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When light illuminates the junction formed between a sharp metal tip and asample, different mechanisms can con-tribute to the measured photo-inducedforce simultaneously. Of particular interest are the instantaneous forcebe-tween the induced dipoles in the tip and in the sample and the force relatedto thermal heating of the junction. A key difference between these two forcemechanisms is their spectral behaviors. The magnitude of the thermal responsefollows a dissipative Lorentzian lineshape, which measures the heat exchangebetween light and matter, while the induced dipole response exhibits adispersive spectrum and relates to the real part of the materialpolarizability. Be-cause the two interactions are sometimes comparable inmagnitude, the origin of the nanoscale chemical selectivity in the recentlydeveloped photo-induced force microscopy (PiFM) is often unclear. Here, wedemonstrate theoretically and experimentally how light absorption followed bynanoscale thermal expansion generates a photo-induced force in PiFM.Furthermore, we explain how this thermal force can be distinguished from theinduced dipole force by tuning the relaxation time of samples. Our analysispresented here helps the interpretation of nanoscale chemical measure-ments ofheterogeneous materials and sheds light on the nature of light-matter couplingin van der Waals materials.

Nanoscale spectroscopic studies of two different physical origins of the tip-enhanced force: dipole and thermal