We introduce a novel type of light-matter interaction force microscopy and spectroscopy, called as "Photo-induced force microscopy''. We visualize the optical interplay between tip and sample junction at molecular level in ambient condition. The linear and nonlinear optical responses such as localized permittivity, stimulated Raman and pump-probe are addressed at space--time limit. The ability of control the force at nano/molecular scale in ambient condition leads to a variety of potential application such as single molecule study,

nano-printing, nano-architecturing with chemical selectivity.

A fundamental nano-structure is the gap formed when two materials are placed in close proximity to one another. If the junction thus produced is in the (sub)-nanometer range, the structure often exhibits unique optical properties when illuminated with light. Because it confines electromagnetic fields to nanoscopic dimensions, the nanojunction provides an exquisite platform for conducting studies on nano-objects and molecules placed in the junction. The response of such a molecule-nanojunction system is especially strong if it supports the accumulation of oscillating charge density along with concentration of the electromagnetic field upon illumination, as is the case when surface plasmon resonances are present. Characterizing the fields in these nanosystems is cumbersome because of the complex nature of near-fields in the junction, yet it is indispensable for the endeavor. In this dissertation we model two examples of illuminated nanojunctions that are extensively used in light-matter interaction experiments: (i) the nanojunction of a plasmonic nanoantenna made of a sphere dimer (nanodumbbell), which we will use to study optical chirality in surface-enhanced Raman scattering, and (ii) the nanojunction of a sharp atomic tip with a planar substrate, with relevance to photo-induced force microscopy.

We develop analytical formalisms and perform full wave simulations to study the electrodynamics of the nanojunction based on various observables in the respective experiments. We find that in the gold nanodumbbells, the nanojunction morphology dominates the near-field behavior and far-field scattering spectrum of the nanodumbbell, regardless of the overall shape of the nanopartciles. Moreover, the asymmetry in the nanojunction can lead to a significant chiroptical response. In the second example, we observe that different scattering mechanisms in the nanojunction affect the field gradient in the nanojunction, which can bring about photo-induced forces that are within the detectable range of a conventional scan probe instrument. We also observe that multiple-scattering mechanisms can lead to a sign reversal in the photo-induced force, a phenomenon that cannot be predicted by simpler models.