Theoretical and experimental investigation of broadband dispersion tailoring of high-order mode in the hybrid microsphere cavity.

The large normal dispersion of the fundamental mode (TE mode) in the whispering gallery modes (WGM) microsphere is detrimental to the visible comb generation. Herein, we demonstrate that this fundamental limitation can be removed by considering the high-order radial modes (TE mode) of the hybrid microsphere cavity (HMC). The studied HMC consists of a high-refractive-index coating (TiO or HfO) and silica microsphere. The simulated electric field energy distribution and measured Q value in our experiment show that optical confinement of the coating effectively excites the TE mode and reduces the free spectral range (FSR) and modal dispersion. In addition, the observed redshift of WGM and decreased trend of FSR are in accordance with simulations. The zero-dispersion wavelength can be linearly shifted to a shorter wavelength or even into the visible region with the reduction of coating thickness or refractive index and larger microcavity, which advances the visible comb generation.