SURFACE MEDIATED NONLINEAR OPTIC EFFECTS IN LIQUID CRYSTALS

by

JESSICA M. MERLIN

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Thesis Advisor: Kenneth D. Singer, Ph.D.

DEPARTMENT OF PHYSICS CASE WESTERN RESERVE UNIVERSITY MAY, 2007 CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the dissertation of

______

candidate for the Ph.D. degree *.

(signed)______(chair of the committee)

______

______

______

______

______

(date) ______

*We also certify that written approval has been obtained for any proprietary material contained therein.

To my parents iv

Table of Contents

Chapter 1 Introduction ...... 1 1.1 The Liquid Crystal Phases ...... 2 1.2 Liquid Crystal Applications...... 6 1.2.1 Liquid Crystal Displays ...... 6 1.2.2 Temperature Sensing...... 8 1.2.3 Photorefraction, Spatial Modulation and Optical Limiting...... 9 1.3 Thesis Organization...... 12 1.4 References...... 14

Chapter 2 Theory & Background ...... 16 2.1 Nematic Liquid Crystal Basics ...... 16 2.1.1 Order Parameter...... 18 2.1.2 Bulk Properties...... 19 2.1.3 Surface Properties...... 21 2.2 Nonlinear ...... 24 2.2.1 Friedericksz Transition...... 24 2.2.2 Adiabatic Following of Light...... 27 2.2.3 Mechanism of the ...... 29 2.2.4 The Photorefractive Effect in Liquid Crystals: Bulk Considerations...... 31 2.3 Surface Mediated Effects...... 35 2.3.1 Photorefractive Measurements...... 36 2.3.2 Electronic Measurements...... 40 2.3.3 Other Interesting Photorefractive Results...... 45 2.3.4 Optical Switching Applications ...... 48 2.4 Device Applications...... 49 2.5 Summary and Conclusions ...... 50 2.6 References...... 51

Chapter 3 Experimental Methods...... 58 3.1 Materials and Sample Geometry...... 58 3.1.1 Sample Geometry...... 58 3.1.2 Materials ...... 59 3.1.3 Properties of PVK-TNF ...... 63 3.1.4 C60 Doped Liquid Crystal Cells ...... 64 3.2 Experimental Apparatus...... 65 3.2.1 Friedericksz Transition Measurement...... 65 3.2.2 Gate Measurement ...... 67 3.2.3 Current Transient Measurement ...... 69 3.2.4 Optical Limiting Measurement...... 70 v

3.3 Summary...... 70 3.4 References...... 72

Chapter 4 Experimental Results: PVA ...... 73 4.1 Friedericksz Transition Shift...... 74 4.2 Optical Gate Effect ...... 77 4.3 Intensity Dependence of the Friedericksz Transition ...... 79 4.4 Optical Limiting...... 81 4.5 Conclusions...... 83 4.6 References...... 84

Chapter 5 Experimental Results: PVK ...... 86 5.1 Demonstration of the Surface Effect...... 87 5.2 Polarity Dependence ...... 89 5.3 Intensity Dependence...... 93 5.4 Thickness Dependence...... 97 5.4.1 Cell Thickness Dependence...... 97 5.4.2 Alignment Layer Thickness Dependence ...... 100 5.5 Electronic Measurements...... 102 5.5.1 Capacitance-Voltage Measurements...... 103 5.5.2 Dependence of the Friedericksz Transition Threshold ...... 103 5.6 Optical limiting and optical switching...... 104 5.7 Discussion & Conclusions ...... 105 5.8 References...... 107

Chapter 6 Summary of Results and Proposed Model, Comparison to the Literature, Discussion & Conclusions, and Future Outlook ...... 108 6.1 Summary of Results and Proposed Model for cells with PVK-TNF...... 108 6.2 Summary of Results and Proposed Model for cells with PVA...... 114 6.3 Disscussion and Comparison to the Literature ...... 116 6.4 Side by Side Comparison of cells made with PVK-TNF and PVA...... 120 6.5 Future Outlook...... 123 6.6 Discussion and Conclusions ...... 125 6.7 References...... 127

vi

List of Tables

Chapter 2 Table 2.1 characteristics for different combinations of liquid crystals and alignment layers...... 37 Table 2.2 Two-Beam coupling figures of merit for various cell types. The range of experimental condition for two-beam coupling suggests that a more direct approach is necessary to evaluate the mechanism as well as the device parameters...... 49

Chapter 3 Table 3.1 Some material characteristics of 5CB...... 60 Table 3.2 Some material properties of PVK...... 62

vii

List of Figures

Chapter 1 Figure 1.1 Graphical depiction of the degree of order for several materials...... 3 Figure 1.2 A schematic diagram of the nematic liquid crystal phase...... 4 Figure 1.3 Graphical interpretation of cholesteric liquid crystal with the rotation of the director, denoted by the red arrows, around the helical axis, the black arrow...... 4 Figure 1.4 A schematic diagram of a) the smectic A and b) the smectic C liquid crystal phase...... 5 Figure 1.5 A schematic diagram of a columnar phase in which the molecules tend to stack like coins...... 6 Figure 1.6 A schematic of the basic elements of a liquid crystal display, a liquid crystal material sandwiched between two polarizers...... 7 Figure 1.7 An example of a liquid crystal spatial light modulator...... 10

Chapter 2 Figure 2.1 Nematic liquid crystals have a preferred orientation known as the director, denoted by n ...... 17 Figure 2.2 Molecular structure of a nematic liquid crystal molecule where X and Y are the terminal groups and A is the linking group...... 18 Figure 2.3 The nematic liquid crystal molecule makes an angle, θ , with the director axis, z ...... 19 Figure 2.4 Depiction of a) splay, b) bend, and c) twist deformations...... 20 Figure 2.5 a) Planar and b) homeotropic alignment of nematic liquid crystal cells where the arrows indicate the preferred orientation of the director...... 22 Figure 2.6 The orientation of the molecule with respect to the bounding surface is defined by a polar, θ, and azimuthal, φ, angle...... 23 Figure 2.7 Two common twist configurations including a) planar twist and b)hybrid twist cells where the arrows indicate the easy direction at the bounding surface...... 25 Figure 2.8 Depiction of the orientation of the director in a planar twist cell where the arrows indicate the rubbing direction...... 27 Figure 2.9 The of incident light(red) is twisted by 90 degrees upon exit...... 28 Figure 2.10 A planar twist cell placed between crossed polarizers...... 28 Figure 2.11 A typical setup for two-beam coupling measurements...... 31 Figure 2.12 A schematic diagram of the process of photorefraction in liquid crystals.32 Figure 2.13 Schematic diagram of model for interface effects proposed by Pagliusi and Cipparrone...... 38 Figure 2.14 Circuit analysis used to model the impedence of the liquid crystal cell...... 42 Figure 2.15 Equivalent circuit used to predict current and voltage characteristics of a liquid crystal cell...... 43 Figure 2.16 Model based on desorption of charge under illumination...... 47 viii

Chapter 3 Figure 3.1 The typical construnction of the planar twist cells where the perpendicular rubbing directions are indicated...... 59 Figure 3.2 The molecular structure of the nematic liquid crystal, 5CB...... 59 Figure 3.3 The molecular structure of the polymer alignment layers, a) PVA and b) PVK...... 61 Figure 3.4 The molecular structure of TNF...... 62 Figure 3.5 An level diagram for a PVK-TNF composite.[8] ...... 64 Figure 3.6 Molecular structure of the fullerene, C60...... 64 Figure 3.7 Schematic diagram of the experimental setup for the Friedericksz transition measurement where the red line indicates the path of the HeNe and the green line indicates the obliquely incident pump ...... 66 Figure 3.8 A typical set of Friederkcsz transition measurements for the case of AC applied field, DC applied field and DC applied field with the pump where the AC voltage is RMS...... 67 Figure 3.9 A typical data set for the gate measurement where the vertical bars indicate the opening and closing of the electronic shutter...... 68 Figure 3.10 An example of the current transient measurement performed below the AC transition, above the DC transition and between the AC and DC transitions...... 69 Figure 3.11 Schematic depiction of the optical limiting measurement...... 70

Chapter 4 Figure 4.1 Plot of the Friedericksz transition threshold for an applied AC field, DC field, and DC with a pumping laser beam...... 74 Figure 4.2 Transmission as a function of a) voltage and b) time where the curves in b) correspond to the vertical lines of the same color in a)...... 75 Figure 4.3 The measured current as a function of time where at t=0, there is an initial jump in current followed by a decay...... 77 Figure 4.4 Plot of transmission as a function of time where the sample is held at a 4V DC, while a pumping beam is switched on and off as indicated in the figure, where the red line corresponds to the current and the black line to the transmission of the HeNe laser...... 78 Figure 4.5 Plot of contrast as a function of input intensity for a typical cell with a photosensitive PVA alignment layer ...... 79 Figure 4.6 Friedericksz transition measured with a pumping laser at various intensities...... 80 Figure 4.7 Friedericksz transition shift as a function of pump intensity...... 81 Figure 4.8 Demonstration of optical limiting capabilities of a twist cell with a PVA alignment layer...... 82

Chapter 5 Figure 5.1 The Friedericksz transition in a cell with the photoconducting PVK layer where the pump power is 1mW or 30 mW/cm2 of 488nm light from an argon ion laser...... 88 ix

Figure 5.2 Transmission and current are measured as function of time in the gate measurement where the pump is an argon ion laser operating at 488nm and an intensity of 26mW/mm2 and the applied voltage is 4.34 V...... 89 Figure 5.3 Transmission versus voltage for a hybrid liquid crystal cell...... 90 Figure 5.4 Voltage at 50% transmission for a hybrid cell where positive and negative correspond to the polarity of the PVK-TNF electrode...... 91 Figure 5.5 Experimental geometry for measurements with PVK-TNF on the a) positive and b) negative electrodes...... 92 Figure 5.6 Results for 25 µm thick, C60 doped hybrid liquid crystal cells of typical attributes...... 93 Figure 5.7 Plot of transmission as a function of applied bias voltage for an applied AC voltage, voltage and DC voltage with several pumping powers ...... 94 Figure 5.8 Friedericksz transition thresholds for several pumping powers...... 95 Figure 5.9 Gate measurements for several values of pump power on a cell containing a PVK-TNF layer...... 96 Figure 5.10 The contrast and time constant calculated for a cell with a PVK-TNF photoconducting layer...... 96 Figure 5.11 The Friedericksz transition threshold was calculated for cells of varying thickness...... 98 Figure 5.12 A plot of Friedericksz transition threshold as a function of pump power for several 50 mµ thick cells with 200nm thick PVK-TNF photoconducting layers...... 99 Figure 5.13 Friedericksz transition measurements on 25µm thick cells of two PVK- TNF thicknesses...... 101 Figure 5.14 Alignment Layer thickness dependence of the Friedericksz threshold voltage where the blue line indicates the AC transition threshold...... 102 Figure 5.15 Capacitance as a function of voltage for a typical cell with a PVK-TNF polymer layer...... 103 Figure 5.16 Friedericksz Transition Threshold as a function of the frequency of the applied AC field...... 104 Figure 5.17 Transmittance as a function of input power for a cell with a PVK-TNF polymer layer...... 105

Chapter 6 Figure 6.1 A schematic diagram of the proposed model for the surface effect...... 109 Figure 6.2 Plot of a) contrast and current and b) Friedericksz transition shift and current as a function of pump beam intensity...... 112 Figure 6.3 The circuit used by Barbero et al. to understand surface effects in liquid crystals...... 113 Figure 6.4 Energy level diagram for photoinduced charge injection from ITO...... 116 Figure 6.5 Cell geometry used in Ono and Kawatsuki...... 117 Figure 6.6 Contrast for two cells: one with just PVA and one with PVK-TNF...... 121 Figure 6.7 Optical limiting in two cells, one with a PVA and the other with a PVK- TNF layer...... 122

x

Acknowledgements

There are number of individuals to whom I would like to express my gratitude for their contributions to my graduate study. In particular, I would like to thank my advisor, K.D. Singer, for his knowledge and support throughout my time spent in his research lab. I would also like to acknowledge the members of my committee, C. Rosenblatt, R. Petschek, P. Mather and I. Shiyanovskaya. In addition to serving on my committee, Irina also spent a lot of her time teaching me physics and lab techniques when I first joined the research lab and continued to support me even after she left. I would also like to acknowledge the ideas and help from our collaborators Y. Reznikov and P. Korneychuk. Also of great benefit was the advice I received in the fabrication of liquid crystal cells from A. Gluschenko, I. Syed, Z. Huang, and R. Wang. R.S. Kurti has been a great source of encouragement since I met him when joining the group and for that I will always be grateful. The opportunity to work with undergraduate and high school students has been very valuable to my learning process and so I would like to acknowledge the hard work of M. Winkler, C. Woodward, E. Chao, K. Poseidon, D. Singer and D. Huang. While at case, I have had the opportunity to take classes from a number of talented professors and so I would like to acknowledge the physics department faculty. I would also like to thank the talented support staff for all the help that they provided to me. I would like to extend my thanks to T.J. Peshek for his advice about class work and lab work along the way. T.P. Eagan has been a source of personal and professional encouragement since I met him. P.D. Anderson has been my best friend and biggest supporter since I met him and I am grateful that we could go through graduate school together. Finally, I would like to acknowledge my parents for encouraging me to pursue my graduate degree and supporting me along the way. xi

Surface Mediated Nonlinear Optic Effects in Liquid Crystals

Abstract

by

Jessica M. Merlin

Liquid crystals have become a significant part of technology, mainly through their

use in the display industry. This is due in part to the fact that the optical properties of

liquid crystals are easily manipulated electronically. It has been recognized that the

optical properties liquid crystals may also be controlled using light. Because of this,

there are other various applications being explored for liquid crystals in photorefraction,

optical limiting and switching, and in spatial light modulators.

Although, the photorefractive effect was reported in liquid crystals over 10 years ago, there is still controversy over the exact mechanism for the reorientation of the liquid crystal director. This difficulty may be due in part to the fact that it is difficult to characterize the effect using photorefractive measurements and figures of merit.

The optical and electronic control of liquid crystals will be studied here using a

Friedericksz transition measurement in a twist cell geometry. This type of apparatus was chosen because it leads to a more direct demonstration of the surface effect. Namely, by studying changes in the Friedericksz transition threshold in a twist cell, a more direct observation of changes in the internal field may be observed.

First a brief introduction to liquid crystals and their role in technology will be presented. This will be followed by a more rigorous discussion of the physics of liquid xii crystals and a review of the important literature. The experimental apparatus and the materials and cell geometry used will be described followed by the results of those measurements. Finally, the results will be considered in terms of a model involving interfacial charge and discussed in the context of previous work. 1

Chapter 1 Introduction

The liquid crystal phase of matter was discovered in 1888 by Friedrich Reinitzer while studying the role of cholesterol in plants.[1, 2] Specifically Reinitzer found that a cholesterol ester had two melting points, the transition from a solid to a cloudy liquid at

145.5C and then to a clear liquid at 178.5C. Liquid crystal research continued strongly until after World War II.[1] Although there is no clear reason why research in this interesting area slowed, some speculation includes the idea that all the “important” liquid crystal physics had already been discovered. It is also postulated that some researchers were still unaware of the new phase of matter and there was a lack of useful device applications.[1] Around 1960, researchers once again became interested in the liquid crystal phases and so finally liquid crystals technologies began to emerge. Most notably, liquid crystals have become the leader of the display industry. In 1968, the first demonstration of a basic liquid crystal display was reported by researchers at RCA.[1]

Within a decade, liquid crystal devices were mainstream in the construction of watches and calculators. Today, liquid crystal displays(LCD) are used in a large number of portable electronic devices. Liquid crystals (LC) have other important applications, such as temperature sensing. Perhaps the progress of liquid crystal display technology may be attributed to the fact that liquid crystals are studied by a broad range of scientists including biologists, chemists, physicists, and engineers at a broad range of institutions, world-wide. 2

1.1 The Liquid Crystal Phases

The liquid crystal phase of matter is intermediate between crystalline solids and isotropic liquids and is sometimes called a mesomorphic phase or mesophase.[3]

Thermotropic liquid crystal phases are accessible by temperature changes as in the case of the cholesterol described above. In contrast, lyotropic liquid crystal phases are achieved by a change in concentration of the liquid crystal molecules. The liquid crystal of importance to this thesis is thermotropic, so here the discussion will be limited to that type of material.

Liquid crystals are named for the fact that the material has properties of both isotropic liquids and crystalline solids. Namely, molecules in the liquid crystal phase flow like a liquid, but maintain some order. The degree of order is less than that of a solid and is generally orientational, but some materials show positional order in one or two dimensions. Figure 1.1 depicts the differences between crystalline solids, isotopic liquids and liquid crystals.

The best way to characterize this phase of a material is by the degree of order.

Figure 1.1 depicts the degree of order for several materials. A crystalline solid has positional order in three dimensions as well as orientational order while an isotropic liquid has neither positional nor orientational order. Figure 1.1 indicates the increasing degree of order of the materials from left to right where the various liquid crystal phases fall somewhere between crystalline solids and isotropic liquids.

3

Figure 1.1 Graphical depiction of the degree of order for several materials.

Figure 1.2 depicts the nematic phase in which there is some degree of

orientational order. The molecules, on average, align along a common direction known

as the director, nˆ , indicated in the figure. The direction of the director is arbitrary in space unless the bounding media is treated for a specific alignment. Nematic liquid crystal phases are formed by either calamitic or discotic molecules meaning rod or disc shaped respectively. The molecules shown in Figure 1.2 are calamitic because of the long rod shape.

Many pure cholesterol esters form a liquid crystal phase known as the cholesteric phase.[3] This phase is shown in Figure 1.3 and may also be achieved by the dissolution of a chiral dopant in a nematic liquid crystal material. This state is characterized by a director that rotates around a helical axis.

4

Figure 1.2 A schematic diagram of the nematic liquid crystal phase.

Figure 1.3 Graphical interpretation of cholesteric liquid crystal with the rotation of the director, denoted by the red arrows, around the helical axis, the black arrow.

In addition to the orientational order observed in the nematic phase, the smectic phase also shows at least one dimensional positional order. This order is due to the layering of the molecules as shown in Figure 1.4. Figure 1.4 a) shows the Smectic A phase in which the director is parallel to the normal of the stacked layers while Figure 1.4 5 b) shows the smectic C phase in which the director is aligned uniformly at some angle to the normal of the layers.

Figure 1.4 A schematic diagram of a) the smectic A and b) the smectic C liquid crystal phase.

Another liquid crystal mesophase is the columnar phase which has orientational order and positional order in 2 dimensions as shown in Figure 1.5. The molecules in this phase tend to stack like coins. 6

Figure 1.5 A schematic diagram of a columnar phase in which the molecules tend to stack like coins.

In addition to the columnar phase, the disc shaped molecules also form nematic phases, known as discotic nematic phases. This phase is similar to the calamitic nematic phase, except for the difference in the shape of the molecules.

1.2 Liquid Crystal Applications

The most common use of liquid crystals in technology is in the display industry, and this is especially true in the last several years. Interestingly, liquid crystals are also found in other everyday objects such temperature sensors with various applications and smart windows. More recently liquid crystals have been studied for their potential in photorefractive devices, spatial light modulators and optical limiters.

1.2.1 Liquid Crystal Displays

The most obvious application of liquid crystals has been in the display industry.

