Cleaner Signals from X-Ray Pulses
Ultrafast dynamics, atomic-scale structure, and exotic states of matter can all be probed using ultrashort x-ray laser pulses, with durations ranging from femtoseconds to attoseconds. But progress on this front has been hampered by the difficulty in knowing how the intensity profiles of individual pulses vary over time. Now Taito Osaka at the RIKEN SPring-8 Center in Japan and his colleagues have overcome this limitation by developing a way to eliminate the background light that typically blurs such profiles [1]. The researchers have implemented the approach for femtosecond pulses and are looking to extend it to attosecond ones.
The team’s method is adapted from so-called optical autocorrelation, whereby a laser pulse is split into two replicas, which are then overlapped with a variable time delay in a nonlinear optical material. This interaction creates an autocorrelation signal whose intensity as a function of the time delay can be used to infer the initial pulse’s temporal profile. In the researchers’ adaptation, two replicas of an x-ray laser pulse cross at a tiny angle in a diamond crystal, which serves as the nonlinear material. This setup causes the resulting autocorrelation signal to travel in one direction, while the unwanted background light produced by each replica alone travels in another.
Osaka and his colleagues demonstrated their approach using high-intensity femtosecond x-ray laser pulses. They obtained background-free autocorrelation signals with attosecond-level timing accuracy. They then used these signals to determine how the pulses changed in profile after passing through a wavelength-filtering device called a monochromator. The team found that the monochromator caused the pulse profiles to become more stable and better defined, in agreement with numerical simulations.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- T. Osaka et al., “Background-free intensity autocorrelation for femtosecond x-ray pulses,” Phys. Rev. Lett. 136, 195002 (2026).