Fabrication of 3d metamaterials using two photon polymerization and selective silver electroless plating

FABRICATION OF 3D METAMATERIALS USING TWO-PHOTON POLYMERIZATION AND SELECTIVE SILVER ELECTROLESS PLATING YAN YUANJUN A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2011 Abstract Three dimensional (3D) metamaterials have unique properties over their 2D counterparts, such as enhanced sensitivity, negative refraction and chirality. However, the fabrication of micron-sized 3D metallic structures is challenging. There are conventional approaches such as aligned layer-by-layer metal deposition, or electroplating with a polymer template. These methods can be time consuming, costly, difficult to carry out, and most importantly, full 3D control is not possible. In this thesis, we have developed techniques that allow for arbitrary 3D metallic structures to be fabricated simply and efficiently, via two steps: a true 3D lithographic micro-fabrication based on two-photon polymerization followed by a selective silver electroless plating step that can conformally coat all sides of the polymer structure surfaces uniformly with silver, while leaving the silicon substrate uncoated. To demonstrate the techniques developed in this thesis, we have fabricated high aspect ratio split-ring resonators that can be used as sensors, and 3D silver helical structures that can be used as terahertz (THz) broadband circular polarizers. The combination of the two techniques has allowed for true 3D metamaterials to be fabricated simply and efficiently. i Acknowledgements My four years of PhD study is a wonderful and unforgettable journey, filled with challenges and excitement. It would never have been possible for me to write this thesis without the support from many people around me, to only some of whom it is possible to give particular mention here. First and above all, I owe sincere and earnest thankfulness to my supervisor Dr. Andrew Bettiol, for his advice, support, encouragement and crazy ideas throughout, and for the friendly and joyful lab environment he has created, which made my experiment hours full of laughter. I would never be able to have such a delightful PhD experience without him. I am obliged to many of my colleagues who helped and supported me at all times. Thanks to Dr. Ren Minqin who introduced me into the CIBA family. Thanks to Dr. Teo Ee Jin, Dr. Chiam Sher-Yi and Dr. Chammika Udalagama for their guidance on experiments, simulations and programming. Thanks to senior graduate students Isaac, Siew Kit and Sook Fun for their valuable advice on research and graduate study. Thanks to Sudheer, Chengyuan and Prashant for their discussions as well as help in doing experiments. Thanks to Aky, Kyle and all other CIBA members, you are all being so nice and sweet! Most importantly, I am truly indebted and thankful to my parents, for raising me up and for the continuous support and encouragement they give me as always. Without your love, I would never be who I am. ii Table of Contents Chapter 1: Introduction 1.1 Motivation and objectives 1.2 Thesis outline . Chapter 2: Review of Metamaterials . 2.1 Introduction 2.2 Two dimensional metamaterials 2.3 Three dimensional metamaterials 12 Chapter 3: Two-Photon Lithography System 21 3.1 Fundamentals of Two-Photon Lithography (TPL) 22 3.2 Optics . 24 3.2.1 Optical setup . 24 3.2.2 Light focusing and Numerical Aperture (NA) 27 3.3 Software development . 32 3.3.1 Slicing of 3D design 32 3.3.2 tpl file coding 33 3.4 Photoresist study 36 3.4.1 Substrate 37 3.4.2 Photoresist studies . 38 iii 3.4.3 3D fabrication with SU-8 2000 . 41 3.4.4 Shrinkage study of SU-8 2000 45 3.5 Summary 51 Chapter 4: Selective Electroless Silver Plating 53 4.1 Electroless silver plating 54 4.1.1 Electroplating method . 54 4.1.2 Electroless silver plating . 56 4.1.3 Advantages of electroless plating for Metamaterials 59 4.2 Selectivity 59 4.2.1 Why selectivity is required and how can it be achieved? . 59 4.2.2 Our method: Radio Frequency (RF) plasma pretreatment 63 4.2.3 Mechanism 65 4.3 Results 67 4.3.1 Coating on 3D structures 67 4.3.2 Roughness optimizations of coated surface 68 4.3.3 Optical characterization of silver coated SRRs 74 4.4 Summary 75 Chapter 5: High Aspect Ratio Split-Ring Resonators 77 5.1 High Aspect Ratio (HAR) Split-Ring Resonators (SRRs) 78 5.1.1 LC resonance of SRR 78 iv 5.1.2 HAR SRR in sensing 81 5.2 Fabrication of HAR SRRs by TPL with an Axicon lens . 84 5.3 Fabrication of HAR SRRs by Proton Beam Writing (PBW) . 89 5.4 Optical characterization of HAR SRRs . 94 5.5 Summary 95 Chapter 6: 3D Silver Helices as THz Broadband Circular Polarizer . 97 6.1 Chiral metamaterials and their properties 98 6.2 Design and fabrication of THz 3D silver helices . 104 6.3 Numerical optimization of THz 3D silver helices . 107 6.4 Proposed optical characterization method . 111 6.5 Summary 113 Chapter 7: Conclusion and Future Outlook . 