Acoustic Signal for Femtosecond Filament Plasma Grating Characterization in Air

Abstract

We have designed the physical model and numerical algorithm for the simulations of acoustic wave propagation from the source induced by relaxation of a femtosecond plasma grating formed by two crossing filaments in atmospheric air. The model is based on the wave equation with the wave velocity depending on the transverse temperature variation. The algorithm is based on the fine resolution (\({\approx }2\,\upmu \text {m}\)) numerical grid employed for the description of the plasma channel substructures in the course of femtosecond filamentation. We have shown that the femtosecond plasma grating emits the superposition of two acoustic signals after plasma recombination. One acoustic signal is represented by an isotropic cylindrical waveform with the characteristic spatial scale equal to the filament diameter (100–200 \(\upmu \text {m}\)) while the other has the spatial scale equal to the plasma grating period in the range \(20{-}40\,\upmu \text {m}\). This second wave propagates in the direction parallel to the axis of plasma grating modulation. Based on the simulations, we suggested the noninvasive method for the plasma grating period and the beam convergence angle detection.