When light illuminates the junction formed between a sharp metal tip and a
sample, different mechanisms can con-tribute to the measured photo-induced
force simultaneously. Of particular interest are the instantaneous force
be-tween the induced dipoles in the tip and in the sample and the force related
to thermal heating of the junction. A key difference between these two force
mechanisms is their spectral behaviors. The magnitude of the thermal response
follows a dissipative Lorentzian lineshape, which measures the heat exchange
between light and matter, while the induced dipole response exhibits a
dispersive spectrum and relates to the real part of the material
polarizability. Be-cause the two interactions are sometimes comparable in
magnitude, the origin of the nanoscale chemical selectivity in the recently
developed photo-induced force microscopy (PiFM) is often unclear. Here, we
demonstrate theoretically and experimentally how light absorption followed by
nanoscale thermal expansion generates a photo-induced force in PiFM.
Furthermore, we explain how this thermal force can be distinguished from the
induced dipole force by tuning the relaxation time of samples. Our analysis
presented here helps the interpretation of nanoscale chemical measure-ments of
heterogeneous materials and sheds light on the nature of light-matter coupling
in van der Waals materials.Comment: 17 pages, 10 figure