115 7.1 Summary of the work carried out for this thesis 115 7.2 Future work 119 Bibliography 123 Appendix A: List of publications . 131 Appendix B: CST Microwave Studio 133 v vi List of figures Figure 2.1 Material parameter space characterized by electric permittivity ε and magnetic permeability μ. From [24]. . Figure 2.2 Basic metamaterial structures to implement artificial electric and magnetic responses. (a) Schematic of periodic wires (with radius r) arranged in a simple cubic lattice (with lattice constant d). (b) Effective permittivity of wire media, acting as dilute metals with an extremely-low plasma frequency. (c) Schematic of split ring resonators, with outer radius r and separation s between the two rings. A magnetic  field penetrating the resonator induces a current ( ), and thus a magnetic moment ( m ). (d) Effective permeability of split ring resonators around the resonance frequency. From [24]. Figure 2.3 The first structure that exhibits negative refraction at GHz. From [4]. 10 Figure 2.4 Advances in metamaterials. The solid symbols denote n < 0; the open symbols denote µ < 0. Orange: data from structures based on the double split-ring resonator (SRR); green: data from U-shaped SRRs; blue: data from pairs of metallic nanorods; red: data from the “fishnet” structure. The four insets give pictures of fabricated structures in different frequency regions. . 11 Figure 2.5 (left) Measured (solid) and calculated (dashed) normal incidence transmittance (red) and reflectance (blue) spectra for two orthogonal polarizations for the multilayer fishnet metamaterial structures when N=1, 2, functional (Ag-MgF2) layers. Insets are SEM images with scale bar 400nm. (right) Refractive index and permeability retrieved from the transmittance data. Solid lines correspond to real part and dashed correspond to imaginary part. From [10]. . 14 vii Figure 2.6 The standard e-beam lithography, deposition and lift-off procedure. The total structure height is limited by the thickness of the photoresist, and the trapezoidal sidewalls also prevent more layers stack to be fabricated. From [33]. 14 Figure 2.7 (left) Processing scheme to make multilayer SRR stack; (right) Fieldemission scanning electron microscopy images of the four-layer SRR structures, oblique view. From [11]. . 15 Figure 2.8 (a) Diagram of the 21-layer fishnet structure with a unit cell of p=860nm, a=565nm and b=265nm; (b) SEM image of the 21-layer fishnet structure with the side etched, showing the cross-section. The sidewall angle is 4.3° and was found to have a minor effect on the transmittance curve according to simulation; (c) Experimental setup for the beam refraction measurement. The focal length of lens is 50mm and that of lens is 40mm. Lens is placed in a 2f configuration, resulting in the Fourier image at the camera position. From [12]. 16 Figure 2.9 (a) Metamaterial design. The white regions are the polymer (SU-8) located on a glass substrate. The sidewalls of the polymer (encapsulated by SiO2 via ALD) are coated with silver. The polarization of the incident electromagneticfield is illustrated on the lower left-hand-side corner. (b) Oblique-view electron micrograph of a structure fabricated by direct laser writing and silver shadow evaporation that has been cut by a focused-ion beam (FIB) to reveal its interior. From [36]. . 17 Figure 2.10 (left) Fabrication procedure of the gold helices: A positive-tone photoresist (blue) is spun onto a glass substrate covered with a 25nm thin film of conductive indium-tin oxide (ITO) shown in green. After 3D DLW and development, an array of air helices in a block of polymer results. After plating with gold in an electrolyte, the polymer is removed by plasma etching, leading to a square array of viii free-standing 3D gold helices. (right) SEM images of the fabricated helices. From [14]. 18 Figure 2.11 (up) Fabrication procedures of the structures. (down) SEM image of the silver coated structures before detached from the glass substrate which was also coated. From [15]. 19 Figure 3.1 Fluorescence from a solution of rhodamine B caused by single-photon excitation from a UV lamp (a) and by two-photon excitation from a mode-locked Ti:sapphire laser operating at a wavelength of 800nm (b). Figure from [48] 23 Figure 3.2 Schematic of the TPL system. 24 Figure 3.3 Microscope and fabrication platform. 26 Figure 3.4 Light focusing by a thin lens. . 27 Figure 3.5 Intensity profile at focal plane in terms of radial optical coordinate v, normalized by maximum intensity: (left) Density plot; (right) 3D plot. . 29 Figure 3.6 Intensity profile at axial plane in terms of axial optical coordinate u, normalized by maximum intensity: (left) Density plot; (right) 3D plot. . 30 Figure 3.7 Intensity distribution contour plots at axial plane for objectives with (left) NA=0.3; (right) NA=0.95. Arbitrary units. . 31 Figure 3.8 A cone design in AutoCAD (left) being sliced into 15 layers (right). . 