Tracking ultrafast hot-electron diffusion in space and time by ultrafast thermo-modulation microscopy

TL;DR

This study employs ultrafast thermo-modulation microscopy to visualize and analyze the rapid and slow diffusion regimes of hot electrons in gold films, revealing distinct electron and phonon-limited thermal diffusion behaviors.

Contribution

It introduces a novel imaging approach to directly observe ultrafast hot-electron diffusion with high spatial and temporal resolution, distinguishing two diffusion regimes in gold.

Findings

Identified initial rapid hot-electron diffusion within a few picoseconds.

Observed a subsequent slower diffusion phase, about 100 times slower.

Simulated the response to confirm hot-electron and phonon-limited diffusion regimes.

Abstract

The ultrafast response of metals to light is governed by intriguing non-equilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them gives rise to nonlinear diffusion behavior on ultrashort timescales. Here, we use scanning ultrafast thermo-modulation microscopy to image the spatio-temporal hot-electron diffusion in a thin gold film. By tracking local transient reflectivity with 20 nm and 0.25 ps resolution, we reveal two distinct diffusion regimes, consisting of an initial rapid diffusion during the first few picoseconds after optical excitation, followed by about 100-fold slower diffusion at longer times. We simulate the thermo-optical response of the gold film with a comprehensive three-dimensional model, and identify the two regimes as hot-electron and phonon-limited thermal diffusion, respectively.