ИСТИНА

One of the most promising sources of THz radiation is the plasma channel of femtosecond filament. THz radiation generated during filamentation has high field amplitude and broad spectrum ranged from 0.1 to 30 THz. Here we use UPPE for numerical investigation of THz pulse generation, which is based on the field approach and takes into account the components oscillating at optical and THz frequencies and derived without assumption of paraxial approximation. For experimental investigation we use Michelson interferometer with a helium-cooled bolometer to study both the high- and low-frequency parts of the THz spectrum. For the polarization studies, we built a setup with separated beams for first and second harmonics so that we could control them independently. According to our numerical modelling, first the high-frequency components with maximum spectral density appear in THz radiation spectrum due to nonlinear response of the bound electrons. Then, the lower-frequency part of the spectrum is formed due to the free electron density increase. Our experiments show increase in the detected bandwidth in comparison with ABCD detection method and observe the THz signal in spectral region governed by the nonlinear response of bound electrons. We study the spatial profile of the emitted radiation and reveal its broadband cone-like shape with maximum spectral intensity and bandwidth propagating at approximately 5° from the beam axis. The experimentally and numerically obtained angular-frequency spectra of THz radiation are in good agreement. THz polarization measured for the case of both linear polarizations of ω and 2ω beams was directed along ω beam polarization. Our simulations show how the pulse shape and polarization of fundamental and second harmonic radiation is distorted during the propagation inside the filament region. Evolution of the 2ω polarization results in generation of THz pulse polarized closer to ω in perfect agreement with our experimental results.