33 Figure 3.9 Binary bitmap designs for a 10μm circle: scaling factor (left) and scaling factor (right). . 34 Figure 3.10 A simple 2D design in its bitmap format (left) and tpl format (right). . 35 Figure 3.11 Sample setups for Si substrate (left) and glass substrate (right). 37 Figure 3.12 Planar structures fabricated in S1813. 38 Figure 3.13 Planar structures fabricated in AZ1518. . 39 ix As the well developed techniques in this thesis have enabled us to design and fabricate metamaterial structures with great ease, more and more functional metamaterial devices are expected in the future. As pointed out by Zheludev [92], the research in metamaterials is moving towards applications such as data storage, displays, waveguides, micro and nano scale lasers, slow light and quantum information. By designing our own “meta atoms”, we will be able to create new materials, new devices, and even new systems that can significantly change the technology. With the future development of advanced fabrication techniques, only our imagination and creativity set the limits. 121 122 Bibliography 1. V. G. Veselago, "Electrodynamics of Substances with Simultaneously Negative Values of Sigma and Mu," Soviet Physics Uspekhi-Ussr 10, 509 (1968). 2. J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, "Magnetism from conductors and enhanced nonlinear phenomena," IEEE Trans. Microw. Theory Tech. 47, 2075-2084 (1999). 3. D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, "Composite medium with simultaneously negative permeability and permittivity," Phys. Rev. Lett. 84, 4184-4187 (2000). 4. R. A. Shelby, D. R. Smith, and S. Schultz, "Experimental verification of a negative index of refraction," Science 292, 77-79 (2001). 5. C. Enkrich, M. Wegener, S. Linden, S. Burger, L. Zschiedrich, F. Schmidt, J. F. Zhou, T. Koschny, and C. M. Soukoulis, "Magnetic metamaterials at telecommunication and visible frequencies," Phys. Rev. Lett. 95, (2005). 6. J. Zhou, T. Koschny, M. Kafesaki, E. N. Economou, J. B. Pendry, and C. M. Soukoulis, "Saturation of the magnetic response of split-ring resonators at optical frequencies," Phys. Rev. Lett. 95, (2005). 7. S. Zhang, W. J. Fan, N. C. Panoiu, K. J. Malloy, R. M. Osgood, and S. R. J. Brueck, "Experimental demonstration of near-infrared negative-index metamaterials," Phys. Rev. Lett. 95, (2005). 8. V. M. Shalaev, W. S. Cai, U. K. Chettiar, H. K. Yuan, A. K. Sarychev, V. P. Drachev, and A. V. Kildishev, "Negative index of refraction in optical metamaterials," Opt. Lett. 30, 3356-3358 (2005). 9. G. Dolling, C. Enkrich, M. Wegener, C. M. Soukoulis, and S. Linden, "Simultaneous negative phase and group velocity of light in a metamaterial," Science 312, 892-894 (2006). 10. G. Dolling, M. Wegener, and S. Linden, "Realization of a three-functionallayer negative-index photonic metamaterial," Opt. Lett. 32, 551-553 (2007). 11. N. Liu, H. Guo, L. Fu, S. Kaiser, H. Schweizer, and H. Giessen, "Threedimensional photonic metamaterials at optical frequencies," Nature materials 7, 31-37 (2008). 12. J. Valentine, S. Zhang, T. Zentgraf, E. Ulin-Avila, D. a. Genov, G. Bartal, and X. Zhang, "Three-dimensional optical metamaterial with a negative refractive index," Nature 455, 376-379 (2008). 123 13. M. S. R. M. S. Rill, C. Plet, M. Thiel, I. Staude, G. Von Freymann, S. Linden, and M. Wegener, "Photonic metamaterials by direct laser writing and silver chemical vapour deposition," Nature Materials 7, 543-546 (2008). 14. J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. von Freymann, S. Linden, and M. Wegener, "Gold Helix Photonic Metamaterial as Broadband Circular Polarizer," Science 325, 1513-1515 (2009). 15. A. Radke, T. Gissibl, T. Klotzbücher, P. V. Braun, and H. Giessen, "ThreeDimensional Bichiral Plasmonic Crystals Fabricated by Direct Laser Writing and Electroless Silver Plating," Advanced Materials 23, 3018-3021 (2011). 16. J. B. Pendry, "A Chiral Route to Negative Refraction," Science 306, 13531355 (2004). 17. A. V. Rogacheva, V. A. Fedotov, A. S. Schwanecke, and N. I. Zheludev, "Giant gyrotropy due to electromagnetic-field coupling in a bilayered chiral structure," Phys. Rev. Lett. 97, (2006). 18. M. Decker, M. W. Klein, M. Wegener, and S. Linden, "Circular dichroism of planar chiral magnetic metamaterials," Opt. Lett. 32, 856-858 (2007). 19. J. B. Pendry, A. J. Holden, W. J. Stewart, and I. Youngs, "Extremely low frequency plasmons in metallic mesostructures," Phys. Rev. Lett. 76, 4773-4776 (1996). 20. S. Linden, C. Enkrich, G. Dolling, M. W. Klein, J. Zhou, T. Koschny, C. M. Soukoulis, S. Burger, F. Schmidt, and M. Wegener, "Photonic metamaterials: Magnetism at optical frequencies," IEEE J. Sel. Top. Quantum Electron. 12, 10971105 (2006). 21. M. Yan, Z. C. Ruan, and M. Qiu, "Cylindrical invisibility cloak with simplified material parameters is inherently visible," Phys. Rev. Lett. 99, (2007). 