Masters thesis of engineering microfabricated functional terahertz reflectarrays and metamaterials

Microfabricated Functional Terahertz Reflectarrays and Metamaterials A thesis submitted in fulfilment of the requirements for the degree of Masters of Engineering Aditi Upadhyay B.Sc (Hons.) School of Electrical and Computer Engineering College of Science Engineering and Health RMIT University August 2015 Declaration I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged; and, ethics procedures and guidelines have been followed Aditi Upadhyay 24/08/2015 ii Acknowledgements I readily acknowledge my indebtedness and gratitude to my supervisor Dr Sharath Sriram for giving me the opportunity of undertaking this study His excellent guidance and constant motivation gave me backbone support during my masters candidature I would also like to sincerely thank Dr Madhu Bhaskaran for her supervision, kind co-operation and valuable ideas on this research project My supervisors have been very supportive and encouraging in all my endeavours I am very grateful to my project collaborators, Dr Withawat Withayachumnankul, Prof Christophe Fumeaux and Prof Derek Abbott from the University of Adelaide for their incredible insights and creative ideas, without which my masters project would not have been possible I also thank members of their research group – Mr Henry Ho, Mr Daniel Headland, Miss Tiaoming Niu and Mr Yongzhi Cheng – for their collaboration and assistance with measurements A special thanks to Mr Dan Smith and Dr Ricky Tjeung from the Melbourne Centre for Nanofabrication for their support and valuable advice This work would not have been possible without access to state-of-the-art equipment and facilities The guidance and patient support from Mr Yuxun Cao, Mr Paul Jones, and Ms Chi-ping Wu of the Microelectronics and Materials Technology Centre is gratefully acknowledged I deeply appreciate the assistance from Dr Jie Tian and Dr Babs Fairchild of the Micro Nano Research Facility I was lent valuable support by Mr Phil Francis from the RMIT Microscopy and Microanalysis Facility I would also like to thank current and former researchers within the School of Electrical and Computer Engineering- Dr Mahyar Nasabi, Mr Andreas Boes, Mrs Robiatun Adayiah Awang and Mr Philipp Gutruf for their assistance in this research project Last but not the least, I am highly thankful to my family and friends, who stood by my side in times of difficulties Their constant support, comprehension and motivation kept me going towards the accomplishment of my goals iii Table of Contents Declaration ii Acknowledgements iii List of Figures, Tables and Flowcharts vii Abbreviations ix Abstract CHAPTER INTRODUCTION 1.1 Motivation and thesis outline 1.1.1 Thesis structure 1.2 Publications 1.2.1 Peer-reviewed Journal Publication 1.2.2 Peer-reviewed Conference Proceedings 1.3 Significant Scientific Contributions CHAPTER TERAHERTZ REFLECTARRAYS 2.1 Introduction 2.2 Polarization beam splitter 10 2.2.1 Design and simulation 11 2.2.2 Reflectarray fabrication 15 2.2.3 Results and discussions 16 2.2.4 Fabrication challenges and solutions 21 iv 2.2.5 Summary 21 2.3 Ultra broadband polarisation convertor 22 2.3.1 Design and simulation 23 2.3.2 Reflectarray fabrication 24 2.3.3 Results and discussions 26 2.3.4 Fabrication challenges and solutions 29 2.3.5 Summary 30 2.4 Polarisation dependent thin-film reflect array 30 2.4.1 Design and simulation 31 2.4.2 Reflect Array Fabrication 36 2.4.3 Results and discussion 39 2.4.4 Fabrication challenges and solutions 40 2.4.5 Summary 41 CHAPTER 42 ULTRA BROADBAND TERAHERTZ ABSORBERS 42 3.1 Introduction 42 3.2 Design and fabrication 43 3.3 Results and discussions 46 3.4 Fabrication challenges and solutions 51 3.5 Summary 52 CHAPTER 53 GRADIENT INDEX METAMATERIALS 53 4.1 Introduction 53 4.2 Beam deflection lens 54 4.2.1 Unit cell design 55 v 4.2.2 Fabrication 57 4.3 Hole lattice metamaterials 58 4.3.1 Hole lattice array element 59 4.3.2 Diffractive Optics 60 4.3.3 Fabrication 61 4.3.4 Fabrication challenges and solutions 63 CHAPTER 65 FUTURE WORK 65 References 67 vi List of Figures, Tables and Flowcharts Fig.1.1.The THz regime of the electromagnetic spectrum located at the interface of microwave electronics and infrared optics……………………………………………… Fig.2.1 Single unit cell of the proposed reflectarray………………………………… 12 Fig.2.2 Structure of one subarray made of 12 dipoles………………………………… 14 Fig.2.3 Instantaneous incident and scattered field distributions from the reflectarray in TE and TM polarizations at THz………………………………………………………… 15 Fig.2.4 Optical micrograph of a small part of the reflectarray……………………… 16 Fig.2.5 Measurement system (a) Photograph of the measurement system (b) Corresponding schematic……………………………………………………………………………… 18 Fig.2.6 Measured normalized amplitude spectra for specular reflection (blue dashed line) and deflection (red solid line)………………………………………………………… 19 Fig.2.7 Radiation patterns at THz for TE and TM polarized incident waves on a logarithmic scale………………………………………………………………………… 20 Fig.2.8 Schematic of the proposed polarization convertor…………………………… 24 Fig.2.9 Schematic of reflectarray structure…………………………………………… 29 Fig.2.10 Fabricated structure- microscopic images for a small area (a) and a unit cell (b) of the