The same film however could be easily
sufficient to shift a Surface Plasmon or waveguide
resonance by a complete width.
3 0 Glass Slide
Fig 1.3 "Prism Coupler Surface Sensor" Experimental Arrangement 2 . OPTICAL EXCITATION OF THIN FILM WAVEGUIDES The explanation of the operation of the "Prism
Coupler Surface Sensor" involves some understanding of the mechanism of planar waveguide mode excitation by an inci dent laser radiation field. In the following sections an approximate theory is first described which provides physical insight into the process; then an exact solution of the arrangement for infinite plane wave excitation is made using a computer simulation program. Even this analysis is only an approximation to the practical situa
tion because a spectrum of plane waves is needed to repre sent a beam of finite width - a qualitive explanation of
the consequences of this is included. Finally the main
observations from previous prism coupling experiments are reviewed because they indicate the important parameters
for monitoring the performance of the device.
2.1 Coupled Mode Theory Of Prism Coupler
Operation
The qualitative mode of operation of the integrated
optical prism coupler is treated in many books (Tamir
1979), but it is helpful to review it here before a more
exact plane wave analysis is undertaken.
A high index film of thickness of order of optical
wavelengths deposited onto a low index substrate with a
low index cladding will have a number of discrete eigen-
modes and associated propagation velocities. The electromagnetic fields of the modes are evanescent into
the upper cladding and lower substrate (Fig 2.1(b)). In
Fig 2.1(a) a laser beam incident on the base of a prism at
3 2 FIG 2.1 COUPLED MODE EXPLANATION OF PRISM COUPLER EXCITATION a sufficiently shallow angle has (under total internal reflection) a radiation field inside the prism and an evanescent field into the air gap below. This evanescent field has a propagation velocity (Vp>) along the interface determined by the angle (9) of incidence of beam and the refractive index of the prism (Np>) as :
Vp = c/ (Ko*Np>*sin( 0 ) ) [2.1]
Where Ko = 2tt/wavelength and c is the speed of light in vacuo.
The expression Np>*sin(0) is a propagation constant normalised to the free space propagation constant Ko - it is subsequently referred to as a "Mode Index". Energy can be coupled from the incident beam inside the prism into the waveguiding film if the latter is placed adjacent to the prism base with an air gap of the order of a fraction of a wavelength (Fig 2.1(c)). This transfer occurs at angles of incidence of the laser beam which match the velocity of the prism evanescent field along the interface to the propagation velocity of the modes of the guide via the evanescent field of the prism, which extends into the waveguide.
Energy coupled into the waveguide structure is also liable to leakage back into the prism subsequently and a juidicious choice of interaction length between the beam and the guiding structure is needed to ensure a sufficient length to enable power build up in the film but not so long that the power has entered and also leaked out. The required interaction length is determined directly by the strength of coupling between the guided mode field and the excitation field in the prism.
34 2.2 Solutions Of Maxwells Equations For The Prism Coupler Geometry
An exact description of the prism coupler electromag netic fields can be obtained by treating the arrangement as a planar dielectric structure with . four media (prism(l), spacing layer(2), guiding layer(3), and clad ding region(4)) and determining the field components which match the boundary conditions and are solutions of Max wells equations - separately for TE and TM polarisations,
(Midwinter 1970 (a)). The guide layer has thickness "2d" and the spacing layer width is "a" in Fig 2.2.
Maxwells equations for non-magnetic isotropic media enforce the wave equation :
V 2 (E)+ [(w2 )/(c2 )]*E = 0 [2.2] Where time dependence of exp(iwt) is assumed.
The total field solution is assumed to propagate parallel to the interfaces as exp(iKx*x) - the field solutions in each separate region will have this form.
Recalling that d/dy (any field) = 0 because of the geometry and matching the components of the curl relations for E and B yields two sets of independent equations.
Both sets contain three equations with one independent field component which identify the choice of incident TM or TE polarisation. Recalling that for the TE mode there is only a y com ponent of E and d/dy is zero converts the wave equation for E to :
d2 (Ey ) /dz2 = (K3c2 - E i * K o 2) *Ey {Ed. = Nd.2 } [2.3]
35 z
CLADDING REGION (4) N4
Fig 2.2 Four Media Planar Structure Representing A Prism Coupler Configuration
UNIT INCIDENT D AMPLITUDE 321
______1 <3) MULTIPLE REFLECTIONS S (2) y (D
'321
Fig 2.3 (a3 Convention Used For Modified Fresnel Coefficients Representation Of Prism Coupler Excitation Using Modified Fresnel Coefficients
36 The transverse propagation constants in the four layers are then defined as:
B* = Ko*\f (Kx2-Ni2 ) Regions (2) and (4). [2.4] G* = Ko*/(Ni.2-Kx2 ) Regions (1) and (3). [2.5]
Where Ko = 2 *Tt/wave length. And Ni is the refractive index of the itix
layer. The nature of the field component variation (exponential or harmonic) in the transverse direction (normal to the film interfaces) is determined by the mag nitude of the exciting plane wave mode index (Kx =
NE,*sin6) relative to the index of the region concerned.
The fields will have a transverse evanescent decay in regions where the refractive index is below the incident mode index and will be harmonic in regions where the refractive index is greater than the incident mode index.
Prism coupler operation is confined to the region of inci dent mode indices for which Kx is below the indices of the prism and guide layers but Kx is greater than the indices of the spacing and cladding regions, so that the fields in the prism(l) and guide(3) are harmonic and the fields in the spacing layer(2) and cladding(4) are evanescent. The second order differential equation above has in each region two independent solutions (both harmonic or both exponential) except for the lower cladding region
(4) where the growing evanescent solution is discarded as unphysical - Fig 2.2 labels these field amplitudes "A" to
"D" .
37 The field solution is (using the notation of Fig
2 .2) :
Ey(3) = Cexp(-iG3 z) + C'exp(iG3 Z) [2.6] {In the guiding layer.} Ey(l) = A exp(-iGxZ) +A'exp(iGxZ) [2.7] {In the prism.}
With similar expressions for the other two regions. Physically A represents the amplitude of an incident plane wave and A' the amplitude of a reflected wave in the prism region. To appreciate the operation of the present device only the total field amplitudes in the guiding layer and the amplitude of the reflected wave need be con
sidered. The former monitors the resonant energy transfer from the prism radiation field into the perturbed "guided mode", whilst the latter has unit amplitude for all angles of incidence but a rapidly changing phase relative to the
input beam when the structure is excited.
The Hx field component is deduced from the cor
responding Ey component via the appropriate curl relation. Application of the boundary conditions SEy = 0 and
SHac = 0 at three interfaces yields six equations with the
seventh unknown the normalisation amplitude of the inci
dent field.
The ratio of guiding layer maximum amplitude to inci
dent amplitude can be expressed as :
C*exp(-i*G3*z)+C'*exp(i*G3 *z) [2.8]
Where :
C =[4iGxB2exp(-(iG3+B4)d)(G3+iB4]/[(K o 3 )(J(G+iH))] [2.9]
C'=[4iGiB2exp(+(iG3-B4)d)(G3-iB4]/[(K o 3 )(J(G+iH))] [2.10]
J = [(-2/(K o3 ))(exp(iGx(d+a))exp(-B4d))] [2.11]
38 G= Gi[exp(B2a)X - exp(-B2a)Y] [2.12] H= Gi[exp(B2a)X + exp(-B2a)Y] [2.13] X= (G32-B2B4)(sin(2G3d))-(G3)(B2+B4)(cos(2G3d). [2.14]
Y= (G32+B2B4)(sin(2G3d))+(G3)(B2-B4)(cos(2G3d). [2.15] The ratio of reflected wave amplitude to incident amplitude is given by: A'/A = exp(2*i*4>). [2.16]
Where tan
P = (exp(B2a )X+exp(-B2a )Y ) [2.18] B = (exp(B2a)X-exp(-B2a)Y) [2.19] And the other parameters have been defined above. From the above, the ratio of the guided field to the incident field is a function of z whose maximum value squared can be considered to be a measure of the power coupled into the guide layer. It is clearly a complicated function of K,c and physically represents the familiar amplitude resonance and response (Midwinter 1970 (a)) as sociated with excitation of any forced oscillation as a function of incident frequency (in this case spatial frequency "Kx"). The corresponding phase curve of the reflected beam can be derived from the expression for tan(cj)) to yield the familiar phase cycle of 360° over the region of excitation. The "Q" of the resonance is deter mined by the strength of coupling between the resonating structure and the prism radiation continuum (or alterna tively the damping of the freely propagating guided mode) caused by the leakage of energy back into the arrangement used to perform the excitation - in this case, radiation damping into the prism.
39 2.3 Theory Of Prism Coupler Operation Using Modified Fresnel Coefficients An alternative exact analysis of the four media planar dielectric structure can be performed by using a technique of modified Fresnel coefficients (Ulrich 1970), which describe the reflectance and transmission of plane waves from a planar layered structure in which an inter mediate layer (2 ) is sandwiched between two semi-infinite media (1) and (3) as in Fig. 2.3(a). The transmission coefficient for a plane wave inci dent from medium (3) is denoted by T 3 2 1 and is a function of the thickness of the middle layer as well as the in dices of all three regions. This modification to the
simple transmission coefficient T33. effectively takes ac
count of the finite thickness of region (2 ) which gives
rise to multiple reflections in this layer.
T321 = h*(l+R32)(l+Rax)(exp(-ie12)/(l+h2*R32 *R2 i) [2.20] Where E12 = K0*Ei*S [2.21]
h = exp(iKoE2*S) E2 = \/(N2 2-£2 ) Ei = \/(Ni2-£2 ) [2.22]
And R 3 2, and R21 are the simple Fresnel coeffi
cients at the 3/2 and 2/1 boundaries. Considering the four layer structure of Fig. 2.3(b)
in which medium (1) also has finite thickness multiple
reflections in this layer need to be summed. A modified
Fresnel coefficient R x 2 3 at the upper boundary (1/2) is also required because of the finite thickness of medium
(2). A summation of all these reflected plane waves with
appropriate amplitudes and phase shifts gives an expres
sion of the total field in the guiding region (1) for any
direction of incident plane wave excitation.
40 Vi(x,z) = V3O)*U(£)*T32i(J3)*A(0,z)*exp(iko0x) [2.23] Where : V 3 (£) is the incident field.
U(J5) = 1/ [ l-RioRi2 3exp( 2iEx) ] [2.24] {represents a sum to infinity of multiple reflections}
A(£,z) = 2cos(koEiZ+Ex+Ei2-<{)io)*exp(i6i+iGi2-icl)io) [2.25] {describes the harmonic field variation in z direction}
Ex = Ko*Ex*W [2.26] The ratio of the field amplitude maximum squared which is a measure of the excitation is given by:
W1/W3 = [Nx 2Q3/4N32Qx ]*4||U(£)||2*||t 32x ||2 [2.27] The ratio of reflected field amplitude to incident amplitude is given by :
V^/V3 = {R3 2 +[h2 (l-R3 2 2 )(Raxo)]/[l+h2 R3 2 R2 xo]}* exp(iKo(£x+E3z)) [2.28]
This expression has modulus unity but a rapidly vary ing phase in the region of excitation. Computer models, one based directly on this analysis and one on the analysis of the previous paragraph yield guided wave field amplitudes and reflected wave phase shifts which are identical - the two methods can in fact be shown to be mathematically identical but the comparison is neither immediately obvious from the expressions, nor fruitful, nor necessary.
Another plane wave method of performing the analysis is by matrix tranformation of the tangential E and H field components across parallel planar boundaries (Hansen 1968); this technique is readily extended to any number of layers and also layers which may have complex refractive indices - it is the usual method of determining the per
41 formance of surface plasmon structures.
2.4 Representation Of Amplitude Field Solutions In The Wavequidinq Laver As A Resonance Phenomena The proximity of the prism can be considered as a perturbation on the isolated waveguiding structure which allows electromagnetic fields to exist in the waveguiding layer over a range of values of propagation mode index (NjpSinG) rather than just at a single discrete value. The closer the prism, the stronger the perturbation , the broader this range. Conversely, increasing the coupling gap between the prism and the guide, will sharpen the an gular resonant excitation to an arbitrarily narrow width - in the limit this leads to the isolated guide with a delta function in angular response. Fig 2.4 illustrates the guided power response as a function of the incident mode index excitation field and indicates the effect of in creasing the spacing layer thickness. Recalling the mechanism of sensing, a weakly coupled
(well isolated) waveguide structure will have a very high angular resolution for resonance position location com pared to the surface plasmon in which the angular width of resonance is determined by the material properties of the metal (in particular the imaginary part of its permittivity) which are responsible for the absorption dip. Angular resonance widths for prism coupler excita tion from early integrated optics experiments are of or der 0.1 degree (probably limited by the laser beam angular divergence) whilst surface plasmon widths may be 0.2 - 2.0
42 FOR INCREASING GAP THICKNESSES degrees.
A device utilising the angular shift in the resonance position has a sensitivity determined both by the mag nitude of the shift produced and the resolution of the smallest measurable change. For the dielectric guide there is no limit to the sharpness of the angular response except that caused by fabrication complications such as absorption, scattering, or non-uniformity of the films.
2.5 Excitation By A Beam Of Finite Width In his paper, Midwinter (1970 (a)) considers the ef fect of reflecting a gaussian beam from the four layer ideal dielectric structure which he previously subjected to plane wave analysis. For a single dielectric boundary there is a displacement of an infinite plane wave of order one wavelength (caused by the phase change at the boundary
- the Goos Hanschen shift). With the waveguide structure, multiple reflections allow the energy to propagate along the interface for much longer distances as the power of the localised beam gradually couples in and leaks out over a length determined by the strength of coupling.
Any beam of finite width contains a range of plane wave components - the interaction of any single component having been previously calculated. In calculating the resultant reflected beam the appropriate phase shifts are applied to the individual components and then these phase shifted plane waves are recombined into an output beam which has a spot profile with a non gaussian intensity distribution over the angular region of excitation of the waveguiding structure.
4 4 This redistribution of energy in the output spot physically represents the effect of part of the spot power coupling into the mode and then coupling out again. This has been demonstrated experimentally using reflection from a high index prism coated with a low index polymer spacing layer (PMMA), on top of which a second polymer guiding layer of higher index (Polystyrene) was fabricated. The reflected beam has a similar gaussian profile to the input HeNe laser source except over a narrow range of incident angles when the structure is excited. The output power distribution monitored using a CCD array at a distance corresponding to the intermediate field (70 cm) is shown in Fig. 2.5 - the orthogonal polarisation was observed to suffer no alteration in power distribution over the range of indicated mode indices. Either side of the resonance an unaltered reflected gaussian profile is obtained but over the region of excitation (determined by the laser beam bandwidth and the structural resonance width in propagation vector space) the spot appears to split into two parts as predicted by Midwinters analysis (1970 (a)). Any discrepancy between Midwinters theoretical plots and the measured profiles (in particular the observation that the dip never goes to zero) is due to the fact that the theoretical plot applies to the near field (i.e at the prism base) whereas the measurements were made in the in termediate field.
Midwinter (1970 (a)) also uses his plane wave phase shift analysis of the reflected finite beam to calculate the power build up in the guiding layer as a function of distance of propagation along the film for various
45 ►is- cn
Rg ZS Spatial Power Redistribution Of The Reflected Spot On Waveguide Structure Excitation
(TE Input Polarisation, 0.8 micron of PMMA and 0.37 micron of Polystyrene). strengths of coupling denoted by a parameter K. The power in the guide as a function of distance always attains a maximum but the height of this maximum is optimised for a particular value of K when approximately 80% of the power is coupled into the guiding region at a position around one and a half spot radii past the beam centre. For a fixed structure and wavelength the coupling parameter K determines the interaction length (beam width) needed for optimum excitation.
2.6 Physical Description Of Power Transfer
Ulrich (1970) takes his exact plane wave theory results and applies the approximation of weak perturbation of the guiding structure by the prism (h<
The functions Amax(£) and T32i(/3) which are slowly varying in £ are approximated to their values at £ = Nm :
47 T(£) ~> Tm (J3) = T3ai(N»)ABla*(Nm)Ua(P) [2.29] {Nm is isolated guide eigenmode index.} {The subscript "max" for A locates the peak value in the z direction.} {Um(£) is the Lorentzian approximation to the
resonance curve.} For a single infinite plane wave component V3(j3) the
(maximum) amplitude in the-waveguide film is given by: Vlx«a3c = v 3 (£)*t(£)*exp(iKo0x) . [2.30]
For any real beam the total field is simply an in tegral of the plane wave expression above. Applying the approximations made previously this yields :
= J* {V3 ( 0 )Um(Nn») Am<*3C(Nm)T3 21 ( 0 ) * exp(iK0x)d£} [2.31] The convolution theorem can be used to convert this integral into the form : Vimax(x) = x
KoKmTmm®~r V3(q)exp[iK0Gm(x-q)dq [2.32] • t
- 0 0
{Gm * (N»p+iKm)Ko} [2.33] Which illustrates how the field in this region at any positon "x" results from the cumulative build up of power
from all previous values "q" less than "x". A similar integral expression can be obtained for the plane wave recombination of the reflected beam at any position along the interface:
x
Va- (x ) = R32[V3 (x)-2KoKxn*J V3 (q)exp(iKoGrti(x-q)dq] [2.34]
-co
The first term identifies the direct reflection from
48 the prism base and the second the energy which has entered the guide at earlier positions and is subsequently leaking back into the prism. An alternative differential formulation describes the power flow at any position along the interface plane in terms of the fields at that position only:
d||Vx(x) [2/dx = 4*C*D«*|V3 (x)||Vx(x)|sin(Wx-W3+<|>3a) - 2h2Dm |Vx(x) || 2 [2.35]
Where Dm = Ko2/h2 [2.36]
C = (Tm— *h*exp(i(|)32) )/2i [2.37]
(j)3 2 = tan-3-(E2 /iE3 ) {TE> [2.38] In this case an expression for the rate of increase
of guided power with distance also contains two terms. The first represents travelling wave interaction between the
incident radiation field and the guided wave field and in
cludes a parameter representing phasematching between the
two wavefronts. The second represents a loss due to
leakage of power from the guide. The maximisation of the
phase term involves adjusting the incident plane wave
direction accordingly so that the multiple reflections of
power in the guide are in phase with the incident plane
wavefronts after reflection in the guide and a maximum in
phasor addition is obtained. If the incident beam does not have plane wave phasefronts at the prism base the curva
ture of the incident fronts mismatch the plane wavefronts
of the guided field and the power transfer is reduced.
