Ultrahigh quality factor (UHQ) microresonators have very long photon lifetimes, enabling high circulating power inside the cavities. As a result of the amplification of the input optical power, these devices are able to excite various nonlinear optical phenomena, such as optical parametric oscillation (OPO) and stimulated Raman scattering. Previously, OPO has been demonstrated using silica toroidal resonators via the Kerr non-linearity. By combining degenerate and non-degenerate four-wave mixing (FWM), frequency combs have been generated with high input power. Additionally, due to the moderate value of the third order nonlinear susceptibility, >5mW of input power was required to obtain a broad (>100nm span) frequency comb.
In this work, we experimentally demonstrate that by coating a silica microtoroid with a Zirconium (Zr) doped silica solgel film, the threshold and comb span improve. Due to the higher third order susceptibility value of the Zr-doped silica layer as compared to silica, the efficiency of the four wave mixing process, especially OPO, has been improved. The frequency comb behaviors of different concentrations of Zr coated devices are experimentally investigated in the near-IR (1550nm), and the comb spans are compared between the various devices. Comb spans in excess of 300nm are achieved with ~mW input powers. In conclusion, the frequency comb performance is dramatically improved by incorporating a thin film of Zr-doped solgel into the silica toroidal cavity.