Ultra-highly sensitive passive near-field microscopy of electromagnetic evanescent waves

Abstract

Electromagnetic (EM) evanescent waves generated on material surfaces without external illumination are of particular interest. If materials are in thermal equilibrium, thermal fluctuation of current and/or charge is induced by stochastic thermal motion of conduction electrons (in metals) or by thermally-activated lattice vibrations (in polar dielectrics), which generates intense thermal EM evanescent waves [1]. If materials are driven out of equilibrium, say, by electric-field-induced currents or by chemical reactions, even stronger EM evanescent waves are induced because the fluctuation of current and/or charge is promoted by activated motion of charge carriers or electric dipoles in non-equilibrium states. Hence, a sensitive passive microscopy of EM evanescent waves with nanometer resolution is expected to provide us with a unique and powerful probe to the local kinetics of material phenomena in equilibrium and non-equilibrium conditions. We have developed such microscopes as described in the above, by incorporating ultra-highly sensitive detectors, called CSIP [2], into passive terahertz (THz) scattering-type scanning near-field optical microscopes (s-SNOMs) [3]. Owing to the high sensitivity, detailed nature of the thermal EM evanescent waves has been clarified through quantitative analysis of imaging of both metals [4, 5] and dielectrics. The spatial resolution reaches a value about 20nm (λ/750) [6]. We also demonstrate the potentiality and powerfulness of our system for probing non-equilibrium kinetics: We show that current-induced hot-electron distribution in narrow metal wires at room temperature is clearly visualized with nanometer resolution. These results show that our systems will find broad application areas in the near future.