convertor…………………………………………………………………………… 25 Fig.2.11 Configurations of the fibre-coupled terahertz system for cross polarization measurement…………………………………………………………………………… 26 Fig.2.12 Responses of the convertor at normal incidence……………………………… 27 Fig.2.13 Responses of the non-optimal convertor at 45° oblique incidence…………… 29 Fig.2.14 A schematic diagram of the unit cell and the layout of the reflectarray……… 32 Fig.2.15 Phase and magnitude responses of the unit cell for both the TE and TM polarized incident waves……………………………………………………………………………… 33 Fig.2.16 Instantaneous scattered fields from the reflectarray in the TE (a) and the TM (b) polarizations at THz…………………………………………………………………… 35 Fig.2.17 Microscope images of a part of the fabricated sample………………………… 36 Fig.2.18 Schematic diagram for the THz-TDS measurement setup…………………… 40 Fig.3.1 The plasmonic absorber a) Schematic of a 2D array carved from a doped silicon substrate The geometric parameters are: px = py = 200 μm, tb = 65 μm, a = 60 μm, l = 160 μm, and ts = 200 μm b,c) Scanning electron images of the fabricated cross structure viewed at 35° from the normal…………………………………………………………………… 44 vii Fig.3.2 Numerically and experimentally resolved spectra a) Reflectance R (ω) and absorbance A (ω) for the 2D cross array absorber b) Reflectance R (ω) and absorbance A (ω) for the bare, doped silicon substrate……………………………………………………… 47 Fig.3.3 Field distributions of the plasmonic absorber at resonance frequencies………… 48 Fig.3.4 Dispersion diagram of the coupled surface plasmon polaritons in the equivalent parallel-plate and rectangular plasmonic waveguides……………………………………… 49 Fig.3.5 Absorption performance as a function of incidence angle θ a) TE polarization b) TM polarization…………………………………………………………………………… 51 Fig.4.1 Illustration of metamaterial unit cell in 3D and 2D……………………………… 56 Fig.4.2 One period of the beam deflection metamaterial, showing lens thickness, l, and lens period width,w 56 Fig.4.3 Screenshot of beam deflection lens……………………………………………… 58 Fig.4.4 a) Construction of array element, d = 60 µm, and t = 250 µm b) Simulated response at THz as a function of hole radius r……………………………………………………… 60 Fig.4.5 Diagram of hole lattice zone plate, with air holes shown as black dots…………… 60 Fig.4.6 a) Simulated near field response of hole lattice zone plate b) Broadband performance of hole lattice zone plate, showing electric field magnitude distribution along the optical axis………………………………………………………………………………… 61 Fig.4.7 a) Photographs of patterned silicon, showing a section of the concentric zone structure b) Zoom in, showing individual holes…………………………………………… 63 Table 2.1 Dimensions of the dipoles for the optimized subarray………………………… 13 Flowchart 4.1 Detailed fabrication steps of gradient index zone plates………………… 62 viii Abbreviations COC Cyclic olefin copolymer DRIE Deep reactive-ion etching DSRR Disk split ring resonator FEM Finite element method GRIN Gradient index IPA Isopropyl alcohol LSPRs Localised surface plasmon resonances PCA Photoconductive antenna PCR Polarization conversion ratio PDMS Polydimethylsiloxane PEC Perfect electrical conductor SEM Scanning electron microscope SPPs Surface plasmon polaritons SRRs Split-ring resonators TE Transverse Electric THz-TDS Terahertz time-domain spectroscopy TM Transverse Magnetic ix Abstract Electromagnetic devices operating from microwave to visible frequencies have already been realised to demonstrate a wide variety of applications However, intriguing electromagnetic phenomena across the terahertz frequencies are yet to be unveiled Terahertz radiation typically refers to the electromagnetic spectrum spanning 0.1-10 THz, which translates to a wavelength range of mm-0.03 mm This spectral band bridging the worlds of electronics and optics has been relatively unexplored and is referred to as „terahertz gap‟ because of accessibility difficulties Metamaterials are artificial composite structures with tailored electromagnetic response They are assemblies of multiple individual elements fashioned from conventional microscopic materials This new class of materials dramatically adds a degree of freedom to the control of electromagnetic waves The emergence of metamaterials coincides with the emerging interest in terahertz radiation (T-rays), for which efficient forms of electromagnetic manipulation are being sought Metamaterials are of particular interest in the terahertz regime, where most natural materials exhibit only weak electric and magnetic responses and hence cannot be utilized for controlling the radiation Beyond the terahertz frequencies, the fabrication of metamaterials can be very challenging with present technologies This thesis emphasizes on implementing and experimentally demonstrating innovative fabrication solutions for micro-scale metamaterials designed to operate in the terahertz electromagnetic regime Microfabrication is a conventional fabrication technique that has been employed to fabricate metamaterials operating at terahertz frequencies A variety of terahertz components based on terahertz metamaterials have been proposed in this thesis- perfect absorbers, quarter-wave plates, half-wave plates to name a few A process has been established to realise subwavelength resonators demonstrating The substrates were then patterned using a