22. N. Fang, H. Lee, C. Sun, and X. Zhang, "Sub-diffraction-limited optical imaging with a silver superlens," Science 308, 534-537 (2005). 23. N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sonnichsen, and H. Giessen, "Planar Metamaterial Analogue of Electromagnetically Induced Transparency for Plasmonic Sensing," Nano Lett. 10, 1103-1107 (2010). 24. Y. M. Liu, and X. Zhang, "Metamaterials: a new frontier of science and technology," Chem. Soc. Rev. 40, 2494-2507 (2011). 25. C. M. Soukoulis, S. Linden, and M. Wegener, "Negative refractive index at optical wavelengths," Science (New York, N.Y.) 315, 47-49 (2007). 124 26. G. Dolling, C. Enkrich, M. Wegener, J. F. Zhou, and C. M. Soukoulis, "Cutwire pairs and plate pairs as magnetic atoms for optical metamaterials," Opt. Lett. 30, 3198-3200 (2005). 27. C. Enkrich, R. Perez-Willard, D. Gerthsen, J. F. Zhou, T. Koschny, C. M. Soukoulis, M. Wegener, and S. Linden, "Focused-ion-beam nanofabrication of nearinfrared magnetic metamaterials," Advanced Materials 17, 2547 (2005). 28. S. Zhang, W. Fan, K. J. Malloy, S. R. J. Brueck, N. C. Panoiu, and R. M. Osgood, "Demonstration of metal-dielectric negative-index metamaterials with improved performance at optical frequencies," J. Opt. Soc. Am. B 23, 434-438 (2006). 29. W. Wu, E. Kim, E. Ponizovskaya, Y. Liu, Z. Yu, N. Fang, Y. R. Shen, A. M. Bratkovsky, W. Tong, C. Sun, X. Zhang, S. Y. Wang, and R. S. Williams, "Optical metamaterials at near and mid-IR range fabricated by nanoimprint lithography," Applied Physics A: Materials Science & Processing 87, 143-150 (2007). 30. M. C. K. Wiltshire, and et al., "Sub-wavelength imaging at radio frequency," J. Phys.: Condens. Matter 18, L315 (2006). 31. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, "Metamaterial Electromagnetic Cloak at Microwave Frequencies," Science 314, 977-980 (2006). 32. S. Zhang, W. Fan, N. C. Panoiu, K. J. Malloy, R. M. Osgood, and S. R. Brueck, "Optical negative-index bulk metamaterials consisting of 2D perforated metal-dielectric stacks," Opt. Express 14, 6778-6787 (2006). 33. A. Boltasseva, and V. M. Shalaev, "Fabrication of optical negative-index metamaterials: Recent advances and outlook," Metamaterials 2, 1-17 (2008). 34. J. B. Pendry, "A Chiral Route to Negative Refraction," (2004), pp. 1353-1355. 35. F. Formanek, N. Takeyasu, T. Tanaka, K. Chiyoda, A. Ishikawa, and S. Kawata, "Three-dimensional fabrication of metallic nanostructures over large areas by two-photon polymerization," Opt. Express 14, 800-809 (2006). 36. M. S. Rill, C. E. Kriegler, M. Thiel, G. von Freymann, S. Linden, and M. Wegener, "Negative-index bianisotropic photonic metamaterial fabricated by direct laser writing and silver shadow evaporation," Opt. Lett. 34, 19-21 (2009). 37. J. G. Fleming, S. Y. Lin, I. El-Kady, R. Biswas, and K. M. Ho, "All-metallic three-dimensional photonic crystals with a large infrared bandgap," Nature 417, 52-55 (2002). 38. T. C. Wang, R. E. Cohen, and M. F. Rubner, "Metallodielectric photonic structures based on polyelectrolyte multilayers," Advanced Materials 14, 1534-+ (2002). 125 39. C. M. Waits, B. Morgan, M. Kastantin, and R. Ghodssi, "Microfabrication of 3D silicon MEMS structures using gray-scale lithography and deep reactive ion etching," Sens. Actuator A-Phys. 119, 245-253 (2005). 40. K. Totsu, K. Fujishiro, S. Tanaka, and M. Esashi, "Fabrication of threedimensional microstructure using maskless gray-scale lithography," Sens. Actuator APhys. 130, 387-392 (2006). 41. M. Campbell, D. N. Sharp, M. T. Harrison, R. G. Denning, and A. J. Turberfield, "Fabrication of photonic crystals for the visible spectrum by holographic lithography," Nature 404, 53-56 (2000). 42. S. Matsui, "Focused-ion-beam deposition for 3-D nanostructure fabrication," Nucl. Instrum. Methods Phys. Res. Sect. B-Beam Interact. Mater. Atoms 257, 758764 (2007). 43. D. F. Tan, Y. Li, F. J. Qi, H. Yang, Q. H. Gong, X. Z. Dong, and X. M. Duan, "Reduction in feature size of two-photon polymerization using SCR500," Appl. Phys. Lett. 90, (2007). 44. C. Reinhardt, R. Kiyan, S. Passinger, A. L. Stepanov, A. Ostendorf, and N. Chichkov, "Rapid laser prototyping of plasmonic components," Appl. Phys. A-Mater. Sci. Process. 89, 321-325 (2007). 45. J. D. Pitts, P. J. Campagnola, G. A. Epling, and S. L. Goodman, "Submicron multiphoton free-form fabrication of proteins and polymers: Studies of reaction efficiencies and applications in sustained release," Macromolecules 33, 1514-1523 (2000). 46. S. Maruo, K. Ikuta, and H. Korogi, "Submicron manipulation tools driven by light in a liquid," Appl. Phys. Lett. 82, 133-135 (2003). 47. W. Haske, V. W. Chen, J. M. Hales, W. T. Dong, S. Barlow, S. R. Marder, and J. W. Perry, "65 nm feature sizes using visible wavelength 3-D multiphoton lithography," Optics Express 15, 3426-3436 (2007). 48. C. N. LaFratta, J. T. Fourkas, T. Baldacchini, and R. A. Farrer, "Multiphoton fabrication," Angew. Chem.-Int. Edit. 46, 6238-6258 (2007). 