This has been mainly due to the ease and low cost of production as well as the light 7 weight and low power consumption of a liquid crystal display. Although early displays utilized either a guest-host effect between a dichroic dye and a liquid crystal or exploited a cholesteric nematic transition, the first twisted nematic liquid crystal display (TN-LCD) was reported in the early 70’s.[4] This type of display is composed of a thin liquid crystal material sandwiched between a set of polarizers as shown in Figure 1.6. The surface of the glass confining the liquid crystal material is treated in such a way so as to induce a parallel alignment at either surface but with the director twisting by 90 degrees between the two surfaces.

Figure 1.6 A schematic of the basic elements of a liquid crystal display, a liquid crystal material sandwiched between two polarizers.

Polarizer A is aligned such that it is parallel to the liquid crystal director at the surface of the cell. Because of the adiabatic following of light, which will be discussed in more detail in Chapter 2, the polarization state of light incident at side A will exit side B with a polarization state that has followed the twist of the director of the liquid crystal.

Using an applied , the molecules may be untwisted causing the polarization state of the light to remain unchanged. The alignment of Polarizer B is crucial to the 8 operation of the device. Two different configurations can be created known as normally white(NW) or normally black(NB) in which the polarizer is parallel and perpendicular to the director at side B, respectively.[5] Figure 1.6 shows the basic elements for a NW mode.

The implementation of the above device into the familiar displays of today has evolved since the 70’s and a description and history of that evolution is outside the scope of this thesis. Many review articles and books have been devoted to a discussion of the interesting history, physics and entrepreneurship in the liquid crystal display field.[2, 4-8]

1.2.2 Temperature Sensing

Temperature sensing applications typically take advantage of the light reflecting properties of chiral nematic or cholesteric liquid crystals.[9] In particular, if under the illumination of white light, the pitch of the liquid crystal will determine which wavelength of light is reflected. This wavelength selectivity is the result of constructive interference of reflected light and occurs when the wavelength of the light and the pitch are of the same order of magnitude. The pitch of a cholesteric or chiral nematic is temperature dependent and so it obvious that a device may be constructed that would function as a thermometer. This type of device may be found in a number of applications that include fever thermometers for the forehead and more popularly, in mood or stress rings.[9] The advantage of this type of thermometer is in the size and cost as they can be easily and inexpensively implemented into applications.

9

1.2.3 Photorefraction, Spatial Light Modulation and Optical Limiting

Photorefraction is a change in the index of refraction under the influence of an

optical field. This effect was discovered by accident in LiNbO3 and LiTaO3 in 1966 by

Ashkin et al. and was initially thought to be detrimental to the nonlinear of devices

utilizing those materials.[10] Later, further research showed that this effect involved very

interesting physics as well as many device possibilities including holographic data

storage. In 1990, Sutter and Gunter demonstrated the photorefractive effect in the

organic crystal, 7,7,8,8-tetracyanoquinodimethane doped 2-cyclooctylamino-5-

nitropyridine.[11] A year later, Ducharme et al. reported the first observation of a

photorefractive polymer system utilizing the guest-host system, a hole transport agent,

diethylamino-benzaldehyde diphenylhydrazone, doped into the host polymer, bisphenol-

A-diglycidylether 4-nitro-1,2-phenylenediamine.[12] Since then, many systems have

been introduced that are generally polymer composites made of several materials, each

with their own special function. The physical details of photorefraction are left for

Chapter 2, but here a brief description of the applications is provided.

The application of liquid crystals in photorefraction has been studied intensely

since the first observation of a photorefractive-like effect in a liquid crystal material in

1994.[13] Although polymer organic materials in general have the advantage of lower

cost and easier production, liquid crystals also offer lower power consumption, making

them an interesting candidate for this type of application.[14] In general, applications of

photorefractive materials include image amplification and processing, correlation and

associative memories, and as holographic storage media.[15] 10

Spatial light modulators are optical components whose purpose is to provide a spatially modulated output in 1-3 dimensions that is based on some electrical or optical input.[16] An example of an optically addressed liquid crystal spatial light modulator is shown in Figure 1.7.[9]

In Figure 1.7, the writing light forms an image on the photoconductor, selectively changing the resistance of the photoconductor according to the intensity pattern.[9] The light is then absorbed by the blocking layer. By varying the resistance across the photoconductor, the effective voltage on the liquid crystal will change because of the series configuration of the cell. When the reading beams are incident from the opposite side of the device, the dielectric will reflect the light based on the orientation of the liquid crystal director.

Figure 1.7 An example of a liquid crystal spatial light modulator.[9] 11

Spatial light modulators have applications in many different technological areas, including displays and fiber optic communication systems.[16] Liquid crystals are an obvious choice for this type of device considering the ease of optical and electronic control of this material. Because there are so many different applications, there are several different types of configurations that have been developed. Dynamic scattering exploits the electrohydrodynamic flow resulting from the conductivity anisotropy of the material.[16] In this case the liquid crystal material produces a lot of scattering due to the

V turbulence of the liquid crystal molecules induced by an applied DC field that is 104 cm or greater.[16] Doping a liquid crystal with a dichroic dye is often employed in the construction of spatial light modulators and is known as the guest-host effect. This effect relies on the fact that the dyed liquid crystal medium will show strong or weak absorption based on the polarization of the light and that the dyes will follow the reorientation of the liquid crystal molecules. There are two field induced effects that are utilized in spatial light modulators known as field-induced nematic-cholesteric phase change and field- induced director axis reorientation as in the device described above. A field induced nematic-cholesteric phase change is simply the untwisting of the director upon application of an electric field of sufficient strength.[17] The thermal effects of an incident laser have also been used to create this device. Finally, micron sized droplets of liquid crystal may be dispersed in a polymer. This method takes advantage of the dielectric anisotropy of the liquid crystal materials. Namely, when under an applied field, the droplets will align and the will match that of the polymer host, allowing light to be transmitted. In the off-state, the droplets have a refractive index that is mismatched to the polymer host and therefore, there is significant light scattering. 12

Because of the widespread use of in research and industry, much effort has been focused on the development of optical limiters.[18] The use of this type of device ranges from protecting expensive semiconductor detectors to protecting the human eye.

In particular, the retina of the human eye may be damaged by visible or near IR light and this damage will depend on several variables including the intensity and wavelength of the incident light and exposure time. A successful optical limiter must attenuate the incident light before it causes damage to the eye or sensor. In the case of the human eye, there is a laser-induced damage threshold(LIDT) where damage occurs which is at a

J fluence of 1 , for visible radiation.[18] cm2

1.3 Thesis Organization

The aim of this thesis is to elucidate the optical and electronic control of liquid crystal devices. Specifically this is addressed through direct measurement of the optical dependence of the DC Friedericksz transitions in 90 degree twisted nematic liquid crystal twist cells. In addition to identifying the physical mechanism, device applications, specifically in optical limiting and photorefraction, will be identified and evaluated.

Chapter 2 is a thorough review of nematic liquid crystal bulk and surface properties with specific attention to electric field effects. The Friedericksz transition will be discussed. The photorefractive effect in organic materials will be considered with reference to a two beam coupling experiment. The mechanism of reorientation of the liquid crystal molecules electronically and optically is explored via a brief review of major contributions to the research in the last 15 years. The main focus will be on 13 surface mediated effects that have been observed by several leading research groups.

Additionally, electronic characterization of liquid crystals will be discussed.

Chapter 3 contains a description of the materials and processes for constructing nematic liquid crystal samples for study. In particular, properties of the polymers used for the alignment layers as well as the specific liquid crystal being used will be examined.

An in depth discussion of the experimental apparatus is included.

The results for cells with various parameters are discussed in Chapters 4 and 5 while an in depth discussion of the physical mechanism for optical reorientation of the liquid crystal molecules and the applications of this effect are left for Chapter 6. In particular, results from cells constructed with a polyvinyl alcohol (PVA) alignment layer are discussed in Chapter 4. This includes the results reported in Ref: [19]. Chapter 5 will cover the use of the photoconducting complex, Poly(9-vinylcarbazole) (PVK) doped with

2,4,7-trinitro-9-fluorenone(TNF) as an alignment layer. In Chapter 6, the model for the neutralization of the accumulated surface charge will be discussed in reference to both types of cells as well as the device quality that was attained. Comparisons to the literature will be investigated with reference to the results presented in this thesis.

Finally, the future outlook of this project will be discussed in Chapter 6. 14

1.4 References

1. Collings, P.J., Liquid Crystals: Nature's Delicate Phase of Matter. 2 ed. 2002, Princeton: Princeton University Press. 2. Kawamoto, H., The history of liquid-crystal displays. Proceedings of the IEEE, 2002. 90(4): p. 460-500. 3. DeGennes, P.G. and J. Prost, The Physics of Liquid Crystals. 2 ed. International Series of Monographs on Physics, ed. J. Birman, et al. 1993, Oxford: Clarendon Press. 4. Chigrinov, V.G., Liquid Crystal Devices: Physics and Applications. 1999, Boston: Artech House. 5. Yeh, P. and C. Gu, Optics of Liquid Crytal Displays. Wiley Series in Pure and Applied Optics. 1999, New York: John Wiley & Sons, Inc. 6. Schadt, M., Liquid crystal materials and liquid crystal displays. Annual Reviews of , 1997. 27: p. 305-379. 7. Scheffer, T. and J. Nehring, Supertwisted nematic (STN) liquid crystals displays. Annual Reviews of Materials Science, 1997(27): p. 555-583. 8. Seiberle, H. and M. Schadt, LC-conductivity and cell parameters; Their influence on twisted nematic and supertwist nematic liquid crystal displays. Molecular Crystals and Liquid Crystals, 1994. 239: p. 229-244. 9. Collings, P.J. and M. Hird, Introduction to Liquid Crystals: Chemistry and Physics. The Liquid Crystal Book Series. 1997, Philadelphia, PA: Taylor & Francis. 10. Ashkin, A., G.D. Boyd, J.M. Dziedzic, R.G. Smith, A.A. Ballman, J.J. Levinstein, and K. Nassau, Optically-induced refractive index inhomogeneities in LiNbO3 and LiTaO3. Letters, 1966. 9(1): p. 72-74. 11. Sutter, K. and P. Gunter, Photorefractive gratings in the organic crystal 2- cyclooctylamino-5-nitropyridine doped with 7,7,8,8-tetracyanoquinodimethane. Journal of the Optical Society of America B, 1990. 7(12): p. 2274-2278. 12. Ducharme, S., J.C. Scott, R.J. Twieg, and W.E. Moerner, Observation of the photorefractive effect in a polymer. Physical Review Letters, 1991. 66(14): p. 1846-1849. 13. Khoo, I.C., H. Li, and Y. Liang, Observation of orientational photorefractive effects in nematic liquid crystals. Optics Letters, 1994. 19(21): p. 1723-1725. 14. Zilker, S.J., Materials design and physics of organic photorefractive systems. Chemphyschem, 2000. 1: p. 72-87. 15. Solymar, L., D.J. Webb, and A. Grunnet-Jepsen, The Physics and Applications of Photorefractive Materials. Oxford Series in Optical and Imaging Sciences. Vol. 11. 1996, Oxford: Clarendon Press. 16. Efron, U., ed. Spatial Light Modulator Techonology: Materials, Devices and Applications. 1995, Marcel Dekker, Inc.: New York. 17. Heilmeier, G.H. and J.E. Goldmacher, Electric-field-induced cholesteric-nematic phase change in liquid crystals. The Journal of , 1969. 51(3): p. 1258-1260. 15

18. Guldi, D.M. and N. Martin, eds. Fullerenes: From synthesis to optoelectronic properties. Developments in Fullerene Science, ed. T. Braun. Vol. 4. 2002, Kluwer Academic Publishers: Dordrecht, The Netherlands. 19. Merlin, J., E. Chao, M. Winkler, K.D. Singer, P. Korneychuk, and Y. Reznikov, All-optical switching in a nematic liquid crystal cell. Optics Express, 2005. 13(13): p. 5024-5029.

16

Chapter 2 Theory & Background

This chapter will introduce the background information that is requisite for

interpretation of the data that will be presented and discussed in Chapters 4-6. First,

nematic liquid crystal physics will be introduced with a description of the order

parameter and the free energy as derived in DeGennes.[1] Next bulk and surface

properties of nematic liquid crystals will be discussed. Then the nonlinear optical

properties of liquid crystals will be introduced. The Friedericksz transition and adiabatic following of light will be introduced with a subsequent discussion of the photorefractive effect with particular attention to the mechanism in liquid crystals. A thorough review of

relevant literature will be given with special attention to surface induced photorefractive

effects in liquid crystals. In addition, important electronic properties and some device

applications will be discussed.

2.1 Nematic Liquid Crystal Basics

The liquid crystal phase is an intermediary state between the solid and liquid phases

that occurs in some materials. This phase exhibits material properties characteristic of

both solids and liquids. Of particular interest are nematic, calamitic liquid crystals which

are characterized by a rod-like shape and one long axis. This type of molecule shows

short range order and, in general, molecules in this phase tend to align along a common

direction known as the director, denoted n , as shown in Figure 2.1. In the absence of 17

external forces, the directors’ polarization is zero and so, nn= − , i.e. this type of material

is centrosymmetric so that the bulk dipole moment vanishes. [1-4].

Figure 2.1 Nematic liquid crystals have a preferred orientation known as the director, denoted by n .

Typical rod shaped liquid crystals have the general molecular structure shown in

Figure 2.2.[5] The main features of this type of molecule are the terminal groups, X , Y , the linking group, A , and the ring systems. The first terminal group often determines

some of the elastic properties and the transition temperature of the liquid crystal. These

groups are usually either an alkyl chain, an alkoxy chain or an alkenyl chain. The second

terminal group often determines the dielectric properties of the liquid crystal, specifically

when there is a positive dielectric anisotropy. The linking group can include a number of moieties such as a single bond between the rings or a diphenylethane, stilbene, tolane, azobenzene, or Schiff’s Base.[5] Finally, the nature of the ring system determines the optical absorption and of the liquid crystal as well as the dielectric and elastic properties. It also plays a role in the birefringence and viscosity of the material.

18

X A Y

Figure 2.2 Molecular structure of a nematic liquid crystal molecule where X and Y are the terminal groups and A is the linking group.

2.1.1 Order Parameter

Liquid crystals can be characterized by an order parameter which indicates the

average order of the bulk material. Consider the liquid crystal molecule shown in Figure

2.3. The angle between the director, n , and the long axis of the molecule, m is given by

θ . Microscopically the scalar order parameter is given by[1, 3, 4]:

1 S = ()()mn⋅⋅− mn 1 2 2.1 1 S =−3cos2 θ 1 2

In a perfectly aligned system, S =1, while in an isotropic liquid, S = 0 . For most

nematic liquid crystals, S =−0.6 0.8 .[3] Higher order terms may be invoked to define the order parameter, but in general these terms are even in order because of the centrosymmetric nature of the material.

19

Figure 2.3 The nematic liquid crystal molecule makes an angle, θ , with the director axis, z .

Macroscopically, the order parameter, Qαβ , can be written in a number of ways and is usually defined by writing the form of one of the static response functions of a material.[1] Any of the response functions including the electric polarizability, magnetic susceptibility or the dielectric function may be chosen. DeGennes writes the macroscopic order parameter for a uniaxial nematic system as[1]:

⎛⎞1 QQTnnαβ=−()⎜⎟ α βδ αβ 2.2 ⎝⎠3

A continuum theory was developed because variations in the macroscopic order

parameter,Qαβ , occur over much longer distances, , than the dimension of the liquid crystal molecules, a .[1] This results in a slight modification to Equation 2.2 :[1]

⎛⎞ 1a QQTnnαβ=−+( )⎜⎟ α ()rr β () δ αβ terms of higher order in . 2.3 ⎝⎠3

2.1.2 Bulk Properties

In the continuum theory outlined by DeGennes, the distortion free energy per volume is derived as: 20

11 22 1 2 FK=∇⋅+⋅∇×+×∇×()nnnnn K() K() 2.4 d 2212 2 3

where K1 , K2 , K3 are the elastic coefficients for the spay, twist and bend deformations respectively[1]. These distortions are shown visually in Figure 2.4. The distortion free energy may be further simplified by assuming a one constant approximation, i.e.

−7 KKKK123≈≈≈, all of which are generally on the order of 10 dynes, leading to

1 22 FK=∇⋅+∇×()()nn 2.5 d 2 { } where surface terms have been ignored[1]. It is valid to ignore the surface terms if the cell of interest is in the strong anchoring regime, meaning that the molecules are rigidly anchored to the surface. Inclusion of surface terms leads to a free energy given by:

1 FKnn=∂∂ 2.6 d 2 α βαβ where it is noted that this is valid in the one constant approximation.

Figure 2.4 Depiction of a) splay, b) bend, and c) twist deformations. Adapted from Ref [3].

In the case of a nematic cell under the application of an applied electric field, E ,

Barbero and Evangelista derive an equation for the free energy[2]:

1122 1 22 1 FK=∇⋅+⋅∇×+×∇×−⋅−⋅()nnnnnnEPE K() K()ε () 2.7 2212 2 3 2a 21

where ε a =−εε ⊥ is the dielectric anisotropy and P is the polarization induced by deformation. This differs from equation 2.4 by two terms which are due to specific phenomena; the first is the usual dielectric energy and the second is due to the flexoelectric effect[2]. This effect is an orientation induced polarization and is analogous to the piezoelectric effect. It consists of an electrical polarization that is induced by a mechanical distortion of the nematic bulk and has been discussed extensively in the literature.[6-10]

2.1.3 Surface Properties

The surface properties of nematic liquid crystals have been well studied, where specific attention has been given to anchoring energy by Barbero[2, 11-30] and others[9,

10, 31, 32]. In the typical cell geometry, the liquid crystal bulk will have its orientation induced by specific treatment of the boundary surfaces. The surface may be treated in a number of ways in order to induce a homeotropic alignment or planar alignment in which the molecules are perpendicular or planar to the glass respectively, as shown in Figure

2.5.

The orientation of the director is obtained by minimizing the free energy. In the case of strong anchoring, where the director is considered to be anchored with infinite strength, there is no need to add a surface energy term to the free energy. Instead the surface energy is left as a boundary condition.[1] This means that the free energy equation may be solved ignoring any terms due to surface energy.

22

Figure 2.5 a) Planar and b) homeotropic alignment of nematic liquid crystal cells where the arrows indicate the preferred orientation of the director.

In the case of weak anchoring, the treatment is more complex. An additional term containing the surface energy must be added to the free energy equation. The surface energy depends on the orientation of the molecules and their relation to the surface, nk⋅ , where k is the direction normal to the boundary. The orientation of the molecule and the surface can be defined by an azimuthal and polar angle, φ and θ respectively as shown in Figure 2.6. The surface free energy can be written as[2]:

FFss= (ΘΦ,,) + W(θφ −Θ −Φ) 2.8 where W is the anchoring energy function.

23

Figure 2.6 The orientation of the molecule with respect to the bounding surface is defined by a polar, θ, and azimuthal, φ, angle.

The anchoring energy is a measure of the energy it takes to rotate the molecules from the easy direction[2]. The expression is dependent on the anchoring condition of the director at the surface. The Rapini-Papoular expression is commonly used to describe the surface energy of a homeotropic film[2]:

1 FW= sin2 θ 2.9 s 2 where the anchoring energy may be measured using several types of optical techniques.

In addition to anchoring energy terms, there are other considerations that are important. A liquid crystal cell is considered a weak electrolyte due to intrinsic impurities. This suggests that there is the possibility of charge accumulation at the electrodes under the application of an applied electric field. Such an accumulation of carriers can shield the liquid crystal bulk from the applied electric field, causing the voltage applied to the cell to be dropped inhomogeneously. This is particularly important 24 for effects that require an applied field such as the Friedericksz transition, or the

Friedericksz transition mediated photorefractive effect, the main subject of this thesis.