thick resist, AZ4562, exposed and developed The required etch depth for this device was 250 µm Two silicon wafers were etched down to ~125 µm and bonded face-to-face to complete the device A mixture of SF6 and C4F8 gas was used in the Deep Reactive Ion Etching (DRIE) technique to etch the samples Fig 4.3 shows a screenshot of the fabricated device These samples are currently being tested at the University of Adelaide 100 µm Fig 4.3 Screenshot of beam deflection lens 4.3 Hole lattice metamaterials The terahertz range has the potential to support high bandwidth short range wireless communications, but due to strong atmospheric attenuation, high gain radiators are required Terahertz radiator gain is commonly boosted using dielectric lenses, however these can have issues including size, weight, and reflection loss Planar optics using arrays of phased elements, typically metal resonators, have been demonstrated as an alternative to dielectric lenses for terahertz beam control However, such structures typically have significant ohmic loss originating from metallic components Examples include arrays of rectangular patches, 58 polarization convertors and shaped holes in a metal sheet In this work, an all-dielectric hole lattice for planar terahertz optics is proposed Since it is non-resonant, and low-loss dielectrics are used, it has higher bandwidth and lower loss than typical metal resonators Dielectric hole lattices have previously been demonstrated in the optical and microwave ranges 4.3.1 Hole lattice array element The array element, shown in Figure 4.4.a), is a block of high resistivity silicon with an airhole in the centre, housed in quartz for structural integrity At subwavelength scales, the air and silicon behave as an artificial dielectric, with effective refractive index depending on hole radius Therefore, transmission phase response may be engineered with careful selection of hole radius, as shown in Figure 4.4.b) Additionally, since the parameter giving rise to the desired phase response is the optical length, one may increase the achievable phase range by simply increasing the thickness of the patterned layer In a manner not unlike in a Fabry-Perot etalon, reflection loss is cancelled out when the silicon layer thickness is a multiple of a halfwavelength, giving near-perfect transmission The in-medium wavelength can be changed by altering the effective refractive index through alteration of hole radius to satisfy this condition Two cases of near-perfect transmission for different phase responses have been indicated in Figure 4.4.b) These Fabry-Perot fringes are the key advantage of the hole lattice zone plate over simpler cut-groove zone plates 59 Fig 4.4 a) Construction of array element, d = 60 µm, and t = 250 µm b) Simulated response at THz as a function of hole radius r Dashed lines denote the Fabry-Perot maxima, which correspond to an approximate 180° phase difference The electromagnetics software package HFSS was used to generate these results 4.3.2 Diffractive Optics If the thickness of the silicon layer is selected such that two Fabry-Perot maxima are phased 180° apart, as in Figure 4.4, one may construct phased diffractive optics with near-perfect transmission Terahertz zone plates may be constructed in this way, in order to boost terahertz antenna gain Fig 4.5 Diagram of hole lattice zone plate, with air holes shown as black dots 60 Such zone plates have concentric zones of alternating large and small hole radii, as shown in Figure 4.5 The response of a hole lattice zone plate, with operating frequency of THz and focal length mm, was simulated using the commercially available electromagnetics software package CST, and results are given in Figure 4.6 These simulations suggest that, although the focal length changes with frequency, the zone plate successfully focuses a beam over a frequency range from 0.5 THz to at least 1.75 THz; a bandwidth of over 1.25 THz Fig 4.6 a) Simulated near field response of hole lattice zone plate b) Broadband performance of hole lattice zone plate, showing electric field magnitude distribution along the optical axis Electric field magnitude is given in linear scale, normalised against the magnitude of the input plane wave 4.3.3 Fabrication One limitation in the fabrication of hole structures is the achievable depth-to-diameter aspect ratio of the holes In order to effectively double the aspect ratio, the silicon layer is processed in two halves, with each having half the thickness of the overall layer They are then bonded together face-to-face The cut groove zone plate is a single layer device, hence the fabrication 61 was comparatively simpler The detailed fabrication steps for both the zone plates are described below: Thin silicon and quartz wafers were diced in 35mm x 35mm dimensions and bonded together with SU8 The bonded substrates were then patterned, exposed and developed The samples were etched using PLASMAPRO 100 ESTRELAS etcher The etch rate of the Bosch DRIE process was 0.4 µm/cycle Etching was processed on two The cut groove sample was timed identical substrates to achieve 125 to etch 62 µm (~155 runs) deep µm depth in each (~312 runs) Post trenches into Si wafer etching, these substrates bonded face-to-face Flowchart 4.1 Detailed fabrication steps of gradient