Energy Coupling Through the Small Gap in Optical Microcavities

In this report, we investigate the influences of the small gap in whispering-gallery modes on energy coupling, resonance quality and frequency. Photon tunneling between an optical resonator and its light-delivery coupler depends strongly on the gap dimension which can vary from zero to size of an optical wavelength involved. An optimal gap dimension is found to exist for maximum energy coupling. The Q factor increases exponentially with increasing gap and saturates as the gap approaches the optical wavelength. The resonance frequency shifts with decreasing gap to a certain value. An optimum gap for sensing applications can be defined at the half maximum energy coupling where both the Q factor and coupling efficiency are high and the resonance frequency is little affected by the gap variation.