We have solved the long-standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal ...that the contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower-frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt down-shift to the plasma frequency. Ring-shaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.
The effective preparation method of epitaxial VO 2 films on the r-Al 2 O 3 substrates based on the MOCVD technique and post-deposition annealing is described. The composition, orientation and ...morphology of the films obtained were investigated by Raman spectroscopy, XRD, EBSD, XPS, SEM and AFM methods. The samples obtained demonstrate high crystal quality and excellent physical properties: sharp metal-insulator (>10 4 resistance change) and intensive optical reflectivity (IR and THz regions) transitions. The model of VO 2 film recrystallization based on the peritectic decomposition of intergrain vanadium oxide phases is proposed.
Intense electromagnetic fields can be obtained now in THz frequency range. THz pulses with electric field intensity above 1 MV/cm are now requested for strong-field applications like particle ...acceleration, short electron bunches measurements, controlling the state of the matter. The advantage of a THz pulse in comparison with the visible frequency range is the almost non-oscillating field that interacts with charged particles more efficiently. The advantage over a direct field is the ability to remotely focus this radiation in space and in time. Except for the huge free-electron lasers facilities, the only way to get THz pulses of MV/cm intensity is the conversion of high-power femtosecond laser pulses. We compare two ways of generating intense THz pulses using the radiation of TW and GW laser systems operating at 800 nm. They are two-color filamentation in gas and optical rectification in nonlinear crystals. The prospects for scaling of THz generation methods for a petawatt laser are discussed.
A material in the form of a multilayer structure based on low-temperature LT-GaAs grown on (111)A-oriented substrates is proposed for fabrication of THz photoconductive antennas. These structures ...contain active LT-GaAs layers and doping acceptor GaAs:Si-based layers. At the optical pump power of 19 mW and the bias voltage of 30 V, a photoconductive antenna based on the optimized {LT-GaAs/GaAs:Si} (111)A structure emits THz pulses with the average power of 2.3 μW at the pulse repetition frequency of 80 MHz; the conversion efficiency is 1.2 × 10
–4
. It is shown that the dependence of the integral power of THz pulses of the antenna based on the {LT-GaAs/GaAs:Si} (111)A structure on the applied voltage is superlinear; the dependence of this parameter on the optical pump power is plotted as a curve with saturation. It is shown that the designed antennas have a practical application in THz spectroscopy of biological solutions.
This paper briefly examines methods for the generation of pulsed terahertz radiation and principles of pulsed terahertz spectroscopy, an advanced informative method for studies of complex biological ...and nanostructured systems. Some of its practical applications are described. Using a number of steroid hormones as examples, we demonstrate that terahertz spectroscopy in combination with molecular dynamics methods and computer simulation allows one to gain information about the structure of molecules in crystals. A 'terahertz colour vision' method is proposed for analysis of pulsed terahertz signals reflected from biological tissues and it is shown that this method can be effectively used to analyse the properties of biological tissues and for early skin cancer diagnosis.
In this letter we propose an approach to obtain directive radiation from wire lasers with subwavelength transverse dimensions and length much larger than the radiation wavelength (wire lasers) based ...on spatial filtering of their radiation using a combination of a spherical lens and a diaphragm. Theoretical modeling based on the antenna model for wire lasers shows that a directive beam with the uniform phase front can be formed when the diaphragm separates the maximum of the image field of the laser created by the lens. We demonstrate spatial filtering of wire laser radiation experimentally using a terahertz quantum cascade laser.
We have carried out the studies aimed at assessing the effect of terahertz radiation on DNA molecules in human blood leukocytes. Genotoxic testing of terahertz radiation was performed in three ...different oscillation regimes, the blood leukocytes from healthy donors being irradiated for 20 minutes with the mean intensity of within the frequency range of . Using the comet assay it is shown that in the selected regimes such radiation does not induce a direct DNA damage in viable human blood leukocytes.
Aqueous solutions of guanylurea hydrogen phosphite (GUHP) have been synthesized. The temperature dependence of equilibrium GUHP concentration in water was obtained. The change in the density of ...aqueous GUHP solutions with a change in their concentration was investigated. The influence of the solution pH on the GUHP solubility in water in the temperature range of +25–45°C was analyzed. The crystallization range of monoclinic GUHP crystals, belonging to the sp. gr.
Cc
, was determined. GUHP crystals were grown using different methods, based on cooling or maintaining a constant temperature, with mixing aqueous solutions or without mixing. The properties of the grown crystals in the optical and THz ranges were investigated.
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