Time-Resolved Measurements of Cumulative Effects in Gas Dynamics Induced by High-Repetition-Rate Femtosecond Laser Filamentation

TL;DR

This study presents the first time-resolved measurements of cumulative hydrodynamic effects in air caused by high-repetition-rate femtosecond laser filaments, revealing stationary density depletions at rates above 10 kHz.

Contribution

It provides novel experimental data on the dynamics of heat deposition and plasma channels in air at high laser repetition rates, advancing understanding of cumulative effects in laser filamentation.

Findings

Stationary air density depletions emerge above 10 kHz.

Refractive index changes correlate with heat deposition.

Filament plasma length varies with repetition rate.

Abstract

The advent of high-average-power, ultrafast ytterbium-based lasers allows us to generate laser filaments at repetition rates ranging from 10s of kHz up to 100s of kHz. At such high repetition rates, the inter-pulse time lies below the time required for the total diffusion of the deposited heat by each laser pulse, leading to cumulative hydrodynamic effects that have so far been rarely studied. Here, we present, to the best of our knowledge, the first experimental time-resolved measurements of these dynamics in air for laser repetition rates between 1 kHz and 100 kHz. We measure the change in the air refractive index caused by the localized heat deposition and the length of the filament-generated plasma channel, with which we can infer the corresponding change in air density. We observe that at repetition rates above 10 kHz, stationary density depletions with vanishing dynamics emerge. Our findings are of wide relevance for the fields of high-repetition-rate laser filamentation and its applications, as well as THz generation from laser-induced plasma sources.