Monitoring Fast Thermal Dynamics at the Nanoscale through Frequency Domain Photoinduced Force Microscopy

In illuminated tip–sample junctions, the absorption of light by the sample is accompanied by local heating and subsequent thermal expansion of the material. In photoinduced force microscopy (PiFM) experiments, thermal expansion is expected to affect the measured photoinduced force through the thermally modulated van der Waals force. Evidence for such thermal contributions in PiFM measurements has been demonstrated in the mid-infrared range, where the primary excitations are molecular vibrational modes. For PiFM measurements in the vis/NIR, where light-matter energy transfer is mediated through electronic excitations, clear experimental evidence of thermal contributions remains elusive. By developing a frequency domain version of PiFM, we retrieve variations in the photoinduced force on the sub-μs time-scales, allowing a direct registration of the thermal relaxation dynamics of the sample after photoexcitation. Our measurements confirm the presence of the thermal contribution to the PiFM signal in the mid-infrared range and provide strong experimental evidence that thermal components also play a role in the forces measured in PiFM in the vis/NIR range of the spectrum.