The results of theoretical and experimental investigations of high-performance ultra-thin metasurface absorbers designed for narrow-band operation at subterahertz frequencies are presented. Implemented in a high-impedance surface configuration with a deep-subwavelength thin ground polypropylene substrate, the absorbers are intended for using as the photo-sensitive layers in wavelength-selective thermal detectors. A compact closed-form equivalent circuit formulation verified by full-wave simulations, as well as the route towards an optimal design of the spectrometric absorber are discussed. In addition, a spectrometric array of metasurface-absorber-integrated pyroelectric detectors with spectral peak sensitivities positioned in the range of 0.094-1 THz and providing the selectivity bandwidth of several per cent is described.
We propose an approach to improve the accuracy and speed of THz-TDS measurements in the low-frequency regime. It is based on applying anti-alias filters to narrow the frequency band of the THz signal that allows increasing the sampling interval in accordance with the Nyquist–Shannon theorem. The concept was verified by studying the transmittance spectra of the reference samples — band-pass THz filters centered at 156 and 376 GHz. We show a reduction of the spectrometer scanning time by an order of magnitude and an increase in measurement accuracy due to eliminating a long-term signal drift.
We prove that groups acting geometrically on delta-quasiconvex spaces contain no essential Baumslag-Solitar quotients as subgroups. This implies that they are translation discrete, meaning that the translation numbers of their nontorsion elements are bounded away from zero.