Ionization-assisted refocusing of femtosecond Gaussian beams

Ionization occurs ubiquitously in intense laser–matter interaction and often leads to rapid decrease in laser intensity via plasma defocusing, shortening the effective interaction length of desired high-field processes. Refocusing of pulses may compensate for this adverse effect. However, it typically relies on Kerr-induced self-focusing and requires sufficiently high power. Here, we present simulations showing the refocusing of intense pulses with an initial Gaussian beam profile in atmospheric pressure gases at relatively low power. We attribute this refocusing to the formation of ring-structure plasmas. We find that tighter focusing leads to stronger refocusing, and the initial chirp of the pulse greatly affects its dynamics due to spatiotemporal coupling of focused broadband pulses. Our results highlight a novel aspect of complex pulse dynamics and can be relevant to applications involving tightly focused ultrafast Gaussian beams.