Spatiotemporal dynamics of femtosecond laser-induced thermal grating scattering of a continuous-wave laser

Laser-induced grating scattering (LIGS) is a robust optical technique for measuring thermodynamic parameters, such as temperature, pressure, and thermal diffusivity, in gaseous and liquid media. While conventional LIGS research primarily focuses on temporally-resolved signal extraction, the spatial dynamics of the signal beam are rarely studied due to its inherent weak brightness. The recent advent of femtosecond LIGS (fs-LIGS) generates a strong, visibly bright signal, providing what we believe to be a novel opportunity to explore these spatial dynamics. In this work, we systematically investigated the influence of pump pulse energy, medium temperature, and relative pump polarization on the spatiotemporal dynamics of the fs-LIGS signal. Our key findings show that both increasing pump laser energy and medium temperature induce a consistent spatial shift of the fs-LIGS signal away from the probe beam, driven by self-focusing effects. Furthermore, while the signal oscillation frequency remains stable with increasing pump energies, high-temperature gradients were found to overestimate thermometry results. Besides, the variation in relative pump polarization, although causing signal intensity fluctuation, does not affect the spatial dynamics or oscillation frequency, thus preserving thermometry accuracy. Based on a simplified analytical model, the above experimental observations were qualitatively explained. These results offer new insights into the interplay between nonlinear optical effects and signal formation, with direct implications for optimizing diagnostic precision in the fs-LIGS technique practice.