2.2 Nonlinear Optical Properties

There are many nonlinear optical properties studied in nematic liquid crystals including and optical reorientation[33, 34]. Here the discussion will involve the effect of light and applied electric field on the reorientation of the liquid crystal director. More specifically, attention will be given to how charge accumulating at the liquid crystal-alignment layer interface contributes to the reorientation of the liquid crystal molecules and thus the Friedericksz transition and photorefractive effect and how impinging light may discharge that accumulated layer.

2.2.1 Friedericksz Transition

Liquid crystal alignment is imposed by surface treatments and can be either planar or homeotropic as shown in Figure 2.5 a) and b) respectively. Other types of cells can be created including planar twist cells and hybrid twist cells shown in Figure 2.7. In general, planar alignment is achieved by rubbing of a polymeric coated substrate with cloth, uni-directionally, while homeotropic alignment is achieved by coating the substrate with a surfactant. Common polymers used to align the nematic liquid crystals include

Polyvinyl alcohol(PVA) and a variety of polymers with imide backbones, polyimides(PI), both of which are commercially available. In some cases certain polyimides will induce homeotropic alignment. In addition, homeotropic alignment may be achieved by overbaking of certain polyimide substrates, such that there is a large pretilt angle 25 associated with that surface. In Figure 2.6, the pretilt angle is the angle with the substrate surface, θ .

Figure 2.7 Two common twist configurations including a) planar twist and b)hybrid twist cells where the arrows indicate the easy direction at the bounding surface.

The Friedericksz transition is a deformation of the liquid crystal alignment that has been imposed by boundary conditions. Generally, the Friedericksz transition is the result of an applied magnetic or electric field and can be observed in various types of cells including homeotropic, planar, and twist variations. The effect takes its name from

V. Friedericksz, who first reported this second order transition in 1927.[1] He showed that the critical field for deformation of the liquid crystal alignment is proportional to the inverse of the thickness of the liquid crystal sample. DeGennes calculates the critical magnetic field for several boundary conditions:

1 2 π ⎛⎞Ki Hti, = ⎜⎟ 2.10 d ⎝⎠χa where the subscript, i , indicates the type of distortion, pure blend, twist or splay, d is the

thickness of the liquid crystal cell and χa is the magnetic susceptibility anisotropy[1]. 26

This can be extended to the specific case of a nematic slab where the easy axis at the

boundaries is twisted by an arbitrary angle, ϕ0 , given by[1]:

1 ⎡ 2 ⎤ 2 KK320− 2 ⎛⎞ϕ HHtt()ϕ0 =+ ()01⎢ ⎜⎟⎥ 2.11 ⎣⎢ K1 ⎝⎠π ⎦⎥ In this thesis, the interest lies in applied electric fields, which can be easily calculated from the equations for the threshold magnetic fields using the transformation,

11 χεHE22→ [1]. For an applied electric field, the critical or threshold field is 28acπ ac given by[1, 3] :

14π 3K E = 2.12 t d Δε for the case of a planar aligned cell with the applied field perpendicular to the alignment plane. It is of interest to note that the electric field depends only on the thickness of the cell and the material properties of the liquid crystal, namely the elastic and dielectric properties. The applied voltage threshold is thus only related to the material properties

because of the relation, VEdtt=⋅. From this equation for the threshold, it is clear that the director will be determined by the interplay between elastic and dielectric properties of the material via K and Δε respectively.

This can be extended to the configuration of interest here, namely a planar twist cell with the applied field perpendicular to the alignment plane as shown in [1]:

1 ⎡ 2 ⎤ 2 KK320− 2 ⎛⎞ϕ EEtt()ϕ0 =+ ()01⎢ ⎜⎟⎥ 2.13 ⎣⎢ K1 ⎝⎠π ⎦⎥

Thus, in the case of strong anchoring, the threshold field will depend inversely on the thickness of the cell. Optically, the Fridericksz transition may be observed by appropriate 27 placement of the director of the liquid crystal cell between crossed polarizers. The cells of interest have a planar twist alignment, depicted in Figure 2.8.

Figure 2.8 Depiction of the orientation of the director in a planar twist cell where the arrows indicate the rubbing direction.

2.2.2 Adiabatic Following of Light

As detailed in Ref [35], an optical switch was created utilizing a 90 degree planar twist geometry. This type of cell has special properties if the measurement conditions are in the waveguide regime, where the pitch is much larger than the wavelength of the probing light. i.e., P >> λ .[3] In this case, light with a polarization direction parallel to the director at the incident surface will have its polarization twisted with the director of the cell. Figure 2.9 demonstrates the twisting of the polarization of the light with the twist of the director.

28

Figure 2.9 The polarization of incident light(red) is twisted by 90 degrees upon exit.

When this type of cell is placed between crossed polarizers, the orientation of the director may be monitored by the transmission of a laser as shown in Figure 2.10 . On the leftmost side of the picture, the polarization of the light is twisted with the director.

The red arrows indicate the twist of the polarization of the light. On the rightmost side of the picture, the director has undergone a Friedericksz transition and therefore the polarization direction of the light does not change, blocking the transmission of light. In this way it is possible to probe the orientation of the director using a laser beam and a pair of crossed polarizers.

Figure 2.10 A planar twist cell placed between crossed polarizers. 29

2.2.3 Mechanism of the Photorefractive Effect

Photorefraction has been well-studied in inorganic crystals for a number of years.

More recently, the focus of materials research for photorefractive materials has moved to organic materials such as organic crystals, polymers and liquid crystals for several reasons. Namely, the cost and ease of production has caused the shift as well as the ability to more closely tune the important photorefractive parameters by altering the material parameters.

Photoconductivity and a field dependent index of refraction are the requisite qualities for a photorefractive material.[36-39] In polymeric composites, photoconductivity is achieved by choosing a combination of polymer and sensitizer which together provide charge generation and transport. The field dependent index of refraction may be achieved by choosing a component which will undergo the electronic

Pockels effect or the orientational electro-optical or some combination of the two. A chromophore is generally chosen to satisfy this requirement because of its nonlinear optical characteristics. Upon application of an electric field, the chromophores will tend to orient along the field, contributing to both effects. Many composites utilize the material, poly(9-vinylcarbazole) (PVK) as the host material while a wide range of choromophores, sensitizers and plasticizers have also been used in PVK composites.[40]

The photorefractive effect is most easily elucidated in reference to a wave mixing experiment. Consider the interference of two coherent laser beams on a photorefractive material. These intersecting beams produce a spatially modulated interference pattern that causes charge excitation in the areas of constructive interference. In most polymers, 30 holes are the mobile carrier and will move either due to diffusion or drift under an external field. The charge carriers become trapped in areas of destructive interference creating a spatially modulated charge distribution and subsequently a modulated space charge field that is shifted from the charge distribution via Poisson’s equation. The

Pockels linear electro-optic effect leads to an index modulation given by [37]:

1 Δ=nnrEx3 () 2.14 pesc2

The electro-optical Kerr effect also contributes to the index modulation:[37]

3 Δ=nrE2 2.15 kk2n

where rk is the Kerr susceptibility. The chromophores are usually used to satisfy this requirement where contributions to the index modulation come from both the linear electro-optic effect and the electro-optical Kerr effect.

In order to verify that the effect is in fact the photorefractive effect, and not some photochromic or local effect, a two beam coupling measurement is usually performed. A typical two beam coupling experiment is shown in Figure 2.11. In the case of a photorefractive medium, asymmetric energy transfer will occur between the pump and the probe beams due to the phase shift between the charge distribution and the space charge field. The intensity pattern formed by two intersecting light beams on photorefractive sample is given by[34, 41]:

IIop =+0 (1cos m( qξ )) 2.16 where q is the grating and defined by[42]:

2π q = 2.17 Λ Λ is the grating spacing, and m is the modulation factor. 31

Figure 2.11 A typical setup for two-beam coupling measurements

2.2.4 The Photorefractive Effect in Liquid Crystals: Bulk Considerations

Khoo et al. reported the orientational photorefractive effect in a nematic liquid crystal cell in 1994.[43] This effect was different than those previously reported in other organic materials and was initially called a “photorefractive-like” or orientational photorefractive effect because the mechanism is a space charge field induced reorientation of the nematic director leading to a modulation of the refractive index. This effect was generally attributed to two mechanisms: the Carr-Helfrich effect and photoinduced conductivity changes in the cell.[4, 34, 41]

Figure 2.12 depicts the Carr-Helfrich process of photorefraction in liquid crystals.

Liquid crystals possess intrinsic impurities and so when under the action of an applied

DC field, a space charge field may be created which leads to reorientation of the nematic director.[4, 34, 41] Additionally, if the impurities are photosensitive or the liquid crystal is doped with photosensitive impurities, the impinging intensity pattern can selectively photo-excite these entities. The result is a space charge density grating that leads to a refractive index grating due to the induced director modulation as in the usual photorefractive effect. 32

Electro-optic response and buildup of refractive index grating

Reorientation of Molecules

Induced Space Charge Field

Figure 2.12 A schematic diagram of the process of photorefraction in liquid crystals.

The space charge field due to the Carr-Helfrich effect is given by[34, 44, 45]:

(σ −σθθ⊥ )sin cos EEΔσ = dc σ sin 2θσ+ ⊥ cos 2 θ 2.18 ()ε −εθθ⊥ sin cos EEΔε = dc ε sin 2θε+ ⊥ cos 2 θ

The photoinduced conductivity changes produce a space charge field given by[34, 41]:

⎡⎤(σσ− d ) ⎛⎞π EqmkTph=−νξ b ⎢⎥cos⎜⎟ q 2.19 ⎣⎦22eσ ⎝⎠ which is similar in form to the usually photorefractive effect. Here the illuminated and

dark conductivity are given by σ andσ d respectively and

DD+ − − ν = 2.20 DD+ + − where D+ and D− are the diffusion constants for positively and negatively charged ions.[41, 44-46] Khoo has shown that the each of these space charge fields significantly contribute to the effect and that in the case of methyl red doped 4’pentyl-4-cyanobiphenyl 33

(5CB), an effect can be observed with no applied field.[46, 47] The dependence of the index modulation on the field is quadratic and given by[4]:

2 Δ=nnE2 2.21

Two types of gratings can be formed in photorefractive media, volume and planar gratings.[48] Volume gratings, also known as thick gratings are characterized by only one diffraction order. In contrast, planar, or thin gratings, have multiple diffraction orders. Gratings formed in liquid crystals are planar and thus, photorefraction in liquid crystal samples is usually measured in the Raman-Nath regime. The Raman-Nath regime

2 ⎛⎞2π is valid for samples where dλ ⎜⎟. When a spatially modulated interference pattern ⎝⎠q is incident on the cells, one expects several diffraction orders. In general interest has been in the first order diffraction efficiency which is defined by;

2 ⎛⎞Δndπ η ∼ ⎜⎟ 2.22 ⎝⎠λ

It was found that by adding a small amount of dopant to the liquid crystals,<−12%, the nonlinear coefficients could be increased by a few orders of magnitude.[41] The dopants have included C60, Methyl Red, Rhodamine 6G and several different types of dyes. As mentioned above, doping 5CB with the azo dye, methyl red, has been well-studied by several research groups and has been show to produce not only an extremely large effect, but an effect that occurs without the need for an applied electric field.[33, 49-55] This implies that the dye molecules are able to produce a large enough torque on the molecules to overcome the intrinsic elastic involved without an applied field. 34

Janossy studied this phenomenon extensively with specific focus on azo and anthraquinone dyes that are dissolved in a liquid crystal host by up to a few percent by mass.[33] Microscopically, the mechanism was attributed to an asymmetric distribution of the dye molecules around the director so that a non-zero torque is exerted on the liquid crystal host molecules occurring only if the interactions between the host and the dye molecules are different in the ground and excited states.[33] Specifically the studies utilized homeotropically aligned samples in Friedericksz transition measurements to study threshold behaviors of the materials with and without dye dopant molecules.

The influence of the dye was initially observed in 1990 by Janossy et al.[50] and subsequently studied thoroughly.[33, 49-57] When a small amount of an anthraquinone dye was added to a nematic host; the optical power necessary to induce a Friedericksz transition in the cell was reduced by two orders of magnitude. The torque on the liquid crystal molecules can be defined by:

opt ⎡⎤22 2 2.23 Γ=ε 00⎣⎦nneo −(nene ×)( ⋅) E

The dye molecules introduce an additional torque on the host molecules given by[33]:

dye 2 Γ=εζ00(nene ×)( ⋅) E 2.24 where ζ is a parameter proportional to the concentration of the dye. This leads to a total optical torque given by[33]:

total ⎡⎤22 2 2.25 Γ=εζ00( +⎣⎦nneo −)()()nene × ⋅ E where in the case of a neat host, ζ = 0. In terms of this formalism, a second dye characteristic coefficient is defined, the molecular coefficient, given by:

ζ ξ = 2.26 αλ 35 where α is the orientational average of the extraordinary and ordinary absorption

ζ coefficients[33]. Additionally, the enhancement factor, , is defined as a figure 22 ()nneo− of merit for the doped cells. This quantity is the ratio of the dye induced and dielectric torques on the host molecules. Experimentally, ζ is determined by a z-scan measurement of the optical Kerr coefficient, where positive and negative values for ζ indicate self focusing and defocusing occurring in the samples respectively.[33] For a negative value of ζ , the dye molecules induce a torque that will align the host molecules perpendicular to the polarization direction of the incident optical beam.

In the azo dyes, an additional observation was made for a change in incident angle for the light. A critical angle exists where ζ changes sign. This was attributed to trans- cis isomerization of the azo dye. The cis and trans isomers contribute to ζ individually, with opposing signs. It was noted that no cis isomers will be excited if the polarization direction of the light is perpendicular to the director.

2.3 Surface Mediated Effects

Although the photorefractive effect described above has been described as a bulk effect, more recently, attention has been given to the effect of surface interactions. In

1996, Bartkiewicz and Miniewicz suggested that the mechanism for holographic recording in liquid crystals also relied largely on edge effects.[42] Since then, many research groups have focused on the surface mechanisms involved in the photorefractive effect in liquid crystals as well as possible device applications. Generally, surface 36 properties have been studied by means of two-beam coupling experiments and some current-voltage measurements.

2.3.1 Photorefractive Measurements

In 1997, Ono and Kawatsuki performed measurements on cells composed of a photoconducting layer, 2,4,7-trinitro-9-fluorenone (TNF) doped poly(9-vinylcarbazole)

(PVK) and an insulating/aligning layer composed of PVA. The cell was filled with the liquid crystal mixture E44.[58, 59] It was suggested that charges are generated in the

PVK layer and then become trapped in the PVA layer influencing the orientation of the molecules at the surface and thus in the bulk of the sample. Two beam coupling was performed on cells containing only the insulating PVA layer as well as cells which contained both the insulating and photoconducting layers. Diffraction patterns were not evident for the cells that had only PVA layers, which the authors suggest supports the photoconducting mechanism. Also, the authors confirmed that the mechanism only occurred in the presence of a DC field. It was determined that the PVA layer was necessary, as a stable grating was not formed in cells with only PVK aligning layers.

In contrast to those results, Mun et al. reported a study of surface effects in nematic cells composed of both a photoconducting layer and the so-called insulating layer in 2001.[60] Two beam coupling was performed in cells whose basic configuration consisted of PVK aligning layers and E7 liquid crystal. Iterations of doping the PVK, E7 or both with fullerene C60 were performed as well as adding a PVA insulating alignment layer between the PVK and E7 for each case. The largest diffraction efficiency was quoted for a cell with both the PVK and E7 layers doped with C60 with no PVA insulating layer. In fact, the diffraction efficiency reported for a similar cell with a PVA 37 layer is about 1/3 lower.[60] This effect was attributed to independent formation of a space-charge field in the PVK and E7 layers when a PVA insulating layer was present while in its absence generated charge could move between the layers. [60] Thus these results suggest that the PVA layer is detrimental to the desired results.

Pagliusi and Cipparone suggested that the sensitivity of the liquid crystal or alignment layer to light are not the only considerations for photorefractive performance but, additionally, a physical or chemical affinity between those two components.[61] The cell configuration used to test this hypothesis consisted of a combination of the nematic mixture E7 or a nematic alkyne compound with the alignment layers PVA and the

Hitachi Polyimide, LQ1800. Diffraction was observed for cells with the combinations of

PVA-E7 and the LQ18000-alkyne compounds, but not for the other combinations, suggesting that the photorefractive behavior is favorable in certain configurations.[61]

The results of the described measurements are shown in Table 2.1.

Liquid Crystal/Alignment Layer Combination Experimental Result

E7 + PVA Diffraction Observed

E7+LQ1800 No Diffraction Observed

Alkyne compound + PVA No Diffraction Observed

Alkyne compound + LQ1800 Diffraction Observed

Table 2.1 Diffraction characteristics for different combinations of liquid crystals and alignment layers. [61]

These results were followed by a more in depth study of the cells made with the

E7-PVA combination, with specific attention to light-induced charge redistribution.[62]

The results of a dark current measurement showed that the PVA does not block hole 38 injection from ITO. Additionally, lack of photo-induced current for the case of a cell composed of just PVA or just liquid crystal indicate that the interface between PVA and the liquid crystal is important. Finally, it was shown that the effect occurs when the PVA surface is at the anode. Pagliusi and Cipparrone proposed a model based on the above data that involves charge accumulation and injection at the alignment layer-nematic liquid crystal interface that is shown schematically in Figure 2.13.

