Design and Fabrication of a Long-range Surface Plasmon Polariton Wave Guide for near-infrared light Diplomarbeit von Johannes Trapp Design and Fabrication of a Long-range Surface Plasmon Polariton Wave Guide for near-infrared light Diplomarbeit im Studiengang Diplom Physik der Technischen Universit¨atM¨unchen angefertigt an der Fakult¨atf¨urPhysik der Ludwig-Maximilians-Universit¨atM¨unchen Arbeitsgruppe Prof. Dr. Harald Weinfurter von Johannes Trapp M¨unchen, den 31. Juli 2011 Erstgutachter: Prof. Dr. G. Rempe Zweitgutachter: Prof. Dr. H. Weinfurter Acknowledgements A work like this thesis can of course not be done alone. Accordingly I would like to express my gratitude to the persons who made this thesis possible and contributed to its success. The first person to thank is Professor Harald Weinfurter, who welcomed me in his group and showed an enormous amount of trust in my endeavours which, in turn, allowed me to work freely. Secondly, great thanks go to Dr. Markus Weber, who explored the world of surface plasmon polaritons with me and always thought of an alternative way to tackle the problems that occurred. Furthermore I would like to thank Philipp Altpeter for his patience to explain to me the variety of machinery and processes in the clean room and who always took the time to help me with his expertise in microfabrication. My office-mate Michael Krug deserves great thanks as well, because he was the first one I turned to for administrative-, computer-, LATEX- or even smaller problems and he helped without hesitation until the problem was solved. The assistance from Dr. Wenjamin Rosenfeld and Daniel Schlenk should not go unmentioned. They contributed to a smart and sleek experimental setup through productive discussions. Of course my gratitude also goes to my parents as they supported me throughout my studies, not only financially but with good advice and as a source of motivation, as well. Finally, I am very grateful to the wonderful Andrea Wahle, who always cheered me up throughout the long patches of unsuccessful fabrication attempts and then celebrated the successive breakthrough with me. Thank you all for your support throughout the year. I Contents 1. Introduction 1 2. Theory of Long-Range Surface Plasmon Polaritons 5 2.1. The permittivity of metals . .5 2.2. Surface plasmon polaritons on a single interface . .9 2.3. Surface plasmon polaritons on a double interface . 13 2.4. Long-range surface plasmon polaritons in metal stripes . 17 2.4.1. Characteristics of the long-range surface plasmon polariton mode................................ 18 3. Wave guide design 25 3.1. Asymmetry in the permittivity of the cladding . 25 3.2. Gold as the wave guide metal . 27 3.3. Outline of the wave guide . 28 4. Wave guide fabrication 31 4.1. Underlying principle of photolithography . 31 4.2. Manufacturing process . 33 4.3. Potential improvements to the quality of the plasmonic wave guide . 41 4.4. The plasmonic wave guide used in the experiment . 43 5. Experimental setup, measurements and results 45 5.1. End-fire coupling . 45 5.2. Experimental setup . 47 5.3. Coupling sequence . 49 5.3.1. Adjustment steps . 50 III Contents 5.4. Observation of the long-range surface plasmon polariton mode . 52 6. Summary and outlook 57 A. Appendix 61 A.1. Difficulties in the fabrication process . 61 A.1.1. Planarisation of the lower BCB layer . 61 A.1.2. Creation of an undercut in the resist layer structure . 63 A.1.3. Clean lift-off of the resist structure . 66 A.1.4. Sawing of the BCB layer . 66 Bibliography 69 IV 1. Introduction Light has always been used as a mean of communication for mankind. Starting with the ancient Greeks that used fire to pass basic messages like the victory in a battle, light | or the absence thereof | carried information for the people. Today's modern telecommunication systems, first and foremost the internet, would not be possible without optical data transfer. At long distances the use of light to encode and transmit information is uncontested. Light guided through optical fibres, that rely on the principle of total internal reflection, exhibits very low losses and it has an inherently high data capacity compared to electrical systems. With the establishment of bright light sources, mainly lasers, optical fibres replaced cop- per wires that were used for long-distance communication purposes before. One might think that, because optical components superseded the electrical ones at long-distances, the replacement of integrated electrical circuits is soon to follow. But despite the superiority of photonic components concerning bandwidth for com- munication purposes, the optical technology has its drawbacks. Optical components use wave guiding features where similar microfabrication techniques are used as in the production of integrated circuits. Although the fabrication does not pose a restriction on the way to optical components with a high circuit density, it is the diffraction limit that does. The theory of diffraction shows that electromagnetic waves cannot be localised with higher precision than the wavelength of the light in the medium. Thus the miniaturisation of integrated optical components that are in use today, is bounded. Therefore other means of guiding light at sub-wavelength scale have to be found. 