Abstract
Optical resonators play a vital role in enhancing the interaction between light and matter, as they allow light to resonate within the structure when the resonator’s perimeter matches an integer multiple of the wavelength. Among various resonator geometries, whispering gallery mode resonators (WGMRs) stand out due to their unique properties. WGMRs exhibit low intrinsic losses, which depend on the material used, and minimal surface scattering losses, achievable through excellent surface quality. These factors contribute to their high Q factors and finesse. WGMRs also confine light through total internal reflection (TIR), leading to compact mode volumes and high energy densities. This high energy density is especially important for driving efficient nonlinear optical phenomena, even under low optical pump power.
In this thesis, a series of nonlinear optical experiments are conducted using crystalline WGMRs with disc-shaped geometries. These resonators offer not only robust environmental stability but also rich nonlinear properties, including second-order χ⁽²⁾ processes. The intrinsic birefringence of these crystals allows for phase matching, further enhancing the efficiency of nonlinear interactions. The combination of high Q factors, compact mode volumes, and intrinsic nonlinearities makes crystalline WGMRs ideal platforms for exploring nonlinear optics in a stable, low-power regime.
To start with, coating high-gain nanocrystals, core/shell CdSe/CdS, on the surface of crystalline WGMRs is explored. To minimize the scattering loss introduced by nanocrystal clusters, polymethyl methacrylate (PMMA) solutions, as the carriers for the nanocrystals, are investigated. The optimal PMMA concentration can increase the Q factor of the coated system by one order of magnitude. However, only enhanced fluorescence is observed and the lasing regime is not reached using our CW pump laser due to the fast Auger recombination.
Next, Raman lasing from a z-cut lithium tetraborate (LB4) WGMR is demonstrated, with a low threshold power of 0.69 mW, made possible by the exceptionally high Q factor (2×10⁹ at 517 nm) of our LB4 WGMR and the material’s high Raman gain. Furthermore, fourth-order cascaded Raman lasing, spanning a spectral range from the pump wavelength at 517 nm to 608 nm, is achieved at an incoupled pump power of approximately 7 mW. This broad spectral coverage holds potential for applications in spectroscopy and related fields.
Broadband second harmonic generation (SHG) at 258 nm, 397 nm, and 780 nm is demonstrated in x-cut LB4 WGMRs. This effect is enabled by cyclic phase matching, resulting from the oscillating refractive index of the TM mode in the x-cut LB4 WGMR. The spatial modulation of the refractive index allows broadband phase matching across the transparency window of LB4. Theoretically, this broadband SHG can extend across a wavelength range from approximately 500 nm to3400 nm.
In the final chapter, both the theoretical and experimental aspects of electro-optic (EO) frequency combs in a z-cut LB4 WGMR are analyzed and explored. First, the EO tuning of the LB4 WGMR is experimentally investigated, showing strong agreement with theoretical predictions. Next, EO comb generation is explored in this high-Q LB4 WGMR when pumped at 1550 nm. Finally, the potential for generating EO combs at visible wavelengths is discussed.