The term representing leakage of power defines an
attenuation constant Km:
dP(x)/dx = -2*Km*Ko*P(x) [2.39]
49 Km*K o represents the nominal bandwidth of the waveguide resonance and the corresponding coupling length is given by :
Lxn = 1/ (Km*Ko ) [2.40] For. optimum coupling of power a finite beam width projected onto the interface which has a length of order
Lm is required (or a beam with a spread in propagation component along the interface direction of order 2*Km*Ko). If the coupling between the guide and the prism is weak, a long interaction length is required to build up the power in the guide but there is a limit to the increase in cou pling obtained by increasing this length because as more power is coupled in at the far end of the beam more of the power already in the guide has leaked out. The interac tion length must be long enough to build up power in the guide but not so long that the power has entered and also leaked out. An alternative interpretation of this is that optimal power transfer requires a matching of the bandwidths of the incident radiation field and the per turbed waveguide structure. The calculated optimum power transfer (80%) occurs for Lm -beam width, variation by a factor of five between these two lengths still affords ap preciable coupling (40%) - indicating that the excitation is relatively insensitive to matching of Lm and beam width.
2.7 "Leaky" Mode Field Distribution Matching
We can further appreciate the above observations by reversing the direction of power flow in the arrangement of a terminated conventional prism coupler - an "output
50 coupler" - such as the right hand prism of Fig. 1.1(c). The incident wave propagating in the isolated "free" guide meets a step perturbation due to the presence of a prism above the guide separated by an evanescent coupling gap of order of fraction of a wavelength. Leakage of power begins abruptly at this position and, provided the pertur bation is small, the propagation direction of the radia tion field corresponds to a mode index very nearly equal to that of the guided mode for the isolated guide. After the abrupt initial leakage the power loss from the guide decreases exponentially as the rate of loss is propor tional to the power remaining in the guide. This gives rise to the leaky beam profile indicated in Fig. 1.1(c) whose central mode index is approximately equal to that of the guided mode and which has a bandwidth determined by the strength of perturbation. This output power coupling is obviously 100% efficient (provided a sufficiently long prism is used).
A reversal of the direction of power flow in the above description corresponds to the situation of waveguide mode excitation considered in this chapter. If a beam of the above profile were incident from the prism it would be completely coupled into the guided mode. A gaussian beam is an approximation to the required leaky mode field if it has the appropriate direction of beam axis and correct nominal bandwidth, but the fundamental mismatch in field shapes will mean that 100% power cou pling cannot be achieved.
A first order improvement in the field shape matching involves tapering the gap linearly (Tangonan 1977, Tien
51 1971) so that the initial coupling at the edge of the prism is weakened producing a less drastic initial field leakage and an output beam profile somewhat more symmetri cal and closer to a gaussian. This arrangement implies theoretically a coupling efficiency of 96%, experimentally 88% was recorded for an incident input gaussian beam. The problem of optimum coupling thus amounts to matching the spatial amplitude distribution of the input beam to the leakage field of the coupler (Ulrich 1971).
An exact calculation of the required spacing gap profile to achieve 100% coupling could therefore be made by maxi mising the overlap integral of the incident gaussian radiation field and the time reversed leaky mode field shape (Tien 1971, Wang et al 1983).
A similar analysis to all of the above could be per formed for the analogous case of an incident radiation field coupling to a waveguide via a periodic surface grat ing perturbation (rather than a prism).
2.8 Prism Coupler Experimental Observations
The theoretical performance of the evanescent cou pling arrangement described in the previous section has been demonstrated experimentally in many early experiments in integrated optics (Tien et al 1970, Harris et al 1970,
Tien et al 1969). The following general observations are made for an arrangement involving a non-terminated prism, an air spacing layer, and an evaporated waveguiding layer: 1/ The structure is only excited over a narrow range of directions of the incident laser beam; and then only strongly excited if the coupling gap is appropriately ad
52 justed. 2/ Excitation is indicated most obviously by light
scattered from the structure into:
(a) The radiation continuum of the substrate.
(b) Azimuthally degenerate modes of the same order as the one excited. (c) Other modes of the structure at all their degenerate azimuths.{"M-lines"} 3/ Because of the loss of power from the excited mode by scattering the relected spot has a dark
band running horizontally across its width.
Observations 2/ and 3/ rely on strong scattering by the evaporated film and are not apparent when polymerised organosilicon films (Tien 1971, Tien et al 1972, Ulrich et al 1973) or solution deposited polymer films are used in stead because these films have such low loss (0.04dB/cm and 0.2 dB/cm respectively) compared to evaporated films
(loss> 5dB/cm). In this case the waveguide excitation is really only readily apparent from the streak of light coupled into the guiding film from an edge teminated prism coupler. With a non-terminated prism there is hardly any reduction in the reflected spot intensity and the mode lines are very faint.
2.9 Device Operation
The above theoretical description of prism-coupler operation and the expected experimental observations form the basis of the optical sensing device to be fabricated. The evanescent field of the excited waveguide mode is used to measure surface changes via shifts in the position of
53 optimum excitation of the structure.
54 3. SILICON OXIDE WAVEGUIDE STRUCTURES 3.1 Initial Choice Of Thin Film Structure
Recalling the general structure of the proposed device (Fig. 1.3) the intention to exploit the resonance sharpness of the prism coupling mechanism, the description of a similar structure used by Midwinter et al (1970 (b)), and the previous comparison with a water clad Surface Plasmon Resonance (SPR) the following thin film arrange ment is proposed initially:
Np = 1.50 Nb = 1.38
Ng = 1.45
Nc = 1.33 W = 0.20 micron
These parameters suggest a typical index for the glass prism, a spacing layer of Magnesium Fluoride (MgF2), a doped silica guiding layer and an aqueous cladding. A comparable sensitivity (for TM mode excitation) to bulk index changes as that obtained with an aqueous clad SPR is possible if a spacing layer of 3.5 micron is used (Giraud
1986). Where sensitivity is nominally defined as the in verse of the smallest bulk index change which displaces the resonance dip by one bandwidth.
The guide material used by Midwinter (Thorium Oxyfluoride - 0.36 micron) had a rather ill-defined index so a doped silica glass is suggested as a sensible alter native. The author attributed good results to the use of an evaporated spacing layer of MgF2 (0.42 micron thick) and, considering the latters reputation of suitability for evaporated films, seems the best choice for the low index
5 5 spacing layer. These chosen values would indicate the evaporation of
MgF2 onto a soda glass substrate with a subsequent 2000 A coating of silica and an aqueous cladding. Anti reflection coating layers and interference thin film fil ters have thicknesses of order of quarter wavelengths i.e a few thousand angstroms - fabrication of a silica layer of this order of thickness (by sputtering) would be per fectly feasible. However a 3.5 micron layer MgFz is an order of magnitude larger than typical deposited films of this material and may thus present some fabrication problems.
3.2 Substrate Preparation
The usual method of glass cleaning involving chromic and sulphuric acid etching seems somewhat too drastic for present purposes considering the importance of microsopic evenness for waveguide fabrication. Instead the soda glass microscope slides (n=1.5104 at HeNe from an Abbe refractometer measurement) were cleaned before deposition of MgF2 using the following procedure which is similar to that described for substrate cleaning for integrated op tics films (Tamir 1979), and commercial thin film vacuum deposition (Holland 1956):
A pre-wash with "teepol" detergent in cold water with a soft nylon brush and cold water rinse to remove bulk in organic contaminants. (This step is found to be indispen- sible - if it is omitted inspection of the slide shows up considerable inorganic residue and dust, on a scale com parable to that of an uncleaned slide, even though all
56 subsequent cleaning steps have been used.)
A cold water wash in dilute "micro" detergent for 15 minutes in an ultrasonic bath to remove the remainder of the inorganic contaminants. Subsequent flushing with hot water to remove excess "micro", then cold water to remove the hot water dissolved contaminants and then distilled water to replace the cold water. The slides are then flushed with methanol to remove water and with IPA before being introduced into an IPA reflux degreaser for 30 minutes to remove organic residue. The slides give a uniform breath figure, have a high coefficient of friction and only specks of dust are ob servable when viewed in transmission using a microscope illuminator, indicating that they are clean to the same degree as the substrates used in the above references. The most immediately obvious contaminants on the glass surface are particles of dust which are very dif ficult to remove, as any liquid cleaning agent will either contain dust itself or attract dust onto the substrate whilst drying, even in the atmosphere of a clean room.
Blow drying in nitrogen is not effective in removing any thing but the largest particles and use of "opti-polymer" coating solutions (which solidify into a film around the dust particles and are then peeled off) cause the surface to charge immediately on stripping and hence dust rapidly reaccumulates. In retrospect, the absence of detail on dust particle removal in the above two references suggests that any problems which may result are not severe enough to merit the effort of removal. This is particularly so
57 for the presently intended design because the MgF 2 is a buffer or spacing layer rather than the guiding film it self - consequently concern with substrate surface defects is not as important as it would be if the waveguidimg films were fabricated directly onto the substrate. Indeed in the above reference (Tamir 1979), the author actually cites growth of a low index buffer layer prior to waveguide deposition as a means of avoiding surface layer defects * The author notes also that this pre-coating layer itself may be the cause of irregularities for ex ample due to crystal grain boundaries.
As an illustration of low loss waveguiding films deposited directly onto ordinary microscope slides which have not been cleaned any more thoroughly than in the above process; plasma polymerised VTMS films 1 micron thick can be fabricated which have losses of only 0.04 dB/cm (Tien et al 1972). The excited modes are visible via scattering due to film imperfections and dust par ticles on the surface of the glass but obviously the ef fect of surface dust particles even with the guide deposited directly onto the glass substrate with no spac ing layer is negligible compared to scattering caused by evaporated crystal grains (greater than 5dB/cm).
Similarly Weber et al (1975) have fabricated monomode dip coated PMMA, Polystyrene, and photoresist films directly onto microscope slides with loss of order 0.2dB/cm and Ul rich et al (1972) using a similar cleaning technique ob tained solution deposited guides of » 1 micron with loss below ldB/cm.
58 Residual surface contaminants condensed from the at mosphere or remaining after cleaning can be removed from the substrate prior to deposition either by raising the temperature of the base material in vacuo or exposing the receiving surface to bombardment of high velocity ions
(Holland 1956). Once the buffer layer has been evaporated under vacuum, exposure to the atmosphere for a period of a few minutes is unlikely to cause gross surface contamination,
though it may cause dust particles to accumulate between
the buffer and guide layer.
3.3 Magnesium Fluoride Deposition
A standard vacuum plant is used for the deposition
of MgF2 and other subsequently evaporated films. The sub strate to be coated is supported on two glass slides so
that the source produces a step at the edge of the film
which can subsequently be measured by a surface stylus in
strument .
MgF2 crystals with size of order a few mm are placed in an open boat of thin molybdenum sheet metal designed to
be somewhat narrower at its centre than at its ends where it connects with the low tension terminals of the coating
unit. The chamber pressure is lowered to 75*10-3 torr by a roughing pump then by a diffusion pump to an evaporation
pressure of order 10-5 torr. In view of the previous remarks on the need for
cleanliness at the interface between the glass substrate
and the spacing layer and the good adhesion and thicker
MgF2 films which can be obtained by substrate heating (to
59 250 °C) - ionic cleaning is considered time-consuming and unnecessary. At power inputs of order 500 W for boats of dimen sions 3.0cm*2.0cm* 1. 5cm the MgF2 slowly melts and evaporates at rates of order 10 - 20 A/s. The deposition is monitored by a quartz crystal monitor placed adjacent to the substrates being coated, for which addition of material on the crystal surface causes a shift in the electromechanical oscillation frequency. It should also be noted in passing that the fact that the crystal and the substrate have different surface microstructures and may be at different temperatures may cause the rates of film deposition on them to differ, which can lead to deposition calibration problems. Although the crystal documentation indicates that a frequency shift of order of 60 KHz can be accomodated it was found that the monitor ceased to func tion at MgF2 film thicknesses which caused shifts of only a few KHz - which meant that the crystal needed to be cleaned after each deposition.
Without resorting to a full investigation it would seem likely that the cause of this difficulty is the ex ceptionally high stress build up on the surface of the crystal due to the MgF2 film, which may move the electromechanical resonance out of the oscillation bandwith - this would also account for the return to functioning after cleaning. Commercial deposition of this material is generally monitored optically by interference techniques and usually for quarter wavelength layers of order of few thousand angstroms so there is no great literature on the subject of deposition of thick films of
60 MgF2 onto quartz crystal oscillators. Above thicknesses of ^lOOO A most dielectric films exhibit columnar growth with column diameter ~100 A. The spaces between these columns are porous and the packing density of the film is governed by,the relative volume oc cupied by the columnar grains and the boundary material, being generally higher for evaporation onto a heated sub strate because this promotes the growth of crystal grains.
Moisture absorption into the pores on exposure of the evaporated films to the atmosphere is responsible for an increase in refractive index, as well as the stress relief which occurs (Ennos 1966). The same process also accounts for the change in crystal monitor reading when the vacuum system is let up to air as the film mass on the monitor increases slightly as water vapour is absorbed. The visible indices of dielectric films are usually quoted to
0.01 implying that variations caused by alteration of deposition parameters make definition of film indices to greater precision than this unfeasible. MgF2 films are polycrystalline, with a grain size and (consequent durability) determined by the substrate temperature during deposition, the rate of deposition and the thickness of the film. In relation to the present device this grain size will affect the amount of scatter ing caused by the unevenness of the interface of the spac ing layer and guiding layer on the scale of optical wavelengths - the theory of which is somewhat involved and not particularly profitable to investigate (Tien 1971).
On an unheated substrate films of thickness greater
than 0.5 micron are liable to cracking under tensile
61 stress irrespective of the deposition rate (though faster rates do reduce the latter somewhat - Ennos 1966). Rais ing the substrate temperature to 300°C both improves the durability of films below 0.5 micron thickness (Holland 1956) and allows films as thick as 1.2 micron to be formed without surface fissures. These observations are accounted for largely by the effect of the substrate temperature on the packing density of MgF2, which increases with tempera ture because of grain growth. A film deposited onto a room temperature glass substrate has a packing density of only 0.85 whilst deposition onto a substrate heated to
300°C raises the packing density to 0.96 which accounts for the 11% difference in thickness between these two films (Ritter et al 1969).
Even the use of a heated substrate (250°C) does not enable the required 3.5 micron spacing thickness of our initially proposed device to be attained. This substrate heating during deposition also promotes growth of crystal grains to sizes of order of film thickness which can cause increased scattering - too much of which may seriously in validate the theoretical model which assumes lossless films. It is apparent therefore that an alteration of the required structure is needed which will allow the use of a spacing layer of MgF2 much less than 3.5 micron thick.
3.4 Reviewing The Design Of The Structure
The requirement of such.a thick spacing layer is re lated directly to the specification of weak coupling be
tween the prism and the waveguide structure. For a chosen
spacing thickness and index the isolation of the guide
62 from the prism is determined by the mode index of the
structure because this fixes the transverse decay constant
B2 = Ko*V’(Kx2-N2 2 ) . One way of raising the value of this mode index is to increase the thickness of the guide
thereby confining more power to this layer - but this also means less power in the sensing cladding on the other side of the guide and so is self-defeating. Combining these
two intuitive observations suggests increasing the mode index instead by using a guide of higher index and cor
respondingly narrower width to compensate for the field confinement. It is necessary in any case to use a guiding
structure with mode index below that of the glass sub
strate (1.5104) in order for the excitation to be acces
sible. A guide of index 1.9 and thickness 700 A (for the
single available mode - the fundamental TE) would need a
MgF2 thickness of only 0.8 micron for the same mode index width as the initially proposed dielectric structure.
An investigation of readily available high index
materials for evaporation suggests Zinc Sulphide or
Silicon Oxide as suitable candidates. The common usage of
Zinc Sulphide in optical coatings would imply that it was
the most suitable, but after some initial difficulties in
attempting to deposit it from open boat sources deposition
of films of Silicon Oxide from a special "baffled box" source was attempted instead. Silicon Oxide is used as a
protective coating for Aluminium mirrors, is smooth , amorphous , and non-hygroscopic. One consequence of choosing a waveguide layer of much higher index than the
substrate prism is that if too thick a film is fabricated
the fundamental mode may have higher mode index than the
63 refractive index of the substrate and so be inaccessible.
A guide thickness of 1080 A for guide index 1.91 and clad dings of 1.38 and 1.0 gives a mode index of 1.5104 so films somewhat thinner will be required. Although a glow discharge facility is available for substrate cleaning before Silicon Oxide deposition, it is
neither necessary (because stable films are obtained using
heated substrates) nor is it convenient for the following
reasons. The chamber pressure needs to be raised to order of
10“3 torr to strike the discharge and if at the same time
the substrate heater filament is at temperatures of order 400 deg C this will cause the heater filament to oxidise.
It is known from previous observations with other
materials (notably Zinc Sulphide - Hunter et al 1978, Rit
ter et al 1969) that the cleaning of nucleation sites is
only effective for 10 or 15 minutes under vacua of 10_s
torr. Since pumping from 10~3 torr to 10~s torr requires such a time period and reheating of the substrate will
require a delay of a similar magnitude, a choice between
substrate cleaning and substrate heating must be made.
3.5 Deposition Of Silicon Oxide
As in the case of MgF2 the physical properties of
this film are sensitively dependent on the deposition parameters; in addition, for Silicon Oxide the
stoichiometry of the film is itself variable as the
process of deposition of pure Silicon Monoxide is compli
cated by oxidation of the film by residual gases in the vacuum chamber (especially oxygen and water vapour). A
64 review of previous work on thermal deposition of this material can be quite confusing as the resultant film depends on the relative importance of deposition parameters in each individual reference.
3.5.1 Summary Of The Reviewed Literature
On Silicon Oxide Deposition The following three parameters are critical in deter mining the properties of deposited Silicon Oxide films :
1/ The residual gas pressure in the system
(especially water vapour and oxygen).
2/ The rate of deposition.
3/ The temperature of the substrate.
