Forbidden light scanning near‐field optical microscopy

Abstract

Near‐field optics (NFO) opens the door to light microscopy techniques with resolutions well beyond the diffraction limit. The richness of optical investigations is now applicable on a near‐molecular level. Among the novel scanning near‐field optical microscopy (SNOM) schemes, the most prominent representatives are aperture SNOM and scanning tunnelling optical microscopy (STOM or PSTM). New experimental and theoretical work has to be performed to study the phenomena specific to NFO. One such example is the angular dependence of light emission in aperture SNOM. The detection of radiation at angles greater than the critical angle of total internal reflection αc = arcsin(n−1), where n is the sample refractive index, can represent a microscopy scheme that combines the respective advantages of both aperture SNOM and STOM. Recent experiments have demonstrated the expected exponential dependence of light intensity on gap width (for fixed emission angle α > αc). The decay length as a function of α is in agreement with the Fresnel description of the evanescent field when total reflection occurs at an interface. These investigations were additionally motivated by calculations based on the multiple multipole method. 1995 Blackwell Science Ltd