Ultrafast Dynamics of Optically Induced Heat Gratings in Metals

Diffusion of heat in metals is a fundamental process which is crucial for a variety of applications of metal nanostructures. Surprisingly, however, ultrafast heat diffusion received only limited attention so far. Here, we show that heat diffusion can be made faster than e-ph energy transfer rate, in which case, it dominates the spatiotemporal dynamics of the temperature. This enables the metals to overcome the conventional limitations of the nonlinear optical response of materials; it can be simultaneously fast and strong. As a specific example, we identify the underlying (femtosecond and few picosecond) time scales responsible for the generation and erasure of optically induced transient Bragg gratings in thin metal films. Further, we show that heat diffusion gives rise to a significant nonlocal thermo-optic nonlinearity; it affects also the nonlinear optical response such that the overall change of the permittivity (hence, reflectivity of the transient grating) has a significant dependence also on the illumination period rather than only on the illumination intensity.