Superfluorescence, free-induction decay, and four-wave mixing: propagation of free-electron laser pulses through a dense sample of helium ions

TL;DR

This study investigates how 24.3 nm free-electron laser pulses propagate through helium gas, leading to various emissions including superfluorescence and free-induction decay, supported by experimental and numerical analysis.

Contribution

It provides the first detailed experimental and numerical analysis of free-electron laser pulse propagation in helium, revealing multiple superfluorescence and decay phenomena.

Findings

Observation of directional emissions at multiple wavelengths

Identification of superfluorescence cascades in helium ions

Detection of free-induction decay signals

Abstract

We report an experimental and numerical study of the propagation of free-electron laser pulses (wavelength 24.3 nm) through helium gas. Ionisation and excitation populates the He$^{+}$ 4$p$ state. Strong, directional emission was observed at wavelengths of 469 nm, 164 nm, 30.4 nm, and 24.5 nm. We interpret the emissions at 469 nm and 164 nm as 4$p$-3$s$-2$p$ cascade superfluorescence, that at 30.4 nm as yoked superfluorescence on the 2$p$-1$s$ transition, and that at 25.6 nm as free-induction decay of the 3$p$ state.