Figure 2.13 Schematic diagram of model for interface effects proposed by Pagliusi and Cipparrone.[62]

First, a DC voltage is applied as suggested in Figure 2.13. It is proposed that holes are injected from the ITO into the PVA on the side of the sample labeled 1 resulting in a reservoir of holes.[62] Negative impurities intrinsic to the liquid crystal will then accumulate at electrode 1. When light is incident on side 1, charge flowing through the interface occurs due to a lowering of the energy barrier between the PVA and the LC allowing charge recombination. The result of the surface charge depletion is an increase on the voltage across the liquid crystal bulk. 39

Further, Kaczmarek et al. addressed the apparent disagreement between Mun et al. and Ono and Kawatsuki.[63] It is noted that the results of the two are similar and could be sample and experiment dependent. Because the opposing results deduced from measurements made by the two groups were two-beam coupling diffraction efficiency and gain, they can not provide direct proof of the mechanism. In fact, more conclusive experiments which probe the internal field as well as experiments in which the effect of light and cell geometry are addressed are necessary to elucidate the surface interactions.[63] Additionally, differences in results may be attributed to sample or experimental condition differences.[63] Kaczmarek et al. performed the usual two-beam coupling experiment on C60 doped PVK(N) cells filled with E7. Three configurations were considered including PVK-C60 on both sides, PVK-C60 on one side and a PI on the other and neat PVK on one side with PI on the other. A light dependent Friedericksz transition was observed in cells with at least one sensitized PVK layer while no threshold was observed for undoped PVK. The observation was attributed to a surface charge layer buildup at the liquid crystal-PVK interface. Additionally, two-beam coupling measurements demonstrated results similar to the previous studies with reorientation occurring in illuminated areas. More recently, that group has reported an intensity dependent Friedericksz transition for planar cells composed as above with PVK-C60 on one substrate and PI on the other filled with the liquid crystal E7.[64]

Some research has directed its focus on device applications of nematic cells with photoconducting layers showing particular interest in and spatial light modulation.[42, 65-74] After observation of the interface effect[42], the use of several types of doped photoconducting alignment layers were demonstrated in the cells[68, 70- 40

74] as well as the introduction of an SiO2 blocking layer between the electrode and photoconducting alignment layers.[65] More recently, Miniewicz et al. used an ellipsometric electroreflectance technique to demonstrate internal electric field changes with the application of a pump beam on a sample with a photoconducting polymer alignment layer.[75] A cell with a TNF doped PVK on one electrode and a polyimide on the other surface was filled with the liquid crystal 1298 MUT, a nematic mixture composed of seven molecules. A typical ellipsometric experimental configuration was used in which a light beam is passed through a polarizer set at 45 degrees and then through a compensator before reflecting off of the sample and traversing another polarizer set at -45 degrees before being measured by a photodiode. AC and DC power are measured at the photodiode as the polarization of the light is changed by the compensator. This ellipsometric measurement was performed with and without a HeNe pump beam. The short axis of the ellipse collapsed into a single line indicating a change in the effective voltage across the liquid crystal bulk with an impinging pump beam.[75]

2.3.2 Electronic Measurements

Attention has been given to the adsorption of charge at the interface between the liquid crystal bulk and the aligning layer, mainly because it is detrimental to the quality of liquid crystal displays. This is due to the fact that the ionic content of the liquid crystal bulk leads to problems with sticking of images to the screen as well as to increased voltage necessary to induce the Friedericksz transition due to the adsorbed charge.[76]

More generally, the interface between an electrode and a liquid may become charged due to either the ionization or the dissociation of surface groups or by the binding or adsorption of charges from the solution to the surface.[77] A layer of counter 41 ions forms to balance the surface charge in which the ions may or not be bound. Bound ions are described as being within the Stern or Helmholz layer while the unbound charge forms what is known as the diffuse electric double layer.[77] It is expected that the potential across the layer has a decay length known as the Debye length and is given by[77]:

−1 1 ⎛⎞2 22 . 2.27 = ⎜⎟∑ ρεε∞iizkT/ 0 κ ⎝⎠i

Kocevar and Musevic have used atomic force (AFM ) to demonstrate the electrostatic force between glass surfaces that are submerged in 5CB and 8CB.[76]

Using AFM, the Debye screening length, dissociated ion concentration, surface electric potention and surface charge density were determined. A Debye screening length of about 100 nm was reported for the studied commercial liquid crystals. Lu et al. measured the Debye screening length using a light scattering experiment and reported lengths of several thousand angstroms.[78]

In an effort to characterize the effects of the electric double layer on the performance of the liquid crystal device, many groups have focused on measuring dielectric characteristics of the liquid crystal cells under various conditions.[79-82]

Sawada et al. have done extensive studies on the properties of the ionic entities in

5CB.[83] Measurements were performed in 20.7 µm thick cells containing no alignment layer and using the liquid crystal, 5CB. Impedence measurements were taken across a temperature range of 23.1°C to 50.8°C where the nematic-isotropic phase transition occurs at 35.4°C. The circuit shown in Figure 2.14 was used to model the liquid crystal cell. Using this model, various cell parameters were extracted including conductivities. 42

Liquid Crystal

Double Layer Double Layer

CLC(ω)

CDL RLC(ω) CDL

C RDL LC RDL

RLC

Figure 2.14 Circuit analysis used to model the dielectric impedence of the liquid crystal cell. [83]

Moreover, some of the research that has been performed on the ionic conductivity of nematic liquid crystals cells has been focused on optimization for the display industry.

In particular, the transport and dynamics of charge carriers have an impact on important properties such as multiplexibility, grey scale, and crosstalk.[84] Schadt has devised a model in which the liquid crystal cells may be modeled using circuit analyses where there are two particular regimes that govern the properties of the cell. [84, 85] These regimes are defined by the type of current transients observed for different values of the quantity

ρLCV where ρLC is the resistivity of the liquid crystal and V is the amplitude of the driving voltage being applied to the liquid crystal.[84] Regime A and B are defined by

9 9 ρLCVmV<Ω10 and ρLCVmV>Ω10 , respectively where current transients follow a simple exponential decay in regime A, while in regime B, the current transients exhibit a more complex behavior. In addition to differences in current transients in the two regimes, the decay of the driving voltage follows a single exponential in regime A, and a 43 double exponential in regime B. These differences are attributed to the formation of a space charge field in regime B and a subsequent inhomogenous charge distribution and non-Ohmic resistivities.[84] Sieberle and Schadt proposed an equivalent circuit that is shown schematically in Figure 2.15.

Alignment Layer Liquid Crystal Alignment Layer ITO

CAL CLC CAL

RITO

RAL RLC RAL

Figure 2.15 Equivalent circuit used to predict current and voltage characteristics of a liquid crystal cell.[84]

In regime A it is assumed that all circuit elements behave linearly.[84] It is also assumed that the current as a function of time is given by:

I (tII) =+∞ 0 exp( − t /τ ) 2.28 where the time constant, τ , is given by:

ε ε AL + LC 2dd τ = AL LC 2.29 11 + 2ρρALdd AL LC LC

The current at t =∞ is given by:

V It()=∞ = 0 2.30 RLC++2RR AL ITO where ρ , d , and ε are the resistivity, thickness, and dielectric constant for the cell component and the subscripts AL and LC indicate alignment layer and liquid crystal 44 respectively. To verify that the model was indeed linear in Regime A, the thickness of the alignment layer and the liquid crystal as well as the resistivity of the liquid crystal were varied. Sieberle and Schadt give the voltage on the liquid crystal bulk as

222 RabVLC 0 VfLC ()= 22 2 2.31 [2RALaRbRab++ LC ITO ] +ω [2 RCaRCb AL AL + LC LC ] where

2 aRC=+1(ω LC LC ) 2 bRC=+1(ω AL AL ) 2.32 ωπ= 2 f which reduces to

RLC VtLC ()=∞ = V0 2.33 RLC++2RR AL ITO as time goes to infinity in this regime. Sieberle and Schadt also cite the expected voltage on the liquid crystal due to the conductivity of the liquid crystal:

⎛⎞ 2 ⎛⎞k33 ⎜⎟φm ⎜⎟+ c ⎝⎠k11 VV()φ ≅+⎜⎟1 2.34 LC⎜⎟4 th ⎜⎟ ⎝⎠ where

2 σ −σεε⊥ ⎛⎞ω − ⊥ + ⎜⎟ σωε⊥⊥⎝⎠0 c = 2 ⎛⎞ω 1+ ⎜⎟ 2.35 ⎝⎠ω0

σ ⊥ ω0 = εε0 ⊥

and the angle φm is the angle of the director at the center of the device. Equating the expressions for the voltage on the liquid crystals, Sieberle and Schadt calculate the 45

applied voltage needed to obtain a specific φm which corresponds to a percent transmission through the cell. This formalism leads to 4 frequency ranges in which different parameters govern the performance of the cells. Using this model, various parameters may be modified in order to optimize the performance of liquid crystal devices.

In Regime B, it is indicated that the above expressions no longer hold, mainly due to the nonlinear behavior of the circuit elements shown in Figure 2.13.[84] The high voltage behavior was determined by measuring the voltage transients of a cell submitted to a short pulse of various amplitudes. At high voltages the transients exhibited a double exponential behavior indicating the buildup of a space charge field in the LC.

More recently, Barbero et al. introduced a new circuit based model for the electronic behavior of liquid crystal cells.[86] Namely, the authors were interested in how variations in surface treatment affected the electrical response of the liquid crystal cell. The model introduces modeling the electric double layer formed at the surfaces as an additional component in the circuit introduced by Seiberle and Schadt.[84, 85] This work focused on the behavior of the current by considering the intrinsic impurities in the liquid crystal and their effect on the current dynamics. It was found that considering the effect of the Debye layer more consistently described the current dynamics than previously reported models.

2.3.3 Other Interesting Photorefractive Results

Several types of dopants, in addition to the dyes and fullerenes mentioned in

Section 2.2.4, have been added to liquid crystals in order to improve the photorefractive effect. G.P. Wiederrecht has led further research into photorefractivity in nematics doped 46 with linked and unlinked donor and acceptor pairs.[44, 87-91] Also, The addition of carbon nanostructures to liquid crystal cells has also been well-studied.[92-94]

In 1999, two beam coupling was reported by Choi et al. with cells composed of

C60 doped E7 aligned by PVK alignment layers.[95] The effect of the PVK was measured by construction of a cell without the layer and it was found that the gain coefficient was several times larger with the PVK layer. This was attributed to the charge transport and trapping properties of PVK.

Lee et al. reported the effect of C60 and carbon nanotubes (CNT) doped into a twisted nematic cell in 2004.[92] Transmission and capacitance as a function of applied voltage as well as a function of time upon application of a single voltage were measured for 3 types of 5CB cells including a neat cell, one C60 doped cell and one CNT doped cell.

The CNT doped cell exhibited a sharper Friedericksz transition which was attributed to the large dielectric anisotropy of the nanotubes. Later sudies by Lee and Lee indicate a diffraction efficiency of 47% for the CNT doped cells.[93] Additionally the same study found that a higher two beam coupling ratio was attained by using a lower molecular weight polymer as the alignment layer.

In addition to the experiments described above, in 2000, J. Zhang et al. reported a surface mediated effect in homeotropically aligned 5CB cells, namely the observation of holographic gratings under applied optic and electric fields.[96] Later, Boichuk et al. presented a model in which it is suggested that the electric field at the surface is reduced due to desorption of ions at the surface when under illumination.[97] This model is shown schematically in Figure 2.16 a-d where only the action of the holes is shown. 47

Figure 2.16 Model based on desorption of charge under illumination.[97]

Initially with no applied optical or electric field, charge is distributed symmetrically in the cell as shown in Figure 2.16 a. Figure 2.16 b shows the result of the applied electric field, namely the breaking of the symmetric distribution of the charge and the subsequent polarization of the cell. When applying only the optical field, the charge undergoes desorption and is brought back to the original symmetry as shown in Figure

2.16 c. Finally if both optical and electric fields are applied to the sample, the ion concentration is enhanced due to the desorption of the ions by the light and the accumulation of the charge under the applied field as shown in Figure 2.16 d.

While there are several models by various authors for the optical effect, all models for the surface effects incorporate the accumulation of charge at the interface and its discharge via the pump beam. The exact effect that the charge has on the orientation of 48 the director is the subject of controversy. This subject will be discussed further in

Chapter 6.

2.3.4 Optical Switching Applications

In 1999, Kim et al. reported the optical switching of a nematic liquid crystal cell aligned with photosensitive polyimide layers.[98] Cells planarly aligned by rubbed polyimides and filled with 5CB were placed between crossed polarizers such that transmission was maximized. The orientation of the liquid crystal molecules was monitored by measuring transmittance as a function of applied voltage. Experiments were performed with and without illumination by a pumping mercury lamp. Both samples showed a voltage threshold for reorientation of the molecules, with the pumped threshold shifted to a lower value.[98] The mechanism was attributed to a photophysical change of the alignment surface, resulting in a reorientation of the liquid crystal molecules.[98, 99]

Recently we reported an all optical switch based on a planar twist cell placed between crossed polarizers.[35] A nematic twist cell was constructed using PVA aligning layers and the liquid crystal 5CB, and placed between crossed polarizers to demonstrate the switiching effect. A weak He-Ne probe beam’s polarization rotates adiabatically so that the transmission is maximized through the crossed polarizers. An externally applied field at the edge of the Friedericksz transition in conjunction with a low power Argon Ion laser pump beam at 488 nm causes a reorientation of the director of the liquid crystal resulting in a transmission drop through the polarizers as the light is no longer adiabatically rotated. The mechanism proposed is based on surface charge accumulation under the action of an external DC field. The accumulated layer is 49 discharged by photo-injection of carriers via the pump beam. This experiment and the results will be discussed in more detail in later chapters.

2.4 Device Applications

In order to analyze the device quality of a liquid crystal cell, it is useful to introduce a figure of merit. In general for photorefractive samples either Δn or the photorefractive gain, Γ , is reported as a measure of the device performance. Table 2.2 lists figures of merits obtained as well as the materials used and the experimental conditions under which the results were obtained that have been reported by several research groups on the different materials. It is difficult to compare the reported data due to the range of materials and experimental setups used.

Alignment Layer Liquid Crystal Δn -1 Experimental Details Reference Γ(cm ) PVK C doped E7 −3 He-Ne 5mW/2mm [95] 60 1.02× 10

PVA C60 doped 5CB −3 He-Ne few mW [46] 1.4× 10 V=1.5V PVA 5CB −3 Argon Ion 514nm [43] 0.16× 10 C60 doped 5CB 2890 Argon Ion 488 nm [100] PVK+PVA E44 10 μm −3 3V P=33mW, BS =3mm [58] 3.6× 10 48 Polythiophene+Disperse E7 2600 Argon Ion 514nm, [70] Red 1 I=100mW/cm2, 10mW/cm2 0.6V/μm PVK-TNF(83:17) 100 nm E7 10 μm 3700 514 nm, p-polarized [68]

Table 2.2. Two-Beam coupling figures of merit for various cell types. The range of experimental condition for two-beam coupling suggests that a more direct approach is necessary to evaluate the mechanism as well as the device parameters.

In addition to holographic applications, this type of cell can be used for other devices including optical limiters and switches. Recent optical limiting research has focused on reverse saturable absorbers and two absorption effects,[101] although 50 some research has been done on liquid crystal materials.[47, 102-108] Recently we introduced an all optical switch that has additional applications in optical limiting.[35]

2.5 Summary and Conclusions

In this chapter the mean field theory outlined in DeGennes[1] was introduced.

This was followed by a description of the important optical effects such as the

Friedericksz transition, adiabatic following of light and the photorefractive effect.

Subsequently, a thorough review of the literature was provided in order to motivate the the results that will be discussed later in the thesis. Finally device applications and figures of merit for liquid crystals were discussed briefly. In the following chapters, a method for a more direct method of extracting the physical mechanism as well as the characterization of the optical switching effect will be presented.

51

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97. Boichuk, V., S. Kucheev, J. Parka, V. Reshetnyak, Y. Resnikov, I. Shiyanovaskaya, K.D. Singer, and S. Slussarenko, Surface-mediated light- controlled Friedericksz transition in a nematic liquid crystal cell. Journal of Applied Physics, 2001. 90(12): p. 5963-5967. 98. Kim, G.H., S. Enomoto, A. Kanazawa, T. Shiono, T. Ikeda, and L.S. Park, Optical switching of nematic liquid crystal by means of photoresponsive polyimides as an alignment layer. Applied Physics Letters, 1999. 75(22): p. 3458- 3460. 99. Kim, G.H., S. Enomoto, A. Kanazawa, T. Shiono, T. Ikeda, and L.S. Park, Application of photosensitive polyimides as alignment layer to optical switching devices of a nematic liquid crystal. Liquid Crystals, 2001. 28(2): p. 271-277. 100. Khoo, I.C., B.D. Guenther, M.V. Wood, P. Chen, and M.Y. Shih, Coherent beam amplification with a photorefractive liquid crystal. Optics Letters, 1997. 22(16): p. 1229-1231. 101. Spangler, C.W., Recent Development in the design of organic materials for optical power limiting. Journal of Materials Chemistry, 1999. 9: p. 2013-2020. 102. Khoo, I.C., J. Ding, A. Diaz, Y. Zhang, and K. Chen, Recent studies of optical limiting, image processing and near-infrared nonlinear optics with nematic liquid crystals. Molecular Crystals and Liquid Crystals, 2002. 375: p. 33-44. 103. Khoo, I.C., H. Li, P.G. LoPresti, and Y. Liang, Observation of optical limiting and backscattering of nanosecond laser pulsed in liquid-crystal fibers. Optics Letters, 1994. 19(8): p. 530-532. 104. Khoo, I.C., R.R. Michael, and G.M. Finn, Self-phase modulation and optical limiting of a low-power CO2 laser with a nematic liquid-crystal film. Applied Physics Letters, 1998. 52(25): p. 2108-2110. 105. Khoo, I.C., M.V. Wood, B.D. Guenther, M.Y. Shih, and P.H. Chen, Nonlinear absorption and optical limiting of laser pulses in a liquid-cored fiber array. Journal of the Optical Society of America B, 1998. 15(5): p. 1533-1540. 106. Khoo, I.C., M.V. Wood, M. Lee, and B.D. Guenther, Nonlinear liquid-crystal fiber structures for passive optical limiting of short laser pulses. Optics Letters, 1996. 21(20): p. 1625-1627. 107. Khoo, I.C., R.R. Michael, R.J. Mansfield, R.G. Lindguist, and P. Zhou, Experimental studies of the dynamics and parametric dependences of switching from total internal to transmission and limiting effects. Journal of the Optical Society of America B, 1991. 8(7): p. 1464-1470. 108. Khoo, I.C., P. Zhou, R.R. Michael, R.G. Lindquist, and R. Mansfield, Optical switching by a dielectric-cladded nematic film. Quantum Electronics Letters, 1989. 25(8): p. 1755-1759.

58

Chapter 3 Experimental Methods

This chapter will present the materials and the sample geometry as well as

introduce the experimental apparatus that was used in the experiments. First the twist

geometry used in all experiments will be described. Details of sample preparation and

fabrication will be given. This will be followed by a description of all the experiments

used to obtain the results given in chapters 4 and 5.

3.1 Materials and Sample Geometry

In order to optimize the performance of the liquid crystal cells and elucidate the

physical mechanism, it is important to choose materials and the cell geometry carefully.

Here, a planar twist configuration was chosen because the device applications exploit the

adiabatic following of the light through the cell. Additionally, the Friedericksz transition

measurement approach provides a clear demonstration of the surface mediated effects. In this chapter, the materials and experimental apparatus will be discussed at length.

3.1.1 Sample Geometry

Figure 3.1 illustrates the typical construction of the planar twist cells. The

polymer alignment layer is cast from solution onto Indium tin Oxide (ITO) glass by spin

coating at a rate of 2000-3000 RPM for 30 seconds. Prior to the deposition of the

polymer layer, the ITO glass was cleaned by ultrasonication in a series of solvents

including Alconox detergent, de-ionized water, acetone, methanol, and isopropanol. 59

After baking for removal of the solvent, the polymer layers are then rubbed uni-

directionally with a cloth covered brick in order to define a preferred alignment direction.

Two polymer coated glass substrates are placed with rubbing directions orthogonal and separated with either mylar strips or glass bead spacers with thicknesses varying from

12.5-50 μm. The cell is then sealed on two sides with epoxy and subsequently filled with the liquid crystal, 4-pentyl 4-cyanobiphenyl (5CB), via capillary action at room temperature. The planar twist configuration is verified by using a light box and crossed polarizers or a polarizing light microscope. The thickness of the alignment layers was measured using a Dektak profilometer.

Figure 3.1 The typical construnction of the planar twist cells where the perpendicular rubbing directions are indicated.

3.1.2 Materials

The molecular structure of the nematic liquid crystal, 5CB, used for all

experiments is shown in Figure 3.2.

