Terahertz Metamaterials and Plasmonics Using Two-Dimensional Materials and Highly Conductive Transparent Oxides
Creators: name: Arezoomandan, Sara
Publisher: name: University of Utah
Year: 2019
Subjects: Electrical engineering
Applied physics

description: Driven by a myriad of potential applications such as communications, medical imaging, security, spectroscopy, and so on, terahertz (THz) technology has emerged as a rapidly growing technological field during the last three decades. However, since conventional materials typically used in microwave and optical frequencies are lossy or do not effectively respond at these frequencies, it is essential to find or develop novel materials that are suitable for device applications in the THz range. Therefore, there is wide interest in the community in employing novel naturally-occurring materials, such as 2D materials, as well as in designing artificial metamaterial structures for THz applications. Here, we combined both of these approaches so to develop reconfigurable THz devices capable of providing amplitude modulation, phase modulation, and resonance frequency tuning. First, graphene is employed as the reconfigurable element in metamaterial phase modulators. For this purpose, we propose the use of unit cells with deep-subwavelength dimensions, which can have multiple advantaged for beam shaping applications. The analyzed metamaterials have one of the smallest unit cell to wavelength ratios reported or proposed todate at THz frequencies. By systematic analysis of the geometrical tradeoffs in these devices it is found that there is an optimal unit cell dimension, corresponding roughly to ~λ/20, which can deliver the best performance. In addition to this, we explored other applications of graphene in metamaterial devices, including amplitude modulation and resonance-shifting. These studies motivated us to analyze what is the most suitable role of graphene from a THz device perspective: is graphene a good plasmonic material? Or it is better suited as a reconfigurable material providing tunability to otherwise passive metallic structures? Our studies show that the Drude scattering time in graphene is an important parameter in this regard. In order to attain strong plasmonic resonances graphene samples with τ &gt;&gt; 1ps are required, which is challenging in large area CVD samples. But graphene is just one example of a wider class of 2D materials. In this work we also studied for the first time the application of 2D materials beyond graphene as reconfigurable elements in THz devices. For this purpose, Molybdenum Disulfide (MoS2) was employed as the reconfigurable element in cross-slot metamaterial amplitude modulators. Our results evidence that smaller insertion loss is possible when employing 2D materials with a bandgap, such as MoS2, rather than a zero-gap material such as graphene. Furthermore, because of a stronger optical absorption active control of the metamaterial properties is possible by altering the intensity of an optical pump. We later investigate and discuss transparent conductive oxides (TCOs), which constitute an interesting choice for developing visible-transparent THz-functional metamaterial devices for THz applications. These materials show a metallic THz response and thus can substitute the metal patterns in metamaterial devices. In our particular studies we analyzed samples consisting of: (i) two-dimensional electron gases at the interface between polar/nonpolar complex oxides having record-high electron density, and (ii) thin-films of La-doped BaSnO3 having record-high conductivity in a TCO. These materials exhibit a flat THz conductivity across a broad terahertz frequency window. As a result of their metal-like broadband THz response, we demonstrate a visible-transparent THz-functional electromagnetic structure consisting of a wire-grid polarizer.