49. K. K. Seet, V. Mizeikis, S. Juodkazis, and H. Misawa, "Three-dimensional horizontal circular spiral photonic crystals with stop gaps below μm," Appl. Phys. Lett. 88, (2006). 50. R. A. Farrer, C. N. LaFratta, L. J. Li, J. Praino, M. J. Naughton, B. E. A. Saleh, M. C. Teich, and J. T. Fourkas, "Selective functionalization of 3-D polymer microstructures," J. Am. Chem. Soc. 128, 1796-1797 (2006). 126 51. X. B. Yin, N. Fang, X. Zhang, I. B. Martini, and B. J. Schwartz, "Near-field two-photon nanolithography using an apertureless optical probe," Appl. Phys. Lett. 81, 3663-3665 (2002). 52. W. H. Teh, U. Durig, G. Salis, R. Harbers, U. Drechsler, R. F. Mahrt, C. G. Smith, and H. J. Guntherodt, "SU-8 for real three-dimensional subdiffraction-limit two-photon microfabrication," Appl. Phys. Lett. 84, 4095-4097 (2004). 53. S. Juodkazis, V. Mizeikis, K. K. Seet, M. Miwa, and H. Misawa, "Two-photon lithography of nanorods in SU-8 photoresist," Nanotechnology 16, 846-849 (2005). 54. R. Houbertz, L. Frohlich, M. Popall, U. Streppel, P. Dannberg, A. Brauer, J. Serbin, and B. N. Chichkov, "Inorganic-organic hybrid polymers for information technology: from planar technology to 3D nanostructures," Adv. Eng. Mater. 5, 551555 (2003). 55. A. Doraiswamy, C. Jin, R. J. Narayan, P. Mageswaran, P. Mente, R. Modi, R. Auyeung, D. B. Chrisey, A. Ovsianikov, and B. Chichkov, "Two photon induced polymerization of organic-inorganic hybrid biomaterials for microstructured medical devices," Acta Biomater. 2, 267-275 (2006). 56. A. del Campo, and C. Greiner, "SU-8: a photoresist for high-aspect ratio and 3D submicron lithography," J. Micromech. Microeng. 17, R81-R95 (2007). 57. J. A. van Kan, A. A. Bettiol, and F. Watt, "Three-dimensional nanolithography using proton beam writing," Appl. Phys. Lett. 83, 1629-1631 (2003). 58. H. Lorenz, M. Despont, N. Fahrni, J. Brugger, P. Vettiger, and P. Renaud, "High-aspect ratio, ultrathick, negative-tone near-UV photoresist and its applications for MEMS," Sens. Actuator A-Phys. 64, 33-39 (1998). 59. G. Liu, Y. Tian, and Y. Kan, "Fabrication of high-aspect ratio microstructures using SU8 photoresist," Microsyst. Technol. 11, 343-346 (2005). 60. C. Becnel, Y. Desta, and K. Kelly, "Ultra-deep x-ray lithography of densely packed SU-8 features: II. Process performance as a function of dose, feature height and post exposure bake temperature," J. Micromech. Microeng. 15, 1249-1259 (2005). 61. A. Ovsianikov, S. Z. Xiao, M. Farsari, M. Vamvakaki, C. Fotakis, and B. N. Chichkov, "Shrinkage of microstructures produced by two-photon polymerization of Zr-based hybrid photosensitive materials," Optics Express 17, 2143-2148 (2009). 62. Y. Y. Cao, X. Z. Dong, N. Takeyasu, T. Tanaka, Z. S. Zhao, X. M. Duan, and S. Kawata, "Morphology and size dependence of silver microstructures in fatty saltsassisted multiphoton photoreduction microfabrication," Appl. Phys. A-Mater. Sci. Process. 96, 453-458 (2009). 127 63. Y.-Y. Cao, N. Takeyasu, T. Tanaka, X.-M. Duan, and S. Kawata, "3D Metallic Nanostructure Fabrication by Surfactant-Assisted Multiphoton-Induced Reduction," Small 5, 1144-1148 (2009). 64. X.-M. Duan, H.-B. Sun, K. Kaneko, and S. Kawata, "Two-photon polymerization of metal ions doped acrylate monomers and oligomers for threedimensional structure fabrication," Thin Solid Films 453-454, 518-521 (2004). 65. S. Y. Chiam, R. Singh, J. Q. Gu, J. G. Han, W. L. Zhang, and A. A. Bettiol, "Increased frequency shifts in high aspect ratio terahertz split ring resonators," Appl. Phys. Lett. 94, (2009). 66. P. M. Dentinger, W. M. Clift, and S. H. Goods, "Removal of SU-8 photoresist for thick film applications," Microelectron. Eng. 61-2, 993-1000 (2002). 67. F. Formanek, N. Takeyasu, T. Tanaka, K. Chiyoda, A. Ishikawa, and S. Kawata, "Selective electroless plating to fabricate complex three-dimensional metallic micro/nanostructures," Appl. Phys. Lett. 88, (2006). 68. W. Dai, and W. J. Wang, "Selective metallization of cured SU-8 microstructures using electroless plating method," Sens. Actuator A-Phys. 135, 300307 (2007). 69. K. Terzaki, N. Vasilantonakis, A. Gaidukeviciute, C. Reinhardt, C. Fotakis, M. Vamvakaki, and M. Farsari, "3D conducting nanostructures fabricated using direct laser writing," Opt. Mater. Express 1, 586-597 (2011). 70. S. Bhattacharya, A. Datta, J. M. Berg, and S. Gangopadhyay, "Studies on surface wettability of poly(dimethyl) siloxane (PDMS) and glass under oxygenplasma treatment and correlation with bond strength," J. Microelectromech. Syst. 14, 590-597 (2005). 71. F. Walther, P. Davydovskaya, S. Zucher, M. Kaiser, H. Herberg, A. M. Gigler, and R. W. Stark, "Stability of the hydrophilic behavior of oxygen plasma activated SU-8," J. Micromech. Microeng. 17, 524-531 (2007). 72. L. J. Gerenser, "Photoemission Investigation of Silver Poly(EthyleneTerephthalate) Interfacial Chemistry - the Effect of Oxygen-Plasma Treatment," J. Vac. Sci. Technol. A-Vac. Surf. Films 8, 3682-3691 (1990). 