2, Universidad de Guanajuato Campus León, Leon, , Mexico
Frequency combs are spectrally broad light sources where each line of the comb is incrementally spaced from adjacent lines. Generation of a comb in silica whispering gallery mode (WGM) optical resonant cavities relies on nonlinear optical effects, most notably the Kerr nonlinearity, which allows for four-wave mixing (FWM) to generate new sideband photons. Because silica has a low Kerr coefficient, considerable input power is necessary in order to obtain a broad spanning comb. An important consideration for obtaining a wide comb is the material dispersion. The overlap of the whispering gallery mode with the comb modes dictated by FWM is crucial for efficient comb generation. The material dispersion plays a role in changing the spacing of whispering gallery modes, limiting the wavelength range before mismatch of the comb modes and resonator modes causes loss to exceed gain. One material that has shown promise in frequency comb generation due to its low dispersion is calcium fluoride (CaF2). Single-crystalline CaF2 WGM resonator-based frequency combs have been demonstrated, but the difficulty in device fabrication limits potential applications.
In this study, we have shown that coating the surface of a silica WGM resonator with a low dispersion material effectively reduces the effective overall dispersion of the material, allowing for higher spanning frequency combs to be generated. We first synthesize CaF2 nanocrystals using a simple chemical co-precipitation method. We then coat them onto the surface of silica whispering gallery mode resonant cavities and show that frequency comb span is enhanced compared to uncoated resonant cavities. The normalized thresholds of the FWM processes in coated devices are unchanged compared to uncoated devices, and are on the order of hundreds of microwatts. At input powers of ~4 mW, the comb span in a CaF2-coated microsphere reaches 300 nm, while a comb of just 30 nm is present in an uncoated silica microsphere at the same power levels. The largest spanning comb on a coated microsphere observed is over 400 nm wide at an input power of ~6 mW.