Direct Optical Field Spatial Modulation by Cumulative Gas Dynamics in High-Repetition-Rate Filamentation

By combining imaging of filament cross-sections from 1 to 100 kHz with multi-physics simulations, we demonstrate that cumulative gas dynamics can actively reconfigure the transverse spatial profile of a filament. Specifically, we identify 100 kHz as the threshold for the onset of direct optical field modulation by cumulative gas dynamics.

Here, the pulse interval (~10 μs) becomes shorter than the thermal relaxation time of air, causing the accumulated convergent airflow to imprint a stable, low-intensity void, a feature that cannot be explained by standard single-pulse nonlinear optics. This hydrodynamic gradient acts as a stable refractive index template that continuously redistributes and distorts subsequent optical fields. These results establish that at 100 kHz, filamentation shifts to a new regime where light-matter interaction is shaped by feedback from the airflow that builds up between pulses.