73. M. Kuemmel, J. Allouche, L. Nicole, C. Boissiere, C. Laberty, H. Amenitsch, C. Sanchez, and D. Grosso, "A chemical solution deposition route to nanopatterned inorganic material surfaces," Chem. Mat. 19, 3717-3725 (2007). 74. F. Watt, M. B. H. Breese, A. A. Bettiol, and J. A. van Kan, "Proton beam writing," Mater. Today 10, 20-29 (2007). 128 75. J. A. van Kan, F. Zhang, S. Y. Chiam, T. Osipowicz, A. A. Bettiol, and F. Watt, "Proton beam writing: a platform technology for nanowire production," Microsyst. Technol. 14, 1343-1348 (2008). 76. J. A. Van Kan, A. A. Bettiol, K. Ansari, E. J. Teo, T. C. Sum, and F. Watt, "Proton beam writing: a progress review," Int. J. Nanotechnol. 1, 464-479 (2004). 77. J. A. van Kan, A. A. Bettiol, and F. Watt, "Proton beam writing of threedimensional nanostructures in hydrogen silsesquioxane," Nano Lett. 6, 579-582 (2006). 78. A. A. Bettiol, C. N. B. Udalagama, J. A. van Kan, and F. Watt, "Ionscan: Scanning and control software for proton beam writing," Nucl. Instrum. Methods Phys. Res. Sect. B-Beam Interact. Mater. Atoms 231, 400-406 (2005). 79. A. A. Bettiol, T. C. Sum, F. C. Cheong, C. H. Sow, S. V. Rao, J. A. van Kan, E. J. Teo, K. Ansari, and F. Watt, "A progress review of proton beam writing applications in microphotonics," Nucl. Instrum. Methods Phys. Res. Sect. B-Beam Interact. Mater. Atoms 231, 364-371 (2005). 80. S. K. Hoi, Z. B. Hu, Y. Yan, C. H. Sow, and A. A. Bettiol, "A microfluidic device with integrated optics for microparticle switching," Appl. Phys. Lett. 97, (2010). 81. E. J. Teo, M. B. H. Breese, A. A. Bettiol, D. Mangaiyarkarasi, F. Champeaux, F. Watt, and D. J. Blackwood, "Multicolor photoluminescence from porous silicon using focused, high-energy helium ions," Advanced Materials 18, 51-55 (2006). 82. S. Tretyakov, I. Nefedov, A. Sihvola, S. Maslovski, and C. Simovski, "Waves and energy in chiral nihility," J. Electromagn. Waves Appl. 17, 695-706 (2003). 83. C. Monzon, and D. W. Forester, "Negative refraction and focusing of circularly polarized waves in optically active media," Phys. Rev. Lett. 95, (2005). 84. E. Plum, J. Zhou, J. Dong, V. A. Fedotov, T. Koschny, C. M. Soukoulis, and N. I. Zheludev, "Metamaterial with negative index due to chirality," Phys. Rev. B 79, (2009). 85. S. Zhang, Y. S. Park, J. S. Li, X. C. Lu, W. L. Zhang, and X. Zhang, "Negative Refractive Index in Chiral Metamaterials," Phys. Rev. Lett. 102, (2009). 86. M. Wegener, and S. Linden, "Shaping optical space with metamaterials," Phys. Today 63, 32-36 (2010). 87. X. C. Zhang, "Terahertz wave imaging: horizons and hurdles," Phys. Med. Biol. 47, 3667-3677 (2002). 129 88. R. Singh, E. Plum, W. L. Zhang, and N. I. Zheludev, "Highly tunable optical activity in planar achiral terahertz metamaterials," Optics Express 18, 13425-13430 (2010). 89. S. Saito, A. Syouji, K. Sakai, K. Fukunaga, M. Mizuno, and I. Hosako, "Broadband terahertz time-domain spectroscopic system with photoconductive antennas," in 2007 Joint 32nd International Conference on Infrared and Millimeter Waves and 15th International Conference on Terahertz Electronics, Vols and 2(Ieee, New York, 2007), pp. 475-476. 90. K. Kitagishi, and Y. Izutani, "Automatically successive waveform acquisition with THz-TDS in a wide frequency range," in 2009 34th International Conference on Infrared, Millimeter, and Terahertz Waves, Vols and 2(Ieee, New York, 2009), pp. 366-367. 91. I. C. Ho, X. Y. Guo, and X. C. Zhang, "Design and performance of reflective ultra broadband terahertz time-domain spectroscopy with air-biased-coherentdetection," in Terahertz Technology and Applications Iii(Spie-Int Soc Optical Engineering, Bellingham, 2010). 92. N. I. Zheludev, "The Road Ahead for Metamaterials," Science 328, 582-583 (2010). 130 Appendix A: List of publications [1] Yuanjun Yan, M. Ibnur Rashad, Ee Jin Teo, Hendrix Tanoto, Jinghua Teng and Andrew A. Bettiol, “Selective electroless silver plating of three dimensional SU-8 microstructures on silicon for metamaterials applications”, Optical Materials Express 1, 1548, 2011 [2] S. Azimi, M. B. H. Breese, Z. Y. Dang, Y. Yan, Y. S. Ow and A. A. Bettiol, “Fabrication of complex curved three-dimensional silicon microstructures using ion irradiation”, Journal of Micromechanics and Microengineering 22, 015015, 2012 [3] Hoi SK, Hu ZB, Yan Y, Sow CH and Bettiol AA, “A microfluidic device with integrated optics for microparticle switching”, Applied Physics Letters, 97, 183501, 2010 [4] Yuanjun Yan and Andrew Bettiol, “Selective electro-less plating of SU-8 microstructures fabricated using two-photon polymerization”, 2010 Photonics Global Conference, p4, 2010 131 132 Appendix B: CST Microwave Studio The simulation studies in this thesis were carried out in CST Microwave Studio. CST Microwave Studio is a software package for electromagnetic design and analysis over a large range of frequencies. The