Several authors indicate how a film of almost pure
Silicon Monoxide (visible index » 2.0) can be obtained on an unheated substrate by rapid (10 A/s) deposition at low pressure (2*10-6 torr), whereas low deposition rates
(lA/s) and high residual pressure (10“4 torr) yield films approximating to amorphous Silicon Dioxide (Holland 1956,
Priest et al 1962, Ennos 1966). However the best qualitative illustration of these observations and the process responsible is given by Hill et al (1967) who demonstrate that it is fundamentally the ratio (N) of the rate of impingement of Silicon Monoxide molecules to that of residual gases which determines the stoichiometry of the film - water and oxygen are almost equally efficient in promoting the higher Oxide and the former is the main residual gas in a continuously pumped vacuum system. For values of N>asl (requiring pressure as low as 2*10_s and rates of lOA/s) films of almost pure Silicon Monoxide are
65 obtained which appear yellow in transmission due to U.V absorption resulting from a slight excess of Silicon. For N<=10“1 transparent films of composition approaching Silicon Dioxide are obtained. (This chemical combination of residual water vapour and oxygen into a Silicon Oxide film is actually used as a "gettering" mechanism in some vacuum systems (Priest et al 1962). Accompanying this variation in stoichiometry is a stress reversal between the limits of pure Silicon Monoxide which (like most other Physical Vapour Deposition dielectrics) has a slight ten sile stress and Silicon Dioxide which has a much stronger compressive stress. For sufficiently thick Silicon Dioxide films (high enough shear force) and for low substrate adhesion (poorly cleaned substrate) the film is likely to buckle spontaneously under compressive stress. Incom pletely oxidised films are susceptible to oxidation on ex posure to the atmosphere as evidenced by an additional compressive stress component and an increased transparency.
A second consequence of high residual gas pressure is that the higher Oxide film produced is loosely packed and therefore porous - this effect which is common to vacuum deposition of most other dielectrics at high residual pressure is generally attributed to very low incident vapour velocity at the substrate due to scattering and consequently reduced likelihood of the molecule finding a low potential energy binding site. This porosity is im portant because it determines the ease of further oxida tion due to water vapour and oxygen adsorption - Silicon
Monoxide films are densely packed and also tend to form a
66 surface protective layer of Silicon Dioxide. They are therefore difficult to oxidise further whereas films with 0:Si ratio >1 are easily oxidised further to Silicon Dioxide (especially if less than 1000 A thick). The additional complications due to substrate heating have been investigated by Panasenko (1979), who reported oxygen deficiency, porosity, and water absorbtion in at tempting to deposit Silicon Dioxide films on a substrate at 350°C, using a residual oxygen pressure of 5*10-4 torr concluding that deposition of dense Silicon Dioxide films would require, a low substrate temperature as well as a high residual oxygen pressure. This same conclusion was reached by Haas et al (1967) who noted that the ef ficiency of Silicon Monoxide gettering of Oxygen is much reduced by an increased substrate temperature.
3.5.2 Experimental Deposition Results
Having carefully documented the previous observations of deposition of Silicon Oxide the results obtained in the present experiments can be explained consistently.
The availability of a second set of terminals in the vacuum system allows the waveguiding film to be deposited on top of the MgF2 spacing layer without having to open the system to atmosphere. The increased stability of Silicon Oxide films when deposited onto a heated substrate makes ion bombardment cleaning unnecessary (and incon venient as noted above). Substrate heating is also slightly facilitated in any case because it can be con tinued from the previous MgF2 deposition.
67 The boat required for Silicon Oxide deposition is of the "baffled box" construction. Such an arrangement is needed for materials which evaporate by sublimation and are consequently prone to "spattering" from a conven tional boat source because of preferential heating of the material in the bottom of the boat. With a "baffled box" construction the evaporant is placed in the bottom of the boat either side of a chimney and is only able to escape from the source by following an indirect path so that the source material does not have a direct line of sight to the substrate - the same basic principal but using a dif ferent geometry is indicated in an early paper on the sub ject of Silicon Oxide spattering by Singh (1978). The lowest pressure available in the vacuum system used was 10~s torr and deposition rates of Silicon Oxide were never greater than 4A/s - the films obtained on un heated substrates (600 A to 3000A) were quite unstable, buckling in places under compressive stress and removed from the glass or MgF2 substrate by water droplets placed on the surface. The mechanism of this buckling process is probably as follows.
Water is readily absorbed into the film because the depostion conditions encourage porosity, as noted above. As well as increasing the stress of the film the water molecules penetrate via the pores to the interface of the film with the substrate. This causes a drastic reduction in the adhesion there and allows the film to detach itself from the substrate. As the water spreads over an increas ing area the film wrinkles but is able to hold itself together because the stress is compressive. The effect
68 also occurs spontaneously at the edges of the slide where water vapour is most likely to enter the interface of film and substrate and also at isolated points in the centre of the film which probably locate unusually large pinholes due to the presence of surface dust particles prior to deposition. Comparing the above experimental values of deposition rate and residual pressure with those required to obtain the Monoxide (according to the above reference) it is ap parent that the pressure used is some 5 times higher and rate some two times smaller, implying a value of the ratio N one order of magnitude too small to yield Silicon
Monoxide. This rough calculation, the measured index of
«1.65 (ellipsometer), and the observation of compressive stress instability all support the conclusions of the pre vious paragraph (3.4) - that the stated deposition condi tions will yield a porous, compressively stressed, inter mediate Oxide.
In an attempt to improve the stability of the films, substrate heating is continued after MgFs deposition and the slide kept at 250°C during the Silicon Oxide deposi tion. The resulting films are noticeably more absorbing for equivalent thicknesses than those deposited on room temperature substrates, have higher indices ~ 1 .9
(ellipsometer), will adhere well even to uncleaned glass substrates and are extremely resistant to removal from the substrate even on addition of a surface water droplet. In fact a 600 A film deposited on a lead glass slide boiled in 0.1 M hydrochloric acid protects the lead glass even though the surface of the exposed substrate has been sub
69 stantially converted to white lead chloride powder. It is noted also that in the same deposition, films on the
shielding beaker whose surface is not heated buckle on ex posure to atmosphere. Again the observations of the pre vious paragraph provide a wholly consistent explanation -
the high substrate temperature dissociates any excess
oxygen which may be temporarily combined so that a film of
reasonably pure Silicon Monoxide is formed. Such films have very slight tensile stress, densely packed structure
and for thicknesses below 1000 A, the shear force is very small. In addition, heating the substrate aids the adhe
sion of most evaporated films as noted earlier. From measurement of the mode indices of excitation of
these films when clad with air and water the value of the
refractive index of the film can be estimated. It was
discovered that the latter decreased from an initial value
of 1.9, to 1.85 after 3 months and to 1.72 after 6 months
indicating subsequent oxidation but on a long timescale. Films which exhibited strong coupling in initial experi
ments were found to give only weak response after being left for one year because the mode index of the structure
had fallen so drastically. It is likely however that this
occurs unevenly through the thickness of the film with
oxidation of outer layers occuring initially and leading to a lower subsequent rate with the interfacial film
responsible for adhesion, altered least of all. This in homogeneity in film refractive index immediately in
validates our step index profile and was probably the ex
planation of some inconsistency in the refractive index
determination discussed above. In addition the variation
70 in time of this effect suggests that another material might be more suitable.
3.6 Excitation Of The Guiding Structure
3.6.1 Experimental Aranaement The waveguiding structure previously prepared on a soda glass slide is used in the experimental arrangement indicated in Fig. 1.3 where the slide can be considered
simply as an extension of the prism by contacting the sur faces with a liquid of index greater than both glass sur faces (Pockrand 1977). It is much more convenient to coat
a microscope slide which can later be discarded than to coat the prism itself from which the films then need to be
removed each time. Apart from being time consuming and
destroying the fabricated structure this is also likely to
cause damage to the prism surface. The indices of the prism (Np) and the slide (Na), at wavelength of 0.6328
micron, were measured on an Abbe refractometer to an ac
curacy of 10~4 : Np = 1.7220 ; Ns = 1.5104.
The incident HeNe laser beam has a 0.5 mm waist at
the laser output mirror, impling a near field range of Zo
= 30cm and full cone angle at 1/e2 in power of 16*10-4 rad. The upper side of the microscope slide is some 25 cm from the laser aperture so the beam has diverged slightly
before reaching the guiding structure. The only refrac tion which will possibly alter the plane wave spectral
width of propagation constant along the interface is that
at the incident prism face - all refractions through the
parallel layers preserve the mode index component parallel
71 to the layers. For normal incidence onto an equilateral prism the effect amounts to an increase of the width from
8*10-4 to 9*10~4 . At incident mode index approaching 1.38 the width is increased to 10*10-4. For quantitative measurements the angle of incidence of the beam is determined by first initialising the zero of rotation by retroreflection of the beam - corresponding to incidence at nominal mode index Njp*sin(60°) onto the waveguide structure (since the prism is equilateral). The accuracy of location of retroreflection and centre of ex citation are typically 0.01° i.e of order 10-4 in mode in dex (Kersten 1975 (a)).
Rotation of the prism causes the position of the spot to move during scanning between the retroreflection in itialisation position and the angle at which excitation occurs - this may amount to a millimetre or so. However for the purposes of qualitive observation this is of no concern because the excitation resonance occurs over a much smaller angular range so that the iluminated region is essentially fixed. If calibration and quantitative measurement are required it is possible to offset the prism axis of rotation to reduce the movement of the spot
(Ulrich et al 1972). A hemispherical prism although ap parently more convenient than a triangular prism is not generally preferred, partly because the invariant location of the refracted spot at the centre of the semicircle is critically dependent on any lateral beam displacement
(Ulrich et al 1973) and also because any advantage to be gained is minimal.
72 3.6.2 Experimental Observation
As noted in the references of chapter one, the ex citation of the structure is most apparent from the scat tering into the radiation continuum out of the films as well as in the appearance of the mode line (the structure has only one resonance) and corresponding reduction in the intensity of the spot. The centre of the resonant excitation is located by rotating the prism until a dark band lies across the centre of the output spot Fig. 3.1(a) (this photograph and all subsequent ones are in negative because the beam was allowed to fall directly onto photographic paper).
The reduction in reflected intensity, scattering due to coupling into the guiding structure and the appearance of mode lines either side of the specularly reflected spot observed with the indicated arrangement are the same ef fects as those observed in early integrated optics experi ments using conventional prism coupling through an air gap into a film deposited onto a low index substrate (Tien et al 1969). These effects are only observed on regions of the slide where the two deposited layers overlap which demonstrates that it is the waveguide structure that is being excited. This conclusion is further supported by the fact that maximum scattering is obtained when the in cident beam is completely polarised in the appropriate direction (TE in this case). Fig. 3.1(b) illustrates how the scattered light measured above the top of the films is found to complement
the reduction in intensity of the reflected beam. The
73 £ * £ £ -» 1.4249 1.4257 1.4265 1.4271 M O O E M E X U24Z
FIG 3.1 (a) MOVEMENT OF LO W INTENSITY BAND ACROSS THE OUTPUT SPOT
Fg ai 74 structures parameters are indicated in the figure and the incident radiation was completely TE polarised. The spac ing layer thickness indicated (0.52 micron) produces a resonance width almost three times that of the laser. The mode index width of the structure is greater than the in cident laser bandwidth yet only part of the incident beam is accepted by the structure as the dark band only oc cupies part of the spot width. This represents the field mismatching of the gaussian beam to the required leaky mode field distribution for 100 percent coupling as well as mismatching due to film unevenness. For a structure with a much narrower width (of order 10-4) the coupling is much weaker although the scattering and mode lines are observable. Curve "A" of Fig. 3.2 in dicates how the reduction in reflected intensity for this structure is negligible when the incident polarisation is completely TE. Curve "B" illustrates that using parallel polarisers inclined at 45° to the film normal produces a measurable reflection dip. The width is however some 30 times broader than expected - this is attributed to unevenness of the film. The observation that the band orientation is not al ways observed to be parallel to the mode lines is an in dication that thickness variations across the area il luminated by the spot are large enough to alter the cou pling angle. The effect was not observed in the papers referenced above because the guiding layer was much thicker, the mode much better confined and consequently the mode index was much less sensitive to thickness varia tion . 75 N g id e ; 1.94 Spachg TOctaess : Q91 rriancn WgMde:83QA Ncfadfcg:tGO 0 .. -i .. - ,-j------1------1 1------1------1------1----- 1.437 1.438 1.439 1.440 1.441 1.442 1.443 1.444I4CDENT MODE NDEX Fig 3.2 Reflected Output Intensity For An Evaporated Structure With Theoretical Mode Index Width 10-4 The theoretical treatment assumed a lossless struc ture but the above observations are actually only apparent because of the scattering of light by the crystal grains in the film. An important consequence of the presence of scattering (or absorption) is that it will cause the resonance to be broadened. Indeed Tien (1971) suggests that the angular width of a well confined waveguide with very weak prism coupling can be used to determine the residual width due to scattering (and absorption) in the film. With the present Silicon Oxide structure it appears that the resonance width is limited by film unevenness before sufficiently weak coupling is attained because the mode index is such a sensitive function of guide thick ness. Location of the centre of the resonance to a few times 10-4 in mode index would require loss less than 80dB/cm (Ulrich et al 1973). The lower limit of the loss in the structure can be estimated by determining the frac tional power loss over a distance of the order of the projected beam width. Typically intensity reductions of the total spot are of order 50% which amounts to 15dB/cm, this is illustrated in Fig 3.3. The total energy in the beam is represented by the area under the curves and the effect of excitation on the reflected spot profile is to reduce this to 50% as well as to cause a lateral shift of the distribution of order of half a beam width. Using an aperture to select a narrow range of plane waves inside the beam spectrum (Fig. 3.1) leads to reduc tions to 3% (over a distance of 2mm) corresponding to a loss of 75 dB/cm for those plane wave components which en ter the guiding structure. If the scattering is con- 77 OUTPUT INTENSITY (ARBITRARY UNITS) Wvgie tutr Prmtr O Fg 3.1) Fig Of Parameters Structure (Waveguide i 33 elce So Poie n Excitation On Profile Spot Reflected 3.3 Fig f Satrn Wvgie Structure Waveguide Scattering A Of 78 sidered as contributing an imaginary part to the mode in dex of "a" the full power bandwidth at half maximum is of order "2a". The above value of loss then indicates a mode index broadening of the resonance of order "2a" = 2*10~4 - which limits the narrowness of the resonance width. The ability of the structure to detect surface changes is illustrated qualitatively by setting the waveguide structure to optimum excitation (maximum scat tering and the dark band at the centre of the reflected spot) and depositing a drop of acetone solvent onto top film. The scattering ceases and the black band disappears for a period of a few seconds after which time the solvent evaporates and the effects reappear without any noticeable hysteresis. Similarly condensation onto the surface of a visibly undetectable water vapour layer destroys the cou pling until the layer has evaporated an reestablished the initial conditions. 3.7 Expected Observations For Ideally Flat Films The 0.5 mm beam waist of the HeNe laser at its output mirror implies a far field angular divergence of order 0.1° and an incident mode index spread (inside the cou pling prism) of order 8*10-4. If this radiation field ex cites a highly lossy waveguide structure with a narrower mode index spread, only a band of the incident plane wave components is accepted and scattered out so the output spot contains a horizontal band of low intensity. The an gular tilt needed to scan the band from the top of the spot to the bottom should correspond to the angular width 79 of the incident beam. If the bandwidth of the incident beam is reduced, the region of low intensity would occupy more of the reflected spot. When the bandwidth of the in cident beam is less than the mode index spread of the structure the beam intensity of the whole of the reflected spot is reduced over an angular width determined by the resonance of the structures. Finally if the films are even across the substrate plane, lateral translation of the slide or rotation about an axis normal to the film planes should not affect the output reflected spot. Any changes observed by the above operations must therefore be attributed to non-uniformity of the structure across the plane of the films. 3.7.1 Focusing The Beam Even with a raw laser beam there is sufficient film unevenness transverse to the beam propagation direction over the illuminated area to obtain an output spot with a vertical band Fig. 3.4(i). Focusing on to a smaller area will cause the band to become more nearly horizontal be cause the the spread in mode indices due to non- uniformities is reduced as the spot illuminates a smaller area. This increased evenness as a result of focusing onto a smaller area is also evidenced by the horizontal orientation of the band irrespective of rotation of the slide or the location of the spot on the slide Fig. 3.4(ii). Another manifestation that the unevenness of the illuminated area is not limiting the interaction is that the required angular tilt to scan the band of reduced in tensity from the top of the spot to the bottom is not 80 i* -9 0 * -9 0 * - 4 5 * -4 5 * c 0 * 0 * 4 5 * 45* * % * 4 90* 90* ■ m (0UNFOCUSED 00 FOCUSED Fig 3.4 Appearance Of The Reflected Laser Spot At Resonance For Focused And Unfocused Beams At Various Azimuths (Parameters Of Fig 3.1) 81 greater than the angular divergence of the beam. 3.7.2 Expanding The Beam Expansion of the spot using a beam telescope enhances the effects from systematic non-uniformity across the plane of the waveguide structure. The cause of the latter is either gradual changes in thickness or in refractive index, rather than point irregularities such as dust par ticles or unevenness on the scale of the crystal grains. With this arrangement, the spread in propagation mode in dex of the incident light is much less than the variation in mode index caused by film uneveness over the area of the enlarged beam spot. Under these conditions a contour pattern of the film surface is obtained as the light couples into the films at positions where the mode indices of the waveguide structure and the incident beam match. The region of low reflected intensity will appear as a straight line only if the mode index variation is uni directional. Otherwise a curved (ocassionally closed) contour is obtained which joins all regions of the film which have the same mode index. Scanning the angle of tilt allows the mode indices (and hence thicknesses) at all points on the surface of the slide to be mapped out in a straightforward manner. The observation that this con tour pattern rotates as the slide is rotated on the prism provides further evidence that the low intensity regions are located in the film itself. 