1 Chapter 1 - Introduction One possible solution are surface plasmon polaritons (SPP). Surface plasmon po- laritons are collective oscillations of a metal's free electrons, that couple to the electromagnetic field in the medium outside the metal, like a dielectric. Because of this coupling, a light wave that excites surface plasmon polaritons becomes bound to the metal surface and can then be guided by it. With the hybrid nature of the surface plasmon polariton the light wave inherits the electron's sensitivity to the metal structure and can now be manipulated on scales much smaller than the free- space wavelength of the light itself. Hence surface plasmon polaritons are a mean of guiding light at a sub-wavelength scale. The major drawback of surface plasmon polaritons is the strong attenuation of the wave that is guided. This is due to the fact, that metals show a strong absorption of electromagnetic radiation in the visible and infrared spectrum. Due to this damp- ing, achievable propagation lengths with surface plasmon polaritons are of the order of only 10 µm, rendering them inadequate for transmission applications. A way to circumvent the strong damping is the use of metal stripes of tens of nanometres thickness and a few micrometre width embedded in a dielectric. This way, not only a single metal-dielectric interface, but four are used, and the damp- ing of metal is reduced significantly. Because the surface plasmon polariton does not exhibit damping in the dielectric around the metal but only in the metal itself, the drastic reduction of the portion of the electromagnetic field inside the metal by reducing the metal's cross-section leads to a tremendous increase in the sur- face plasmon polariton's propagation length. Furthermore, the oscillations at the four surfaces couple to form two well defined modes that span across the whole metal stripe. One of them exhibits propagation length in the millimetres range and is therefore called long-range surface plasmon polariton mode or syncopated long- range surface plasmon polariton (LRSPP). With a propagation length of several millimetres the long-range surface plasmon polariton is suitable for the application in photonic integrated circuits. Wave guides based on the principle of long-range surface plasmon polaritons have 2 first been realised in 2000 by Charbonneau et al. [1]. Gold stripes of 8 µm width, 20 nm thickness and 3,5 mm length where clad in SiO2 and excited with light of 1550 nm wavelength, soon other groups followed. For example Nikolajsen et al. [2] showed in 2002 that a LRSPP (or plasmonic) wave guide can be build with gold stripes in a polymer cladding, and, in 2008, Park et al. [3] succeeded in the con- struction of a plasmonic wave guide with a propagation loss lower than one dB/cm at a wavelength of 1,31 µm. But long-range surface plasmon polaritons can be used for more than just straight wave guides, also Y-splitters and directional couplers have been built (e.g. [4]). Even more sophisticated devices, for example a Mach- Zehnder interferometric modulator, a directional-coupler switch and an integrated power monitor have been realised [5]. All these devices have been built for telecom wavelengths (free-space wavelength: 1,31 µm { 1,55 µm), but also the visible and near-infrared spectrum are of interest. In the visible, LRSPP wave guides could be used, for example, to efficiently collect single photons from diamond nanocrystals. Another interesting application would be the demonstration of LRSPP-interference (Hong-Ou-Mandel effect) on the basis of a plasmonic beam splitter. In this thesis the fabrication and test of a LRSPP wave guide for a free-space wave- length of 785 nm is presented. Starting with the necessary theory and numerical simulations to understand the phenomenon of long-range surface plasmon polari- tons, the requirements for a wave guide are then derived and rendered into a design for a plasmonic wave guide. The fabrication process of the wave guide, mostly done in a clean room, is disclosed in detail in the next chapter. Subsequently, the ex- perimental setup and the measurement scheme are described before the results are presented. The last chapter gives an outlook on possible applications and the further direction of the development of the LRSPP wave guide. 3 2. Theory of Long-Range Surface Plasmon Polaritons In this chapter the theoretical background of long-range surface plasmon polaritons is presented. Starting with the model of electrons in a noble metal | the Drude- Sommerfeld-Theory | the basic properties of surface plasmon polaritons at a single metal-dielectric interface are derived and discussed, closely following [6] and [7].