Quantum Theory for Near-Field Nano-Optics

Abstract

Near-field optics and related techniques are based on the electromagnetic interactions of matter in the quasi-static regime, where the electromagnetic fields in a mode coupled with matter play the fundamental role. Here, the quasi-static regime implies that the dominant electromagnetic interactions between matter take place across a distance much smaller than the optical wavelength. Recent developments in microfabrication provide a diversity of probe tips with the potential to pick the local fields out of the sea of macroscopic electromagnetic interactions [1]. In the near-field regime, the optical properties of matter and associated near-fields depend strongly on the sizes and shapes of the matter involved. In fact, the optical response of matter is determined by internal electronic processes including the interaction with optical fields consistent with both electronic and electromagnetic boundary conditions. The resulting scattered fields reflect the properties of these internal processes in the illuminated objects, especially when they are observed in an optical near-field. The scattered fields exhibit asymptotic behavior in the far-field limit as propagating optical waves of a retarded nature that carry electromagnetic energy out of the object [2]. In this case, the optical response of matter can be represented in macroscopic quantities, such as dielectric functions, which enable us to reproduce the macroscopic electromagnetic boundary conditions correctly [3]. In contrast, when an observation is carried out in a subwavelength vicinity of objects, the scattered field involves strong near-fields showing steep decay, which reveal the details of optical interactions taking place inside the objects. Theoretical descriptions of optical near-field processes should therefore be based on detailed studies of the electromagnetic interaction of matter at the microscopic level.