Decreasing ultrafast X-ray pulse durations with saturable absorption and resonant transitions

TL;DR

This paper investigates how saturable absorption and resonant transitions in copper can shorten ultrafast X-ray pulses, using computational plasma simulations to understand the underlying physics and proposing fluorescence as an alternative method.

Contribution

It provides a detailed computational analysis of the mechanisms behind pulse shortening in copper due to resonant transitions and suggests a new approach using fluorescence for shorter pulses.

Findings

Resonant $K$--$M$ transitions cause copper to become opaque again.

Transient transparency leads to shortened X-ray transmission signals.

Fluorescence can be used as an alternative source for shorter X-ray pulses.

Abstract

Saturable absorption is a nonlinear effect where a material's ability to absorb light is frustrated due to a high influx of photons and the creation of electron vacancies. Experimentally induced saturable absorption in copper revealed a reduction in the temporal duration of transmitted X-ray laser pulses, but a complete understanding of this process is still missing. In this computational work, we employ non-local thermodynamic equilibrium plasma simulations to study the interaction of femtosecond X-rays and copper. Following the onset of frustrated absorption, we find that a $K\text{--}M$ resonant transition occurring at highly charged states turns copper opaque again. The changes in absorption generate a transient transparent window responsible for the shortened transmission signal. We also propose using fluorescence induced by the incident beam as an alternative source to achieve shorter X-ray pulses. Intense femtosecond X-ray pulses are valuable to probe the structure and dynamics of biological samples or to reach extreme states of matter. Shortened pulses could be relevant for emerging imaging techniques.