Photo-induced force microscopy (PiFM) detects photo-induced molecular polarizability of feature sizes down to the molecular level by mechanical detection of the force gradient of the interaction between the optically driven molecular dipole and its mirror image in a metal coated AFM tip. Thus, PiFM not only excites the sample with near-field but also detects the response in near-field, a feature truly unique among tip-enhanced optical microscopy techniques. The near-field detection allows PiFM to be significantly easier and more robust to operate than the techniques relying on far-field detection. Another major advantage is the absence of far-field background signal, leading to excellent signal-to-noise ratio even with low excitation power and from extremely thin samples (as thin as ~ 1 nm thick). Yet another advantage is that PiFM relies on non-contact or light tapping AFM mode, which (1) prevents even the softest samples from damage and (2) achieves higher spatial resolution than AFM topography due to the steeper functional dependence of dipole-dipole force on the tip-sample distance.