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

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 detailed account of changes in opacity and emergence of resonances is still missing. In this computational work, we employ nonlocal 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-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 rays 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.

Automated summary

Saturable absorption is a nonlinear effect that hinders a material's ability to absorb light due to a high influx of photons and the creation of electron vacancies. This computational study investigates the interaction between femtosecond x rays and copper using nonlocal thermodynamic equilibrium plasma simulations. The results show that copper's saturable absorption reduces the temporal duration of transmitted x-ray laser pulses and leads to changes in opacity and the emergence of resonances. The authors also propose the use of fluorescence induced by the incident beam as an alternative source for achieving shorter x-ray pulses.