2015: Quantum-mechanical theory of THz radiation generation by extended gas media

A unique non-perturbative quantum-mechanical theory has been applied for description of the terahertz (THz) emission in Ar and Ne atomic gases interacting with arbitrary polarized two-colour ω + 2ω radiation of Ti:Sa and Cr:F lasers. It is shown that due to dispersion effects the amplitude of the response THz field varies periodically with the travelled distance. These spatial variations manifest themselves in the angular spectra of THz emission and, as a result, the integral efficiency of the THz emission depends significantly on the medium geometry. The origin of such behavior is in the variation of the relative phase of ω and 2ω components of the laser field which result in oscillatory time dependence with period equal to a half of the oscillation period of the second harmonic field. The influence of relative phase between fundamental and second harmonic pulses on the THz intensity has been investigated quantum-mechanically for the case of two different gases (Ne and Ar), two different seed lasers (Ti:Sa and Cr:F), and two angles between the polarization vectors of two-color laser field components (0 and π/2). The results of simulations show that the optimal phase is π/2 for all described cases. On the assumption that each atom of the medium emits independently, as well as taking into account the spatial distribution of the laser field intensity, the angular spectra of THz radiation generated in atomic ensembles of different lengths and widths have been calculated.

The results of simulations show that the angular spectra demonstrate the cone structure at low frequencies (~1 THz) and the additional sideband lobes appear in the high frequency range (a few tens of THz and above). The profile of angular spectrum depends on the geometric form of the gas medium. Note that the cone structure of the THz emission in the ionization-free regime, when the density of ionized electrons is negligible small, allows us to propose a new interpretation of cone structure of filament emission, because it is usually assumed that the cone structure of filament emission is due to the specific phase-matching conditions associated with the Kerr and plasma nonlinearities.

The results of this work have been published in the paper: S.Yu. Stremoukhov and A.V. Andreev, “Spatial variations of the intensity of THz radiation emitted by extended media in two-color laser fields”, Laser Phys. Lett. 12, 015402 (2015).