Ponderomotive-expulsion: toward creating an electron-free volume

TL;DR

This paper demonstrates a method using two laser pulses to clear free electrons from a focal volume, with experimental and simulation results showing partial success and discussing improvements for complete electron removal.

Contribution

The study introduces a pump-probe approach to expel electrons from a laser focal volume, advancing methods for creating electron-free regions in laser-matter interactions.

Findings

Pump pulse expels electrons effectively when timed 300 fs before probe.

Electron yield is suppressed with optimal pump-probe delay, confirming ponderomotive expulsion.

Simulations suggest inward electron expulsion due to Airy rings, indicating complex dynamics.

Abstract

We describe a demonstration of a prototype approach to clear the laser focal volume of free electrons and disable their atomic and molecular sources. Employing two temporally separated, copropagating pulses, we exploited a pump-probe setup in our experiment. The pump ionized a low-density gas and expelled free and nascent electrons from its focal volume. The probe, traversing the same focal volume, expelled any remaining free and probe-induced nascent electrons. We gauged the effectiveness of the approach by capturing the spatial distribution of ejected electrons with image plates while we varied the relative intensity and time delay between the pump and probe. When we injected the pump 300 fs before the probe, we found the electron spatial distribution significantly altered and the yield suppressed, proving ponderomotive expulsion works. However, the yield was enhanced when we set the temporal spacing between the pump and probe to 150 fs. Simulations show the enhancement is due to Airy rings of the focused pump expelling electrons inward toward the propagation axis. Our results show that the complete removal of focal-volume electrons was inhibited by spatial overlap fluctuations and the stronger probe generating ionization outside the cleared-volume of the pump. We discuss ways to mitigate these impediments and propose alternate two-beam arrangements to achieve more efficient focal-volume clearing.