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The hyperbolic materials are strongly anisotropic media with a permittivity/permeability tensor having diagonal components of different sign. They combine the properties of dielectric and metal-like media and are described with hyperbolic isofrequency surfaces in wave-vector space. Such media may support unusual effects like negative refraction, near-field radiation enhancement and nanoscale light confinement. They were demonstrated mainly for microwave and infrared frequency ranges on the basis of metamaterials and natural anisotropic materials correspondingly. For the terahertz region, the tunable hyperbolic media were demonstrated only theoretically. This paper is dedicated to the first experimental demonstration of an optically tunable terahertz hyperbolic medium in 0.2–1.0 THz frequency range. The negative phase shift of a THz wave transmitted through the structure consisting of 40 nm (in relation to THz wave transmitted through substrate) to 120 nm bismuth film (in relation to both THz waves transmitted through substrate and air) on 21 µm mica substrate is shown. The optical switching of topological transition between elliptic and hyperbolic isofrequency contours is demonstrated for the effective structure consisting of 40 nm Bi on mica. For the case of 120 nm Bi on mica, the effective permittivity is only hyperbolic in the studied range. It is shown that the in-plane component of the effective permittivity tensor may be positive or negative depending on the frequency of THz radiation and continuous-wave optical pumping power (with a wavelength of 980 nm), while the orthogonal one is always positive. The proposed optically tunable structure may be useful for application in various fields of the modern terahertz photonics. [ABSTRACT FROM AUTHOR]

The terahertz frequency range is promising for solving various practically important problems. However, for the terahertz technology development, there is still a problem with the lack of affordable and effective terahertz devices. One of the main tasks is to search for new materials with high sensitivity to terahertz radiation at room temperature. Bi1−xSbx thin films with various Sb concentrations seem to be suitable for such conditions. In this paper, the terahertz radiation influence onto the properties of thermoelectric Bi1−xSbx 200 nm films was investigated for the first time. The films were obtained by means of thermal evaporation in vacuum. They were affected by terahertz radiation at the frequency of 0.14 terahertz (THz) in the presence of thermal gradient, electric field or without these influences. The temporal dependencies of photoconductivity, temperature difference and voltage drop were measured. The obtained data demonstrate the possibility for practical use of Bi1−xSbx thin films for THz radiation detection. The results of our work promote the usage of these thermoelectric materials, as well as THz radiation detectors based on them, in various areas of modern THz photonics. [ABSTRACT FROM AUTHOR]

Recent years, the need to create tunable THz devices emerged due to rapid development of THz photonics. As a rule, such devices are based on semiconductors and phase change materials, and the use of hyperbolic media based on these materials is a promising approach to realize terahertz wave near-field control. In this work, the dispersion of complex effective permittivity tensor of a layered structure consisting of 40 nm, 70 nm and 120 nm semimetallic Bi films on a top of 21 μ m mica dielectric substrate, calculated on the basis of THz time-domain spectroscopy method, is used to theoretically estimate such effects as Purcell enhancement, Van Hove singularity, and Compton effect. The influence of continuous-wave optical pumping onto the optical properties of Bi-based hyperbolic medium and related effects is also studied in this work. The studied effects may be used in THz radiation enhancement, thermoelectric engineering and thermophotovoltaics, optical and electronic detection, molecular sensing, and even in development of spasers. The obtained results prove the possibility of application of Bi-mica composite in a development of fast-acting terahertz hyperbolic materials which are easy to produce due to their simple structure. • The optical control of the Purcell factor in Bi-film was studied for the first time. • The optical pumping effect on Van Hove and Compton effects in Bi-film was studied. • The Purcell factor enhancement in Bi-film was theoretically investigated. • The slow light mode presence,the change in THz wave and electrons pulse were studied. • The ability of using Bi with optically controlled parameters in photonics is shown. [ABSTRACT FROM AUTHOR]