Figure 3.2 The molecular structure of the nematic liquid crystal, 5CB. 60

This liquid crystal is commercially available and has the trade name K15. This is an alkylcyanobiphenyl liquid crystal and is composed of an aromatic ring system in which two phenyl groups are linked by a single bond.[1] This configuration is known as a biphenyl ring system. The first terminal group is composed of a long hydrocarbon tail, an alkyl group(pentyl). The second terminal group is composed of a polar cyano group.

Some important material properties of 5CB are listed in Table 3.1. This liquid crystal is in the nematic phase at room temperature.

Dielectric Constants ε& , ε ⊥ 19.7 , 6.7 [2, 3]

514.5nm [3]

Refractive Indices ne , no 1.5442, 1.7360 632.8nm 1.5309, 1.7063

Elastic Constants k1 , k2 , k3 6.4 , 3 , 10 [2, 3]

()10−12 N

Table 3.1 Some material characteristics of 5CB.

Choosing the alignment layer is crucial in determining the performance of the liquid crystal devices studied here as well as understanding the surface effects. Two different polymer alignment layers were used, both a photosensitive and a photoconductive material. The photosensitive material chosen was polyvinyl alcohol(PVA), shown in Figure 3.3 a) , because it has been well-studied and is reported to have a strong anchoring capability with 5CB. This material was purchased from Gallade chemicals, a distributor for EMD Chemicals. The PVA was dissolved in de-ionized water and subsequently mixed by ultrasonication. The mixed solution was then filtered 61 and cast by spin-coating onto ITO glass as described above. The concentration of the

PVA in the de-ionized water is one factor in determining the thickness of the cast layer.

For layers <100nm , the solution was approximately 0.5% by mass of PVA to the de- ionized water. The substrates were baked at a temperature of 80 degrees for 2 hours to remove any remaining solvent.

Figure 3.3 The molecular structure of the polymer alignment layers, a) PVA and b) PVK.

Poly(9-vinylcarbazole)(PVK) , shown in Figure 3.3 b) was chosen as a photoconductive layer. This material was purchased from Sigma-Aldrich. The PVK was dissolved in a 5:1 solution of toluene and cyclohexanone. The concentration of PVK in the solvent solution was varied between ~2%-6% to achieve substrates with layer thicknesses ranging from ~100nm-600nm. The spin coated substrates were baked at 90 degrees for 30 minutes and then 180 degrees for 60 minutes. The electronic properties of this polymer have been studied extensively.[4-6] Material properties of PVK are listed in

Table 3.2. 62

Properties of Poly(N-vinylcarbazole)(PVK) Reference

Dielectric Constant(at 10^6 3.0 [7]

Hz)

Volume Resistivity ()Ωcm 1016 [7]

Surface Resistance()Ω 1014 [7]

Refractive Index 1.69-1.70 [7]

Table 3.2 Some material properties of PVK.

Charge transport in PVK is unipolar with holes being the mobile carrier and proceeds by a hopping mechanism. In order to utilize the photoconductive properties of

PVK, a sensitizer, 2,4,7-trinitro-9-fluorenone(TNF) , was added to the polymer solution before it was cast onto the ITO coated glass. The complex of PVK and TNF will be referred to as PVK-TNF. The molecular structure of TNF is shown in Figure 3.4 and was purchased from Accustandard. The ratio of the sensitizer to the polymer was 2% by mass.

Figure 3.4 The molecular structure of TNF.

63

In the case of the TNF doped PVK samples, a thin layer (<100nm) of PVA was additionally cast by spin coating on top of the PVK layer. This was crucial to the stability of the cells as the PVK twist cells typically showed poor alignment within 1 hour without the additional PVA layer.

3.1.3 Properties of PVK-TNF

As indicated above, TNF was added to PVK to make the composite photoconducting in the visible. PVK-TNF composites have been studied for a number of years because of its photoconductivity, especially in photorefractive applications. The important feature of both PVK and TNF is the delocalization of π-electrons above and below the plane of the molecule. Typically, the TNF couples to the carbazole side chain of PVK to form the charge transfer complex. The result of this complex is an absorbance shoulder that extends across the visible spectrum, a characteristic that is not observed in either of the neat materials.[8] An energy level diagram for a PVK and TNF composite is shown in Figure 3.5 , adapted from Ref [8] . The model shown in the diagram suggests that the first excited state, CT1, of the PVK-TNF complex is the creation of an ion pair, with the carbazole unit of PVK becoming positively ionized and the TNF becoming negatively ionized.[8] 64

Figure 3.5 An energy level diagram for a PVK-TNF composite.[8]

3.1.4 C60 Doped Liquid Crystal Cells

In order to verify the polarity dependence of the devices, several liquid crystal

cells were doped with fullerene, C60. The molecular structure of C60 is shown in Figure

3.6. The material is dissolved in the 5CB at a concentration of less than 0.1% by mass.

Homogeneity of the solution is ensured by mixing the C60 and 5CB with a magnetic stir bar.

Figure 3.6 Molecular structure of the fullerene, C60. 65

3.2 Experimental Apparatus

Typically surface mediated effects in liquid crystal cells have been characterized by photorefractive measurements. Here a Friedericksz transition measurement configuration is used for all measurements, i.e. a twist cell is placed between crossed polarizers. The different measurement techniques are described below.

3.2.1 Friedericksz Transition Measurement

Figure 3.7 shows a schematic diagram of the experimental setup used to measure the Friedericksz transition in the twist cells. A weak probe beam from a HeNe laser with wavelength, 632.8 nm, polarized parallel to the first polarizer, is incident upon the liquid crystal cell. The rubbing direction, and thus the director, is parallel to the polarization direction of the first polarizer. If no voltage is applied or the applied voltage is below the

Friedericksz transition threshold, the light rotates with the liquid crystal director because it is operating in the adiabatic regime.[1] The light exits the cell with its polarization parallel to the rubbing direction on the exit boundary of the cell and the polarization direction of the polarizer and is collected by a photodiode. The intensity is then read as a voltage on a Keithley 2000 multimeter.

66

Figure 3.7 Schematic diagram of the experimental setup for the Friedericksz transition measurement where the red line indicates the path of the HeNe and the green line indicates the obliquely incident pump laser.

If a voltage above the Friedericksz transition threshold is applied, the director rotates such that it is almost perpendicular to the substrate, to nearly a homeotropic alignment. In this case, the light does not rotate adiabatically and upon exiting the cell has a polarization which is perpendicular to that of the second polarizer. In this case, light exiting the cell will not pass through the second polarizer. The cell has two states, on, at maximum transmission, the director of the cell twists by 90 degrees between the two boundaries and off, above the Friedericksz threshold, the molecules are almost perpendicular to the substrates, at minimum transmission.

The measurement entails monitoring the transmission through the crossed polarizer setup as a function of applied bias voltage. Data from typical Friedericksz transition measurements is shown in Figure 3.8. Transmission is monitored as a function of applied AC or DC voltage. In some cases a more powerful (mW power) pump beam with wavelength of 532nm is incident obliquely on the sample throughout the measurement.

This measurement was used extensively to study the physical mechanism of charge accumulation and discharge via a pumping laser at the electrodes. By changing 67 the thickness of the liquid crystal cell or the alignment layer, changes in the screening field can be monitored. In addition to changing the material thickness, the polarity dependence can be studied by doping the liquid crystal with fullerene, C60.

Friedericksz Transition 10

9 ac 8 DC DC w/pump 7 6 5

4 3 2 1 Transmission (arb units) 012345678 Bias Voltage (V)

Figure 3.8 A typical set of Friederkcsz transition measurements for the case of AC applied field, DC applied field and DC applied field with the pump where the AC voltage is RMS.

It was found that the DC threshold was shifted to a higher voltage than the AC threshold and that the pump beam could shift the DC threshold to a lower value. Detailed results will be discussed in chapter 4 and chapter 5 for PVA and PVK respectively.

3.2.2 Gate Measurement

The gate measurement monitors transmission of the probe beam as a function of time while the pump beam is switched on and off. This measurement also utilizes the experimental apparatus diagramed in Figure 3.7. In this measurement, the sample is biased at a DC voltage which is near the edge of the Friedericksz transition threshold. A typical gate measurement is shown in Figure 3.9. The vertical blue bars indicate the opening and closing of an electronic shutter. 68

Gate Measurement 10 0.77 8 0.76

6 A) 0.75 μ

4 0.74 Transmission

Transmission Current 2 0.73 Current (

0 0.72 0 50 100 150 200 250 300 350 Time (s)

Figure 3.9 A typical data set for the gate measurement where the vertical bars indicate the opening and closing of the electronic shutter.

At the start of the measurement, a voltage is applied to the cell. At t = 0 , transmission through the sample and the crossed polarizers is a maximum. At some time later, the pump beam is applied to the sample via the electronic shutter, indicated by the first vertical bar in Figure 3.9, and the transmission through the sample drops to a minimum. When the optical field is removed, the sample returns to its maximum value, indicated by the second vertical bar. Therefore, this measurement clearly depicts the optical switching action of the device.

In addition to demonstrating the optical switching effect, a device figure of merit, the contrast, as well as the switching time may be extracted from this measurement. The contrast is given by:

II− Contrast = max min 3.1 IImax+ min

69

In addition to monitoring the transmission, the current was measured as a function of time as the pump beam is applied and removed. This measurement was performed in order to detect changes in current with and without the action of the laser beam. Figure

3.9 also shows the current measurement when the electronic shutter is activated at the vertical bars.

3.2.3 Current Transient Measurement

Current transient measurements were performed in order to verify the role of current in the liquid crystal cells under an applied electric field. The experimental setup consists of monitoring the current upon application of an applied bias. Specifically, this experiment was performed in three regimes, 1) below the AC Friedericksz transition threshold, 2) above the DC Friedericksz transition threshold and 3) between the AC and

DC Friedericksz transition thresholds. The current was monitored using a Keithley electrometer. A typical current transient measurement for the three regimes is shown in

Figure 3.10

10 9 8 1V 7 4V 5V 6 5

4 3 2

Transmission (arb units) 1 0 0 50 100 150 200 250 300 Time (s)

Figure 3.10 An example of the current transient measurement performed below the AC transition, above the DC transition and between the AC and DC transitions. 70

3.2.4 Optical Limiting Measurement

The optical limiting measurement is a slightly modified version of the experiment pictured in Figure 3.7, as shown in Figure 3.11. The pump beam is rerouted to follow the path of the probe beam and becomes both the pump and probe beam. The output power is measured as a function of input power.

Figure 3.11 Schematic depiction of the optical limiting measurement.

From this measurement, a second figure of merit can be extracted, the transmittance,

Poutput Transmittance = 3.2 Pinput and is plotted as a function of input voltage. This figure of merit indicates the quality of this device for applications in sensor and eye protection. The results of this experiment will be detailed in Chapters 4 and 5.

3.3 Summary

In the chapter, several experimental measurements were used that utilized the same basic apparatus, a twist cell placed between crossed polarizers. This cell geometry 71 and experimental apparatus was chosen very carefully. By selecting a nematic twist cell for this experiment, the adiabatic following of light may be exploited. Coupling the cell geometry with the experimental setup provides an easily measured Friedericksz transition as well as a useful demonstration of the device applications. Cells using the different polymer layers were measured and those results will be presented in Chapters 4 and 5 and discussed in terms of a theoretical model in Chapter 6. 72

3.4 References

1. Collings, P.J. and M. Hird, Introduction to Liquid Crystals: Chemistry and Physics. The Liquid Crystal Book Series. 1997, Philadelphia, PA: Taylor & Francis. 2. Yeh, P. and C. Gu, Optics of Liquid Crytal Displays. Wiley Series in Pure and Applied Optics. 1999, New York: John Wiley & Sons, Inc. 3. Blinov, L.M. and V.G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials. Partially Ordered Systems. 1994, New York: Springer-Verlag New York Inc. 4. Mort, J., Polymers as electronic materials. Advances in Physics, 1980. 29(2): p. 367-408. 5. Ostroverkhova, O., Nonlinear optical probes and processes in polymers and liquid crystals, in Physics. 2001, Case Western Reserve University: Cleveland, Ohio. 6. Weiser, G., Absorption and electroabsorption on amorphous films of polyvinylcarbazole and trinitrofluorenone. Physica Status Soldi A, 1973. 18: p. 347-359. 7. Domininghaus, H., Plastics for Engineers: Materials, Properties, Applications. 1988, Munich: Hanser Publishers. 8. West, D. and D.J. Binks, Physics of Photorefraction in Polymers. Advances in Nonlinear Optics. 2005, Boca Raton: CRC Press.

73

Chapter 4 Experimental Results: PVA

Initial experiments focused on 90 degree nematic planar twist cells in which PVA was used as a polymer aligning layer for the liquid crystal. This arrangement was selected because it produces high quality and stable cell alignment. Some of this data was previously reported in Ref [1].

The mechanism of optically induced director reorientation has been mainly studied by photorefractive measurements [2-17]. Here, the Friedericksz transition measurement will be introduced to elucidate the surface effects to more directly probe changes in the internal field of the liquid crystal cells. From the Friedericksz transition measurement, evidence of a screening field will be provided via the shift of the DC transition toward the AC transition when pumped optically. This will be further examined by describing the results of the measurements of the current and transmission through the cell and crossed polarizers as a function of time for different values of applied DC voltage. This measurement directly demonstrates the changes in internal field.

Subsequently, the optical gate measurement will used to illustrate the switching effect as well as monitor changes in the current under the pumping laser. The intensity dependence of the Friedericksz transition will be discussed. Finally, the optical limiting in a photosensitive twist cell will be demonstrated.

74

4.1 Friedericksz Transition Shift

The Friedericksz transition measurement was chosen as the basis for most of the experiments reported here because it provides a more direct demonstration of the surface effects than a typical photorefractive measurement provides. The results of these measurements may be used to determine the physical mechanism of the surface effect.

That information may be used as a guide for their use in various applications.

Figure 4.1 shows a plot of the transmission of the HeNe laser beam(~15µW) as a function of applied voltage for an AC field, DC field, and DC field with a pumping laser beam. Here, the 1 kHz AC voltage is RMS and the pump laser is an argon ion laser operating at 488nm with an intensity of 730mW/cm2. The cell used was 25 µm thick with a PVA alignment layer < 100nm thick. AC transition measurements were also performed with the pump laser, but showed no effect that is dependent on the incident light intensity.

a) Friedericksz Transition 10 AC 9 DC DC with Pump 8 7 6 5

4 3 2 1 0

Transmission (Arbitrary Units) 012345678 Bias Voltage (V)

Figure 4.1 Plot of the Friedericksz transition threshold for an applied AC field, DC field, and DC with a pumping laser beam.

75

Clearly, Figure 4.1 leads to several observations about the surface effect. The shift of the DC transition (red line) from the AC transition suggests that the voltage drop across the liquid crystals is different in the two cases. This demonstrates that there is a screening field in the DC case due to charge accumulation at the surfaces. When the liquid crystal cell is pumped optically, the DC transition shifts toward the AC transition.

This suggests that the pump laser acts to lower the screening field.

In order to further understand the exact cause of the shift between the AC and DC transition as well as the shift of the DC towards the AC transition when under the action of a pumping laser, a second experiment was performed. In Figure 4.2, the Friedericksz transition of Figure 4.1 is pictured along with a plot of the transmission as a function of time. The sample used is the same as that used for the results in Figure 4.1. In this case, the transmission of the HeNe is measured as a function of time with a specific voltage applied at t=0.

a) Friedericksz Transition b) 10 10 AC 9 DC DC with Pump 8 8 1V 7 3V 6 6 5V

5

4 4 3 2 2 1 0 0 Transmission (Arbitrary Units) (Arbitrary Transmission Transmission (Arbitrary Units) Transmission 012345678 0 50 100 150 200 250 300 Bias Voltage (V) Time (s)

Figure 4.2 Transmission as a function of a) voltage and b) time where the curves in b) correspond to the vertical lines of the same color in a).

76

At first, 1V is applied to the cell while the transmission of the HeNe laser is monitored as a function of time. The transmission stays constant and at its maximum value because the voltage is below the measured AC transition, as indicated by the pink vertical bar in Figure 4.2 a). This result is expected because in the AC case, the threshold only depends on the elastic and dielectric constants of the material. When a DC voltage below this value is applied, no change in transmission should be observed because the field on the liquid crystal bulk is not large enough.

If a voltage above both the AC and DC transitions is applied, the transmission is expected to drop to a minimum value and then maintain that value. This is the case of the purple vertical bar at 5 volts in Figure 4.2 a) which corresponds to the purple curve in

Figure 4.2 b). Here the transmission drops and rises only slightly as a function of time.

This small rise corresponds to the intersection of the vertical bar and the DC Friedericksz curve in Figure 4.2 a).

Finally, when a voltage between the AC and DC transition is chosen, here 3V, the transmission drops initially, but over time it increases to the maximum value of the transition. This suggests that initially, the field on the liquid crystal bulk is sufficient for director reorientation, but there is a screening field that increases as a function of time under the applied voltage. Figure 4.2 b) actually shows the buildup of the internal field.

Figure 4.3 shows the current as a function of applied field measured simultaneously to the curves in Figure 4.2b). Initially, there is a jump in the current at t=0, followed by a decay in time while the voltage is applied. This indicates that upon the application of the voltage, there is movement of charge carriers related to the buildup of the field demonstrated in Figure 4.2b). 77

5

1V 4 3V 5V

3

2

1 Current (microamps)

0 0 50 100 150 200 250 300 Time (s)

Figure 4.3 The measured current as a function of time where at t=0, there is an initial jump in current followed by a decay.

4.2 Optical Gate Effect

A third experiment, the optical gate experiment, was performed for two reasons.

The first is to demonstrate the use of this liquid crystal device as an optical switch. The second reason for performing this experiment is to investigate changes in the current upon application of the optical pumping beam when under a constant applied DC voltage.

This optical switching effect is demonstrated in Figure 4.4 where transmission of the

HeNe laser is plotted as a function of time. Initially the sample has a bias applied to it, which is near the Friedericksz transition threshold. In this case, the sample is biased at

4V. The pump beam(488nm Argon Ion laser) is incident on the sample via an electronic shutter which is open and closed at certain times as indicated in Figure 4.4. The pump 78 beam in Figure 4.4 is an argon laser operating at 488nm with a pump power of 72mW, an intensity of 23 mW/mm2.

10 0.58 Transmission Current 0.57 8 0.56 0.55 6 0.54 pump on 0.53 4 pump off 0.52 2 0.51 Current (microamps) Current Transmission (arb units) 0.50 0 0 50 100 150 200 250 300 Time (s)

Figure 4.4 Plot of transmission as a function of time where the sample is held at a 4V DC, while a pumping beam is switched on and off as indicated in the figure, where the red line corresponds to the current and the black line to the transmission of the HeNe laser.

The drop of the transmission of the HeNe laser indicates that the application of the pump laser lowers the screening field, thus increasing the field across the liquid crystal cell. The rise in the current suggests that when the laser is pumping on the sample, it is injecting some photocurrent. Therefore, Figure 4.4 demonstrates the optical gate effects as well as depicts the change in the internal field upon application of the pump via a photocurrent. By performing the gate measurement at a number of pump intensities, the contrast may be calculated via Equation 3.1, and plotted as a function of the pump laser power or intensity as shown in Figure 4.5. 79

1.0

0.8

0.6

0.4 Contrast 0.2

0.0 0 5 10 15 20 25 30 Intensity (mW/mm)2

Figure 4.5 Plot of contrast as a function of input intensity for a typical cell with a photosensitive PVA alignment layer

The contrast increases with increasing pump intensity and saturates at higher pumping powers. The reason for this behavior will be discussed in Chapter 6.

