Metallic nanocavities sustain localized surface plasmons which enable deep subwavelength light concentration and manipulation. The nanocavity in between two nanoparticles, i.e., a dimer, is a typical geometry for light concentration, with typical electric field enhancement of 10² - 10³ under resonant excitation. The smaller the gap distance is, the larger the field enhancement will be. However, direct probing of the field enhancement is prohibited since the probe (e.g. a scanning tip) cannot be inserted into the tiny gap of the nanocavity. In this talk, I will talk about how to probe the field enhancement within a subnanometer gap in a nanoparticle over mirror (NPOM) configuration using Surface enhanced Raman spectroscopy (SERS). The strong field enhancement in the gap can also be used to amplify the interaction between light and exciton. Furthermore, I will also show that the nanocavity can be used as a plasmon sensor with unprecedented sensitivity.