We present the novel type of microscopy – photo-induced force microscopy (PiFM) [1, 2] and its applications. The photo-induced force, enhanced by the plasmonic field, depends on the local response of the media, i.e. local polarizability as well as the local absorption which resulted in the chemical selectivity with a spatial resolution of a few nanometers [3]. The advantage of this approach is that any optical transitions linear and nonlinear of molecules can be probed directly as a far-field free manner. This new form of microscopy operates in a regime of non-contact/tapping mode which is away from the hard-contact mode. In this work, we present theoretical and experimental analysis of linear and nonlinear optical responses such as Raman vibrational molecular states [2], the excited state absorption [4] in single molecular clusters, IR vibrational transition [5] and surface plasmon polaritons in thin gold film [6] in photo-induced force microscopy. The ability to apply AFM's for nanometer scale optical spectroscopic analysis will open new opportunities in materials science and biology by investigating the chemical and optical nature of individual molecules.

REFERENCES
1. I. Rajapaksa, et al., “Image force microscopy of molecular resonance: A microscope principle'', Appl. Phys. Lett. 97, 073121 (2010).
2. J. Jahng, et al., “Linear and Nonlinear Optical Spectroscopy at the Nanoscale with Photoinduced Force Microscopy'', Acc. Chem. Res., 48 (10), 2671-2679 (2015).
3. J. Jahng et. al, “Quantitative analysis of sideband coupling in photoinduced force microscopy”, Phys. Rev. B, 94, 195407 (2016).
4. J. Jahng et. al, “Ultrafast pump-probe force microscopy with nanoscale resolution”, Appl. Phys. Lett, 106, 083113 (2015).
5. D. Nowak, et. al., "Nanoscale chemical imaging by photoinduced force microscopy," Sci. Adv. 2, e1501571 (2016).
6. J. Jahng et. al, “Visualizing surface plasmon polaritons by their gradient force” Opt. Lett., 40 (21), 5058-5061 (2015)

Figure 1: Schematic of photo-induced force microscopy