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In this paper, we report our studies on field effect transistor (FET) THz detectors operating in the non-resonant mode based on the Dyakonov-Shur plasma wave detection theory, where the quantum capacitance dominates. The influence of quantum capacitance in detector response is theoretically developed and numerically simulated at low and high frequencies. Fundamental constraints in the upper frequency limit are also analyzed for FET THz detectors based on various materials, showing advantages of GaN for 8 - 20 THz applications. Experiments at microwave and THz frequencies have been carried out for GaN based devices showing agreement with the theory.

The role of gated and ungated two dimensional (2D) electron plasma in THz detection by high electron mobility transistors (HEMTs) was investigated. THz response of GaAs/AlGaAs and GaN/AlGaN HEMTs was measured at 4.4K in quantizing magnetic fields with a simultaneous modulation of the gate voltage U_GS. This allowed us to measure both the detection signal, S, and its derivative dS/dU_GS. Shubnikov - de-Haas oscillations (SdHO) of both S and dS/dU_GS were observed. A comparison of SdHO observed in detection and magnetoresistance measurements allows us to associate unambiguously SdHO in S and dS/dU_GS with the ungated and gated parts of the transistor channel, respectively. This allows us to conclude that the entire channel takes part in the detection process. Additionally, in the case of GaAlAs/GaAs HEMTs, a structure related to the cyclotron resonance transition was observed.

We have further developed widely-tunable monochromatic THz sources. These sources are based on difference-frequency generation (DFG) in GaSe and GaP crystals. Using a 47 mm long GaSe crystal the output wavelength was tuned in the range from 66.5 to 5664 μm (from 150 to 1.77 cm^-1) with the peak powers reaching 389 W. This record-high power corresponds to a conversion efficiency of ~0.1%. On the other hand, using a 20 mm long GaP crystal the output wavelength was tuned in the range 71.1–2830 μm whereas the highest peak power was 15.6 W. The advantage of using GaP over GaSe is obvious: crystal rotation is no longer required for wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within the bandwidth of as narrow as 15.3 nm. Most recently, we implemented a new scheme for detecting THz waves based on upconversion at room temperature, i.e. by mixing the THz wave with an infrared laser beam, we observed the upconverted signal at a wavelength just slightly longer than that of the infrared laser. To date the detectable THz power is just an order of magnitude higher than that for a bolometer. This scheme allows us to measure the pulse energy density, wavelength, linewidth, and pulse width of a THz beam at room temperature. Using our widely-tunable monochromatic THz beam, we directly measured the absorption spectra of three different families of the homologues of the chemical vapors.

Recently, a reproducible reactor-status effect was measured for the decay of ${}^{\mathrm{\text{22}}}$Na. However, the effect was not observable for the ${\beta }^{\mathrm{\text{-}}}$ decay of ${}^{\mathrm{\text{60}}}$Co. Since no systematic cause for this effect was found, the possibility exists that it is caused by an interaction of reactor antineutrinos with the source nucleus. The observed effect is 18 orders of magnitude larger than for the antineutrino capture of a free proton. A possible explanation is a final-state interaction between the incoming antineutrino and a neutrino in the source nucleus. The effect has still to be confirmed by an independent measurement. In this paper, the consequences for applications in monitoring nuclear facilities are discussed, as well as the consequences for fundamental physics and the opportunities for resolving some outstanding questions about neutrino properties.