Experimental demonstration of attosecond hard X-ray pulses

TL;DR

This paper reports the first experimental evidence of attosecond pulse generation in the hard X-ray regime using a free-electron laser, enabling ultrafast, high-resolution probing of electronic phenomena.

Contribution

It demonstrates the direct generation and measurement of attosecond hard X-ray pulses, extending attosecond techniques to higher photon energies and shorter timescales.

Findings

Attosecond pulses ranging from 100 as to 400 as were observed.

Generated pulses are nearly transform-limited with a bandwidth-duration product of about 2 fs·eV.

This work extends attosecond X-ray capabilities to probe matter at atomic scales and ultrafast times.

Abstract

We present the first direct experimental confirmation of attosecond pulse generation in the hard X-ray regime with a free-electron laser. Our experiment is based on measurements of a nonlinear optical phenomenon known as amplified spontaneous emission (ASE) from 3d transition metals. By analyzing the yield of the collective X-ray fluorescence induced by ultrashort pulses at the Linac Coherent Light Source, we identify the generation of attosecond pulses and shot-to-shot fluctuations in their duration, ranging from 100 as to 400 as. The observed product of bandwidth and pulse duration for 100 as pulses is approximately 2 fs$\cdot$eV, indicating the generation of nearly transform-limited pulses. Our results extend the photon energy reach of attosecond techniques by one order of magnitude, providing the ability to simultaneously probe matter on the time-scales of electronic phenomena and with atomic spatial resolution. Furthermore, attosecond hard X-ray pulses can outrun the fastest radiation damage processes, paving the way to single-shot damage-free X-ray measurements.