Measurement properties of electric field intensity distribution of whispering gallery mode with near-field optical probe

The distribution of the external electric field intensity of the whispering gallery mode (WGM) can be measured using a scanning near-field optical microscopy (SNOM) probe. As probing the resonator influences the optical resonance state, the measurement properties of the SNOM probe are worth studying. In this study, the measurement mechanism of WGMs using a glass SNOM probe was analyzed numerically. A probe with a nanometric-diameter is generally preferable. The results showed that the high-contrast measurement was possible; however, the signal was weak. Using a tip with a diameter equal to half the resonant wavelength, the signal strength was maximized with the same high contrast level as the nanoprobe. Interestingly, the measurement mechanism was different depending on tip size. With the nanoprobe, the interaction with the WGMs varied depending on the sensing locations; therefore, the resonant states were modulated during the measurements, which may have induced unexpected mode hopes. The resonance state was steady during measurements using a probe tip with a diameter of half the resonance wavelength. Although the mechanisms were different, the measured electric field intensity distributions were the same for both tip diameters.