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We report measurements of radiation transmission in the 0.220--0.325 THz frequency domain through GaN quantum wells grown on sapphire substrates at room and low temperatures. A significant enhancement of the transmitted beam intensity with the applied voltage on the devices under test is found. For a deeper understanding of the physical phenomena involved, these results are compared with a phenomenological theory of light transmission under electric bias relating the transmission enhancement to changes in the differential mobility of the two-dimensional electron gas.

Experimental results of direct measurement of resonant monochromatic terahertz emission optically excited in InGaAs transistor channels are presented. The emission is attributed to two-dimensional plasma waves excited by photogeneration of electron-hole pairs in the channel at the frequency $f_0$ of the beating of two cw-laser sources. The presence of resonances for the radiation emission in the range of $f_0\pm 10$ GHz (with $f_0$ from 0.3 up to 0.5 THz) detected by a Si-bolometer is found. Numerical results support that such a high quality of the emission resonances can be explained by the approach of an instability in the transistor channel., Comment: Under review in Phys. Rev. B

Electric field gradient effects induced by an asymmetrically in-plane shaped GaAs/AlGaAs modulation-doped structures of various design are investigated within 4-300 K temperature range. It is demonstrated that current-voltage characteristics of such structures at low, 4-80 K, temperatures exhibit well-pronounced asymmetry arising due to a presence of two different gradients of the electric field in a two dimensional electron gas. This phenomenon is caused by both, different accumulation of two-dimensional electrons due to asymmetrical shape of the structure and nonlocality in the electron drift velocity. Experiments are illustrated by a phenomenological model and Monte Carlo simulation. Possible applications of the effect to detect electromagnetic radiation of GHz and THz frequencies are discussed as well., Comment: 9 pages, 13 figures; version 2 -- greek letters in figures corrected

On the basis of the equivalence of the energy balance deacription at micro- and macro-level we propose a quantum generalization of the viscous friction law for a macroscopic Langevin equation describing thermal fluctuations without the zero point contribution. This equation recovers the classical case in the limit h -> 0. In particular it satisfies the quantum regression theorem and resolves several anomalies appearing in the quantum extension of the fluctuation dissipation theorem., Comment: 5 pages, 0 figures, submitted to Phys.Rev.Lett. on 09.05.2003

In the attempt to derive the regression theorem from the fluctuation dissipation theorem several authors claim the violation of the former theorem in the quantum case. Here we pose the question: does it exists a quantum fluctuation dissipation theorem (QFDT) in its conventional interpretation? It is shown that the relation usually called as the QFDT is the condition of detailed macroscopic energetic balance. Following this interpretation the existing conflict between the two theorems in the quantum case is removed., Comment: 13 pages, 3rd Int. Conf. on Unsolved Problems on Noise (will be published in Proceedings)

It is shown that the quantum fluctuation dissipation theorem can be considered as a mathematical formulation in the spectral representation of Onsager hypothesis on the regression of fluctuations in physical systems. It is shown that the quantum fluctuation dissipation theorem can be generalized to an arbitrary stationary state., Comment: 8 pages