Abstract
First, we cover the history and background of optical resonators and WGMRs to establish the basis for the rest of the manuscript. A Fabry–Pérot resonator is treated as an example of a simple two-dimensional optical resonator before explaining the complex three-dimensional WGMRs. Next, we describe the various experimental methods used to conduct experiments in this work. These methods include fabricating a resonator via different materials, shaping the symmetry parameters such as radius of the disc (R) and the minor radius (r). Furthermore, we shed light on coupling techniques, specifically the prism coupling method. Testing the newly fabricated resonator is explained, i.e., how to measure its Q-factor and free spectral range (FSR). We also discuss other methods such as experimentally determining the polar (p) mode numbers and thermally tuning WGMs. Furthermore, we explore a novel method of coupling to a WGMR by using trapezoidal prisms; this method can be used to determine the ratio of radii (R/r) of a disc-shaped WGMR and to fine-tune the distance between a prism and resonator. The WGMR used to demonstrate this method is composed of YLF which makes it the first of its kind and exhibits a high intrinsic Q-factor of 1x10^9. After covering the theoretical and experimental background of disc-shaped WGMRs, a literature review is presented to put a focus on the significance and applications of these WGMRs and to give the reader an idea about the state of the art.
Ti:sapphire is a widely used active medium in the solid state laser industry, mainly due to its broad gain bandwidth and benefiting from the material stability of sapphire. In this thesis, we demonstrate a disc-shaped WGMR carved out of a Ti:sapphire crystal, as an alternative approach to the standard laser cavity. Merits of WGMRs as laser cavities include their small monolithic nature, small mode volume and high Q-factor. The fabricated Ti:sapphire WGMR despite the titanium doping exhibits high Q-factors of 1x10^8 at 1550nm, and 5x10^7 at 795nm. By pumping this resonator with a green laser at 516.6nm, a record low lasing threshold of 14.2mW is observed with a slope efficiency of 34%. The observed lasing can be both multi-mode and single-mode. This is the first demonstration of a Ti:sapphire whispering-gallery laser (WGL).
Furthermore, we discuss a novel method of utilizing the Ti:sapphire WGL as an amplifier. This method is based on the pump-probe technique, i.e., the gain in Ti:sapphire is evaluated in the near infrared region by using a probe laser with a central wavelength of 795nm along with the 516.6nm pump laser. This method results in decreasing linewidth of the modes excited with the probe laser, consequently increasing their Q.
We demonstrate a widely tunable source of coherent infrared light based on optical parametric oscillation (OPO). High Q-factor crystalline WGMRs are a suitable candidate for observing OPO with high efficiency. WGMRs can be fabricated from suitable nonlinear crystals to experimentally observe OPO. Here, we investigate OPO in a disc-shaped WGMR fabricated from MgO-doped LN, tailor-made for pumping with 516.6nm. Widely tunable parametric modes are generated owing to the type of excited mode and temperature and degenerate OPO is observed at 1033.2nm. At optimal phase matching conditions this degenerate OPO can lead to the formation of a frequency comb.
Finally, we report strong frequency shifts, observed experimentally in the modes of a LN WGMR when a dielectric is moved into close proximity to it. The mode frequencies of the WGMR are shifted by planar substrates -- of refractive indices ranging from 1.50 to 4.22 -- by moving them close to the resonator rim. Both blue- and red-shifts in the frequency of the modes are observed, as well as an increase in mode linewidth, when substrates are moved into the evanescent field of the WGM. We compare the experimental results to a model previously predicted in theory and provide an additional intuitive explanation. This explanation is based on Goos-Hänchen shift for the optical domain, with applications to dielectric structures ranging from meta-surfaces to photonic crystal cavities.