82 3.8 Quantitative Measurement Of Mode Indices The shift in mode index which is to be used in deter mining the magnitude of the adlayer perturbation is caused by the interaction between the power in the sensing evanescent field and the surface layer. Because the amount of power in the sensing cladding depends on the strength of the excitation it might appear that the shift obtained would be a function of the coupling efficiency - making the interpretation of the measurement very complicated. Fortunately this is not the case because whereas the above reasoning applies to the total power, the power respon sible for the measured shift is that coupled into the guiding structure. The field distributions set up by each of the accepted plane wave components is exactly that described in the theory of Chapter 2, and experimentally an arbitrarily narrow range of plane wave components in the reflected beam can be isolated by using a small aper ture in the far field. Experimental measurements made un der these conditions will therefore correlate directly with the predictions of the plane wave anlaysis. That the direction of the plane wave component which is at the centre of the excitation does not depend on the coupling strength can be demonstrated by simply expanding or focusing the beam and observing that the band is still across the centre of the spot (provided the lens or ex pander axis is correctly aligned along the optical bench). 3.9 Suitability Of Silicon Oxide Films Quantitative agreement between the measurements on a fabricated device and the theory given in Chapter 2 83 requires a much better knowledge of the film parameters than is available for the Silicon Oxide waveguiding struc ture. Refractive index inhomogeneity, variations over a period of time in this quantity as well as scattering due to crystal grains all cause uncertainty in the exact characterisation of the present sensing structure. Similar problems of initialisation are also encountered in ap plications using the Surface Plasmon excitation where the value of the real part of permittivity ranging from -16 to -18 (or a surface adlayer of 10 A of silver sulphide) can cause a variation in mode index of order 6*10~3. In order to obtain reassuring quantitative agreement between the plane wave theory and an experimental device it would be desirable to fabricate a more nearly "ideal" structure without regard to its performance as a sensor per se. 84 4. FABRICATION OF "LOSSLESS” POLYMER STRUCTURES 4.1 Choice Of Deposition Technique A review of early integrated optics waveguide fabrication (Tamir 1979) suggests several alternative deposition techniques to vacuum evaporation of which the most amenable would appear to be solvent casting of polymer films. Solution deposited films of epoxy and or ganic polymers were considered an improvement on the ear liest integrated optics waveguides made from evaporated layers which had much higher losses (Harris et al 1970, Tien et al 1971). As well as integrated optics waveguide fabrication polymer film deposition by spin coating is a frequently used technique for obtaining thin (a few thousand angstrom) polyimide insulating layers for electronic device fabrication and also for deposition of photo resist films (Weber et al 1975). Polymer films produced by this technique are generally of good optical quality:- non absorbing, non scattering, amorphous, having smooth surfaces and can be conveniently fabricated with thicknesses in the range of tenths of a micron to sevaral micron. Apart from being much less time consuming to prepare than evaporated films, they can also be considered as reasonable practical approximations to the ideal, nominally "lossless" layers implied by our theoretical treatment. The intention is therefore to fabricate a two layer polymer structure on the surface of a glass slide; the first of relatively low index as the spacing layer and the second of higher index as the waveguide. Readily deposited polymers which have been found 85 suitable not only for integrated optics but also for op tical fibre transmission as well as bulk optics (Dislich 1979), include PMMA (nominal index » 1.49) and Polys tyrene (nominal index » 1.58 - though this plastic is somewhat birefringent in thin film form). It is desirable to choose as low an index as possible for the spacing layer because this will lead to a larger power percentage in the sensing cladding. Organic solids with indices much lower than PMMA do exist (fluorinated hydrocarbons - e.g PTFE visible index 1.35), but these materials are espe cially renowned for their difficulty of deposition due to insolubility. Fluorinated hydrocarbon cladding films have been used to coat Silica fibres by sol gel deposition (Dislich 1979, Suzuki et al 1974) but they are much too thick (several micron) and uneven for present require ments . Recalling how previous measurements of intensity change on excitation of the waveguiding structure actually relied on scattering, one may wonder how it will be pos sible to detect the excitation of an almost lossless polymer structure (save by residual scattering). Although there is no amplitude change of the reflected beam, the infinite plane wave analyses of Chapter 2 - Midwinter (1970), Ulrich (1970) predict a rapid and substantial phase change of one complete cycle in the reflected wave relative to the incident wave as the resonance is scanned and it should be possible to utilize this to make the cou pling apparent. 86 4.2 Fabrication Details The Spin Coating Technique / Several authors have analysed the theoretical dynamic thickness distribution to be expected during the process of spin coating a substrate with a single phase liquid (D'Olivera 1981, Washo et al 1972). The modelling of the deposition of a solid from an evaporating solvent (a two phase system) is much more complicated and the above analyses seem a poor approximation. Density variations of the resultant film due to different concentrations of the deposited solid across the substrate, as well as gradual thickness variations caused by solvent evaporation will not be accounted for at all by the above theoretical treatments. The density and thickness distribution of the solid residue are therefore best determined experimentally if there is substantial solvent evaporation during the spinning process. Films deposited from viscous solutions or volatile solvents tend to harden during spinning and are con sequently very uneven, having a coloured speckled ap pearance with a very rough surface profile. Ulrich et al (1972) and Weber et al (1975) note that a slowly evaporat ing solvent is essential in spin coating so that the semi- liquidus film remaining on the substrate after rotation has ceased has a surface which can be smoothed out by sur face tension. With very dilute solutions it may be neces sary to terminate the rotation while the material is still liquidus in order to obtain a sufficiently thick film. On all but the largest of substrates this can lead to long range uneveness due to the same surface tension 87 effects at the perimeter of the substrate. It is there fore necessary to use a sufficiently dilute solution to obtain smooth surface coverage, but not so dilute that the substrate needs to be brought to rest while the film is still very liquidus. The more volatile the solvent used, the narrower is the range of suitable dilutions. Xylene is a commonly used solvent in photoresist spin coating (because of its high boiling point - 140°C) and is also suitable for Polystyrene (Tamir 1979, Ding et al 1983) and PMMA (if dissolved in hot solvent). The solu tions used in the present depositions were obtained by dilution of saturated solutions (0.17 gm per cm3) of the solid polymers in Xylene. The glass substrate on the rotation table is covered completely with the solution and rotated at angular speeds of order of thousands of revolu tions per minute for a period of several seconds. The initial investigation of the films deposited by the spinning process was performed using Silicon wafer substrates in order to use the strong interference colouration to make film thickness variations immediately visible (this has in fact been used directly as a detec tion technique for biological adlayer thicknesses Sandstrom et al 1985). The large difference in indices between film and substrate gives a strong substrate reflection and almost all of the incident light con tributes to the interference colouration, which makes thickness non-uniformities due to gradual variations or local perturbations from dust particles, readily apparent. An estimate of the upper limit of film thickness variation across the Silicon wafer can be made from the absence of a 88 single interference order:- Considering that a thickness variation as small as 250 A is sufficient to cause a reflected colouration change from red to blue, the fabricated films must be even to the order of 100A which is only a few percent for a waveguiding film 3700 A thick. Uneveness due to disrup tion of the films by dust particles occurs with 0.8 micron thick PMMA films but these are isolated, occasional non uniformities; there is no apparent disruption by dust par ticles of the 0.37 micron guiding layer film. Towards the end of the deposition process the colouration of these sub-micron films changes rapidly as the thickness decreases by solvent evaporation and this can conveniently be used to determine when the film is in the process of hardening - the spinning is terminated when these interference colours begin to appear. Reviewing the structure it is intended to fabricate unfortunately reveals a serious problem in depositing Polystyrene from X y l e n e - namely that the substrate will have already been pre-coated with the PMMA spacing layer which is soluble in Xylene. It is perfectly feasible therefore to use Xylene to deposit the initial PMMA film, but another solvent which does not dissolve PMMA is needed for the subsequent Polystyrene deposition. Of the possible available and readily usable solvents satisfying the above criteria indicated from a study of the Polymer Handbook, carbon tetrachloride is chosen. Un fortunately it has a rather low boiling point and con sequently there are problems with over-rapid evaporation as noted above:- as the material is being spread across 89 the substrate the film is hardening, which leads to very- uneven films as demonstrated by the visible red/green speckle indicating rapid thickness variations of order of a quarter wavelength differences (250 A or so). In an at tempt to reduce the rate of solvent evaporation the films were spun in an atmosphere of pure solvent. However even with this arrangement very short spin times are required to avoid very rough films, and this leads to patchy films and very poor reproducibility of thickness. The Polymer Handbook suggests that PMMA is insoluble in cold Xylene (our PMMA solutions were made by dissolving the solid in hot Xylene and subsequently diluting this solution ). To investigate the solubility of PMMA in cold- Xylene a previously spun film of PMMA (thickness of order 0.8 micron) on a Silicon substrate is covered with sol vent and after a few seconds removed by spinning to simu late the deposition of a subsequent film from a Xylene solution. The film remaining exhibits an uneven coloration with estimated thickness changes of order 500 A. Polymer films can be hardened by baking out any residual solvent, D'Olivera (1981) indicates a temperature of 70°C to remove trichloroethylene, Swalen et al (1977) suggest baking at 60°C for removing a range of common sol vents (it is important not to raise the temperature near to 250°C as this results in depolymerisation). In the present fabrication process hardening was at tempted by baking at 160° C for 30 minutes to remove the remaining solvent from the film and the experiment of coating with Xylene solvent was repeated. Films treated in this way show no detectable colour change across the 90 surface of the film ; so the baked PMMA film appears either to be immune to attack from cold Xylene, or else is evenly removed across the film plane. It should be noted that in any case the mode index location of the resonance is only weakly dependent on the thickness of the PMMA spacing layer. 4.2.1 The Proposed Structure An isolated polystyrene waveguide (visible index=1.58) of width 0.37 micron, and with cladding in dices of 1.49 (PMMA) and 1.00 (air) has a TE mode index of 1.5115. It would not be possible to excite this resonance through a soda glass substrate (index of 1.5104 at HeNe), so high index lead glass substrates are used instead (Schott SF10 index = 1.7220 at 6328 A). A Polystyrene layer of 0.37 micron thickness is readily deposited, has two fundamental resonances (one for each orthogonal polarisation), and would require a PMMA spacing thickness of order 0.8 micron to give a resonance width for TE slightly narrower than the raw HeNe laser beam plane wave spectrum and for the TM slightly broader. For isotropic materials, the mode index spacing between TE and TM modes is greatest for the fundamental modes and for the thinnest possible guide thickness which is able to support these two modes. For the parameters indicated the separation in mode indices between the two resonances is several times the width of the broader (TM) resonance. 4.2.2 Determination Of Film Refractive Indices A technique of modified total internal reflection 91 (Ding et al 1983, Leclerc et al 1984) is used in order to verify the quoted nominal values of refractive indices for PMMA and Polystyrene (1.48-1.50 and 1.58 respectively). The measurement is performed by fabricating a film of a few micron on top of a prism of higher index and coupling into leaky modes of the structure at mode indices just below the film index. A decrease in specular reflected intensity occurs as the excited resonance couples energy to all the other resonances which results in "m-lines". The positions of these excitations give a sensitive measure of the film index (a few parts in 10-4) but only a rough estimate of the film thickness (to 5 percent) the limit of very thick films corresponds to the operation of the Abbe refractometer. Considering that the ellip- someter can only measure the index of its own reference sample to a few parts in 10-3, the above "Quasimode" tech nique is obviously superior for determining the indices of films which can be prepared in thicknesses of several op tical wavelengths (although its estimation of film thick ness is correspondingly poor). Furthermore the ellip- sometric technique is by nature of its "modus operandi" unsuitable for the measurement of birefringent films (such as Polystyrene). Finally use of the "Quasimode" technique with an expanded beam generates an immediate visual con tour of any variations in index across the film plane (assuming that the film is sufficiently thick that the mode index is independent of variations in film thickness). Using this technique a film index of 1.4980 and birefringence of 10-*4 was measured for a 3 micron sample 92 of PMMA (baked at 160 °C) - Swalen et al (1977) quotes 1.4856 and birefringence Sn = 2*10-4 at the same wavelength (6328 A). The variation of refractive index across the film plane is less than 2*10-4 over 1 cm. Spin deposited Polystyrene films have a birefringence at least an order of magnitude larger than PMMA unless the film stress which is the cause is released by annealing (Sosnowski et al 1972) or drying in an atmosphere of the solvent (Ding et al 1983). The dependence of film stress birefringence on the volatility of the solvent used and the subsequent drying procedure (as well as the omission of the operating wavelength in quoted literature) accounts for the variation in the quoted values of TE and TM in dices for this material - N-t« = 1.582, N-tm = 1.588 (Leclerc et al 1984); N-t® = 1.586, Ntm = 1.589 (Sosnowski et al 1972). At HeNe the indices determined in the present experiments for a 3 micron (baked) Polystyrene film were Nta = 1.5848 and Ntm = 1.5874. These measured indices for Polystyrene and PMMA are therefore in broad agreement with previously quoted values. It is intended to use the location of the resonances of the air clad structure to initialise the indices of the polystyrene waveguide layer. 4.3 Locating The Resonances Experimental1v Having fabricated the "lossless" polymer structure the detection of the excitation when there is only a shift in phase of the reflected plane waves (and no amplitude reduction) needs to be re-considered. Actually when the sample is used in the arrangement of Fig. 1.3 93 (with a high index liquid between the slide and the prism N = 1.7224) some scattering of light from the films is ob served at the two angles corresponding to TE and TM ex citation, this gives rise to a decrease of order 20% in the reflected intensity (Fig. 4.1(b)). Detection of the phase shift on reflection suggests some form of inter ference between the incident and reflected beams - Midwin ter et al (1970 (b)) have used such a technique to produce a set of interference fringes in an experimental demonstration of his plane wave theory predictions. 4.3.1 Detection Of Excitation By Phase Shifts Instead of a single polarisation incident field phase reference, a much neater solution is to include an equal amplitude of the orthogonal polarisation in the incident beam and to use this as the phase reference. Any phase shift between two orthogonal polarisations results in a change in the ellipticity of polarisation. By definition, at the centre of the TE phase resonance the TE polarised reflected wave has undergone a phase shift of 180 deg relative to its phase off resonance whilst the phase of the TM component is varying only very slowly. As the in cidence angle is tuned the polarisation ellipticity will vary becoming plane polarised orthogonal to the input at one particular angle. To be exact the variation in phase of both polarisations during resonance scanning must be included as well as any initial offset in phase between TE and TM before resonance is excited. The previous plane wave analysis automatically takes into account both of these effects and can be used directly to obtain the phase 94 difference between the two polarisations. From such a calculation a minimum in reflected intensity is expected if the input and output polarisers are aligned parallel; actually with the present arrangement (Fig. 1.3), a mini mum is observed with crossed polarisers because of the ex tra k phase shift of the TE component on reflection from the plane mirror. 4.4 An Alternative Babinet Compensator The introduction of a variable phase shift between two orthogonal polarisations (with no amplitude alteration) is an arrangement sometimes required in opti cal work - usually a "Babinet" compensator plate of crossed uniaxial crystal wedges is used. It should be ap parent from the above that the resonant waveguide reflec tion performs an identical function, where the variation is achieved by prism rotation. It does however require a spatially coherent beam and that the structure's resonance be much wider than the incident beam mode index spread. An additional drawback is the change in beam direction which the reflection introduces (though this can be offset to a great extent). Nevertheless such a device would be rela tively inexpensive to fabricate, thickness variations across the slide would be unimportant for a sufficiently thick guide layer, and there would be no other restric tions on the choice of suitably robust films. 4.5 Transmission Through Output Plane Polariser The phase difference between TE and TM polarisations can be obtained immediately from the plane wave analyses 95 and yields an expression for the ellipticity "e" of polarisation of the reflected light (the ratio of the semi-major to semi-minor axes) -(Zaininger 1964), (Azzam 1977) : e = tan (0/2) [4.1] Where : 0 is the phase lead of TM on TE. The semi major and semiminor axes are always inclined at 45° to the TM direction because the TE and TM components are of equal magnitudes. The power transmission of such an elliptically polarised beam through a linear output polariser is determined by the folllowing expression : P = Pmax/2 [1 + sin( 2a) cos (0) ] [4.2] Where : Pmax is the power tansmission when the inci dent beam is plane polarised parallel to the polariser axis. a is the angle of the polariser to the TM direction. And cj> is the phase lead of TM on TE with 0 = 0 cor responding to polarisation in the same quadrant as the polariser axis. 4.6 Experimental Demonstration Taking note of the above precautions for polymer film deposition, the two layer structure proposed in section 4.3.1 and indicated in the inset of Fig. 4.1(a) is fabri cated. The thickness of the polymer layers is determined by removing the film from an area of the substrate with a scalpel blade and measuring the exposed step with an electronic stylus instrument (to an accuracy of a few percent). The predicted dips in reflected intensity 96 caused by waveguide excitation for such a profile is plotted in Fig. 4.1(a). The plot has two minima, iden tified as the fundamental TE and TM guided modes in the presence of the prism (the TM resonance occurring at lower mode index and having a broader bandwith than the TE). In fact Midwinter et al (1970) make a brief note (without explanation) of this same effect of intensity reduction using parallel polarisers in their paper adding that this reflectance dip makes the arrangement "a sensitive angle sensor." There may be some concern that setting the incident ‘ polariser at 45° to the normal to the plane of the films will not result in equal amplitudes of incident TE and TM polarisations in the waveguide substrate (as assumed by the theory) because of differences in transmission coeffi cients across the several intervening boundaries. With the "lossless" polymer structure in position the angular position which produces a plane polarised output can be obtained by locating the minimum transmission through a second polariser. The polariser direction defined in this way will be orthogonal to the input polariser direction only if the intervening differences in TE and TM amplitude transmissions are negligible. With the polymer structure the directions are orthogonal at least to the accuracy of location of polarisation directions (a few degrees). This uncertainty in polariser directions limits the (absolute) accuracy of location of the centre of resonance but since only mode index shifts are measured, the exact definition of the centre of a resonance is not so important. 