4.3 Intensity Dependence of the Friedericksz Transition

In order to further characterize the effect of the pump beam on the discharge of the screening field, intensity dependent Friedericksz transition measurements were performed. Figure 4.6 shows Friedericksz transition measurements in which the pump laser is the argon laser operated at 488nm is used in addition to the HeNe probe for various intensities of the pump laser on a 25 µm thick liquid crystal cell. 80

12

No Pump 10 11 mW 21 mW 8 31 mW 41 mW

6

4

2

0 02468 Transmission (ArbitraryUnits) Bias Voltage (V)

Figure 4.6 Friedericksz transition measured with a pumping laser at various intensities.

Again, by applying the pumping laser during the Friedericksz transition, the threshold voltage is shifted. Another important feature of Figure 4.6 is the decrease in maximum transmission for the different intensities. This is not an intensity dependent feature, rather transmission drops as a function of time if a field is being applied to the cell. In fact, visually one can observe the accumulation of a dark deposit on the surface of the cells. This prevents long term measurements on a single cell. The only cells to demonstrate this behavior were those that had only a PVA layer and build up accumulated faster in cells with thicker PVA layers.

In addition to these effects, changing the thickness of the liquid crystal or the alignment layer changes the Friedericksz transition threshold, but this is difficult to quantify due to the buildup on the cell surface. Changing the liquid crystal cell gap 81 thickness may change the number of impurity ions or the distribution of the electric field.

This buildup is only observed in cells that have only PVA polymer layers.

Figure 4.7 is another demonstration of the optical switching of the cell. Here the threshold is calculated as the voltage at which the transmission drops to 80% of the maximum.

5.0

4.5

4.0

3.5 80%Transmission

3.0 0 10203040 Pump Power (mW) Friedericksz Transition Threshold (V)

Figure 4.7 Friedericksz transition shift as a function of pump intensity.

4.4 Optical Limiting

From the gate measurement, it is easy to see how this type of liquid crystal device could be used in an optical limiting application. By placing the twist cell between crossed polarizers such that it is normally transparent and opaque upon the application of a application of intense light. An experiment was performed in order to test the optical limiting capabilities of the cell. In this case, there is no need for the probe laser and only 82 the pump beam is used. The liquid crystal cell and crossed polarizers are treated as the device. The laser power is measured before and after the device for a range of different pump powers. The normalized transmittance (ratio of output to input) is shown as a function of input power in Figure 4.8.

Optical Limiting 1.0

0.8

0.6

0.4

0.2

0.0 Normalized Transmittance Normalized 0.1 1 10 100 Input Power (mW)

Figure 4.8 Demonstration of optical limiting capabilities of a twist cell with a PVA alignment layer.

The curve shown in Figure 4.8 is indicative of the optical limiting capabilities of this type of sample. More importantly, these experiments lay the foundation for the creation of a useful device that may be constructed by altering various cell parameters, such as the addition of a photoconducting alignment layers. In fact, a photoconducting alignment layer leads to some interesting results and is the subject of Chapter 5.

83

4.5 Conclusions

Based on these results, it is clear that the mechanism of director reorientation by optical means is at least in part a surface effect. The shift between the AC and DC transitions is indicative of the charge accumulation at the surfaces. The buildup of the internal screening field when a DC field is applied was demonstrated. The optical control of the Friedericksz transition was demonstrated by the shift of the DC transition when pumped as well as through the gate and optical limiting measurements. The gate measurement provided evidence of an injected photocurrent when the pump beam is applied. The action of this photocurrent is to reduce the screening field by neutralizing accumulated charge. Due to a buildup on the surface of the cell after a voltage is applied for several hours, there were limitations on the various types of cells that could be measured. A model based on these results will be discussed in Chapter 6.

The goal of these measurements was to provide more direct evidence of the surface effect. The next step is to begin to optimize the performance of the cells as well as probe the mechanism in more depth. Because the effect is photo induced, a photoconducting polymer layer was added to the cells and is the topic of Chapter 5.

84

4.6 References

1. Merlin, J., E. Chao, M. Winkler, K.D. Singer, P. Korneychuk, and Y. Reznikov, All-optical switching in a nematic liquid crystal cell. Optics Express, 2005. 13(13): p. 5024-5029. 2. Bartkiewicz, S., K. Matczyszyn, A. Miniewicz, and F. Kajzar, High gain of light in photoconducting polymer-nematic liquid crystal hybrid structures. Optics Communications, 2001. 187: p. 257-261. 3. Bartkiewicz, S., K. Matczyszyn, J. Mysliwiec, A. Miniewicz, B. Sahraoui, C. Martineau, P. Blanchard, P. Frere, J. Roncali, and F. Kajzar, Liquid crystal panel with dye doped PVK layer for real time holography processes. Molecular Crystals and Liquid Crystals, 2002. 374: p. 85-90. 4. Bartkiewicz, S., K. Matczyszyn, J. Mysliwiec, O. Yaroshchuk, T. Kosa, and P. Palffy-Muharey, LC Alignment controlled by photoordering and photorefraction in a command substrate. Molecular Crystals and Liquid Crystals, 2004. 412: p. 301-312. 5. Bartkiewicz, S. and A. Miniewicz, Mechanism of optical recording in doped liquid crystals. Advanced Materials for optics and electronics, 1996. 6: p. 219- 224. 6. Becchi, M., I. Janossy, D.S.S. Rao, and D. Statman, Anomalous intensity dependence of optical reorientation in azo-dye-doped nematic liquid crystals. Physical Review E, 2004. 69: p. 051707-1-051707-6. 7. Dyadyusha, A., M. Kaczmarek, and G. Gilchrist, Surface screening layers and dynamics of energy transfer in photosensitive polymer-liquid crystal structures. Molecular Crystals and Liquid Crystals, 2006. 446: p. 261-272. 8. Kaczmarek, M., A. Dyadyusha, S. Slussarenko, and I.C. Khoo, The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers. Journal of Applied Physics, 2004. 96(5): p. 2616-2623. 9. Kajzar, F., S. Bartkiewicz, and A. Miniewicz, Optical amplification with high gain in hybrid-polymer--liquid-crystal structures. Applied Physics Letters, 1999. 74(20): p. 2924-2926. 10. Lee, W. and C.C. Lee, Two-wave mixing in a nematic liquid-crystal film sandwiched between photoconducting polymer layers. Nanontechnology, 2006. 17: p. 157-162. 11. Lee, W., C.-Y. Wang, and Y.-C. Shih, Effects of carbon nanosolids on the electro-optical properties of twisted nematic liquid-crystal host. Applied Physics Letters, 2004. 85(4): p. 513-515. 12. Miniewicz, A., K. Komorowska, J. Vanhanen, and J. Parka, Surface-assisted optical storage in a nematic liquid crystal cell via photoinduced charge-density modulation. Organic Electronics, 2001. 2: p. 155-163. 13. Mun, J., C.S. Yoon, H.W. Kim, S.A. Choi, and J.D. Kim, Transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers. Applied Physics Letters, 2001. 79(13): p. 1933-1935. 85

14. Mysliwiec, J., A. Miniewicz, and S. Bartkiewicz, Influence of light on self- diffraction process in liquid crystal cells with photoconducting polymeric layers. Opto-Electronics Review, 2002. 10(1): p. 53-58. 15. Ono, H. and N. Kawatsuki, Orientational holographic grating observed in liquid crystals sandwiched with photoconductive polymer films. Applied Physics Letters, 1997. 71(9): p. 1162-1164. 16. Ono, H. and N. Kawatsuki, Real-time holograms in liquid crystals on photoconductive polymer surfaces. Optics Communications, 1998. 147: p. 237- 241. 17. Pagliusi, P. and G. Cipparrone, Surface-induced photorefractive-like effect in pure liquid crystals. Applied Physics Letters, 2002. 80(2): p. 168-170.

86

Chapter 5 Experimental Results: PVK

The optically induced surface mediated effects that were demonstrated in the cells with PVA alignment layers in Chapter 4 led to the results that will be discussed in this chapter. Various results using an additional layer of the photoconductor PVK doped with TNF will be discussed. PVK was chosen because it has been well studied for its photoconducting properties.[1] Also, it has been used as an alignment layer in various studies that report the surface photorefractive-like effect in liquid crystal cells.[2-7]

Although, it is possible to make cells using PVK as an alignment layer, these cells are usually only stable for less than one day. If a thin layer of PVA is cast on top of the photoconducting PVK layer, the cell stability substantially improves. Adding the layer of

PVA otherwise showed no observable changes in the performance of the liquid crystal cells within experimental uncertainty. This is in contrast to some reports in which PVA degraded[4] and enhanced[6, 7] the photorefractive effect in these cells. This issue will be discussed further in Chapter 6.

First, the Friedericksz transition measurements for an AC field, a DC field and a

DC field with a pumping laser will be presented. Complimentary to these results, transmission and current as a function of time from a typical gate measurement will be reported. These results will demonstrate the surface mediated optical effect in cells that have PVK-TNF layers and actually show that the performance is enhanced in comparison to the results from Chapter 4. Next, the polarity dependence (i.e. the direction of the electric field) will be examined by utilizing cells which have PVK-TNF layer on one 87

substrate, in a hybrid cell geometry. These experiments provided intuition in the

development of the theoretical model that will be introduced in Chapter 6. Subsequently,

the intensity dependence of the effect will be explored by measuring the Friedericksz

transition and gate effects using pump beams of various intensities. A plot of contrast

and switching time, both of which are calculated from gate measurements, will be

presented. Then, the thickness dependence of the PVK-TNF layers as well as the liquid

crystal bulk will be discussed. In addition to the optical effects, some electronic

measurements will be presented. Finally, optically limiting and switching results will be

discussed in terms of figures of merit.

5.1 Demonstration of the Surface Effect

In this section, the surface effect will be demonstrated by first showing the

Friedericksz transition shift. Then, gate measurements will be presented in order to

provide a second demonstration of the effect.

The surface effect is demonstrated in Figure 5.1 by the shift in the DC

Friedericksz transition toward the AC transition for a cell with the PVK-TNF layer. The

AC voltage is 1kHz and the pump beam is from an argon ion laser operating at 488nm. It

is important to note that although the shift is similar to that which occurs in cells with the

PVA layer, it may be achieved at a pump power of an order of magnitude less. Here, the

transition is completely shifted with a pump power of ~30 mW/c m2 while in Figure 4.1 pumping with the same laser at a pump power of ~730 mW/c m2 produces only a partial

shift of the transition. 88

Friedericksz Transition 0.8 NoPump 0.7 Pump 0.6 AC Bias

0.5

0.4

0.3

0.2

0.1 Transmission (arb units) 0.0 0 5 10 15 20 25 30 Bias Voltage (V)

Figure 5.1 The Friedericksz transition in a cell with the photoconducting PVK layer where the pump power is 1mW or 30 mW/cm2 of 488nm light from an argon ion laser.

The curves in Figure 5.1 indicate that the effect is stronger with the addition of the

PVK-TNF layer when compared to the samples discussed in Chapter 4. This suggests that the photoconductivity of that layer enhances the surface effect. Efficient charge generation and transport may be achieved in the PVK-TNF when subject to an incident optical field. When compared with PVA, this accounts for the difference in the

Friedericksz transition shifts for the two types of cells. The difference in possible mechanisms for the two types of cells will be discussed in more depth in Chapter 6.

Gate measurements also reveal the surface effect as shown in Figure 5.2. Here a pump power of ~ 25 mW/cm2 is applied to a cell held at 4.34 V DC. Figure 5.2 also shows that switching effect is more efficient and faster in cells with PVK-TNF layers when compared to data in Chapter 4.

89

Gate Measurement 10 0.77 8 0.76

6 A) 0.75 μ

4 0.74 Transmission

Transmission Current 2 0.73 ( Current

0 0.72 0 50 100 150 200 250 300 350 Time (s)

Figure 5.2 Transmission and current are measured as function of time in the gate measurement where the pump is an argon ion laser operating at 488nm and an intensity of 26mW/mm2 and the applied voltage is 4.34 V.

These results suggest more efficient neutralization of the ion accumulation layer

(Debye layer) at the surface with the addition of the PVK-TNF layer. From the results shown in Figures 5.1 and 5.2, it is clear that it is advantageous to add a layer of PVK doped with TNF to the liquid crystal cells in order to enhance the nonlinear optical effect.

5.2 Polarity Dependence

The liquid crystal director responds to the square of the electric field, as shown in equation 2.7, so at first glance it may seem as if the direction of the applied field should not affect the performance of the cells. That is not the case here because this effect is presumed to be due to intrinsic ionic impurities in the liquid crystal bulk. Upon application of a DC field, the impurities accumulate at the counter electrodes. Testing the polarity dependence of the effect should reveal important information about the sign of 90 charge carriers involved in the optical effect as well as the role of the photoconducting layer.

In order to determine the importance of the direction of the applied electric field, several different types of cells were created. The basic cell construction consisted of one

ITO electrode coated in a PVA alignment layer and a second ITO electrode coated in a

PVK-TNF layer with an additional thin (<80nm) layer of PVA, which will be denoted as a hybrid cell. The samples were filled with the liquid crystal 5CB which in some cases had additional dispersed impurities.

Friedericksz transition measurements of a hybrid cell with neat 5CB are shown in

Figure 5.3. The pump laser used for these measurements was a cw frequency doubled

Nd:Yag at 532nm. From Figure 5.3, there is a clear dependence of the Friedericksz transition on the polarity of the PVK electrode. In particular, when PVK is the positive electrode, there is little shift in the transitions with pump power. In contrast, when PVK is the negative electrode, the transition is shifted significantly toward a lower threshold at pump powers ~ 1 mW or 10 mW/cm2.

PVK is Positive PVK is Negative 0.50 0.50 0.45 DC 0.45 DC 0.050mW 0.050mW 0.40 0.10mW 0.40 0.10mW 0.20mW 0.20mW 0.35 0,50mW 0.35 0,50mW 1.0mW 1.0mW 0.30 0.30 0.25 0.25 0.20 0.20 0.15 0.15 0.10 0.10 0.05 0.05 0.00 0.00

Transmission (Arbitrary Units) 02468101214

Transmission (Arbitrary Units) Units) 02468101214 Bias Voltage (V) Bias Voltage (V)

Figure 5.3 Transmission versus voltage for a hybrid liquid crystal cell.

91

The polarity dependence trend is reproducible in every hybrid cell that was tested.

The polarity effect is more obvious from the calculated threshold as a function of pump power as shown in Figure 5.4 where the Friedericksz transition threshold is plot as a function of pump power. Here, the Friedericksz transition threshold is chosen as the voltage at which the cell reached 50% of the maximum transmission in the Friedericksz transition measurement.

10 9 Positive 8 Negative 7 6 5

4 3 2 50% Transmission 50% 1 0 0.00.20.40.60.81.0 Pump Power (mW) Friedericksz Transition Threshold (V)

Figure 5.4 Voltage at 50% transmission for a hybrid cell where positive and negative correspond to the polarity of the PVK-TNF electrode.

There are several significant observations from Figure 5.3 and Figure 5.4 to be noted:

• The DC transition occurs at a lower voltage when PVK-TNF is the positive electrode. • There is little shift with increasing pump power when PVK-TNF is the positive electrode. • When PVK-TNF is at the negative electrode, the optical shift effect is clearly demonstrated.

92

The first bullet point suggests that there is a larger screening field when the PVK-

TNF is at the negative electrode. This could be due to injection of holes from the PVA electrode that accumulate at the PVK-TNF interface. This is possible because it has been suggested that PVA does not block hole injection from ITO.[8]

Figure 5.5 shows the experimental situations corresponding to the results in

Figure 5.3. The second and third bullet points suggest that the effect is more efficient when PVK-TNF is the negative electrode, the situation shown in Figure 5.5 b). In this case, positive impurities accumulate at the negative electrode. Since PVK-TNF is considered to be preferentially a hole conductor, this result suggests that photogeneration and transport of holes toward the ITO in the PVK-TNF layer is the neutralization mechanism.

Figure 5.5 Experimental geometry for measurements with PVK-TNF on the a) positive and b) negative electrodes.

Further experiments were performed on C60 doped cells. The Friedericksz transition measurements in those cells are shown in Figure 5.6. 93

PVK Negative Electrode PVK Positive Electrode C60 doped C60 doped 0.5 0.5 AC AC DC DC DC w/0.5mW Pump 0.4 DC w/0.5 mW Pump 0.4

0.3 0.3

0.2 0.2

0.1 0.1

0.0 0.0 Intensity (Arbitrary Units) Intensity (Arbitrary Units) 0 5 10 15 20 0 5 10 15 20 Bias Voltage (V) Bias Voltage (V)

Figure 5.6 Results for 25 µm thick, C60 doped hybrid liquid crystal cells of typical attributes.

These results are similar to those in Figure 5.3 where PVK-TNF must be at the

negative electrode in order for the cells to be photoactive at low (~mW) pump power.

The results of the polarity experiments suggest that the interface between the

polymer and the liquid crystal is important. Additionally, the results also indicate that the

neutralization of positive ions at the negative PVK-TNF electrode is an important part of

the mechanism. These results will be used to introduce the physical mechanism that will be discussed in Chapter 6.

5.3 Intensity Dependence

The intensity dependence of the effect is most easily demonstrated by

measurement of the DC Friedericksz transition while pumping at several pump powers.

Figure 5.7 shows a plot of the Friedericksz transition for a cell submitted to an AC

voltage, a DC voltage and a DC voltage additionally pumped at several laser powers

ranging from 25 µW to 2 mW. In this case, the pump laser was a frequency doubled cw

Nd:Yag. This data set reflects typical results for a 50 µm thick nematic twist cell 94 containing a 200nm thick PVK-TNF alignment layer covered with a PVA layer. The transitions are shifted to lower voltages with increasing pump power. This result is reasonable because the cross section for photogeneration increases with intensity.

0.5

0.4 AC DC 0.025mW 0.3 0.050mW 0.075mW 0.10mW 0.15mW 0.2 0.20mW 0.50mW 1.0mW 0.1 2.0mW

0.0 0 5 10 15 20 25 30 Transmission (Arbitrary Units) Bias Voltage (V)

Figure 5.7 Plot of transmission as a function of applied bias voltage for an applied AC voltage, voltage and DC voltage with several pumping powers

The Friedericksz transition threshold was calculated as described in Chapter 4 and is shown in Figure 5.8. Specifically, the voltage at which the transmission reached 50-

90% of the maximum was calculated for several pump powers. There is a clear indication from Figure 5.8 that the Friedericksz threshold shift has an intensity dependence that begins to saturate at high pump intensity.

95

20 18 50% 60% 16 70% 80% 14 90% 12 10 8 6 4 2 0 0.00.40.81.21.62.0 Pump Power (mW) Friedericksz Transition Threshold (V) Threshold Transition Friedericksz

Figure 5.8 Friedericksz transition thresholds for several pumping powers.

Further, gate measurements for several values of pumping power are shown in

Figure 5.9. The cell used in this data set is identical to that used for the data in Figures

5.7 and 5.8. With increasing pump power, the drop in transmission, as well as the time it takes to drop, increases. From Figure 5.9, it is clear that the effect saturates for higher intensities. 96

Gate Measurements 0.45 0.40 0.35 0.050mW 0.30 0.075mW 0.10mW 0.25 0.15mW 0.20mW 0.20 0.25mW 0.50mW 0.15 0.75mW 1.0mW 0.10 1.5mW 2.0mW 0.05 3.0mW 4.0mW 0.00

Transmission (Arbitrary Units) (Arbitrary Transmission 0 50 100 150 200 250

Time (s)

Figure 5.9 Gate measurements for several values of pump power on a cell containing a PVK-TNF layer.

