Phase-shift and Amplitude Analysis Reveal Stages of Nanoparticle-Assisted Photothermal Annealing of Polydimethylsiloxane

In nanoparticle-assisted photothermal microscopy, absorption of radiation by a nanoparticle is followed by non-radiative relaxation which in turn leads to changes in the surrounding medium temperature, pressure, and density. Under harmonically modulated irradiation, the finite heat diffusion rate causes a phase delay between the thermal oscillation at a location in the medium relative to that at the nanoparticle surface. The phase delay averaged over the probe laser volume can be measured concomitantly with the amplitude of detected probe power modulation. In this study, we show that, in conjunction with the more widespread modulation amplitude measurement, the photothermal phase provides a complementary, sensitive probe of thermally-induced changes in the local medium properties. Moreover, with the help of simulations it is possible to extract from phase/amplitude data the temperature-dependent properties of the medium as shown on a widely used, technologically important polymer resist -- polydimethylsiloxane (PDMS).