97 REFLECTANCE (%) 1.504 - 40 -4 - - 80 - 400 20 00 .0 158 .1 152 .1 156 .1 160 .2 1.624 1.522 1.620 1.518 1.516 1.514 1.512 1.510 1.508 1.506 i - ° m I 41 a T AD M OAIAIN EOACS O N I LD OYE STRUCTURE POLYMER AIRCLAD AN FOR RESONANCES POLARISATION TM AND TE (a) 4.1 FIG \ \ \ ■i I < < OE NE SPREAD INDEX MODE ■ < 'I * I ,....V, / LASER BEAM 100 0 /1 3 DEG __ / 1 " L J / ' / 1.720 ' / IH I CLADDING AIR WITH HOEIA OUTPUT THEORETICAL ERCIE NE PROFILE INDEX REFRACTIVE 70 A •7700 1.408 .87 (TE) 1.5807 .95 (TM) 1.6965 70 A 3700 OE INDEX MODE 1.00 REFLECTANCE (%) 4.6.1 Experimental Agreement With Theoretical Curves The experimental curve obtained for the structure (Fig. 4.1(b)) was used to initialise the indices of the polystyrene layer by using values for N«a and N-t^n chosen to fit the corresponding theoretical curve of Fig. 4.1(a) - so the agreement of these two plots is somewhat mislead ing. Having initialised the structure, the bulk sensing capability of the device can be determined by replacing the air cladding with distilled water of nominal index 1.33. This allows a comparison between the predicted response of the water clad structure Fig. 4.2(a) and the measured curve of Fig. 4.2(b). A shift of both resonances to higher mode indices is predicted (because of the in crease in cladding index). A narrowing of the resonance widths is also expected (because of increased mode index and hence more rapid evanescent field decay in the spacing layer, as well as a decrease in the field amplitudes in this region because of power transfer to cladding). Finally the separation of the resonances should decrease (because of a smaller Q ratio ((Nj./N^)2 ) between the TE and T M . The measured curve of Fig. 4.2(b) illustrates all of these characteristics and correctly locates the mode in dices of both polarisations - the agreement is very reas suring. The reduction in the depth of the minima is due to increased mismatch between the laser mode index spread and the much narrower bandwidth of the resonances. The above curves also provide a measure of the 100 .0 156 .0 150 .1 154 .1 158 .2 152 .2 MD INDEX MODE 1.524 1.522 1.520 1.518 1.516 1.514 1.512 1.510 1.508 1.506 1.504 REFLECTANCE (%) -20 40 -4 - 60 - 80 - -100 i I 4. a T AD M OAIAIN EOACS O A AUOS LD OYE STRUCTURE POLYMER CLAD AQUEOUS AN FOR RESONANCES POLARISATION TM AND TE (a) .2 4 FIG ■ i i ■ ■ 1.720 N <— /N ERCIE NE PROFILE INDEX REFRACTIVE 7700 A 1.498 1 .95 (TM) 1.5955 .87 (TE) 1.5897 i ■ i i i i ■ i i i 3700 A 3700 1.333 IH QEU CLADDINp AQUEOUS WITH O E NE SPREAD INDEXMODE HOEIA OUTPU THEORETICAL AE BEAMLASER ____ i ___ I ____ / V / \ i __ l I ' I I ILi____i \ ' ____ i ____ i / __ *| ILi ____ I ------1 ------l 100 o to ------1------u - i ... _____ I I_____i_____i______I_____l_____1 ... ______| l_____I______I_____I______I_____I_____I_____I_____I______I__1 ______i i 1.504 1.506 1.508 1510 1.512 1.514 1.516 1.518 1.520 1.522 1.524 MODE INDEX FIG 4 .2 (b ) EXPERIMENTAL REFLECTANCE FORTHE STRUCTURE OF FIG 4.2(a) average percentage power in the cladding from the mag nitude of the mode index shift which results from changing the cladding index from 1.00 to 1.33. Making the ap proximation that the mode index shift is simply the clad ding index change weighted by the percentage of power in this region the following expression can be used : Pclad = (Mode Index Shift)/(Cladding Index Change) Where Pclad is-the average cladding power percentage. The TM mode index shift is 114*10“4, indicating an approximate average cladding power percentage of 3.5%. The calculated values for the structure when air clad and water clad are 1.9% and 5.3% respectively. The TE mode index shift is 69*10“4, indicating an ap proximate average cladding power percentage of 2.1 %. The calculated values for the structure when air clad and water clad are 1.5% and 4.4% respectively. Location of the resonance mode index to 10“4 implies an accuracy of determination of the bulk index to (3- 4)*10-3 . Evaporation of the water drop leaves a surface residue so that the output intensity does not return to its initial value as well as causing surface disruption of the polymer films as evidenced by increased scattering and substantial broadening of the resonances. The condensation of a water vapour layer on the sur face produces similar effects to those with Magnesium Fluoride/Silicon Oxide arrangement of Ch 3 save that the present structure is not so sensitive. The theoretical curves of Fig. 4.1(a) and Fig. 4.2(a) are not reproduced exactly in the experiments because the 103 former are derived from an infinite plane wave analysis of the structure whereas the latter observations sum the ef fect of the structure on the range of plane waves in the beam. The experimental curve is a complicated convolution of the plane wave structural response with the plane wave spectrum of the incident gaussian beam - the mismatch in field shapes between the leaky mode of the reversed struc ture and the incident gaussian beam profile indicates that this convolution can never produce a zero in reflectivity of the total spot (80 percent reduction is the maximum). Besides the above, any film unevenness on the scale of the beam width will cause a reduction in coupling ef ficiency from that predicted for an ideally plane parallel guide - this will especially be a problem if a narrow laser bandwidth is used because this would require an ex panded beam covering a larger area of the film. This means that there will be a trade-off between a sharpening of the resonance coupling of the total spot by using a wider beam and a broadening due to the increased film unevenness over the larger area. A gradual thickness variation of the film therefore places a lower limit on the angular resolution of the device in attempting to reduce the angular resonance width by using a longer in teraction length (a wider beam). 4.7 Predicted Appearance Of The Output Spot The output power distribution of the beam reflected from the waveguiding structure is determined from a summa tion of the phase shifted (and polarisation altered) plane wave components (Chapter 2). As with scattering from the 104 evaporated guide, a reduction in output power intensity is obtained on excitation of the structure but in this case the plane wave components are removed by the output polariser after excitation rather than in the films them selves . Considering only the gaussian beam divergence in the plane normal to the films (ignoring transverse divergence), the beam contains a range of mode index values (K^c/Ko = NE>*sin(9)) which determine the strength of excitation of the structure. If the plane wave component on axis is at the centre of the phase resonance it will suffer a phase change of 180° (relative to TM polarisation) and be completely absorbed by the output polariser. Components off-axis will receive phase changes either side of 180° and generate elliptically polarised light which will be partially transmitted. The far field output spot is a fourier transform spectrum of the plane wave beam components in a real 3 dimensional gaussian beam (with a range of Ky/Ko values). An intensity reduction of plane waves around a particular Kx ~ NjpSinQ (and any Ky/Ko) will give rise to a horizon tal band across the spot. This is indicated in Fig. 4.3.(ii) ; Fig. 4.3. (i) and (iii) indicate the appearance of the output spot with the polarisers absent and parallel respectively. The form of the output spot must be in variant to azimuthal rotation of the slide on the prism provided the films can truly be considered to be plane uniform over the area of the spot, as is the case in Fig. 4.4 using a focused spot. Even with the unfocused beam the dark band is nearly horizontal for all azimuths - in- 105 • £ • (0 No Potarisers (9 Crossed Pofarisers GO P aralel Poferisas FIG 4 .3 APPEARANCE OF THE LASER SPOT ON REFLECTION FROM A POLYMB? STRUCTURE FOR DIFFERENT POLARISER ARRANGENENTS < a m — 9 cf - 4 ^ 0' 4Sk -m * 4 5* 90* FIG 4.4 INVARIANCE OF THE OUTPUT SPOT FOR AZIMUTHAL ROTATION - THE BAND OF LOW INTENSITY ALWAYS RUNS HORIZONTALLY 106 dicating that the mode index variation across the film plane is much less drastic than for the silicon oxide guides. 4.8 Experimental Observations Of The Output Spot (a) The appearance of the output is determined by the relative magnitudes of the spread in mode index of: 1/The ideal structure. 2/The incident beam. 3/The film, due to uneveness (over the beam spot area). For a sufficiently small beam spot size, the third effect is negligible and a horizontal band is obtained (for all azimuths) when the beam mode index bandwidth is greater than that of the structure as in Fig 4.4. The percentage of the spot width occupied by the band is a direct measure of the ratio of the structure mode index width relative to that of the beam and the angular tilt needed to scan the band from the top of the spot to the bottom is a measure of the angular beam divergence. A decrease in intensity of the whole spot occurs over a range of angles determined by the structures mode index spread when the latter is much larger than that of the beam. Weakening the coupling and using a wider beam (to maintain strong excitation) can produce a non-horizontal band whose direction varies as the slide is rotated azimuthally, both of which indicate non-negligible thick ness variation. In the limit of a very broad beam with a 107 narrow angular spectrum the output drop in reflected in tensity is a surface profile of the variation in mode in dex across the film as in Fig. 4.5. (b) In the case of strong excitation of highly scat tering films, maximum contrast is obtained by rotating the input polarisation completely to TE (or TM). For detec tion by polarisation change (with "lossless'' polymer guides) using equal amplitudes of TE and TM incident light, maximum contrast is obtained with the output and input polarisers crossed and at 45 ° to the normal to the films. With some films samples the best contrast is ob tained neither with a single incident polarisation nor with equal incident TE and TM polarisations (and crossed polarisers) but at some other input and output settings. This implies that for such films there is a significant contribution from both scattering and phase variation. 4.9 Explanation Of The Effect Of Non-Uniformity As considered above (4.5.2 (a)) the effect of non uniformity is particularly apparent if large beam widths are used - the output beam has regions of low intensity which appear to be located in the film. This prevents very sharp angular resonances from being obtained because these require a long beam interaction length i.e a wide beam. In this situation the incident bandwidth is much narrower than the spread in mode index due to thickness variation over the laser beam width, so the coupling oc curs at different positions in the film where the thick ness of the film matches the mode index of the structure 108 3630 A 3700 A , 1cm FIG 4 . 5 SURFACE CONTOUR OF A POLYSTYRENE GUIDE - FILM THICKNESSES ACROSS THE SAMPLE ARE INDCATED 3790 A 109 to the incident laser mode index (assuming that thickness rather than refractive index is responsible for the variation). Altering the mode index of the incident beam will cause the region of reduced intensity to move across the slide - the rate of displacement relating directly to the unevenness of the film (i.e how rapidly the mode index changes with position). In this limit a contour pattern of the surface is mapped out directly as the prism angle is scanned. Fig. 4.5 demonstrates this effect for the polymer structure (with a 1.2 micron PMMA spacing layer) - a particularly uneven region of the film has been chosen close to the edge of the slide. The situation is quite analogous to any optical interference arrangement from an approximately plane parallel structure where fringes of equal inclination or fringes of equal thickness are ob tained depending on the relative magnitudes of the varia tion in film thickness over the beam area and the range of plane wave components in the beam. In the intermediate region an output spot with a band running at an angle to the horizontal is obtained. A horizontal band across the output spot will be ob tained even in the presence of thickness variations in the film plane provided they are parallel to the direction of propagation. The vertical movement of the band across the spot when the prism is tilted will be slower than that corresponding to scanning of the plane wave components in the beam (as would be the case for a parallel layered structure). This is important in the light of the at tempted attainment of very narrow resonance widths - i.e although a narrow band can be obtained within the laser 110 spot the fringe is partially located in the film so that the movement of the fringe across the detector aperture is much slower than the rate of mode index scanning. The an gular tilt required to move the band from the bottom of the spot to the top in this case is larger than the an gular plane wave spectrum of the beam. I l l 5. OPTIMISATION OF THE SENSING STRUCTURE In this chapter the choice of waveguide structure which optimises surface sensitivity is investigated and the most suitable materials for use in practice are sug gested - the performance of a proposed device is compared to that of the surface plasmon sensor. The possible use of multimode structures is also considered. 5.1 Sensitivity Of The Polymer Structure The polymer structure of the previous chapter demonstrated the use of resonant coupling to detect a change in bulk cladding index in the region above the guide by means of the surface evanescent field. The sen sitivity of the structure however is very poor when com pared to the SPR, the same bulk index change would produce a shift 33 times larger than for the present structure. The reason for this is indicated by Fig. 5.1 which il lustrates that there is a very much smaller percentage of waveguided power in the sensing region in the case of the polymer structure because the relatively thick polystyrene guide confines the mode strongly inside this layer. The diagram also suggests that an alternative waveguide struc ture of increased index and decreased width might produce a more favourable power distribution for sensing. 5.2 Choice Of Laver Thickness The following observations indicate the existence of a guide thickness which optimises the percentage of power in the sensing cladding (assuming a prior choice of guide 112 Fig 5.1 SCHEMATICAL POWER DISTRIBUTIONS FOR SURFACE PLASMON AND DIELECTRIC WAVEGUIDING STRUCTURES 113 index) : For an asymmetric structure with a buffer layer of higher index than the cladding and guide thickness just at cut off, all of the power is in the buffer layer as the field extends to infinity (assuming an isolated guide). Increasing the guide width rapidly puts power into the guiding and cladding regions but excessive guide thick nesses will confine the power strongly to this region leaving little in the sensing cladding. It is apparent therefore that there is a maximum in the percentage of power in the cladding region somewhere between cut-off and strong guide confinement. This is demonstrated in Fig. 5.2(a) (cladding power percentage versus guide thickness) and Fig. 5.2(b) (the same ordinate versus mode index) - for an isolated waveguide with indices of 1.38, 2.34, 1.0. The optimum ocurrs at thickness 499 A or mode index 1.5064 with a value of 20.4 percent. The cladding power falls to 0.4 percent at mode index 1.38001 (near the cut-off thickness of 250 A) and to 1.0 percent at mode index 2.3999 (near bulk propagation in the guide at thickness 3645 A). The power decay into the sensing cladding (normal to the film plane) has a 1/e distance of 447 A. The choice of guide width to maximise sensitivity is not simply a matter of optimising the total percentage cladding power since the field confinement also drasti cally affects the overlap integral of the field perturba tion by the adlayer. For any specified adlayer perturba tion there is a unique combination of guide index and thickness which will give the largest mode index shift. The exact choice of guide index and thickness for maximum 114 CLADDING POWER (%) .5 .5 .5 .5 .5 MODEINDEX 2.15 1.95 1.75 1.55 1.35 s Fnto O Wvgie oe Index Mode Waveguide Of Function A As i 52 b Cadn Pwr Percentage Power Cladding (b) 5.2 Fig s Fnto O Gie Thickness Guide Of Function A As 115 sensitivity is therefore determined by the magnitude of the perturbation itself. Given that the proposed sructure is not designed to detect just one particular adlayer per turbation and that some non-optimisation in sensitivity is perfectly tolerable, an approximate calculation of the most suitable structure can be made as follows. An appropriate initial constraint to obtain the strongest interaction overlap integral is the requirement that cladding evanescent field 1/e decay length matches the thickness of the adlayer. This fixes the value of the cladding decay constant delta (for the wavelength selected) and consequently the mode index of the structure (0): delta = - N c 2 ) [5.1] There is obviously a continuum of values of guide in dex and thickness which can give this value of £ (Fig. 5.3). Since the sensing field shape is identical for all these cases it follows that the optimum sensitivity will be obtained by maximising the cladding power percentage and this will determine uniquely the most suitable index and thickness. Actually the variation of cladding power percentage with index (and corresponding thickness) con strained by fixed is monotonic (Fig. 5.4) so that the design requires the highest possible guide index and smal lest thickness which can be fabricated practically. Since the highest possible polymer indices are of order 1.7 (Dislich et al 1973, Dislich 1979) and are not especially suited to thin film fabrication, high index evaporated in organic materials such as Zinc Sulphide (n»2.34) are the preferred alternative. Zinc Sulphide can be readily 116 i 54 ldig oe Pretg A A ucin f ud Index Guide Of Function A As Percentage Power Cladding 5.4 Fig CLADDING POWER (%) THICKNESS (A) i 53 obntos f ud Tikes n Index And Thickness Guide Of Combinations 5.3 Fig hc Gv ACoe Vle f oe Index Mode Of Value Chosen A Give Which ih oe ne Fixed Index Mode With 117 GUIDE INDEX GUIDE INDEX deposited as robust films with small grains (to give a relatively smooth top guiding surface). Having fixed the guide index at the largest possible value available practically the guide thickness which max imises the cladding power percentage can be determined. The mode index and the sensing field evanescent decay are then also determined. Fig. 5.2(b) indicates a value of 1.5064 for mode index or 498 A for thickness. Since this mode index is uncomfortably close to the substrate index to be used (1.51 soda glass) a somewhat thinner guide of 400 A is proposed which has a correspondingly lower mode index (1.4404), longer sensing decay length (486 A) and a slightly lower cladding power percentage (19%). 5.3 Sensitivity To Guide Width Variation The variation in mode index with guide thickness is an important function to investigate because it determines how stringent the requirement of evenness of the deposited layer is. Fig. 5.5(a) and Fig 5.5(b) indicate a rela tively broad maximum in sensitivity of the structures mode index to guide width variations. The maximum occurs at mode index 1.5327 or thickness 536 A and the magnitude is of order 7*10-4 per A. Because the turning point is so broad the value at mode index 1.5064 (499 A)is also 7*10-4 and has fallen only to 6*10~4 at 1.4404 (400 A). It is apparent also from comparison of Fig. 5.2(a) and Fig. 5.5(a) that the maximum in cladding power and sensitivity to waveguide thickness variation follow each other closely. The value of thickness which causes maximum guide width variation sensitivity actually locates the 118 i 55 a Md Idx estvt A A ucin f ud Width Guide Of Function A As Sensitivity Index Mode (a) 5.5 Fig i 55b Md Idx estvt s Fnto O Md Index Mode Of Function A As Sensitivity Index Mode 5.5Cb)Fig MODE INDEX SHIFT (10“4 > PER A 119 HCNS (A) THICKNESS UD LAYER GUIDE INDEX MODE point of inflection on the curve of mode index versus thickness (Tien 1971) where the differential of mode index with guide thickness (d£/dw) is steepest. It follows im mediately that choosing the guide thickness in this region will make the fabricated device highly susceptible to. thickness variations in the high index layer across the film plane over the beam spot area. Tiefenthaler et al (1984 (b)) have made calculations of the effect of adlayer perturbations by considering the latter as an effective increase in the guide thickness - their curves of mode in dex sensitivity to thickness also demonstrate the maxima shortly after cut-off thicknesses. The fact that this width locates a turning point in d0/dw means that the magnitude of the sensitivity is slowly varying in this region. 5.4 Comparison With A Critical Angle Sensor If the thin waveguiding layer and the spacing layer are dispensed with altogether the device obtained is iden tical to the Abbe Refractometer which uses the critical angle between the prism and cladding as a sensitive measure of the index of the latter. This arrangement in which the field extends to infinity in the cladding is a poor sensor of surface adlayers because only a small frac tion of the (infinite) region of integration will be per turbed by a finite surface layer. This observation demonstrates that the function of the spacing and guiding layers is fundamentally to provide surface confinement of the fields so that thin film adlayer perturbations in this region will drastically affect the coupling condition. 