index zone plates 62 were DRIE was performed using the Bosch process, where a mixture of SF6 and C4F8 gas was alternatively permitted into the high vacuum chamber and ionized by applying RF power The face-to-face bonding of the two substrates of hole lattice zone plate was done by using superglue (which can be dissolved in acetone and removed, if need be) Figure 4.7 shows the screenshot of the fabricated zone plates The characterisation of these samples is currently in process 200 µm 100 µm Fig 4.7 a) Photographs of patterned silicon, showing a section of the concentric zone structure b) Zoom in, showing individual holes 4.3.4 Fabrication challenges and solutions  The support structure adopted in the initial design was Cyclic Olefin Copolymer (COC) During the etch runs, the sample was burnt after a few cycles because COC is an insulator which couldn't withstand the system heat (plasma temperature of approximately 250°) Hence, quartz was adopted as the mechanical support for the silicon wafers, it was also found to be compatible with the design dimensions Conventionally, the non-resonant structures are less sensitive to the substrate properties This is evident from the fact that the substrate medium did not affect the operation of the silicon layer, which is one of the benefits of using an all-dielectric design 63  The preliminary tests for the bonded samples showed poor performance Misalignment in the face to face bonding of substrates could have been a probable reason behind this Hence, double thickness wafers (~250 µm) were used to re-fabricate the designs This eliminated the need to bond the wafers, thus making the process more precise and accurate The measurements of the GRIN metamaterials in presently underway 64 CHAPTER FUTURE WORK This thesis aimed to explore state-of-the-art fabrication approaches and techniques to realise terahertz metamaterials and metallic resonators on flexible substrates During the course of this investigation, while many research questions were answered, the author feels that there still exist numerous opportunities for continuing research in alignment with those presented in this thesis A few significant topics which can serve as the focus of future research programs are listed below: The limited bandwidth behaviour of the terahertz polarisation beam splitter could be improved by considering multi-layer approaches developed in the microwave regime The challenging adaption of these techniques to the terahertz regime will be the topic of future investigations These reflectarrays could be applied as optical components for polarization discrimination or as polarization demultiplexers for terahertz communications  The proposed design of polarisation convertor (half wave retarder) can also be used to invert the handedness of the elliptically or circularly polarized waves and can be scaled to operate at higher frequency ranges  Owing to the flexible characteristics, similar transmit reflectarray configurations can be designed to be mounted onto surfaces of cylindrical or spherical devices for terahertz imaging and communications In addition, the proposed concept of reflectarray can be extended to polarization-dependent reflective and transmissive deflection by adding more layers of resonant elements onto the backside of the sample 65  The plasmonic absorber demonstrated at terahertz frequencies can potentially be implemented in many applications, such as in bolometric imaging and terahertz communications In addition, due to its ability to capture nearly the entire incident electromagnetic energy at terahertz frequencies, the plasmonic absorber can serve as a useful component in terahertz communications for suppressing multipath reflections The bandwidth can be extended further with alternative designs to accommodate mode resonance modes  Due to the subwavelength thickness and the high focusing strength, the presented GRIN metamaterials are an important step towards compact THz imaging systems with high spatial resolution Furthermore, the gradient index metamaterials exhibit low dispersion at broadband THz frequencies, which can open up opportunities for developing new class of adaptive THz optics by extension of the concept to tunable metamaterials The dearth of strong terahertz characteristics in natural materials has provided a great impetus driving the research area of terahertz metamaterials However, the potential of metamaterials is still clogged by the limitations of designs and fabrication techniques Further research will alleviate the reliance on naturally occurring materials, by offering a wider range of customizable characteristics from artificial structures This leads to a number of opportunities in developing new devices for terahertz applications, where the use of existing materials lacks strong electromagnetic interactions The rapid development of terahertz metamaterials will propel 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improving the practicability and flexibility of controlling the direction of terahertz radiation, the terahertz reflectarrays have been realised... Microfabrication of highly efficient devices working at broadband terahertz frequency ranges  Fabrication of micro devices to enable polarisation control at terahertz frequencies CHAPTER TERAHERTZ