Nonlinear behavior in hybrid optical resonators*

Whispering gallery mode optical resonators have found numerous applications spanning from basic science through applied technology due to their intrinsic ability to amplify an input optical field. Because these devices confine light at the periphery, the circulating optical power directly interacts with both the cavity material and cavity surface. Therefore, gain can be either directly doped into or coated onto the cavity. Here, three different hybrid devices are fabricated from silica resonators.

We have synthesized a suite of metal-doped silica (Zr, Ti) using the sol-gel process and fabricated devices using UV-lithography. The intercalation of the metal in the silica matrix increases the Raman gain coefficient by changing the polarizability of the silica. As a result of the increase, the Raman lasing threshold and Raman efficiency both decrease. The threshold is sub-microWatt and the efficiency is over 40%.

We have synthesized a suite of organic, highly nonlinear optical small molecules to enable high performance Raman lasers and frequency combs. By grafting the small molecule directly to the device surface, the overall nonlinearity of the device is increased. Using this strategy in combination with high Raman gain small molecules, we have demonstrated ultra-low threshold Raman lasers. When combining this approach with high Kerr coefficient materials, we can achieve high efficiency Kerr frequency combs.

By combining metal nanorods with small molecule coatings, we can push the small molecule enhancement further. By coating an optical cavity with small molecule functionalized metal nanorods, a plasmonic-photonic mode is formed. This hybrid mode focuses the optical field into the organic layer, improving the light-nanomaterial interaction. Using this strategy, wide span frequency combs with low thresholds are demonstrated. Additional nonlinear behaviors, such as Anti-Stokes/Stokes generation, are also observed at low input powers.