Broad-Bandwidth Photothermal Microscopy for Real-Time Studies of Nanoparticle-Assisted Melting and Resolidification

Interfacial effects and dependence of physical properties on size and shape can make phase change process strikingly different at nanoscale from those in bulk matter. A light-absorbing nanoparticle embedded in a material can thermally trigger phase transition which is tightly localized in space and time. Here, we present the concept of a broad-bandwidth photothermal microscopy, which enables monitoring such phase change processes with high spatial and temporal resolution. When a heating beam is absorbed by the nanoparticle, intensity of scattered probe light changes depending on physical state of the matrix. By measuring the transient intensity change vs time, we identified different dynamics of heat diffusion in solid/liquid state of fatty acids: faster in solid due to higher thermal conductivity. In addition, phase change in the vicinity of the nanoparticle and growing thickness of the melted layer under stronger heating light was experimentally observed for the sample. In order to have a deeper understanding of the results, a simulation model was developed and it showed good semi-quantitative agreement with the experimental results.