Using the gate measurement, two different figures of merit can be calculated, the contrast and the switching time. Figure 5.10 shows the contrast and switching time as a function of pump power for a cell with a PVK-TNF polymer layer.

1.0 10

0.8 8 Time Constant (s)

0.6 6

0.4 4 Contrast

0.2 2

0.0 0 01234 Pump Power (mW)

Figure 5.10 The contrast and time constant calculated for a cell with a PVK-TNF photoconducting layer. 97

It is clear from Figures 5.8 - 5.10 that there is an intensity dependence for the optical effect that saturates at high power (~1mW). This behavior will be explored further in Chapter 6 in terms of the theoretical model.

5.4 Thickness Dependence

The thickness of both the cell and the alignment layer was studied in order to determine the origin of the physical mechanism. These two parameters were chosen to be varied because changing them will change the value of the electric field across the liquid crystal bulk and the surface. The objective here was to use this data to quantify the surface effect. This task was met with some difficulty due to problems with reproducibility among samples. The following sections will discuss this issue in terms of changes in alignment layer and liquid crystal cell gap thickness.

5.4.1 Cell Thickness Dependence

According to the continuum theory discussed in Chapter 2, the Friedericksz transition threshold should not be dependent on the cell thickness. The Friedericksz transition threshold voltage is only dependent on the properties of the liquid crystal, the elastic coefficients and the dielectric anisotropy via equation 2.12. This holds true as samples submitted to an AC voltage do not show a thickness dependence. There is a thickness dependence on the DC electric field, suggesting that voltage across the liquid crystal is dropped inhomogeneously.

Consider that a volume of liquid crystal will have some number of intrinsic impurities of charge, Q, in that volume. The voltage, V, across the liquid crystal bulk is then related to the charge by the capacitance, V=Q/C where the capacitance is given by 98

C=εA/d giving V= Q d/εA. Now consider 2 cells that are identical except that one is

twice the thickness of the other, denoted thin and thick. This means that Vthick=4Vthin or that the voltage across the thick cell is 4 times the voltage across the thinner cell. The factor of 4 results from a factor of 2 for doubling the cell thickness and another factor of

2 for doubling Q when doubling the volume of the cell.

Figure 5.11 illustrates the Friedericksz threshold voltage for several intensities for cells of three different cell gap thicknesses. All cells in this figure contain a 200nm thick PVK-TNF layer. The thickest cells, made with 50 µm mylar spacers, have a lower

DC threshold and are more easily shifted with the application of the pump beam as is apparent from Figure 5.11. This is in contrast to the expected result.

PVK-TNF Cell Thickness Dependence 20

18 12.5μm 25 μm 16 25 μm 14 50 μm 50 μm 12 10

8 6 4 2 0 Friedericksz Threshold (V) 02468101214161820 Pump Power (mW)

Figure 5.11 The Friedericksz transition threshold was calculated for cells of varying thickness.

The 12.5 and 25 µm cells in general have a threshold that is larger than the 50 µm

thick cells as is shown in Figure 5.11, Further investigation of the reproducibility of the 99

Friedericksz transition threshold for a single type of liquid crystal cell is shown in Figure

5.12.

20 57μm 18 58μm 16 61μm 62μm 14 64μm 12 10 8 6 4 2 0 0.00.51.01.52.0 Pump Power (mW) FriederickszTransition Threshold (V)

Figure 5.12 A plot of Friedericksz transition threshold as a function of pump power for several 50 mµ thick cells with 200nm thick PVK-TNF photoconducting layers.

The cells in Figure 5.12 were made with 50µm thick mylar, but the actual thicknesses were measured using an interferometric technique and are indicated in the legend of the figure. Based on this result, it is difficult to quantify the thickness dependence within experimental error. This difficulty may be alleviated by preparation of the cells in a clean room environment in order to prevent extraneous ionic impurities from getting into the cell during fabrication. Because the of the reproducibility issues presented in Figure 5.12, no quantitative results may be drawn, so it is not clear whether there is a thickness dependence at all.

Although it is difficult to quantify the effect, general trends are observed. Namely, thicker cells ~50µm always have an observable Friedericksz transition. In addition, Shen 100 et al. performed extensive research on Ohmic contacts and found that injection efficiency is a function of cell thickness.[9] In particular, the greater the distance between electrodes, the more Ohmic the contact. This could suggest that injection is part of the mechanism. Another important observation is that the transitions in thicker cells are much less sharp. If the Friedericksz transition threshold occurs at a voltage greater than

10-15 V, the transition is very slow and sometimes difficult to measure because of scattering, possibly due to electrohydrodynamic effects. Since there is great difficulty in reproducibility at this point, cell gap thickness dependence is still undetermined.

5.4.2 Alignment Layer Thickness Dependence

As mentioned above, changing the liquid crystal layer thickness has no effect on the AC voltage threshold in the samples. This is also the case for changes in alignment layer thickness. Changing the thickness of the alignment layer does have an effect on the

DC Friedericksz transition threshold voltage where the DC threshold voltage is greater for thicker alignment layers. This effect has been observed in other aligning layers such as polyimides and PVA[10] and so it is not unique to PVK alignment layers. Figure 5.13 shows AC and DC Friedericksz transitions for cells of different alignment layer thicknesses. 101

PVK-TNF ~150nm PVK-TNF ~550nm 0.5 0.5

0.4 0.4

0.3 0.3 AC AC DC 0.2 DC 0.2 0.19 0.22mW 0.52 0.50mW 1.01 0.1 0.1 1.0mW 2.0 2.1mW 0.0 0.0

Transmission (Arbitrary Units) 0 5 10 15 20 0 5 10 15 20 Transmission (Arbitrary Units) Bias Voltage (V) Bias Voltage (V)

Figure 5.13 Friedericksz transition measurements on 25µm thick cells of two PVK-TNF thicknesses.

From Figure 5.13, it is clear that the transition threshold voltage increases with increasing photoconductor layer thicknesses in agreement with the literature. Figure 5.14 shows the Friedericksz threshold voltages for several cells as a function of pumping intensity. At a pump power of greater than 1 mW, the curves overlap. For lower pump powers the thinner cells have a lower threshold and the DC transition is greater than 20V for the two larger thicknesses.

Based on Gauss’s law, the electric field shouldn’t depend on the thickness of the alignment layer. This is the case at higher pumping powers, but not below 1 mW or with no pumping beam at all. It is possible that the barrier for charge injection increases as the thickness of the alignment layer increases. It is also possible that there is a change in photogeneration efficiency in the photoconductor with changes in layer thickness.

102

PVK-TNF Layer Thickness Dependence 20 18 500nm 350nm 16 200nm 14 12 10

8 6 4 2 0 Friedericksz Thershold (V) Thershold Friedericksz 0246810121416 Pump Power (mW)

Figure 5.14 Alignment Layer thickness dependence of the Friedericksz threshold voltage where the blue line indicates the AC transition threshold.

Based on the poor reproducibility of the threshold voltages among similar cells, it is difficult to make any conclusions about the thickness dependence of either the cell gap or the alignment layer. In order to clarify these results, further measurements are necessary.

5.5 Electronic Measurements

Measurements were performed in order to examine the electronic properties of the twist cells, namely, capacitance-voltage measurements and the frequency dependence of the Friedericksz transition. These measurements were performed in order to be able to compare them with the electronic results given in Chapter 2. 103

5.5.1 Capacitance-Voltage Measurements

The capacitance as a function of voltage for several cells with PVK-TNF layers was measured. The result of a typical C-V curve for a 50 µm thick liquid crystal cell with a 200nm PVK-TNF layer is shown in Figure 5.15.

1.0 Forward Reverse 0.8

0.6

0.4

0.2 Capacitance (nF) Capacitance

0.0 0 5 10 15 20 25 Bias Voltage (V)

Figure 5.15 Capacitance as a function of voltage for a typical cell with a PVK-TNF polymer layer.

All of the cells measured showed similar results when the capacitance per volume was considered. The hysteresis observed in Figure 5.15 is characteristic of the ionic charge impurities in the liquid crystal cells.

5.5.2 Frequency Dependence of the Friedericksz Transition Threshold

The frequency dependence of the Friedericksz transition was measured and is shown in Figure 5.16 for several cells. The results shown here are for a 25µm thick cell with a 200nm thick PVK-TNF polymer layer. 104

4.0

3.5

3.0

2.5

2.0

1.5

1.0 80% Transmission 0.5

0.0 0.1 1 10 100 1000 10000 100000

Friedericksz Transition Threshold Transition Friedericksz Frequency (Hz)

Figure 5.16 Friedericksz Transition Threshold as a function of the frequency of the applied AC field.

This frequency dependence shown here agrees with that reported in Ref. [11, 12].

The threshold increases dips at 10 Hz and then increases sharply as it gets closer to 0 Hz.

The low frequency behavior occurs because ionic impurities are mobile and accumulate at the electrodes.

5.6 Optical limiting and optical switching

As illustrated through the results in this chapter, cells with PVK-TNF layers exhibit superior quality in terms of the surface effect when compared to the PVA cells.

The contrast and switching time for a cell with PVK-TNF layers was shown in Figure

5.10. It is clear when compared to the contrast shown in Figure 4.5 that the effect occurs at a pump power that is one order of magnitude lower in cells with only PVA polymer layers. Additionally, a plot of the transmittance as a function of input power, as shown in

Figure 5.17, shows a similar result when compared to Figure 4.8. 105

0.5

0.4

0.3

0.2

Transmittance 0.1

0.0 0.01 0.1 1 10 100 Power Input (mW)

Figure 5.17 Transmittance as a function of input power for a cell with a PVK-TNF polymer layer.

The optical limiting and switching results will be discussed further in

Chapter 6 where a side-by-side comparison between PVA and PVK-TNF will be discussed in terms of the theoretical models.

5.7 Discussion & Conclusions

The surface effect was demonstrated via the optically induced Friedericksz transition threshold shift. Using hybrid cells, it was found that PVK-TNF should be at the negative electrode in order to see the photo activated effect and that possibly the mechanism is the optical neutralization of positive ions at the negative electrode. The intensity dependence was demonstrated by Friedericksz transition and gate measurements. Next, the dependency of the Friedericksz transition threshold on the thickness of the polymer layers and the liquid crystal bulk was explored by changing cell parameters. Although, quantitative results were not achieved, qualitative results include 106 the increase in the DC threshold with increasing polymer layer thickness and the decrease of the DC threshold with increasing liquid crystal cell gap. Electronic measurements including C-V measurements and frequency dependent Friedericksz transition threshold measurements were performed and the results are indicative of charge accumulation at the surface of the cells under applied DC fields. Finally, the optical switching and limiting performance was compared with the results from the PVA photo-sensitive cells.

These results, along with those in Chapter 4, will be summarized and discussed in terms of a proposed model in Chapter 6. 107

5.8 References

1. West, D. and D.J. Binks, Physics of Photorefraction in Polymers. Advances in Nonlinear Optics. 2005, Boca Raton: CRC Press. 2. Lee, W. and C.C. Lee, Two-wave mixing in a nematic liquid-crystal film sandwiched between photoconducting polymer layers. Nanontechnology, 2006. 17: p. 157-162. 3. Miniewicz, A., F. Michelotti, and A. Belardini, Photoconducting polymer-liquid crystal structure studied by electroreflectance. Journal of Applied Physics, 2004. 95(3): p. 1141-1147. 4. Mun, J., C.S. Yoon, H.W. Kim, S.A. Choi, and J.D. Kim, Transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers. Applied Physics Letters, 2001. 79(13): p. 1933-1935. 5. Mysliwiec, J., A. Miniewicz, and S. Bartkiewicz, Influence of light on self- diffraction process in liquid crystal cells with photoconducting polymeric layers. Opto-Electronics Review, 2002. 10(1): p. 53-58. 6. Ono, H. and N. Kawatsuki, Orientational holographic grating observed in liquid crystals sandwiched with photoconductive polymer films. Applied Physics Letters, 1997. 71(9): p. 1162-1164. 7. Ono, H. and N. Kawatsuki, Real-time holograms in liquid crystals on photoconductive polymer surfaces. Optics Communications, 1998. 147: p. 237- 241. 8. Pagliusi, P. and G. Cipparrone, Charge transport due to photoelectric interface activation in pure nematic liquid-crystal cells. Journal of Applied Physics, 2002. 92(9): p. 4863-4869. 9. Shen, Y., A.R. Hosseini, M.H. Wong, and G.G. Malliaras, How to make Ohmic contacts to organic semiconductors. ChemPhysChem, 2004. 5: p. 16-25. 10. Dyadyusha, A., M. Kaczmarek, and G. Gilchrist, Surface screening layers and dynamics of energy transfer in photosensitive polymer-liquid crystal structures. Molecular Crystals and Liquid Crystals, 2006. 446: p. 261-272. 11. Schadt, M., Liquid crystal materials and liquid crystal displays. Annual Reviews of Materials Science, 1997. 27: p. 305-379. 12. Seiberle, H. and M. Schadt, LC-conductivity and cell parameters; Their influence on twisted nematic and supertwist nematic liquid crystal displays. Molecular Crystals and Liquid Crystals, 1994. 239: p. 229-244.

108

Chapter 6 Summary of Results and Proposed Model, Comparison to the Literature, Discussion & Conclusions, and Future Outlook

The results in this thesis have suggested that the mechanism for optical

reorientation of the liquid crystal director is related to the accumulation and neutralization

of ions at the surfaces of the cell. Here, a summary of those results will be presented and

discussed in terms of a model for the nonlinear surface effect. Also, it is necessary to put

these results into perspective with respect to the other reported surface effects in liquid crystal cells. This will additionally serve as a guide for further experimentation in characterizing the effect and designing devices. Also, the results of these experiments will be put into perspective by considering the future outlook of liquid crystals for use in optical limiting, photorefraction and other applications. Finally, a summary of the thesis will be presented.

6.1 Summary of Results and Proposed Model for cells with PVK-TNF

In this section the results from cells containing the PVK-TNF layers, from

Chapters 5, will be summarized. These results will be used to introduce and support a

proposed model for the optical surface mediated effect.

Results were reported for several measurements in nematic twist cells that contain

TNF doped PVK layers in Chapter 5. Several important observations were noted: 109

• The effect occurs at optical pump powers that are over an order of magnitude lower than the cells containing PVA layers discussed in Chapter 4. • During the gate measurement, there is an observable jump in current upon application of the electric field. • The direction of the electric field matters, namely the optical effect is more efficient when PVK-TNF is at the negative electrode. • The optical effect saturates at high powers, greater than 1mW. This is evident in all intensity dependent measurements including optical limiting, gate and Friedericksz transition measurements. • Thickness of the alignment layer and cell gap may matter, although this was difficult to quantify due to problems with reproducibility among cells. • The performance of the cells is the same within experimental uncertainty for cells that have an additional layer of PVA as those without that layer. • The voltage-induced deposit that was present in the cells with PVA was not observed in any cells with a PVK layer.

In Chapter 2, there were several models and theories introduced that describe optical and electrical effects in liquid crystals. What the results in this thesis have suggested, thus far, is that the optical effect coupled to electric characteristics of the cell is key to the optically mediated surface effect. Based on these results, there are several conclusions that may be drawn and incorporated into a theoretical model. Figure 6.1 is a schematic diagram of the model proposed here.

a) b) c)

Figure 6.1 A schematic diagram of the proposed model for the surface effect.

110

An electric field is applied to the liquid crystal cell as shown in Figure 6.1 a)

denoted by Ea . In response to Ea , intrinsic impurity charge carriers move toward the counter electrodes as shown in Figure 6.1 b). The accumulated charge carriers at the

liquid crystal/alignment layer interface results in a screening field, Es , that is opposite in

direction to the applied field. This results in a total field on the liquid crystal bulk, Et .

The current transient measurements in chapter 4, in Figures 4.2 and 4.3, illustrated the accumulation of the charge carriers at the electrodes. Namely, when a cell was

subject to a voltage that was between the AC and DC Friedericksz thresholds, the

transmission dropped at first and then increased with time. This provides direct evidence

of the accumulation of the charge at the electrodes. It is hypothesized that those charge carriers are positive ionic impurities intrinsic to the cell, based on the results of the polarity measurements shown in Figure 5.3, namely that the optical effect occurs when the photoconductor is at the negative electrode.

This screening field due to the accumulated charge at the interfaces screens the liquid crystal bulk from the applied field and results in an increase in the DC Friedericksz transition threshold from the AC Friedericksz transition threshold. The total field on the

liquid crystal is then given by the resultant field, Et , which is the difference between the

applied and screening fields. Reorientation of the director occurs when Et is sufficient to

overcome the elastic energy anchoring the molecules to the surface. The shifting of the

Friedericksz transition threshold was demonstrated for both types of cells in Figure 4.1

and Figure 5.1.

So far, only the Friedericksz transition shift under the action of a DC electric field

has been considered. In order to understand the behavior under illumination, pictured in 111

Figure 6.1 c), let us consider the data more carefully. It has also been shown that the

Friedericksz transition may be shifted toward the AC threshold optically, and that this shift is responsible for the optical switching action in the gate experiment. This shift toward the ac transition voltage suggests that the mechanism involves the optical neutralization of the accumulated charge due to the DC bias. Results for a gate measurement were reported in Figure 5.2 where both current and transmission were monitored as a function of time while the cell was held at a constant voltage and the pump was switched on and off. This figure showed a jump in the current which suggested that part of the mechanism is a photo induced current, indicating that particular attention should be paid to the behavior of the current.

Let us examine the behavior shown in Figure 5.2. The current jumps to a certain value when the light is on, which is intensity dependent, and then holds fairly constant until the light is switched off. At that time the current drops to the original value and again holds rather constant. This suggests that current is always flowing in the cell even after the optical switching has occurred, indicating a dynamic neutralization process. It is also interesting to plot the contrast and Friedericksz transition shift with the current as a function of intensity as in Figure 6.2 a) and b). The value for the current shown in both plots is obtained by averaging the value of the current measured during the gate measurement when the pump beam is applied. The Friedericksz transition shift is defined as the difference between the value of the DC transition threshold under illumination and the AC transition threshold. In these specific plots, the cells used have a PVK-TNF alignment layer that was ~200nm thick and a thin PVA layer with a cell gap of 50 µm.

These plots indicate that the photocurrent is monotonically increasing while the contrast 112

in the gate measurement and the shift of the Friedericksz transition saturate at

approximately the same intensity. We note, then, that the saturation occurs when the

Friedericksz transition has returned to nearly its value under AC field.

a) b)

0.6 14 16

16 (nAmps) Current 12 0.5 (nAmps) Current

10 0.4 14 8 0.3 14 6

Contrast 0.2 4

12 80% Transmission 12 0.1 2

0.0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.00.51.01.52.02.53.03.5 Pump Intensity (mW/mm2) Pump Intensity (mW/mm2) Friedericksz Transition Threshold (V)

Figure 6.2 Plot of a) contrast and current and b) Friedericksz transition shift and current as a function of pump beam intensity.

