Scanning Near‐Field Gap‐Mode Microscopy

Since the pioneering days of scanning near-field optical microscopy, an important goal has always been to combine the scanning probe microscopy techniques with spectroscopic means for characterizing the chemical structure of materials with nanometer spatial resolution. This turned out to be a demanding but fascinating endeavor, as the nanoscale optical and spectroscopic phenomena manifest distinctive characteristics different from those observed at the macroscopic scale. This challenge continues to stimulate worldwide researchers from physics, chemistry, and engineering streams to investigate light–matter interactions at dimensions much smaller than the wavelength of electromagnetic radiation (d ≪ λ). Widespread applications of such investigations are demonstrated, for example, in material sciences (maximizing photovoltaic energy conversion), in engineer sciences (pursuing quantum devices at the quantum limit with single photons), or in life sciences (as local optical sensors to observe chemical processes in living cells). With the development of nanophotonics and nanotechnology, the understanding of light–matter interaction in the subwavelength regime has been considerably deepened and more and more new microscopic spectroscopy techniques have been invented. In this chapter, we will introduce an effective high-resolution scanning near-field optical microscopy method – scanning near-field gap-mode microscopy (SNGM), which combines optical spectroscopy and scanning probe microscopy. In the following sections, we lead you through the principles, the theoretical background, the experimental realization, and the applications of this technique. Our goal is to develop an intuitive picture of this fascinating ultramicroscopic technique and to discuss the physical processes that allow us to extract optical information from a sample with nanometer spatial resolution and sensitivity down to the single-molecule level.