Dark-field imaging as a non-invasive method for characterization of   whispering gallery modes in microdisk cavities
  Whispering gallery mode microdisk cavities fabricated by direct laser writing are studied using dark-field imaging and spectroscopy in the visible spectral range. {Dark-field imaging allows us to directly visualize the spatial intensity distribution of whispering gallery modes. We extract their azimuthal and radial mode indices from dark-field images, and find the axial mode number from the dispersion relation. The scattering spectrum obtained in the confocal arrangement provides information on the density of optical states in the resonator. The proposed technique is a simple non-invasive way to characterize the optical properties of microdisk cavities. 
D. A. Baranov, K.B. Samusev, I. I. Shishkin, A.K. Samusev, P.A. Belov,   A. A. Bogdanov
physics.optics