120 5.5 Choice Of Spacing Laver Thickness And Index Having selected the guide layer thickness to optimise sensitivity, the spacing layer width is chosen to provide the required isolation (narrowing in mode index space) of the resonant excitation. For the 400 A guide of index 2.34, buffer index 1.38 and cladding index of 1.0 the required thickness of spacing layer for a resonace width of order 10-4 in the fundamental TE mode is 0.95 micron. The diagram of power distribution between the layers of the polymer structure (Fig. 5.1) indicates that a high buffer layer index leads to a large percentage of power in this region which is unavailable for sensing on the other side of the guide. If the cladding medium is an aqueous solution rather than air, the power percentage in the sensing region is increased substantially (from 20% to 35%) because it is directly related to the asymmetry of the structure. It is apparent that the sensitivity of the device is increased by reducing the spacing layer index to as low a value as possible. There seems little oppor tunity for this with any readily deposited polymer films (Dislich et al 1973, Dislich 1979) as none of these have indices significantly lower than PMMA. Fluorocarbons such as PTFE (and Teflon FEP) which do have low indices (1.35), cannot be deposited from solvents and although deposition by plasma polymerisation and sputtering has been attempted, the techniques are somewhat ill-defined and the resulting films far from ideal. They have however been deposited from aqueous particle suspension, by drying and subsequent fusing in a "sol gel" process (Suzuki et al 121 1974) but such films were used as cladding materials and were several micron thick. For an air clad structure the improvement in cladding power percentage as a result of lowering the buffer index (from 1.38 to 1.35) is actually not very impressive. Fig. 5.6(a) indicates an increase of only 2 percent in cladding power because a guide with cladding indices of 1.35 and 1.00 and another with cladding indices 1.38 and 1.00 have comparable asymmetries. Fig. 5.6(b) demonstrates this further for an aqueous clad structure (index 1.33) with values of buffer layer indices of 1.38,1.35, and 1.33. In the limit of a symmetric structure with buffer and cladding indices equal (1.33) the maximum approaches the limit of 50 percent (for a guide of infinetesimal width with the power equally divided between the claddings on either side). It is apparent from this, that for sensing with an air cladding there is little to be gained by reducing the buffer index from 1.38 to 1.35 but for aqueous claddings the improvement may be worthwhile. Given their relative ease of deposition and their low indices, vacuum evaporated films would appear to be the most suitable buffer layer materials (despite possible scattering problems). There are several evaporated films with lower indices than Magnesium Fluoride (LiF : 1.33- 1.37, CaF2 : 1.28-1.38, NaF : 1.33, Cryolite : 1.33) but they are unsatisfactory for a variety of reasons such as porosity, variable composition, inhomogeneity, and (for adlayer detection in aqueous solution) because of hygros- copicity and consequent solubility when water clad. A spacing layer of Magnesium Fluoride is therefore preferred 122 CLADDING POWER (%) i 56b Te fet f ud Assymetry Guide Of Effect The 5.6(b) Fig n ldig oe Percentage Power Cladding On 123 NDEX D IN ODE M even though this means more power in the spacing layer and unavailable for sensing than would be obtained with any of the above films. 5.6 Proposed Fabricatable Structure Fig. 5.7 illustrates the refractive index profile of the proposed evaporated film sensing structure and also for comparison the polymer structure of the previous chap ter. The choice of guide thickness of 400 A means that the TE mode index resonance is less than the index of soda glass so that soda glass microscope slides can be used as convenient substrates. This thickness choice also indi cates a spacing isolation of Magnesium Fluoride (0.95 micron) which is feasible practically. Films of Zinc Sul phide only attain bulk refractive index above 1000 A, so at 400 A there is likely to be a decrease in index through the film as the substrate interface is approached - this variation will invalidate a step index profile somewhat. The following brief calculation gives some indication of the improvement in sensitivity of this optimised struc ture compared to the polymer device. A bulk index change of 0.33 produced a mode index shift of 67*10~4 for the polymer structure. A shift of the same magnitude for the optimised improved structure could be obtained for an adlayer film of only 100A and nominal index 1.5. 124 i 57 SIAIN FTE MRVMN I FEDDSRBTO FR UFC SENSING SURFACE FOR DISTRIBUTION FIELD IN IMPROVEMENT THE OF ESTIMATION 5.7 Fig REFRACTIVE INDEX SN AALBE ILCRC MATERIALS DIELECTRIC AVAILABLE USING NORGANI S R E Y A L IC N A G R O IN EVAPORATED 5.7 Possible Use Of TM Fundamental Mode The table below lists the appropriate mode indices for the TE mode with air and aqueous claddings and the buffer thickness required to obtain resonance widths of 10-4 (for a guide thickness of 400 A) : Mode Index Spacing Thickness Air Clad 1.4404 0.9 micron Water Clad 1.5098 0.7 micron In the case of the water clad structure, a fundamen tal TM mode resonance is also available for the same guiding structure. The presence of this second resonance is interesting because it would possibly enable two ad- layer film parameters (thickness and index) to be deter mined. The following table indicates the mode index of the resonance, the power distribution and the buffer thickness required to obtain the same resonance width as for the TE mode. Buffer Mode Index Index Spacing Pbuff Pguide Pclad 1.38 1.3852 1.9 pirn 70% 0.4% 19.6% 1.35 1.3639 1.5 jim 62% 0.6% 36.6% To obtain a sufficiently narrow TM resonance width (using the same guide thickness for TE) would require a thickness of Magnesium Fluoride spacing which cannot be practically realised. The coupling between the prism and the guide is determined by the evanescent field decay through the spacing layer whose transverse decay constant is equal to : 126 Kappa = V(02-Nt>2 ) [ 5 .2 ] Increasing 0 (by using a thicker guiding layer for the TM mode) would be one way of reducing the coupling strength and enabling a narrower spacing layer to be used, but this also decreases the percentage of power in the sensing region. Reducing the buffer index (to increase the transverse field decay rate) or using a material which can be fabricated in films of sufficient thickness are more attractive alternatives. 5.8 Theoretical Performance Compared With The Surface Plasmon Resonance 5.8.1 Resonance Width The usual thickness of silver film selected in ap plications using the SPR is that which produces a minimum of zero intensity in the reflected plane wave at the resonance angle. This corresponds physically to the matching of the leakage into the prism of the non-isolated SPR with the power loss of the mode. Fig. 5.8 illustrates the reflected plane wave response from a SPR arrangement for a range of thicknesses of the metal layer. Fig. 5.9 (i-iv) illustrate the mode index width of the air clad SPR using a silver film compared with that of the "optimised" air clad dielectric structure described above as the spacing layer of the latter is increased. The width of the SPR is fixed by loss in the metal and the choice of zero reflected intensity at resonance. In the case of the dielectric structure however, the resonance width decreases very nearly exponentially with increasing spacing thickness - the limit of infinite thickness iden- 127 REFLECTANCE (%) i 58OTCL ELCIIY F 3 HS SYSTEM PHASE 3 A OF REFLECTIVITY OPTICAL 5.8 Fig IH MDL LYR F OPE RFATV INDEX REFRACTIVE COMPLEX OF LAYER MIDDLE A WITH R CLADDI G IN D D A L C IR A 128 eEim) - 00 .0 6 -1 : ) Re(Efilm mEim : 53 .5 0 : Im(Efilm) i 59 IET OPRSN F UFC PLASMON SURFACE OF COMPARISON DIRECT 5.9 Fig REFLECTANCE (%) - REFLECTANCE (%) 129 (iv) REFLECTANCE (%) I REFLECTANCE (%) -80 -60 -40 * 6 0 - , J - w (1.0234) 1.4300 \ \ \ \ (1.0334) 1 1.4400 \ \ / / / AAEES S RVOUSLY PREVIO AS PARAMETERS (1.0434) 130 UFC PLASMON SURFACE ckness s e n k ic h t 1.4500 XET PCN LAYER SPACING EXCEPT DIELECTRIC DIELECTRIC (1.0534) 1.4600 S : INFINITE ------OE INDEX MODE tifies the isolated guide with a delta function eigenvalue of mode index. 5.8.2 Sensing Field Decay Lengths The sensing field decay rate is determined directly from the mode index of the excitation (J3) and the cladding index (Nc ) by relation [5.1] : Transverse Decay Constant = V(£2 - Ne2) The SPR mode index is only slightly above the index of the cladding, Fig. 5.9 indicates a value of 1.0334 in the case of an air cladding. The reason for this is that almost all of the modal power (99.6 percent) is in the region above the. metal film. Fig. 5.8 also indicates that the position of the minimum varies by less than a half width (5*10-3) for metal thicknesses from 450 A to 700 A. It follows that even for non optimised choice of metal film thickness the decay constant is of order 0.2 - for HeNe wavelength (0.6328 Jim) this gives a power decay length of order 2000 A. In the case of the dielectric structure the value of £ can be varied widely between the limit of guide index « 2.34 and the upper cladding index « 1.38. Even for this lower limit the difference between mode index and air cladding is 10 times larger (0.38) than for the SPR imply ing at least a factor of 3 decrease in the decay length (600 A). At the upper limit of mode index (2.34) the decay length is 20 times shorter (300 A). The actual cal culated values for the power 1/e decay length normal to the film for the devices indicated above are as follows : 131 Surface Plasmon D i e l e c t r i c Transverse Decay Constant 0.261 1.037 (Dimensionless) [1/e] Field Fall Off Distance 1930 A 486 A (0.6328 micron) 5.8.3 Theoretical Mode Index Shifts Although an exact calculation of the shift in resonance mode index due to an adlayer can be made by developing a five planar layer model, for sufficiently small shifts a perturbation calculation is perfectly adequate. An indication of the range of validity of the latter is that calculations made with it for shifts of or der 10-2 in mode index yield values in agreement with those using the exact theory to within a few percent (Fig. 5.10). The perturbation calculation is preferred because it provides better physical understanding and because it is much easier to invert the formula to deduce the film parameters from measured shifts using this type of cal culation. Assuming a step index adlayer perturbation, the theory of Yariv and Yeh (1984) can be used for the TE mode. The perturbation calculation cannot be used for the TM mode for reasons given on page 428 of the above reference, namely that the presence of two guided electric field components in the TM case, invalidates one of the theoretical assumptions ((E.Grad)E = 0). A second useful approximation to make is that the structure can be considered as a free guide. This will be 132 MODE INDEX SHIFT f h Deeti Prubto Calculation Perturbation Dielectric The Of i 51 Ilsrto O Te Breakdown The Of Illustration 5.10 Fig 133 a valid assumption provided the spacing thickness chosen is large enough to isolate the guide so that the effect of the prism on the excitation mode index is only a few parts in 10-4. 5.9 Predicted Resonance Shift Comparison fa) Air Clad Evaporated Lavers Using experimental data from a paper by Pockrand (1978), the shifts in resonance mode index of an air clad SPR caused by build up of a Lithium Fluoride adlayer are determined. The curve of mode index shift versus adlayer thickness labelled "Surface Plasmon" in Fig. 5.11 is plotted from these values. The corresponding mode index shifts for the nominally optimised dielectric structure for the same perturbations can also be determined, the resulting graph is labelled "Dielectric Structure" on Fig. 5.11. The ratio between the magnitudes of the shifts for small perturbations is approximately 1/2. For bulk index changes (assuming the change does not alter the field shapes) a ratio of 20/99 (i.e approximately 1/5) would be predicted because this is the ratio of cladding power per centages. For smaller (finite) adlayer thicknesses the closer surface confinement of the dielectric cladding field compensates somewhat for reduced total power in the sensing medium and a somewhat larger ratio (1/2) than 1/5 is obtained. From the Fig. 5.11 it appears that a 50 A layer will produce a mode index change for the dielectric of order 50*10-4. Recalling that it is possible to locate the 134 MODE INDEX SHIFT i 51 Eprmna Md Idx hf O A Strongly Perturbed A Of Shift Experimental Index Mode 5.12 Fig TUTR UIG ansu Furd N Zn Sulphide Zinc Magnesium USINGAND Fluoride STRUCTURE ufc PlasrrionSurface For Corresponding Curve And niae Deeti Structure Dielectric Indicated N A AR CLADDING AIR AN AND 135 THICKNESS(A) ADLAYER centre of the resonance to within a fraction of the laser beam divergence (of order 10-4 in mode index) this implies detection limits on an atomic scale. The SPR is already well-known to have sensitivity of this order. It is in teresting to note that complete coverage by a film only 10A thick is easily measured by both of the above struc tures whereas detection of the presence of such a small interface perturbation is at the limit of most ellip- sometric and surface stylus instruments. Fig. 5.12 is a plot of the measured mode index shift caused to the SPR by adlayer perturbations one order of magnitude larger than above using experimental data from another paper by the same author Pockrand et al (1977). Because of the exponential decay of the sensing field a convex saturating curve is expected whereas the experimen tal curve is concave for the thickness region considered. The explanation for this anomaly is that the cladding field shape can be altered sufficiently drastically by the adlayer for the perturbation assumption of fixed zero order field shapes to be invalidated. Again, this is easily appreciated when it is recalled that for the SPR almost all of the modal power is in the cladding region. The dielectric structure is much less susceptible to per turbation breakdown because a considerable percentage of the power is anchored in the guiding and buffer layers. (b^ Water Clad Biological Adiavers Fig. 5.13 illustrates a similar comparison of SPR ex perimental mode index shifts and predicted dielectric shifts, this time for structures with aqueous claddings 136 MODE INDEX SHIFT FIG FIG 5.13 OPRSN F IVR UFC PAMN SHIFT PLASMON SURFACE SILVER OF COMPARISON SN Mgeim loie N Zn Sulphide Zinc AND Fluoride Magnesium USING DAE IDX OIAL 1.5 NOMINALLY INDEX ADLAYER N A AUOS CLADDING AQUEOUS AN AND IH DEETI STRUCTURE DIELECTRIC A WITH DAE TIKES (fo THICKNESS ADLAYER 137 and biological adlayer build up by deposition from the solution onto the sensing surface using data from Flan- nagan (1984). The ratio of resonance shifts is very nearly the same as that for the air clad structures above (1/ 2). Considering the possibility of increasing the angular resolution of the dielectric structure over the SPR by a factor of much more than two, suggests that the former could be the more sensitive of the two devices for measurement in aqueous solution. 5.10 Use Of Bimodal Structures As mentioned previously, the fact that the aqueous clad Dielectric Structure may support two (fundamental) modes, one TE polarised the other TM polarised providing two measurements suggests the possibility of determining two adlayer parameters. The use of this arrangement is illustrated theoretically in more detail as follows : An adlayer of unknown thickness and index is deposited onto the structure indicated in the inset of Fig. 5.14 and produces mode index shifts of 0.0508 and 0.0698 in the TE and TM resonances respectively. For the TE mode the curve of the continuum of possible combina tions of index and thickness which could produce the measured change can be determined by inversion of the perturbation formula - this yields plot A in Fig 5.14 of adlayer thickness versus adlayer index. In a similar man ner the continuum of possible combinations of these two parameters which would produce the TM shift can be deduced (by inversion of the exact calculation) to yield plot B of 138 FIG 5.14 ILLUSTRATION OF THE USE OF TWO DIFFERENT MODE RESONANCES TO DETERMINE TWO ADLAYER PARAMETERS 139 Fig. 5.14. The intersection of the two curves locates uniquely the value of both index and thickness of the ad- layer . The angle between the two curves at the point of in tersection is a measure of how sensitive the determination of the parameters is to inaccuracies in measurement of mode index shifts (i.e how accurately the parameters can be determined). The intersection of the curves occurs with a large angle between the tangents when the field shapes for the two modes are widely different and a small angle when they are nearly degenerate indicating the inde pendence of the measurements. As the sensing field shapes become similar the two curves intersect at shallower and shallower angles until, for identical decay rates of the two evanescent fields, they become completely degenerate and it is not possible to determine the two parameters in dependently. Physically, the situation corresponds to using a short range TE sensing field which is more strongly perturbed by effects close to the surface and a complimentary TM sensing field which has a much longer range and therefore records more "bulk-like" changes (Fig. 5.15) . The above observations are further illustrated by Fig. 5.16(a) in which the error in adlayer thickness determination caused by an error of 10-4 in both TE and TM measurements is plotted as a function of the thickness it self . For very thin films the Sp-t™ shift is very small so the 10-4 is a large percentage error implying that the thickness uncertainty is large. For thick films the TE shift is almost saturated and a change of 10-4 represents 140 2 .3 4 Widely Differing 2 .3 4 A FIG 5.15 A WAVEGUIDING STRUCTURE WITH A THIN GUIDING LAYER AND LARGE INDEX DIFFERENCES 141 ADLAYER THICKNESS ERROR ADLAYER (A) i 51 () ro I Alyr hcns Determination Thickness Adlayer In Error (a) 5.16 Fig u T 1‘4 ro I T Ad M eoac Location Resonance TM And TE In Error 4 10‘ To Due u T 1 Err n E n T Rsnne Location Resonance TM And TE In Error 10 To Due Fig 5.16 5.16 Fig G d Err n dae Idx Determination Index Adlayer In Error 3 142 a large thickness uncertainty. For intermediate film thicknesses the percentage errors of the TE and TM measurements are comparable and the total error has a min imum. Fig. 5.16(b) illustrates that the error in refrac tive index determination of the adlayer is a monotonically diminishing function of thickness because the mode index shifts for both TE and TM are monotonically increasing functions of the latter. A possible alternative to using several modes would be to use two or more wavelengths which is equivalent to scaling the sensing field decay distance of the structure. In theory this could be used to deduce the refractive in dex distribution profile normal to the film. 5.11 Use Of Higher Order Modes It might be suggested that a thicker guide which sup ported more than just the fundamental TE and TM modes could be used to improve the accuracy of determination of the nature of the adlayer perturbation. There is probably little to be gained from this however - firstly because the corresponding TE and TM modes degenerate for thick guide layers and secondly because for all modes more field is confined in the guide when the width of this layer is increased. 5.12 Practical Fabrication Of Two Mode Structure In order to be able to fabricate a structure to per form the sensing function indicated above it would be necessary to arrange for different thicknesses of spacing layer for the TE and TM modes in order to have strong and 143 sharp coupling for both. Unfortunately the implied thick ness for the TM mode using a Magnesium Fluoride spacing layer is somewhat excessive in the light of previous at tempts at fabricating films of this material. An alternative material such as Sodium Fluoride could be used to obtain the necessary thickness but this material is somewhat suspect with regard to index in homogeneity normal to the film plane besides being very hygroscopic with an index dependent on water vapour pres sure of the ambient atmosphere. This behaviour would also prevent its use with a water cladding - but it may well be possible to use this material with an air clad Zinc Sul phide guide to demonstrate the use of "crossed" sensing fields, although it is anticipated that the quantitative agreement between theory and experiment may not be very impressive. 144 6. EXPERIMENTAL % FABRICATION OF THE NOMINALLY OPTIMISED STRUCTURE The previous chapter indicated that ZnS was likely to be the most suitable material for the practical attainment of a nominally optimised sensing structure. In chapter 3 an attempt was made to use ZnS as the high index guiding layer in the initially fabricated structure but it was discovered that the films produced were unsatisfactory. After reviewing the mechanism and method of deposition which indicate in hindsight the reasons for previous dif ficulties a different deposition process is subsequently selected and suitable films are obtained. Having fabri cated the structure its performance and limitations are reviewed and attempts to overcome the latter are made. 6.1 Deposition Of ZnS Films By Evaporation The required 400 A film of ZnS is deposited onto soda glass substrates using a variety of sources in a standard vacuum chamber - bearing in mind that the evenness of the film is of the utmost priority. Quarter wavelength films of ZnS at 0.6328 micron are 700 A thick so it would appear feasible to fabricate a film of this material 400 A thick. Initial attempts using ZnS granules and then ZnS pow der in an open Molybdenum boat produced visibly uneven films, and obvious boat reaction as the refactory metal blackened (possibly due to formation of Molybdenum Sulphide). Some other rather peculiar deposition effects were also observed such as growth of films on metallic surfaces in the chamber and on the rear surface of the substrates rather than the lower side directly facing the 145 source. An alternative "Howitzer" source developed by Turner (1951) of Bausch and Lomb (initially to reduce substrate heating due to long firing times in multilayer deposition) and utilised by many subsequent workers (Cox et al 1958, Hunter et al 1978) to obtain satisfactory ZnS films was used instead. The source consists of a tungsten hairpin filament protruding through a ceramic disc surrounded by a steel cylinder. The powdered ZnS charge is loaded into the cylinder so that on heating the evaporation occurs from the centre of the cylinder outward. The success of this type of source in producing much more acceptable films is related to the following observations which also help to clarify some of the problems with the initial at tempts : - 1/ There is no contact of the material with the tungsten filament and consequently no reaction of the Zinc Sulphide powder cylinder with the metal hairpin. 