Observation of Negative Effective Thermal Diffusion in Gold Films

Abstract

Ultrafast light-induced spatiotemporal dynamics in metals in the form of electron and/or phonon heating is a fundamental physical process that has tremendous practical relevance. In particular, understanding the resulting lateral heat transport is of key importance for various (opto)electronic applications and thermal management but has attracted little attention. Here, by using scanning ultrafast thermo-modulation microscopy to track the spatiotemporal electron diffusion in thin gold films, we show that a few picoseconds after the optical pump there is unexpected heat flow from phonons to electrons, accompanied by negative effective thermal diffusion, characterized by shrinking of the spatial region with increased temperature. Peculiarly, this occurs on the intermediate time scale, between the few picosecond long thermalization stage and the many picosecond stage dominated by thermoacoustic vibrations. We accurately reproduced these experimental results by calculating the spatiotemporal photothermal response based on the two-temperature model and an improvement of the standard permittivity model for gold. Our findings facilitate the design of nanoscale thermal management strategies in photonic, optoelectronic, and high-frequency electronic devices.

Spatiotemporal Microscopy: Shining Light on Transport Phenomena

“…The crossover from fast to slow diffusion occurs after a few picoseconds, in agreement with typical interaction timescales, after a short period of apparent negative diffusion. [62] The group of Hui Zhao used point-scanning microscopy to study the diffusion of electronic heat (hot carriers) in graphene, finding diffusivities between 5,500 and 11,000 cm 2 s -1 . [43,64] Such large diffusivities are expected given the large charge mobility of graphene (>10,000 cm 2 V −1 s −1 ), and the direct connection between charge mobility and diffusivity (see Section 3).…”

“…Apparent negative diffusion is observed around a few picoseconds (right image, adapted with permission. [ 62 ] Copyright 2023, ACS). c) Pre‐time‐zero pump‐probe SPTM allows for observing diffusion with an effective delay time of ≈13 ns (inverse of laser repetition rate), in this case yielding the thermal diffusivity of suspended MoSe 2 .…”

“…The crossover from fast to slow diffusion occurs after a few picoseconds, in agreement with typical interaction timescales, after a short period of apparent negative diffusion. [ 62 ]…”