Metamaterial based Terahertz Devices

Abstract

Terahertz electromagnetic waves refer to electromagnetic waves with wavelengths ranging from 1mm (0.3 THz) to 10μm (30 THz). Terahertz waves possess the unique property of being able to penetrate fabrics, plastics and other visually opaque materials while being nonionizing and harmless to human body. Therefore, THz waves have attracted huge interest for their possible applications in non-destructive testing (NDT), bio and security imaging systems. Realization of practical THz systems for above applications requires THz sources, detectors and devices that can efficiently manipulate THz waves, such as modulators, filters, polarizers, and so on. This thesis is concerned with design and realization of such devices to control free space THz waves using metamaterials. Metamaterials refer to artificially fabricated materials which exhibit characteristics that are not available in naturally occurring materials. In this work, two categories of metamaterials are explored. First category consists of dynamic or reconfigurable metamaterials that can enable tunable control of the amplitude, phase and polarization of the electromagnetic waves. Second category consists of static broadband metamaterials that exhibit unique phase and polarization characteristics. We first demonstrate a dynamic metamaterial consisting of split ring resonator (SRR) based metamaterial realized on GaAs/AlGaAs semiconductor heterostructures. These heterostructures support a 2-dimensional electron gas (2DEG) whose carrier concentration can be controlled through applied electric field. The SRR is designed such that heterostructure at the SRR split gaps forms a high frequency varactor which tunes the unit cell capacitance and resonance frequency of the metamaterial. The fabricated device exhibits an amplitude modulation depth of 45% percent at 0.58 THz, with a modulation speed up to 50 MHz. The above results demonstrate the potential of III-V heterostructures based metamaterials as high speed metamaterial platform compatible with commercial semiconductor foundry process for large area fabrication. Secondly, we explore dynamic metamaterials based on Vanadium dioxide (VO2). VO2 is a correlated electron material which can exhibit insulator to metal transition (IMT) resulting in large electrical conductivity variation. We demonstrate a cut wire resonator based metamaterial containing VO2 thin film patches such that IMT in VO2 results in 90° phase modulation in the transmitted wave. Based on this, we demonstrate a linear to circular polarization switching device. We further show that in single layer plasmonic metasurfaces, the realizable phase modulation is limited to much less than 180°. We then analytically and experimentally show that above limitation can be overcome in metal-insulator-metal (MIM) type metamaterials that can realize any phase modulation up to 180°. The above results have important implications for realizing flat beam steering or beam shaping devices at THz frequencies. We also explore static metamaterials with unique phase and polarization characteristics. We study cross polarization metasurface based MIM resonators. We theoretically demonstrate that an incident polarization on above cavity structures results in co- and cross-polarized reflections, which have a 90° phase difference given the metasurface satisfies certain symmetry. This 90° phase difference exists at all wavelength, independent of exact shape of metasurface, type of substrate and thickness. Furthermore, we demonstrate that similar behavior also exists in another type of metamaterials called hybrid complementary metamaterial. The above metamaterials enable compact, flexible, broadband polarization control devices that cannot be realized using conventional materials.