These results suggest an explanation for the switching effect. When the pump

beam is incident on the cell, a photocurrent flows in the photoconducting PVK-TNF

layer. Based on the fact that PVK-TNF is an efficient hole conductor when stimulated by

light as well as the polarity dependence results , it is proposed that holes generated by the

light near the negative electrode drift toward the ITO electrode in response to the applied

electric field. The current flowing in the cell creates a negative space-charge cloud at the

interface that neutralizes the positive impurities accumulated at that interface. The accumulated layer at the negative electrode is then reduced causing a current flow in the sample. The neutralization of the impurities results in a lower screening field and thus, a shift in the DC Friedericksz transition toward the AC Friedericksz transition. The fact that the current increases monotonically with intensity, while the optical properties

saturate, and the shape of the current in the gate experiment suggest that the 113

neutralization of the Debye layer is a dynamic process involving a concurrent flow of

charge and an accumulation of a neutralizing space charge layer. This is reminiscent of a

space charge limited current in thermionic emission, though obviously the source of

current differs. The degree of neutralization depends on the magnitude of the generated

photocurrent.

This part of the model is supported by the intensity dependence of the

Friedericksz transition threshold shift. In the gate measurements and Friedericksz

transition measurements, the pump beam causes a drop in transmission or a shift in the

Friedericksz transition. This results in a measured photocurrent, I ph , as depicted in

Figure 5.2.

From the model and results in this thesis, the dynamics of the current is clearly

important to the surface mechanism. Barbero et al. recently reported on the importance of the Debye layer in the electrical response of a liquid crystal cell.[1] A model was presented in which the liquid crystal cell is modeled by resistors and capacitors as shown

in Figure 6.3. A similar circuit model suggested by Seiberle and Schadt[2, 3] was

discussed in Chapter 2. Barbero et al. add a resistance and capacitance due to the Debye

layer as shown in the figure.

Alignment Layer Double Layer Liquid Crystal Double Layer Alignment Layer

R0 CS CI CB CI CS

R R S RI RB RI S

Figure 6.3 The circuit used by Barbero et al. to understand surface effects in liquid crystals.[1] 114

Experiments by Barbero et al. were performed on 5CB liquid crystal cells that were planarly aligned by a 10 nm thick layer of PVA.[1] Step and saw tooth voltages with an amplitude of 0.7 V were applied to the cells while the current was monitored as a function of time. That voltage was chosen so that the liquid crystal cell maintained planar alignment and chemical equilibrium. Application and removal of step voltages resulted in a relaxation feature in the current that was attributed to the time evolution of the charge density at liquid crystal polymer interface. It was shown that including the

Debye layer in the analysis is crucial to explain the current dynamics in the liquid crystal cells, that is, the relaxation time of the current is due to the accumulation of charge at the interfaces. The dynamics of the current clearly impact the time it takes to neutralize the accumulated charge.

In order to extend the model from Barbero et al. to the results in this thesis, we

note that the resistance of the alignment layer and the capacitance and resistance of the

Debye layer are intensity dependent. These are also implicitly time dependent, so that the

equations will need to be solved self-consistently in order to predict the dynamic

switching behavior. It is also interesting to note that in Figure 5.2, the time constants for

the current and the optical switching are different. Thus, solving the dynamic circuit

model will not necessarily predict the optical dynamics.

6.2 Discussion of Results for cells with PVA

The model presented in Section 6.1 was based on results obtained in the PVK-

TNF cells, for the most part. Some interesting physics was in fact learned using the cells with only PVA layers. In addition to the optical control of the Friedericksz transition and 115 the current transient measurements, there were observations unique to the PVA cells including the accumulation of a dark deposit on the surface of the cells. This deposit only occurs in cells that have PVA only. The dark deposit observed on the cells after exposure to electric fields indicates that there is some surface chemistry occurring. This could possibly be due to adsorption of ions at the surface. It is important to reiterate that this deposit only occurs in cells with only PVA layers, suggesting that the ITO-PVA-LC-

PVA-ITO configuration is necessary to observe it.

Based on the results in the PVA cells, a theoretical model was previously reported in Ref. [4]. This model involved a similar mechanism to that described in Section 6.1, that is, neutralization of the charge accumulation optically is the result of some photo injected species. No polarity dependence measurements have been performed on the

PVA cells, nor is the majority carrier known, so the exact mechanism for the neutralization of that charge has not been determined.

It is important to note that it is not clear exactly why this effect occurs at all in the cells with just PVA. In general, PVA is not known to be a photoconductor. There are actually several possible scenarios for the neutralization of the accumulated charge including photo-induced charge injection from the liquid crystal or PVA to the ITO or vice versa. One possible mechanism for photo induced charge injection can be deduced from the energy level diagram shown in Figure 6.4. Electrons can be excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital

(LUMO) in the liquid crystal or polymer. The electron can then fall to the Fermi level of the ITO resulting in the injection of a hole into liquid crystal or polymer. The work function of ITO is ~4.7 eV.[5] 116

ITO LUMO

EF

Energy HOMO

E0

Figure 6.4 Energy level diagram for photo induced charge injection from ITO.

Finally, the optical effect occurs at pump powers that are over an order of magnitude lower in the cells with PVK-TNF cells. This may also be attributed to the fact that PVA is not a photoconductor. Further research is necessary in order to establish the precise mechanism of charge neutralization in the cells with PVA layers.

6.3 Discussion and Comparison to the Literature

Several different theories for the surface mediated effects have been reported and were presented in Chapter 2. Here, the results of these studies will be compared to the results reported in this thesis.

Several groups have studied the effect of using both a PVA and PVK layer in the liquid crystal cells. Within experimental uncertainty, we have found that cells with and without the PVA layer show a difference only in the lifetime of the stability of the twist cell. Other groups have found the PVA to be both necessary[6, 7] and detrimental[8]. 117

Ono and Kawatsuki demonstrated holography in planarly aligned liquid crystal samples

using a wave mixing geometry.[6, 7] The samples were constructed as suggested in

Figure 6.5 where only the polymer-liquid crystal interface is shown.

Figure 6.5 Cell geometry used in Ono and Kawatsuki.

Cells of the type shown in Figure 6.5 were subject to an applied electric field that

caused the liquid crystal director to rotate such that it was perpendicular to the substrates.

Subsequently, the cells were illuminated by the interference pattern formed by two HeNe

laser beams. It was proposed that charge generated in the photoconducting PVK-TNF

layer was subsequently trapped in an insulating PVA layer. The space charge field

created in the insulating layer leads to reorientation of the director at the surface. This

study suggested that the effect requires the use of an insulating layer in order to observe

the all optical effect. We have demonstrated that although it is difficult to produce very stable cells that do not have a PVA layer, the optical effect reported in this thesis was observed in cells that only have PVK-TNF layers, in contrast to these results.

Further research using PVA layers suggested that the layer was detrimental to the optical effect. Specifically, Mun et al. demonstrated a 1/3 drop in diffraction efficiency when PVA layers were added to the liquid crystal cells.[8] The authors presented a model in which the interference pattern of the two laser beams results in charge generation in both photoconducting layers and the liquid crystal itself. Sample 118

parameters were iterated including variations in which the PVK or the liquid crystal, or

both were doped with C60. In some cases, the PVA was added between the liquid crystal

and PVK layer. It was proposed that current paths were created along the areas of

constructive interference in a typical wave mixing geometry. In situations where only the

liquid crystal was doped, it was suggested that charge generated in the liquid crystal moves via current paths and becomes trapped in the photoconducting layer. The addition

of the PVA layer results in a lower diffraction efficiency because it blocks charge injection from the liquid crystal to the PVK layer. In cells where the PVK was doped with the C60, charge is generated in the photoconducting layer and moves in response to

the applied field or by diffusion and finally becomes trapped in the same layer. The

addition of the PVA layer resulted in an initial sharp increase in diffraction efficiency,

followed by a rapid decrease and finally reached a steady state after several minutes.

This behavior was explained by suggesting that there is charge accumulation in the PVK

layer at the interface with the liquid crystal on one side and then at the ITO on the other

side. The decrease to steady state was rationalized by the eventual diffusion of the charge.

The major conclusion of the study by Mun et al. is that the PVK layer serves as a

charge trapping layer. Charge may become trapped in the PVK layers either by generation in the PVK layers themselves or generation in the liquid crystal followed by

movement along the current paths. It was suggested that PVA was detrimental to the

effect, blocking the charge injection between the LC and the PVA.

The comparison of these two studies to each other and to the results presented

here is difficult, mainly because the experimental conditions vary between them, but

there are some interesting results to consider. Ono and Kawatsuki showed that PVA was 119

necessary to observe the optical effect while Mun et al. find it to be detrimental.

Furthermore, some research has shown the optical control of the nematic director in cells

that do not contain any photoconducting layer, as in the data presented in Chapter 4,

although, in general, the various photorefractive figures of merit are somewhat

diminished when compared to those with the photoconducting layer. The results from

Ono and Kawatsuki, Mun et al. and the research in this thesis were all performed using

different liquid crystals, E7, E44, and K15 respectively. The use of PVA in the results

presented here is just to improve alignment and stability of the liquid crystal cells, therefore, the layer of PVA is cast from a dilute solution in water and results in a thickness of less than 100 nm while the PVK-TNF layers were 200nm thick. The thickness was not reported by Ono and Kawatsuki for their measurements, but Mun et al. used polymer layers that were 1µm thick. Here we have shown that the PVK-TNF cells shown no difference in performance when coated with the PVA layer. Although this is in conflict to the other reported results, this could be due to different experimental conditions in the different cases.

Other research has suggested that the liquid crystal and polymer layers must have a chemical or physical affinity based on results where the effect was only observed in certain alignment layer and liquid crystal combinations.[9] This could also contribute to the conflicting results.

The second important observation to consider is the intensity dependence of the effect coupled with the order of magnitude enhancement in the PVK-TNF cells. This is most likely due to the intensity dependence of the charge generation efficiency in PVK-

TNF. The saturation of the intensity dependence was also reported in Ref. [10]. 120

Additionally, PVK-TNF is a well-know photoconductor while PVA is photosensitive at

best, which reveals why the cells with the photoconducting layer are more efficient.

Finally, there is little discussion that can be devoted to changes in the polymer layers and cell gap thickness. Because of the reproducibility issues only qualitative results may be reported in these experiments. Ref. [10] observed a change in the DC

Friedericksz threshold with increasing thickness of the alignment layer, but offered no clear explanation of this behavior. Specifically, increasing the thickness of the layer increases the DC Friedericksz transition threshold. Similar observations were made here for both PVA and PVK-TNF layer thickness. This may suggest that charge injection may play some role in the optical effect because barrier potentials may increase with the thickness of the polymer layers.

The next step in further characterizing the surface mediated effect is to study how the electronic and optical effects are coupled. Namely, the effect of the intensity of the pump laser on the current dynamics needs to be determined so that the dependence of the resistance and capacitance of the Debye layer on the current may be further understood.

6.4 Side by Side Comparison of cells made with PVK-TNF and PVA

The results reported in Chapters 4 and 5 suggested that the optical effect is much stronger in cells which have a PVK-TNF layer incorporated into them. Figure 6.6 shows a comparison of the contrast obtained for two types of cells, a cell that has a PVA layer and a cell that has a PVK-TNF layer. 121

1.0 PVA PVKTNF 0.8

0.6

0.4 Contrast 0.2

0.0 0.01 0.1 1 10 Intensity mw/mm2

Figure 6.6 Contrast for two cells: one with just PVA and one with PVK-TNF.

There are two interesting features to be noted from the values in Figure 6.6. First, the cell with PVK reaches a maximum contrast at a lower pump power than a cell with the PVA layer. This may be attributed to the photoconductivity of the PVK-TNF polymer layer. In addition to that feature, the contrast for the PVK-TNF cell reaches it maximum such that the curve is steeper. Both of these attributes are important for optical switching applications.

In addition to better performance in optical switching, the PVK-TNF also shows more promise in optical limiting, as is illustrated in Figure 6.7. Optical limiting was observed using an argon ion laser operating at 488nm for both cells shown in Figure 6.7. 122

Optical Limiting

1.0 PVA PVK-TNF 0.8

0.6

0.4

0.2

0.0 Normalized Transmittance 0.01 0.1 1 10 100 Input Power (mW)

Figure 6.7 Optical limiting in two cells, one with a PVA and the other with a PVK-TNF layer.

The ratio of the normalized transmittance is constant below 0.1 mW power for the

PVK-TNF cell and below ~1 mW for the PVA cell. The PVK-TNF cell drops to about

20% transmittance at ~1 mW and ~10 mW for the PVK-TNF cell. It is likely that exploring other photoconductors could result in even more efficient optical limiting devices.

Both the results for the contrast and the optical limiting measurements suggest that adding a photoconducting layer to the cells enhances the surface mediated effect.

The fact that cells with PVA layers show the effect suggests that there may be multiple mechanisms involved in the surface mediated effects that occur on different time scales and at different intensity thresholds.

123

6.5 Future Outlook

There are several possible improvements to the experiments here as well as additional experiments that may more clearly define the surface effect. Firstly, the reproducibility of the DC measurements needs improvement. This could be achieved by preparation of the cells in a clean room environment. Also, the control of the ionic impurities in the cells may lead to more conclusive results as well as better reproducibility.

In order to more clearly understand the mechanism in the case of cells with PVA, there are some additional experiments that may be performed. Some initial attempts were made at blocking injection from electrodes in the cells with PVA, by placing a passivation layer between the electrodes and the alignment layer. Preliminary results suggested that the passivation layer could not fully block the optical effect. In fact, increasing the thickness of the passivation did not do much either. This suggests two conclusions, either the passivation layer needs to be replaced with something that blocks charge more efficiently, or that hole injection from ITO is not the only mechanism for the surface mediated effect in cells with only a PVA layer. Mechanisms that include charge injection may also be studied by using an electrical contact with a different work function. Clearly, further study of the charge at the surface is necessary to determine whether injection in either direction from any species is contributing to the effect. In particular, this may be achieved by altering electrode conditions and monitoring changes in the operation of the device.

There are a few experiments that may be performed to further clarify the mechanism in both PVA and PVK. Namely, using liquid crystals that are used in some of 124

the relevant literature may allow for a more direct comparison of results. Additionally,

some ellipsometric electroreflection techniques have been used by Miniewicz et al. to

study internal field changes in liquid crystal cells. [11] Implementing a similar

spectroscopic technique may help quantify changes in the electric field. This might be

particularly useful in understanding alignment layer and liquid crystal cell gap thickness

dependence where the DC Friedericksz transition can not be measured. Finally, although

the twist cells provide an easy demonstration of the optical effect, it may be interesting to

measure planar cells, as would be most useful in photorefractive materials. This may be

particularly interesting if the difficulties in using the PVK-TNF as an alignment layer is

due to weak anchoring at the PVK-TNF electrode. I suspect that this may be a factor

because in some preliminary results, cells made with PVA only on one side and PVK-

TNF only at the other side initially showed a nice twist alignment that relaxed to planar alignment within an hour of filling the cell. Additionally, rubbed PVK-TNF results in a

director orientation that is perpendicular to the rubbing direction, suggesting that the

alignment is more likely a chemical or physical bond than due to steric interactions.

Implementation of some or all of these suggestions may lead to a better

understanding of the physical mechanism of the surface mediated effect. Further study

may also produce cells that perform well enough to be used in real devices for eye or

sensor protections. This may be achieved by finding a suitable photoconductor that not

only both shows the optical effect, but is also provides strong planar anchoring for the

liquid crystal. This could involve use of other well-studies photoconductors like

polycarbonate(PC) doped with triphenyl diamine (TPD). 125

6.6 Discussion and Conclusions

Chapter 1 presented a brief description of liquid crystals and their common uses

in technology, most notably in liquid crystal displays. In Chapter 2, all of the theory necessary to understanding the surface mediated effects was developed starting with

DeGennes formulation of the order parameter and liquid crystal free energies. This was followed by contributions to the theory by Barbero et al. and others interested in surface

terms in the free energy equation. Subsequently, a full review of the relevant literature

was presented including measurements that pertain to bulk and surface effects in liquid

crystals as well as electronic measurements. Chapter 3 introduced the materials and cell

geometry as well as the experimental apparatus used in the experiments presented in this

thesis. Results obtained using cells with PVA alignment layers were reported in Chapter

4. This was followed by further measurements in cells that had an additional

photoconducting polymer layer in Chapter 5. Here, in Chapter 6, a review of the results

and the theoretical model was presented along with comparison to the literature and the

future outlook of liquid crystals for use in photorefraction, optical switching and limiting.

There are several theories for the surface mediated nonlinear optical effects in

nematic liquid crystal cells that are being studied for their use as photorefractive

materials and other device applications. Here a twist cell used in a Friedericksz transition

geometry provides more direct evidence of the surface effect than the studies where two

beam coupling measurements are used to characterize the cells. The theory presented

here involves the neutralization of accumulated charge by an optical pump beam. Based on the results in this thesis, liquid crystal cells show promise for uses in optical

switching, limiting and photorefraction. This is mainly due to the easy manipulation of 126

the materials and parameters in the cells. The next step is to first improve the

reproducibility of the effect. This is most likely accomplished by careful preparation of

the cells. Once reproducibility has been achieved, the effect could be more easily

characterized for differing cell parameters leading to a more clear explanation of the surface effect. 127

6.7 References

1. Barbero, G., G. Cipparrone, O.G. Martins, P. Pagliusi, and A.M.F. Neto, Electrical response of a liquid crystal cell: The Role of Debye's Layer. Applied Physics Letters, 2006. 89: p. 132901-1-3. 2. Schadt, M., Liquid crystal materials and liquid crystal displays. Annual Reviews of Materials Science, 1997. 27: p. 305-379. 3. Seiberle, H. and M. Schadt, LC-conductivity and cell parameters; Their influence on twisted nematic and supertwist nematic liquid crystal displays. Molecular Crystals and Liquid Crystals, 1994. 239: p. 229-244. 4. Merlin, J., E. Chao, M. Winkler, K.D. Singer, P. Korneychuk, and Y. Reznikov, All-optical switching in a nematic liquid crystal cell. Optics Express, 2005. 13(13): p. 5024-5029. 5. Sworakowski, J. and J. Ulanski, Electrical properties of organic materials. Annual Reports in the Progress of Chemistry Section C, 2003. 99: p. 87-125. 6. Ono, H. and N. Kawatsuki, Orientational holographic grating observed in liquid crystals sandwiched with photoconductive polymer films. Applied Physics Letters, 1997. 71(9): p. 1162-1164. 7. Ono, H. and N. Kawatsuki, Real-time holograms in liquid crystals on photoconductive polymer surfaces. Optics Communications, 1998. 147: p. 237- 241. 8. Mun, J., C.S. Yoon, H.W. Kim, S.A. Choi, and J.D. Kim, Transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers. Applied Physics Letters, 2001. 79(13): p. 1933-1935. 9. Pagliusi, P. and G. Cipparrone, Charge transport due to photoelectric interface activation in pure nematic liquid-crystal cells. Journal of Applied Physics, 2002. 92(9): p. 4863-4869. 10. Dyadyusha, A., M. Kaczmarek, and G. Gilchrist, Surface screening layers and dynamics of energy transfer in photosensitive polymer-liquid crystal structures. Molecular Crystals and Liquid Crystals, 2006. 446: p. 261-272. 11. Miniewicz, A., F. Michelotti, and A. Belardini, Photoconducting polymer-liquid crystal structure studied by electroreflectance. Journal of Applied Physics, 2004. 95(3): p. 1141-1147.

128

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