2/ The source is highly energy efficient as the radiation from the filament evaporates material from the inner surface of the packed ZnS cylinder without heating the whole charge and causing subsequent unnecessary substrate heating. 3/ Because ZnS evaporates by sublimation, heating the solid in an open boat can cause spattering, as the material at the bottom of the boat vaporizes first. This problem is avoided in the "Howitzer" source because the filament produces heating from the top surface. 146 Even with this source however it was found that the deposited films were patchy and uneven, suggesting that the film grows preferentially at certain places on the substrate. Ritter (1976), Hunter et al (1978), Cox et al (1958), Holland (1956) detail the mechanism of ZnS deposi tion which gives an appreciation of the difficulties involved:- The ZnS charge dissociates on heating (as demonstrated by mass spectroscopy of the deposition vapour - Ritter et al 1969). The Zinc atoms nucleate on the substrate and the abundance of sulphur at high vapour pressure means that stoichiometric recombination occurs almost immediately. The condensation rate decreases dras tically with increasing substrate temperature (Macleod 1986) because the sticking coefficient of Zinc is strongly temperature dependent. Substrate temperature strongly af fects deposition - high temperatures are desirable for ad herent and durable films but above 150° it is almost im possible to get any film growth on a glass substrate (Cox et al 1958, Hunter et al 1978, Ritter et al 1969). Sub strate precoating with ThCU which is known to be an effec tive nucleant prevents the previous irregularities of deposition (Ritter et al 1969, Macleod 1986). This func tion can alternatively be performed by the glow discharge cleaning (Ritter et al 1969, Cox et al 1958, Hunter et al 1978, Macleod 1986). In view of the above, a fifteen minute Oxygen plasma discharge cleaning is included in the deposition proce dure, ensuring that the time elapsed between cleaning and 147 deposition is no longer than ten minutes (as the exposed nucleation sites rapidly become contaminated). This precaution yields films with no immediately visible unevenness - the explanation for this is probably that glow discharge cleaning will even out any possible dif ferences in densities of nucleation sites across the sub strate plane. The deposition rate is not an important parameter in determining the optical properties of the film (Hunter 1978). 6.2 Properties Of ZnS Films 6.2.1 Microstructure ZnS films deposited by either sputtering, thermal evaporation, electron beam evaporation (EBE), or atomic layer epitaxy (ALE) are of interest in electroluminescent devices where the microstructure of the deposited film may affect the light emitting properties of the device fabricated. For the present device the microstructure will possibly be important in terms of the resulting in homogeneity and scattering. Theis et al (1983) have studied Transmission Electron Micrographs of ZnS films prepared by the above methods which yield information on their microstructurei Thermally evaporated and EBE films deposited onto amorphous substrates have very similar structure, a highly disordered fine grained structure for the first 2000 A with subsequent columnar growth in a direction determined by the incident vapour flow - initial grain size parallel to substrate » 100A (independent of the temperature), in creasing to * 300 A at 2000 A thickness but with no 148 specific orientation. This rate of increase is larger for higher substrate temperature. Later columnar grains begin to grow. Annealing for 3 hours at 500 °C produces no visible microstructural change, but electron beam heating does give rise to grain growth. For a 0.5 micron thick film the final grain size can be 5-10 times that in the initial layer. The films generally have roughness of or der 10% over a wide range of thicknesses. ZnS films deposited by ALE differ most appreciably in that they have no fine grained initial region so that the columnar grains extend through the film right to the substrate surface this behaviour is attributed to the low nucleation density for this process because of the high substrate temperature used (500 °C). The roughness of these films is on a similar scale to that of evaporated or sputtered films. The stoichiometry of the films is governed by the process of ZnS decomposition to Zn and S2 , nucleation of Zn to the substrate and recombination to ZnS. There is a sulphur deficiency for thick films deposited onto heated sub strates (140 °C) because of re-evaporation by this ele ment. This effect is not noticeable in thin films because the large surface area of crystal grain boundaries traps the sulphur so that each Zn combines with a Sulphur - in thicker films this "holding" effect is much reduced be cause of increased crystal grain size. 6.2.2 Stress Stability ZnS films are known to develop considerable compres sive stress for all but the thinnest films - the magnitude of this stress is down by a factor of 2 on the tensile 149 stress developed in MgF2 films (Ennos 1966). Low deposi tion rate and heated substrate reduce the compressive stress of the ZnS film produced - (the "Howitzer" source was actually designed to keep the substrate cool so that the compressive stress of the film would remain large). The presence of this stress is demonstrated by wrinkling of such films when the adhesion between the film and sub strate is broken. Films deposited onto soda glass sub strates were stable in the atmosphere, but addition of a water droplet onto the surface caused buckling of the film over an increasing area of the substrate - in a matter of seconds the film detaches itself from the substrate and the stress is relieved as the film wrinkles. This be haviour is highly undesirable in the context of one of the possible applications of the proposed device involving detection of biological adlayers deposited from aqueous solution. The mechanism probably involves the seeping of the liquid through pinholes in the film which apart from increasing the films compressive stress, also severely weakens the binding of the film at the substrate interface and allows the compressive stress to buckle up the film (as with the Silicon Oxide films). It is important to note that the deterioration of the ZnS film caused by the water cladding is not due to water solubility (as original workers with this material thought). In fact pieces of film only 400A thick can float on the surface of water, for periods of days without any noticeable decrease in thickness (evidenced by a con stant reflectance colouration). Some older supplier catalogues suggest that ZnS is hygroscopic but the uptake 150 and removal of water is quite reversible with no dissolv ing of the film - ZnS is used extensively in thin film op tics even on unsealed surfaces with no special reference to any hygroscopicity. 6.2.3 Stability Of Zinc Sulphide On Magnesium Fluoride It is a matter of some good fortune that the ZnS films in the "Prism Coupler Surface Sensor" are not to be deposited directly onto soda glass substrates but onto previously deposited MgF2 films which because of their very high tensile stress more than offset the tendency of compressive buckling of ZnS films (Ennos 1966). For a ZnS film of thickness around 400 A (order of a half quarter wavelength thickness) the compressive stress is of order 2000 kg/cm2 . A MgF2 film of 9000 A (of order eight times that of a quarter layer.) has a tensile film stress greater than 3500 Kg/cm2 which more than offsets the ZnS compres sive stress (Ennos 1966). Even if water reaches the in terface between the two films through pinholes in the up per film, the ZnS is stable because of the compensation of tensile stress from the MgF2 in this situation. This stabilisation of films with opposing stress has been well appreciated for this combination of materials since Turner (1951) first fabricated multilayer stacks. The latter was able to obtain multilayer films of total thickness 70 micron by alternating layers of ZnS and MgF2 whereas a 2 micron film of MgF2 disintegrates. It is possible there fore to obtain guiding ZnS films on top of MgF2 spacing layers which are stable to deposition of water on their 151 surfaces. Removal of the liquid by evaporation over a period of order 30 minutes however can cause the resonance to be substantially broadened (wider than the beam mode index spread) with a correspondingly weakened excitation, indicating that some surface disruption does occur on this longer time scale. 6.2.4 Refractive Index Inhomoqeneitv Theoretical modelling of the optical properties of very thin films is required when optical measurements are to be used to characterise a sample. This is explicitly the "Modus Operandi" of ellipsometry (Zaininger 1964, Az- zam 1977) and also of the "Prism Coupler Surface Sensor" presently being considered, where a step index profile is assumed. The fact that there are no sharp interfaces is ac counted for in both of the above devices by simply averag ing over the macroscopic beam area. Another problem resulting from the granular microstructure of the film is that evaporated films only obtain values approaching their bulk refractive indices above a minimum thickness - this will cause refractive index inhomogeneity in a direction normal to the film plane. For very thin films (several hundred angstrom) the microstructure is stongly dependent on the deposition conditions as well as on the substrate surface. Harris et al (1979) suggest that the critical thickness is around 1000 A since ZnS films have a density which increases initially in the first 500 A and levels off around 1000 A. Netterfield (1976) indicates an index increase from 2.32 to 2.34 through a film 1000 A thick. 152 It is apparent therefore that for a film of this material as thin as a few hundred angstrom, a step index profile is not a valid model. The films fabricated in the present discussion (400 A) will have a lower average index than the bulk and a gradual increase in index through the film. Thin film measurement using SPR excitation on silver films also has similar problems of initialisation of the sensing structure. Growth of a surface Oxide layer several angstrom thick and gradual tarnishing due to Sil ver Sulphide formation throughout the film complicates calibration, reduces the sensitivity, and eventually renders the film useless (Abeles 1976). 6.3 Estimation Of The Error Due To Refractive Index Inhomoqeneitv A straight forward calculation can be made of the likely error caused by the refractive index inhomogeneity by taking the limiting values of guide index and calculat ing the response of the structure to the same adlayer in both cases : A 400 A thick film of ZnS with index 2.32 has a mode index (with air and MgF2 claddings) of 1.4342. If the film index were actually 2.34 the calculated thickness for the same mode index would be 390 A. The mode index shifts for these two structures on application of an adlayer of index 1.5 and thickness of 500 A would be 523*10~4 and 528*10-4 respectively - only 1% error. This implies that the use of the mode index of the bare struc ture to initialise the film thickness compensates for uncertainties in the former - the sensitivity of a 153 thinner/thicker guide layer with slightly higher/lower in dex is held constant. It follows that variations in index of this order throughout a film introduce negligible error into the initialisation. 6.4 Film Evenness The attainment of very sharp angular resonance widths is limited by the mode index variation of the guide layer over the beam spot area. The calculations of the previous chapter demonstrated how the choice of guide layer width of 400 A meant that the mode index of the structure was very sensitive to variations in this quantity d(modeindex)/d(guidewidth) ~ 7*10~4/A. Evenness to 10-4 would therefore require an average thickness variation over 2mm beam spot of less than 1/7 A (i.e 0.03% for a 400 A thick film) - which is quite a stringent requirement. Statistical thickness variations on an atomic scale or on the scale of crystal grains are unimportant except in so far as they may limit the narrowness of the resonance width by scattering broadening or reduce the contrast in the output. It is much more important to avoid systematic thickness variation across the substrate plane over dis tances of order of the coupling length i.e on the scale of millimetres. As an illustration of how easily the resonance width is broadened by film unevenness:- a struc ture with a theoretical width of 10-4 will be 20 times wider if there is a variation of only 3 A over the spot area. The plane wave component selected by the aperture simply couples into the film at a slightly different posi tion on the substrate as the prism is tilted causing the 154 intensity to remain low for a larger range of angles than is implied by the structures width. 6.5 Buffer Thickness Variation The following calculation demonstrates that excep tionally large variations in the spacing layer thickness are needed to produce mode index shifts of order 10-4. A waveguide structure with nominal indices 1.38, 2.34, 1.0, guide thickness 400 A and spacing thickness 8000 A has mode index 1.4404. The same structure with a spacing film of 5000 A has mode index 1.4403. 6.6 Measured Thickness Variation The curve of Fig. 6.1 predicts the expected geometri cal thickness variation produced by point source evapora tion onto a planar substrate (Holland 1956) with . the source at a distance of 23 cm perpendicularly from the substrate plane. The measured thickness variations of a waveguiding ZnS film on 0.65 micron of MgF2 are also in dicated on Fig. 6.1. The variations were measured by using an expanded beam which contained a plane wave spectrum much narrower than the spread in mode index due to thickness uneveness over the width of the beam. Fig. 6.2(a) plots similar measurements made on another sample in two perpendicular directions across the substrate plane demonstrating that the variation can even be in the op posite sense to that expected from the source evaporation geometry and also that the vapour flow is anisotropic. The reflected output contains a contour pattern (Fig. 6.2(b(i), b(ii)) which identifies film thicknesses which 155 400 A - Theoretical thickness distribution from point 407 source (at 23cm from substrate) 406 B - Theoretical thickness distribution from 2mm 405 ring source (at 23cm from substrate) 404 C - Experimental curve 403 Spacing Layer : 0.65 micron MgF2 402 401 400 399 398 397 396 395 _J___ !__ I___ l____I___ I____1 I I I l l l I 12 10 8 6 4 2 0 Z 4 6 8 10 12 14 Distance across film plane (nun) FIG 6.1 EVAPORATED ZINC SULPHIDE THICKNESS DISTRIBUTION O62( UFC RFE FEAOAE IC UPD FLM SULPHDE ZINC EVAPORATED OF PROFLE SURFACE ) (a 6.2 TO Film thickness (Angstrom) itne cos im ln (mm) plane film across Distance FEB.2 (b) (b) FEB.2 OTU RLE O ZN SLHD FILM SULPHIDE ZINC OF RELIEF CONTOUR f scattered of pattern Contour light viewed light from above the fromabove filmsurface filmwith theof but above As oaig regions locating hcns 477A thickness ema film at beam thickness 470A reflected erae in Decrease intensity of intensity i ______(HD (ID CD 5mm i match the incident beam mode index as explained in Ch 4 (and Fig. 4.5) - a pattern of scattered light above the film surface maps out (in negative) the same contour Fig 6.3b(iii). The film thickness measurements were taken along directions perpendicular to the contours because this direction identifies the maximum gradient of the thickness changes across the plane. From the plots it is readily apparent that the variation is much larger than predicted from the deposition geometry. In addition, regions of maxima and minima in film thickness can be located at different substrate positions for a single film deposition. This latter observation also excludes the possibility that the cause of the non uniformity is refractive index variation (as a result of differing grain densities across the film) because these too would follow the vapour incidence deposition geometry. Similarly thickness variation as a result of substrate temperature variation is invalidated by this observation. The root cause of this (presently excessive) uneven ness appears to be the low sticking coefficient of ZnS vapour which has been reported in the literature (Hunter 1978). Using a simple arrangement in the deposition cham ber, both sides of a microscope slide substrate have been simultaneously coated; the lower side directly from the source and the top side by reflection of the vapour from an aluminium plate. Taking only first reflections into ac count and recalling a prior observation that the vapour sticks better to aluminium than to glass a lower limit of 30% reflection of the vapour from glass surfaces can be deduced. The consequence of this low sticking coefficient 158 is that the vapour will reflect from the surrounding cham ber walls onto the substrate surface to produce a compli cated thickness variation with a repeatability dependent on the proximity of these reflecting surfaces. In the deposition chamber set up it is usual to place a cylindrical glass shield between the source and sub strate to prevent excessive coating of the chamber walls over a period of time. The above observation suggests that this shield is a liability which is likely to in crease the uneveness of the resultant film. It was sug gested that the sticking coefficient might be much higher for the vapour impinging on a surface precoated with the ZnS. Repeating the above experiment but this time with the glass reflection plate pre-coated with the material unfortunately produces no drastic decrease in reflection coefficient. Removal of the source shield improves the evenness only marginally, implying that reflections of the vapour from the chamber walls are not inconsiderable. Although ZnS has been deposited for decades for use in thin film filters, the above effect is usually over looked completely for two probable reasons. Firstly be cause it does not lead to any serious deterioration in performance - fundamentally because such devices operate with light propagation normal to the film planes rather than along them and are consequently rather insentive to film thickness variations. Secondly, the physical dimen sions of the coating plant may reduce the effects of vapour reflection to negligible proportions. A useful corollary of this limitation imposed on the fabrication of the desired structure is that a very 159 straightforward and sensitive method of obtaining a visual representation of the evenness of evaporated (or sputtered) films of high index has been identified. The surface profile indicated in Fig. 6.2 (b) demonstrating measurements of film thickness variation on the scale of angstroms is quite impressive and it would be difficult to imagine any other method of comparable performance, com bining such ease of operation and low cost. 6.7 The Source Of Scattering To gain some appreciation of the origin of the scat tering in the MgFz/ZnS waveguide arrangement an alterna tive structure with ZnS deposited onto PMMA was fabri cated. Initially the existence of the waveguide resonance was only apparent when crossed polarisers were used imply ing that it is the MgF2 film which causes the scattering in the purely evaporated structure. After several days the structure scattered light quite appreciably probably as a result of structural changes in the polymer layer. Deposition of an inorganic evaporated film onto a plastic coated substrate was not quite a straightforward task and raised the following interesting observations. 6.8 Deposition Of ZnS Onto PMMA Having previously overcome problems with low sticking coeficient of ZnS vapour on glass by plasma cleaning and use of the "Howitzer" source it is discovered that deposi tion of a 600 A film on a glass substrate leaves an ad jacent plastic covered slide with no measurable coating (with plasma cleaning included). The film can be recorded 160 as growing on the crystal monitor surface without any deposit actually present on the plastic substrate. The solution to this problem is simply to coat the monitor crystal surface with the same plastic film so that the growth of the film is correctly calibrated.. Fig. 6.3 il lustrates how the monitor reading varies with time and ex plains the previous problem - there is a very slow initial growth until a thickness of order 30 A has been acheived which nominally represents complete monolayer coverage of the plastic with ZnS. Thereafter the deposition involves growth of ZnS on ZnS which is a much more rapid process. The curvature of the plot in this region suggests a gradually decreasing rate of ZnS deposition which cor responds to the increasing diameter of the ZnS cylinder surrounding the "Howitzer" filament. Similar calibration problems (though on a much less drastic scale) occur whenever different materials are coated on the substrate and the monitor (Macleod 1986). 6.9 The Fabricated Structure (1) Stability It has been possible to fabricate a thin 400 A ZnS layer onto a MgF2 spacing layer which is stable in the at mosphere and when water clad. Evaporation of the water cladding however causes the coupling resonance to become very broad and faint as the interface of the ZnS film to the MgF2 is disrupted. Attempted cleaning of the sensing surface by rubbing with cotton wool soaked in acetone can remove the thin ZnS film from the MgF2 spacing layer if the abrasion is not sufficiently light. 