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While terahertz spectroscopy can provide valuable information regarding the charge transport properties in semiconductors, its application for the characterization of low-conductive two-dimensional layers, i.e., σ_s <  < 1 mS, remains elusive. This is primarily due to the low sensitivity of direct transmission measurements to such small sheet conductivity levels. In this work, we discuss harnessing the extraordinary optical transmission through gratings consisting of metallic stripes to characterize such low-conductive two-dimensional layers. We analyze the geometric tradeoffs in these structures and provide physical insights, ultimately leading to general design guidelines for experiments enabling non-contact, non-destructive, highly sensitive characterization of such layers.

This paper discusses the terahertz electromagnetic response of metallic gratings on anisotropic dielectric substrates. The metallic gratings consist of parallel gold stripes. Utilizing numerical simulations, we observe that it is possible to excite a series of resonant modes in these structures. These modes are affected differently by the different indices on the anisotropic substrate. An analytical model is discussed to show that modes associated with transmission peaks are due to the excitation of (a) Fabry–Pérot modes with polarization along the grating and/or (b) waveguide modes with polarization perpendicular to the grating. It is observed that the resonance associated with the TM_1,1mode is a narrow linewidth resonance which, in some particular circumstances, becomes nearly independent of substrate thickness. Therefore, from the spectral position of this resonance, it is possible to extract the out-of-plane component of the substrate refractive index with very small uncertainty. Based on this observation, we demonstrate the refractive index characterization of several lossless semiconductor substrates through frequency-domain polarized terahertz transmission measurements in the frequency range of 0.2–0.6 THz at normal incidence. The reliability of the technique is demonstrated on well-known materials, such as high-resistivity silicon and sapphire substrates. This technique is also applied for the characterization of a Fe-doped β-Ga₂O₃single-crystal substrate.

Terahertz waves spanning over the 0.1 to 10 THz region of the electromagnetic spectrum have attracted significant attention owing to a variety of potential applications such as short‐range high‐speed data transmission, noninvasive screening and detection, materials characterization, spectroscopy, etc. This has resulted in massive strides in the development of essential system components such as broadband terahertz sources, detector arrays with high responsivity, as well as modulators. In parallel to this, spurred by the isolation of graphene in 2004, a tremendous interest in 2D systems has led to the rapid exploration and development of a library of atomically thin materials. These can exhibit a myriad of electrical and optical functionalities stemming from semiconducting, insulating, semi‐metallic, or superconducting behavior. In this context, since the early 2010s, 2D materials have been actively explored for active control of terahertz electromagnetic radiation. This paper aims to provide a concise overview of the pioneering efforts as well as the latest progress in these two overlapping research areas. In particular, the discussion is focused on the application of graphene and transition metal dichalcogenides in optically and electrically actuated terahertz amplitude and phase modulators. Furthermore, it provides an outlook on the technological prospects and challenges in these devices.