Enhanced coupling to whispering gallery modes by two-photon absorption induced by a highly focused field

The authors report on enhanced coupling to whispering gallery modes in fluorescent polystyrene microspheres using two-photon absorption induced by a highly focused field. Due to the highly confined excitation nature under focused evanescent illumination achieved by a circularly obstructed beam, the whispering gallery modes can be excited within a small volume near the perimeter of the microsphere. As a result, the visibility, the Q factor, and the degree of polarization of the fluorescence spectra induced in the microsphere are enhanced by 60%, 37%, and five times, respectively.