161 800 700 600 500 400 300 200 100 0 , Fjg6.3 Buid Up Of ZnSFIn Onto A PMMA Coated Substrate (2) Guiding Laver Unevenness Refractive index inhomogeneties through the film, though present, do not seriously deteriorate quantitative measurement using this structure. Film thickness variations across the plane of the film prevent very sharp mode index coupling widths (of or der 10-4) being realised as a result of partial fringe location in the film. With the present films, widths one order of magnitude larger seem more reasonable. This precludes the need for a thick MgF2 spacing layer (0.9 micron), a thickness of 0.6 micron which corresponds to a width of 12*10“4 being perfectly adequate. In fact the strength of excitation by the raw laser beam (mode index width 8*10~4) of a structure with a width broadened to this magnitude is considerably enhanced. This improves the contrast of the reflectance dip which is important be cause the latter is obscured considerably by scattering. Fig 6.4 illustrates the broadening of the resonance width for a structure fabricated with a MgF2 thickness of 1.2 micron, which has a theoretical width of 3*10~4 - it is apparent that the experimental width is larger by an order of magnitude. The inset of Fig 6.4 demonstrates that the band of plane waves removed from the beam corresponds to the structures width - the full spot represents a mode in dex spread of 8*10~4 so a band occupying 1/3 of this is of order 3*10~4 . If the structure is water clad so that the width is substantially decreased to 8*10~6 the excitation is not observable (the expected mode index is 1.4636 so the resonance is accessible). 163 OUTPUT INTENSITY ( Arbirary UrAs) i 64 raeig f h Rsnne uv De o im Unevenness Film To Due Curve Resonance The Of Broadening 6.4 Fig (3 ) Limiting Resolution Due To Scattering Loss In the case of the SPR the dip in reflected intensity is a result of power absorption in the non-ideal metal, which can be represented as an imaginary part of the nor malised propagation constant (P/Ko). The magnitude of the latter relates directly to the width in mode index (or angle) of the resonance. For silver the value is around 30*10-4, whilst for SPR on gold the resonance width is approximately 3 times larger (with the same cladding) be cause the latter is more strongly absorbing. It follows that the width of the scattering resonance for the dielectric will have a width limited by the scattering loss of the waveguide structure (provided that the films can be fabricated perfectly plane parallel). The plot of Fig. 6.4 implies a maximum intensity reduction of order 5% which is a loss of 13 dB over a nominal distance of 2 mm i.e 65 dB/cm (TE input polarisa tion - no output polariser). This suggests a contribution to the resonance width of order 2*10“4 which would prevent narrowing beyond this value - as discussed above other factors limit the sharpness first. This value compares sensibly with the losses of multimode Zinc Oxide guides with grain sizes comparable to the wavelength used by Tien (1971) - 60dB/cm. In the same paper the author notes that Zinc Sulphide multimode guides have losses of 5dB/cm mostly due to absorption. In the present device the loss is attributed mainly to scattering from the granular structure of the MgF2 buffer layer and the sizeable power percentage in this region. Tien et al (1970) propose this 165 same technique for determining the loss of integrated op tical waveguides by excitation from a well isolated prism coupler. In the case of the SPR the loss mechanism of absorp tion in the metal film causes a smooth reduction in the reflected spot intensity. For the scattering, inorganic, "Prism Coupler Surface Sensor" the reduction in intensity of the output spot is much less organised, depending as it does on the statistical grain size of the evaporated film to scatter those plane wave components which enter the guiding layer. It is noticable how the contrast of the region of low intensity varies across the film plane of a sample, probably as a result of varying crystal grain size (in the MgFz) caused by uneven temperature distribution during deposition. (As an example of the latter - a 1.2 micron MgF2 film was observed to be visibly cracked on the extremes of the slide where the substrate heater was un able to raise the temperature sufficiently to promote grain growth). 166 7. "PRISM COUPLER SURFACE SENSING" APPLICATIONS Having discussed the design and operation of the sensing device, some examples its use (without the use of input and output polarisers) are presented in this chap ter . 7.1 Qualitative Illustration Of Surface Sensing Figs 7.1 and 7.2 illustrate the response of the sur face sensing structure to a condensed layer of water vapour which subsequently re-evaporates - the device parameters are indicated on the diagrams. To obtain these plots the structure is positioned for optimum excitation with the reflected intensity reduction appearing as a horizontal band across the output spot. An aperture nar rower than this low intensity region of the spot is placed in front of a receiving photodetector. Any alteration of the waveguide structure by surface adlayers produces a sharp rise in intensity as the structure moves off resonance, removal of the perturbation (in this case by re-evaporation of the water vapour layer) results in a reduction of the intensity (Fig. 7.1). The structure only returns gradually to its initial level as the residual water vapour evaporates (Fig 7.2). The timescale for this process can be immediately obtained from the plot and would provide a direct measure of the retaining capacity of the deposited layers to various other vapours and also their rates of re-evaporation if the deposition atmosphere were carefully calibrated. The exact shape of the curve of output intensity at the spot centre versus time depends on several factors 167 OUTPUT INTENSITY (Arbitrary Units) Output intensity (arbitrary units) . Retrto T Iiil nest Lvl s eiul tr aor Evaporates Vapour ater W Residual As Level Intensity Initial To estoration R 7.2 IE CSeconds) TIME :- the variation in thickness of the perturbing adlayer with time, the exponential sensing decay field and the gaussian dependence of spot intensity with beam propaga tion constant. The plots do not give an indication of the sensitivity of the structure to the perturbation as the magnitude of the perturbation itself has not been measured. Tiefenthaler et al (1984 (a), (b)) have recently demonstrated similar effects using a terminated grating and an Si02/Ti02 waveguiding layer. The authors were also able to distinguish between water surface adlayers and ab sorption in the pores of the waveguide by using two dif ferent modes and they concluded that by saturating the film with water vapour, 3 times as much could be taken into the bulk than was formed as a surface adlayer. Both the device described in these references and that fabricated above would form the basis of an inexpen sive, disposable, easily used humidity sensor or an ar rangement for determining the susceptibility of various evaporated films to adsorption of different vapours as well as the retaining capacity for the latter. Alterna tively, treatment of the surface by a chemical monolayer could be used to detect the presence of specific vapour species and their rate of binding to the surface. Deposition of acetone solvent onto the surface of the structure causes an initial rapid increase in intensity and subsequent restoration of the dark band on a time scale of a few seconds - the subsidiary intensity rise possibly corresponds to condensation of a water vapour layer caused by cooling of the surface after the initial- 170 Output intensity (arbitrary units) i 73 eoiin f ctn Ot Te esr ufc Wt Sbeun Evaporation Subsequent With Surface Sensor The Onto Acetone Of Deposition 7.3 Fig rapid acetone evaporation (Fig 7.3). The dark band returns to its initial position to within 10~4 in mode index, in dicating the absence of any residue deposited from the solvent. This illustrates the possible application of the device to determining the purity of a solvent. Under ambient conditions the pores of an evaporated film will contain water vapour. When this is removed the effective refractive index of the films decreases. This is illustrated in Fig. 7.4 (plot A) in which dry nitrogen blown onto the surface of the structure causes an increase in the reflected output above the initial minimum setting because the structure is moved off resonance. Plot B on the same diagram and with the detector sensitivity to mode index variation shows the effect of blowing dry nitrogen onto a structure with a guiding layer of Silicon Oxide. These two structures have very similar power distributions between the layers and nearly identical experimental resonance widths (30*10“4) yet the response of the SiO structure is 4 times smaller. The comparison demonstrates that films of SiO are much less porous than films of ZnS because the removal of water vapour (from both the guiding layer and the MgF2) using the same Nitrogen stream is much less effective. Again, Tiefenthaler et al (1984 (a) and (b)) have recently demonstrated similar effects using a terminated grating and an Si02/Ti02 waveguiding layer suggesting applications in humidity and gas sensing. 7.2 Quantitive Demonstration Of Thin Film Surface Sensing To demonstrate the device performance quantitatively, 172 Output intensit (arbirary units) i 74 eosrto O Te ifrne n ooiy ewe Eaoae Fls f n Ad SiO And ZnS Of Films Evaporated Between Porosity In Difference The Of Demonstration 7.4 Fig 1 2 3 4 5 6 7 8 9 100 90 80 70 60 50 40 30 20 10 0 TIME TIME (Seconds) a solid adlayer is deposited instead of a condensed vapour layer and a plot of reflected intensity versus mode index of the incident beam is obtained Fig. 7.5 (plot A). The adlayer film is removed in situ and a second curve ob tained Fig. 7.5 (plot B) . The shift in mode index of the minimum can be used to determine the adlayer thickness if a value for its index is assumed. In this case the ad layer is a Polystyrene film less than 100 A thick (index in the visible nominally 1.58) deposited by spin coating from a very dilute solution - this can be removed com pletely in situ by flushing the surface with acetone as demonstrated in a prior experiment. The absolute value of the mode index of the bare structure is used to initialise the waveguide thickness (388 A with given values, taking nominal index 2.34 for Zinc Sulphide). The calculated thickness of the adlayer is only 38 A which is difficult to detect (let alone measure) at all reliably by ellip- sometry or surface stylus but is easily recorded by the present device. A second, somewhat thicker film was coated onto the structure and predicted to be 120 A, the same film film on a silicon wafer was determined to be 150 A thick (possibly due to a layer of 30 A of silica from thermal oxidation), surface stylus indicates 110 A (with error of 10 A). 7.3 Demonstration Of The Resolution Of the Device. To further illustrate the performance of the device and its practical resolution Fig. 7.6 plots the effect of depositing adlayers of PMMA of increasing thickness onto 174 Fig 7.5 Shift Of Waveguiding Structure Resonance Caused By A 40A Polystyrene Adlayer OUTPUT INTENSITY (Arbitrary Units) i 76 eeto O Icesn Alyr hcnse O PMMA Of Thicknesses Increasing Adlayer Of Detection 7.6 Fig the evaporated structure indicated in the diagram. The resonance curve for the uncoated structure is first plotted; the slide is then removed from the prism coated with the plastic adlayer and repositioned on the, prism as close as possible to its original location. The new resonance position is located with the dark band across the spot centre and the detector aperture is positioned at minimum intensity - the new resonance curve is then plotted. The plastic film is then removed in situ by flushing with acetone and the minima of the bare substrate relocated. Any offset between the measurements of the two minima for the bare structure due guiding film uneveness and incorrect positioning of the slide can then be ac counted for. Recalling that the guide itself is only 423 A thick, even an adlayer thickness of 100A is a sufficiently large perturbation to change the coupling strength and make the contrast of the intensity reduction in the output spot very poor. Adlayer films of thickness of order of a few tens of angstroms are therefore used. The adlayer thick nesses indicated in Fig. 7.6 were deduced from the measured shift in the minima. Because the layers used are so thin, it is difficult to obtain an independent estimate of their absolute values to compare with those determined in the present experiment. As a demonstration that the values deduced are internally consistent, Fig 7.7 is in cluded which plots the deduced film thickness against the speed of rotation used in the spin deposition. The ex perimental points lie to a fair approximation on a smooth curve whose shape is characteristic of the spin deposition 177 THICKNESS (A) i 77 acltd im hcnss ess pe O Rtto I Si Deposition Spin In Rotation Of Speed Versus Thicknesss Film Calculated 7.7 Fig process (the same function is obtained with films thick enough to be measured on a stylus instrument). Fig 7.6 indicates that measurement of film thicknesses (for com plete uniform coverage) of order of tens of angstroms with resolution of order of one angstrom are perfectly possible i.e comparable surface sensing performance to that of the SPR. 7.4 Comparison Of Dielectric Structure And SPR Sensitivities In order to make a general comparison of the sen sitivities of the dielectric structure and the SPR, a simple, nominal figure of merit which can be used is the ratio of the shift in the resonance position to the nominal width of the resonance. Figs 5.11, 5.12, 5.13 in dicated that for both air and aqueous cladding the mag nitude of the resonance shift for the SPR is approximately twice that of the dielectric structure for perturbation by thin films a few hundred angstroms thick. The unbuffered SPR with an air cladding has a resonance width of order 0.2 degrees whilst the minima of Fig 7.6 for the dielectric structure correspond to widths of order 0.1 de grees (compare mode index spread with those of Figs 4.1, and 4.2). For sensing with an air cladding it appears that the figures of merit are nearly equal - the SPR has a shift twice as large, but a resonance twice as broad as the dielectric. Because the experimental resonance width of the dielectric structure is limited by film evenness the nar rowest resonance available with this device using an 179 aqueous cladding is also 0.1 degrees. For the aqueous clad SPR however, the theoretical width is approximately 2.0 degrees (Flannagan 1984) a factor of twenty larger than the value in air. Recalling that the SPR shift with an aqueous cladding is only twice as large as the shift with the dielectric device (Fig 5.13), the dielectric device would appear to be an especially attractive alter native for aqueous sensing suggesting a figure of merit one order of magnitude larger. 7.5 Suggestions For Future Work (a ) Agueous Biosensing Although the above measurements, for the dielectric structure were performed with air cladding above the device structure, there is no difficulty in using an aqueous solution which deposits a surface organic adlayer as suggested in the previous references to biosensing. It should be acknowledged that although the structure does not wrinkle immediately on application of an aqueous ad layer and the water clad resonance is perfectly observ able, there is a deterioration in the surface smoothness of the deposited structure over a period of order 30 minutes as evidenced by the increasing broadening of the resonance and the poorer contrast of the region of low in tensity. (b ) Use Of Other Possible Thin Film Materials It has already been noted that a spacing layer with a index lower than that of Magnesium Fluoride would increase the power percentage in the sensing region slightly. 180 Given the deposition difficulties with fluorinated hydrocarbon films, the investment does not appear worthwhile. Reduction of scattering might be possible by attempting to decrease the crystal grain size of the MgFa by changing the deposition conditions. Obtaining sufficiently even films is likely to be a problem whatever depositon material is chosen because of the high sensitivity of the structure to this parameter which relates closely to the need for high sensitivity to adlayer perturbations (Fig. 5.2, Fig. 5.5). Zinc Sulphide seems to be the best readily available material for the high index guide layer although Zinc Selenide which is hard, durable, non-hygroscopic and transmits from 0.5 jum to 22^J.fY)yii ~ 2.5 in the visible could possibly be used. This material is used in multilayer coatings for laser mirrors in the infra red, deposited by CVD. (c ) Use Of Different Wavelengths In theory the excitation of the structure by a con tinuum of wavelengths could be used to determine the refractive index profile (normal to the film surfaces) of the perturbation adlayer because the decay of the evanes cent sensing field is scaled by the wavelength of radia tion used. If only two laser wavelengths are used then it is possible to determine both the adlayer thickness and index independently. The accuracy of determination of these parameters by this method improves with the separa tion of the two wavelengths used as this leads to sensing fields with widely differing field shapes. (d ) Alternative Excitation Structure In principle it is possible to replace the prism with 181 a grating on the opposite side of the slide to the waveguide structure so that the incident beam is dif fracted into the slide. If the grating is terminated so that subsequent reflections ocurr off the planar lower surface, the beam is totally internally reflected and can be monitored by a detector attached to the end of the slide - the optical bench is then effectively reduced to a microscope slide of several centimetres which is rotated in the path of a fixed laser beam. (e^ Grating Coupling Via An Embossed Grating Tiefenthaler (b) et al (1984) have made use of grat ing coupling excitation of thin film waveguides for detec tion of surface adlayers. In their device a hard inor ganic guide layer of Si0 2 /Ti0 2 is deposited by a "sol gel" process which allows the coupling grating to be embossed into the surface of the guide Lukosz (1983). The struc ture has a measured width of 14*10-4 which requires 9 monolayers of water (27 A) to switch between coupling states. The authors suggest that on the assumption of a fabricatable device width of 10“4 a single monolayer of water (3 A) would be sufficient to switch the device. Fur ther, if a reflectivity variation of 1% were detectable (as in other reflectometry techniques) an equivalent 0.03 A would be be measurable equivalent to 1/10 coverage by a monolayer. The attainment of such a narrow resonance width will however be limited, in exactly the same way as for the prism, by the uniformity of the waveguiding film it is possible to fabricate in practice. One implied disadvantage of prism coupling (compared to grating coupling) is the difficulty of large-scale, in 182 expensive, production repeatability if the sensing struc ture is to be fabricated onto a prism. Recalling the ex perimental arrangement of Fig. 1.3 shows that this dif ficulty can be easily overcome by depositing the structure onto a microscope slide which can be contacted repeatedly onto the prism with index matching oil. The choice is then between fabrication of a two-layer buffered waveguide arrangement and a single embossed guide layer onto the substrate glass microscope slide. 7.6 Conclusions It has been possible to fabricate on soda glass microscope slides a "Prism Coupler Surface Sensor" (based on the principle of optical coupling into a thin film waveguiding structure) which is capable of detecting sur face coverage by layers of the order of angstroms thick. This sensing structure can be simply mounted onto the face of a prism in an inexpensive optical arrangement. For films less than 500 A thick the sensor provides a better estimate of thickness than is available with ellipsometric techniques or surface stylus. In its mechanism of opera tion the device closely resembles that of the SPR on low loss metal films which has previously found wide applica tion for surface sensing. Unlike the SPR for which the propagation mode index is insensitive to metal thickness variation (because the guiding occurs primarily at a single interface), the mode index of the dielectric structure becomes critically de pendent on the guide thickness when the design is op timised for high surface sensitivity. 183 Theoretically the resonance width of the excitation can be decreased arbitrarily. Practical deposition of thin films however is subject to slight systematic non uniformity which combines with the sensitive mode index dependence on guide thickness to become the dominant fac tor limiting the attainment of very narrow resonances. For all but the most insensitive structures the ap pearance of the reflected laser spot relates to the thick ness variation of the high index guide layer - in the limit of a highly expanded incident beam, a surface profile of the latter is obtained. This indicates that the corollary of film unevenness limiting the resonance sharpness is that a straightforward and sensitive method of film thickness monitoring is identified - in the case of ZnS on MgF2 resolution on the scale of angstroms is possible. Qualitative demonstration of surface detection of vapour condensation and evaporation, and detection of in visible residues left by evaporated solvents suggest numerous applications. The experiments with dry nitrogen blown onto the sensing surface could possibly be developed to monitor gas flow rates or alternatively could be used to obtain information on the porosity of the evaporated films themselves. Invisible polymer adlayer films whose presence is un detectable by ellipsometry or surface stylus can be detected and measured easily by the "Prism Coupler Surface Sensor". Even with broadened resonances, films of order tens of angstroms can be measured with resolution of order of one angstrom. 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