Theory of resonant SNOM (scanning near-field optical microscopy): Breakdown of the electric dipole selection rule in the reflection mode

A theoretical study is reported about SNOM with the use of light in resonance with the quantized levels of a mesoscopic sample. The microscopic variation of electromagnetic field and the coherence of matter state are properly treated by solving the microscopic Maxwell equations and Schrodinger equation self consistently. A remarkable effect, i.e., the breakdown of the dipole selection rule, is shown in the reflection mode where a probe tip is used for both illumination of the sample and collection of the signal. Namely, both dipole-active and higher-multipole-active levels give comparable signal intensities for appropriate probe tip positions.