user can design graphically any shape of structures and their material properties. After boundary conditions and meshing parameters are defined, the solvers calculate Maxwell’s equations numerically for a given range of frequencies. The user can retrieve results such as Scattering parameters (Sparameters), electric field and magnetic field distributions, etc. A key feature of this software package is that it offers three different solvers: transient solver, frequency domain solver and eigenmode solver. The most efficient tool is the transient solver, which can obtain the entire broadband frequency behavior of the simulated device from a single calculation run. For applications such as connectors, transmission lines, filters and antennae, this solver is usually used. However, when the simulated structure is smaller than the shortest wavelength of interest, the transient solver is less efficient. In this case, the frequency domain solver is usually employed. An advantage of the frequency domain solver is that it supports both hexahedral and tetrahedral meshes, whereas the transient solver only supports hexahedral meshes. The frequency domain solver also contains fast alternative algorithms for calculating S-parameters for strongly resonating structures. When the operating modes inside a device are required for the calculation, the eigenmode solver is the best choice. For our simulation of SRRs, both the transient solver and the frequency domain solver were tested and verified to work. The frequency domain solver is more favorable 133 because it calculates all S-parameters for both polarizations in a single simulation run, whereas the transient solver simulates the two polarizations separately because their boundary conditions are different. Another advantage of the frequency domain solver is that it can easily model circular polarization. This is especially useful for applications that require left and right circular polarized incident light instead of parallel and perpendicular linear polarized light, such as the 3D helical structures that will be described in Chapter 6. Therefore, for all the simulations carried out in this thesis, the frequency domain solver is used. Figure 7.1 Modelling and parameters configuration of the SRR in CST Microwave Studio with Frequency domain solver. Figure 7.1 is the modeling and parameters configuration of the HAR SRR in Chapter 5. A general procedure for simulations in CST Microwave Studio with frequency domain solver is now described: 134 1) Set the unit and frequency range to be calculated. 2) Model the structure: It can either be designed graphically in CST or imported from external format such as dxf. Important dimensions such as the structure length, width and height, can be defined as variables. 3) Assign material properties to each part of the structure and the environment: For example, our silver coated SRR consists of an SU-8 structure, 100 nm silver coating on all sides of the SRR, and a Si substrate. The environment material is set to vacuum. 4) Define boundary conditions: We want to set the boundary conditions such that the SRR repeats itself infinitely in X and Y directions. The boundary conditions for transient solver and frequency domain solver are different. For the frequency domain solver that we adopt, both X and Y boundary conditions are set to be “unit cell”. The boundary condition for Z direction is set to be “open”. 5) Define input, output and their modes: The input and output are located before and after the structure, respectively. Incident light is launched from the input and different modes can be chosen. For our SRR, only two orthogonal polarizations are considered, TE and TM. For the helices discussed in Chapter 6, left circular polarization (LCP) and right circular polarization (RCP) are defined instead. The output is where the detector is located. Modes also have to be defined, just like the input. 6) Generate the mesh: As Maxwell’s equations are solved numerically, a proper mesh size is important. Smaller meshes give higher accuracy, but the processing time increases. Larger meshes reduce the calculation burden, while sacrificing the accuracy. 135 7) Calculate S-parameters with frequency domain solver: The S-parameters reveal the relation between transmitted fields and the incident fields. In the SRR simulation, parallel polarization is assigned as mode and the perpendicular polarization as mode 2. According to the definitions of S-parameters, S11 represents the transmittance for parallel polarization and S21 represents the conversion from parallel to perpendicular polarizations when light transmits through the structure. Likewise, S22 and S12 represent the transmittance and conversion for perpendicular polarization, respectively. 