This chapter explores electromagnetic-matter interactions from photon to extinction length scales, i.e., nanometer of X-ray and above. Starting with Casimir-Polder effect to understand interactions of metals and dielectrics at near-atomic distance scale, it stretches to larger wavelengths to explore optomechanics and its ability for energy exchange and signal transduction between PHz and GHz. This range is explored with near-quantum sensitivity limits. The chapter also develops the understanding phononic bandgaps, and for photons, it explores the use of energetic coupling for useful devices such as optical tweezers, confocal microscopes and atomic clocks. It also explores miniature accelerators as a frontier area in accelerator physics. Plasmonics—the electromagnetic interaction with electron charge cloud—is explored for propagating and confined conditions together with the approaches’ possible uses. Optoelectronic energy conversion is analyzed in organic and inorganic systems, with their underlying interaction physics through solar cells and its thermodynamic limit, and quantum cascade lasers.