Four-wave mixing in molecular gases under filamentation of the collimated femtosecond beam

Abstract

The four-wave mixing process during a single-color femtosecond filamentation in the molecular gas is observed experimentally. The role of the seed is represented by the selfshifted to infrared region Raman bullet and the new blue-shifted component burns up as a result of the interaction between the Raman bullet and the reservoir radiation. The blueshifted component propagates along the beam axis. The theoretical analysis of the four-wave mixing process synchronism shows that the on-axis forward propagation of the blueshifted component occurs when the plasma concentration is higher than a certain threshold (3.3 × 10 16 cm −3 at the fundamental wavelength of 800 nm).

Laser-induced plasma influence onto intrapulse four-wave mixing under femtosecond filamentation in air

“…We analyze the phase synchronism between the Raman, blue-shifted and reservoir radiation propagating in air with a small addition of free electrons generated due to the multiphoton/tunnel ionization in the filament [15]. Hollow and filled cone regimes of blue shifted components generation are shown in the case of low and high plasma density.…”

“…In this paper we will try to clarify the origin of such satellites in air. We have suggested the preliminary theoretical interpretation [15] that the plasma density may determine the frequencyangular spectrum of the blue-shifted component. If the plasma density is low, the emission of the blue component is possible only into the hollow cone.…”

“…As a result the blue-shifted component arises. In the theoretical analysis provided in [15] we have shown that the FWM process contributes to the conical emission in the visible range, which accompanies the filamentation in air [16][17][18]. Filamentinduced conical emission builds up from several sources such as the dispersion of the nonlinear Kerr medium [7,19], the angular-frequency self-phase modulation induced by the joint action of the Kerr and the plasma nonlinearities [18].…”

“…The Raman bullet can initiate the four wave mixing (FWM) process from the radiation reservoir [15]. As a result the blue-shifted component arises.…”

“…After a certain threshold (approximately of 3.3 × 10 16 cm −3 at the fundamental wavelength of 800 nm) is exceeded, the FWM signal is emitted both into the cone and along the filament axis. The experimental conditions (energy, beam quality, etc) in [15] correspond to the under threshold regime of the blue component propagation, however, the experimental technique used in [9,15] is spatially selective, and mainly accesses the on-axis part of radiation. As a result, the blue-shifted component emission into the cone was not detected.…”