Background-free Tracking of Ultrafast Hole and Electron Dynamics with XUV Transient Grating Spectroscopy

TL;DR

This paper demonstrates a background-free XUV transient grating spectroscopy technique to directly observe ultrafast electron and hole dynamics in germanium with high temporal resolution, avoiding complex data deconvolution.

Contribution

The authors implement XUV-TGS with tabletop high-harmonic generation, enabling direct visualization of ultrafast carrier decay times and extraction of complex refractive index evolution without Kramers-Kronig reconstruction.

Findings

Direct measurement of separate ultrashort decay times of electrons and holes.

Achieved up to 34% reflectivity change via the real part of the refractive index.

Minimal impact (~0.5%) from imaginary part changes on reflectivity.

Abstract

Extreme ultraviolet (XUV) transient absorption (TA) and transient reflectivity (TR) spectroscopies enable element-specific insights into attosecond-timescale processes in solids. XUV transient grating spectroscopy (TGS) is an emerging tool that combines the advantages of both absorption and reflectivity while offering intrinsically background-free detection. Here, we implement XUV-TGS by generating a transient grating in germanium solid using two few-cycle near-infrared pulses and probing it with an attosecond XUV pulse, produced via tabletop high-harmonic generation. The spectrally resolved, diffracted XUV pulses directly visualize the separate ultrashort decay times of both photoexcited electrons and holes, without the need for iterative deconvolution. By combining XUV-TA and -TG spectroscopy, we extract the evolution of the complex refractive index, \~n, without the need for Kramers-Kronig reconstruction, as required in XUV-TR, allowing us to extract the roots of the induced optical response. We find reflectivity changes of up to 34% via the real part of \~n, whereas changes in the imaginary part only result in a variation in reflectivity of around 0.5%.