Junghoon Jahng et. al. have published a theoretical and experimental analysis of the dominant forces measured in PiFM in a paper titled “Gradient and scattering forces in photoinduced force microscopy.” They conclude that both optically induced dipole-dipole gradient force (Fg) and scattering force (Fs) are present in a typical PiFM setup. Whereas Fs is insensitive to the tip-sample distance, Fg shows a z-4 dependence. Experiments conducted on various samples confirm that Fg is strongly dependent on the polarizability of the sample, which enables spectroscopy imaging via force detection. It is therefore important to conduct PiFM with as small a tip-sample distance as possible in order to produce the largest contrast based on sample’s polarizability.