Sergey S. Zhukov

In this work, the optical parameters of a 550 μm thick deuterated triglycine sulfate (DTGS) crystal in the range of 0.3–1.5 THz were investigated in low and high fields by time-domain spectroscopy. The X and Z axes of the refractive index ellipsoid were identified, and the refractive indices and extinction coefficients were determined. In addition, it was shown that, in strong THz fields, a phonon A-mode with a frequency of about 40 cm⁻¹ can be excited in the DTGS crystal when the THz wave vector coincides with the polar axis of the crystal and the polarization of the THz radiation coincides with the X-axis of the crystal. The use of the observed effect and the determination of the mode excitation parameters can be twofold. First, they show the limits of the THz field for non-invasive action on the crystal. Second, they present the mode excitation conditions that can be used, for example, in controllable absorbers.

В данной работе методом спектроскопии во временной области исследованы оптические параметры кристалла DTGS толщиной 550 мкм в диапазоне 0,3–1,5 ТГц в слабых и сильных полях. Идентифицированы оси X и Z эллипсоида показателя преломления, определены показатели преломления и коэффициенты экстинкции. Кроме того, показано, что в сильных ТГц полях в кристалле DTGS может возбуждаться фононная A-мода с частотой около 40 см⁻¹ при совпадении волнового вектора ТГц с полярной осью кристалла, а поляризация ТГц излучения – с осью X кристалла. Использование наблюдаемого эффекта и определение параметров возбуждения мод может быть двояким. Во-первых, показаны пределы ТГц поля для неинвазивного воздействия на кристалл. Во-вторых, представлены условия возбуждения мод, которые могут быть использованы, например, в управляемых поглотителях.

Dynamicallyprogrammable metasurfaces capable of manipulating terahertz (THz) wavefronts in various manners depending on external controls are highly desired for next-generation wireless communication systems and new tools for THz diagnostics. Such metasurfaces may utilize the insulator-to-metal transition in VO₂, which can be induced both electrically and optically. Optical control is especially convenient for individual addressing to each meta-atom, but it is hampered by the high optical switching threshold of VO₂. We experimentally realize VO₂-based THz metasurfaces with hybrid electro-optical control when the metasurface is brought close to the transition point by an almost-threshold current, and then is easily switched by unfocused continuous-wave light. We were able to control the metasurface THz transmission by 0.4W/cm² near-IR light, while purely optical switching required tightly focused light with an intensity of >3×10⁵W/cm². After correcting for the fact that a tightly focused spot dissipates heat easier, we estimate that the optical switching threshold reduction due to the electric current alone is ∼2 orders of magnitude. Finally, coating the metasurface with Au nanoparticles further reduced the threshold by 30% due to plasmonic effects.

Structural or crystal asymmetry is a necessary condition for the emergence of zero-bias photocurrent in light detectors. Structural asymmetry has been typically achieved via p-n doping, which is a technologically complex process. Here, we propose an alternative approach to achieve zero-bias photocurrent in two-dimensional (2D) material flakes exploiting the geometrical nonequivalence of source and drain contacts. As a prototypical example, we equip a square-shaped flake of PdSe₂ with mutually orthogonal metal leads. Upon uniform illumination with linearly polarized light, the device demonstrates nonzero photocurrent which flips its sign upon 90° polarization rotation. The origin of zero-bias photocurrent lies in a polarization-dependent lightning-rod effect. It enhances the electromagnetic field at one contact from the orthogonal pair and selectively activates the internal photoeffect at the respective metal-PdSe₂ Schottky junction. The proposed technology of contact engineering is independent of a particular light-detection mechanism and can be extended to arbitrary 2D materials.

Eumelanin, the human skin pigment, is a poly-indolequinone material possessing a unique combination of physical and chemical properties. For numerous applications, the conductivity of eumelanin is of paramount importance. However, its hydration dependent conductivity is not well studied using transport-relaxation methods. Furthermore, there is no such work taking into account the simultaneous control of humidity as well as metal ion concentration. Here we present the first such study of the transport and relaxation characteristics of synthetic eumelanin doped with various Cu ion concentrations while controlling the humidity with a frequency range of 10⁻³ Hz–1 MHz. We found that Cu ions do not cause the appearance of additional relaxation processes, but partially slow down those present in neat eumelanin. In addition, considering previously published work, the key relaxation process observed in doped and undoped materials is associated with the moisture-induced synthesis of uncharged semiquinones and a corresponding increase in the overall aromaticity of the material.