8) Parameter sweep (optional): In some cases, the structure parameters need to be adjusted in order to optimize their functionality (e.g. height study shown in Section 5.1.2). The parameter of interest can be set as a variable within the domain. During calculation, the solver sweeps the whole range and calculates multiple values for the parameter. The user then can analyze the results and find out the optimal conditions. 136 [...]... less than one pitch of left-handed helices, (B) two pitches of left-handed helices, and (C) two pitches of right-handed helices For wavelength longer than 6.5μm, the glass substrate becomes totally opaque Hence, transmittance cannot be measured From [14] 103 Figure 6.6 Design of the THz 3D silver helix 104 Figure 6.7 Normal view (left) and oblique view (right) of the 3D helices in SU-8... properties of metamaterials, and reviews some of the recent developments in metamaterial research and fabrication techniques Chapter 3 and Chapter 4 form the second part of the thesis, the technical development In Chapter 3, our in-house developed two- photon lithography (TPL) system is discussed in detail, in terms of optical setup, software programming, photoresist studies, and a detailed study of SU-8... aims to address some of these issues through the development of a novel and effective technique for fabricating true 3D metamaterials The technique consists of two steps: a 3D fabrication step utilizing an SU-8 photoresist as a polymer template, and a 2 selective metallization step The capabilities of the newly developed technique are demonstrated by applying the technique to two metamaterial applications... conventional 2D metamaterials, true 3D fabrication of metallic structures is much more challenging because it cannot be achieved using standard lithographic and metal deposition techniques 1.1 Motivation and objectives One of the most challenging aspects of metamaterials research is fabrication In the early experimental reports, planar techniques, such as electron beam or UV lithography and metal deposition,... dimensional metamaterials Real world applications of metamaterials that utilize properties such as negative refraction [4], superlensing [30] and invisibility cloaking [31], require bulk samples, or 3D metamaterials For the microwave frequencies, 3D metamaterials can be easily fabricated using standard circuit board technology More recently, new fabrication 12 technologies have enabled 3D metamaterials. .. near-infrared (NIR) and the optical range The unit cell, or “meta-atom”, also progressed from two- dimensional (2D) to three-dimensional (3D) , due to the development of more sophisticated fabrication techniques In this chapter, the basic properties of metamaterials are discussed, followed by a brief history of the development of metamaterial research, focusing on different designs and their fabrication techniques... spectra of the SRRs under both parallel and perpendicular polarizations in the following scenarios: (A) Neither SU-8 or Si are coated with silver; (B) Both SU-8 and Si are coated with silver; (C) Only SU-8 is coated with silver Results indicate that only in scenario (C) is resonance present 60 Figure 4.6 Silver coatings on (left) Si and (right) SU-8 surfaces following the electroless silver plating. .. have resonance at 2THz, and SU-8 SRRs with 100nm silver coating behaves similar to the silver SRRs, with a slight frequency shift 72 Figure 4.18 SEM image of the crosssection of coated sample Thickness is measured to be 100nm 73 Figure 4.19 (a) An array of double split ring resonators fabricated in SU-8 on Si and coated with Ag using selective electroless plating (b) Transmission... 2.5 The fabrication techniques they used were standard EBL, metal and dielectric deposition with e-beam evaporation, and a lift-off procedure Although up to 3 functional layers (7 actual layers) were demonstrated, fabricating even thicker bulk metamaterials using this approach becomes increasingly difficult This is because in a standard deposition and lift-off procedure, the total thickness of the deposited... stack using FIB, which is capable of cutting nanometre-sized features with a high aspect ratio It had 21 layers with 30 nm of Ag and 50 nm of MgF2 alternately Direct measurement of the angle of refraction verified that the structure had a negative refractive index (Figure 2.8) 15 Figure 2.8 (a) Diagram of the 21-layer fishnet structure with a unit cell of p=860nm, a=565nm and b=265nm; (b) SEM image of . FABRICATION OF 3D METAMATERIALS USING TWO-PHOTON POLYMERIZATION AND SELECTIVE SILVER ELECTROLESS PLATING YAN YUANJUN A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF. Shrinkage study of SU-8 2000 45 3.5 Summary 51 Chapter 4: Selective Electroless Silver Plating 53 4.1 Electroless silver plating 54 4.1.1 Electroplating method 54 4.1.2 Electroless silver plating. characterization of HAR SRRs 94 5.5 Summary 95 Chapter 6: 3D Silver Helices as THz Broadband Circular Polarizer 97 6.1 Chiral metamaterials and their properties 98 6.2 Design and fabrication of THz 3D silver