The ultrafast response of metals to light is governed by intriguing nonequilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them leads to nonlinear diffusion behavior on ultrashort time scales. Here, we use scanning ultrafast thermomodulation microscopy to image the spatiotemporal hot-electron diffusion in thin gold films. By tracking local transient reflectivity with 20-nm spatial precision and 0.25-ps temporal resolution, we reveal two distinct diffusion regimes: an initial rapid diffusion during the first few picoseconds, followed by about 100-fold slower diffusion at longer times. We find a slower initial diffusion than previously predicted for purely electronic diffusion. We develop a comprehensive three-dimensional model based on a two-temperature model and evaluation of the thermo-optical response, taking into account the delaying effect of electron-phonon coupling. Our simulations describe well the observed diffusion dynamics and let us identify the two diffusion regimes as hot-electron and phonon-limited thermal diffusion, respectively.

We generate a photonic hook using a pulsed wave and study the heat-associated effects of a spherical nanoparticle.

Diffusion of heat in metals is a fundamental process which, surprisingly, received only a little attention from the research community. Here, we study heat diffusion on the femtosecond and few picoseconds time scales. Specifically, we identify the underlying time scales responsible for the generation and erasure of optically-induced transient Bragg gratings in metal films. We show that due to a interplay between the temporally and spatially nature of the thermo-optic response, heat diffusion affects the temperature dynamics in a partially indirect, and overall non-trivial way. Further, we show that heat diffusion affects also the nonlinear response in a way that was not appreciated before.1 In comparison, the source in the TTM has just the temporal profile of the absorbed power. 2 Note that the eTTM does not capture the increase of rate of energy transfer to the lattice during the thermalization time, as discussed in [18]; however, this effect should have, at most, a modest quantitative effect on the issues discussed in the current work.

MXene, a recently developed 2D material, has attracted considerable attention because of its graphene‐like but highly tunable properties. It appears that the metallic properties of MXene titanium carbide are pronounced...