Results 1 - 10 of 2154
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[en] In a nonstationary (self-modulation) regime, terahertz quasi-Cherenkov generators can operate at MHz–GHz repetition rates and megawatt power levels. Shot noise, intrinsic to electron beams, is the cause of statistical fluctuations of radiation in the generators. The shot-noise related spread of instability growth time imposes appreciable limitations on the possibility of coherent summation of electromagnetic oscillations from several quasi-Cherenkov generators operating in a nonstationary mode.
[en] The THz water content index of a sample is defined and advantages in using such metric in estimating a sample's relative water content are discussed. The errors from reflectance measurements performed at two different THz frequencies using a quasi-optical null-balance reflectometer are propagated to the errors in estimating the sample water content index.
[en] We have previously demonstrated that terahertz pulsed imaging is able to distinguish between rat tissues from different healthy organs. In this paper we report our measurements of healthy and cirrhotic liver tissues using terahertz reflection spectroscopy. The water content of the fresh tissue samples was also measured in order to investigate the correlations between the terahertz properties, water content, structural changes and cirrhosis. Finally, the samples were fixed in formalin to determine whether water was the sole source of image contrast in this study. We found that the cirrhotic tissue had a higher water content and absorption coefficient than the normal tissue and that even after formalin fixing there were significant differences between the normal and cirrhotic tissues' terahertz properties. Our results show that terahertz pulsed imaging can distinguish between healthy and diseased tissue due to differences in absorption originating from both water content and tissue structure.
[en] We have fabricated resonant terahertz metamaterials on free-standing polyimide substrates. The low-loss polyimide substrates can be as thin as 5.5 μm yielding robust large-area metamaterials which are easily wrapped into cylinders with a radius of a few millimeters. Our results provide a path forward for creating multi-layer non-planar metamaterials at terahertz frequencies. (fast track communication)
[en] We numerically investigated the effect of chemical potential on Dyakonov–Tamm waves (DTWs) guided by a graphene-coated structurally chiral medium in the terahertz and the visible spectral regimes. Only one DTW can propagate in a specific direction in wide angular sectors, but multiple DTWs can propagate in a specific direction in narrow angular sectors, in both spectral regimes. Although the phase speed of a DTW depends weakly on the chemical potential in both spectral regimes, the propagation distance can be strongly dependent on the chemical potential in the terahertz regime but not in the visible regime. This difference can be attributed to the real part of the surface conductivity of graphene, which varies significantly with the chemical potential in the terahertz regime but not in the visible regime. (paper)
[en] Tunable high-power THz-wave radiation is achieved via a compact eudipleural THz-wave parametric oscillator. The maximum THz-wave output is 1.164 V at 1.755 THz when the pump energy is 90 mJ. In the experiments we find that the maximum output of THz-wave moves to the high frequency band as the pump energy increases and this phenomenon is reasonably explained. The polarization characteristics of the THz-wave are analyzed. (authors)
[en] Two 2D techniques for visualising the field of pulsed THz radiation (‘shadow’ and ‘interferometric’), which are based on the linear electro-optical effect with application of a ZnTe detector crystal 1 × 1 cm in size, are compared. The noise level and dynamic range for the aforementioned techniques are analysed and their applicability limits are discussed. (thz radiation)
[en] The study of the gas discharge in quasioptical beams of electromagnetic radiation of the terahertz (THz) frequency band is attractive for fundamental and applied research. The study of this discharge was made possible by the emergence of unique and reliable sources of radiation of the THz frequency band. Electrovacuum radiation sources of THz band (gyrotrons) have been created at the Institute of Applied Physics (IAP RAS) (Nizhny Novgorod) with a power order of 100 kW in pulsed mode of operation (pulse duration ∼ several dozens of µs) and a kilowatt in continuous mode, and investigations of gas discharge phenomena in various gases have been started. Even the first experimental studies of the discharge in noble gases (argon) showed a significant difference in the THz discharge from the discharge at lower frequencies, even in the dynamics of discharge glow. The maximum of the discharge glow was observed after the end of the THz pulse and the afterglow duration was hundreds of microseconds. This paper is devoted to the theoretical and experimental study of the breakdown thresholds of various gases by radiation at 263 and 670 GHz and the study of discharge dynamics in noble and molecular gases under the action of 670 GHz radiation. (paper)
[en] Experimental results on the density of states and on the acoustic modes of glasses in the THz region are compared to the predictions of two categories of models. A recent one, solely based on an elastic instability, does not account for most observations. Good agreement without adjustable parameters is obtained with models including the existence of nonacoustic vibrational modes at THz frequency, providing in many cases a comprehensive picture for a range of glass anomalies