Cooling times in femtosecond pump-probe experiments of phase transitions with latent heat

Abstract

Ultrafast pump-probe experiments can reveal the physics of complex materials by triggering a reaction with short laser pulses and then looking for the response. However, the repetition rate in such experiments is limited because the sample needs to cool down back to the initial state between subsequent laser excitations. Here we investigate by theory and experiment how this cooling rate depends on the shape and dimensionality of the material. Using VO2 as the example, a strongly correlated material with an ultrafast phase transition with large latent heat, we report more than six orders of magnitude of change between the cooling times of freestanding thin films, thin films on a substrate, and bulk materials. A numerical latent-heat model reproduces all results and can therefore be generally used to predict the speed of the back reaction in almost all kinds of phase-change materials.