High-Q Microcavities: Optomechanical Nonlinearities, Measurement Techniques and Applications

• Main Author: Rokhsari Azar, Hossein
• Format: Others
• Published: 2006
• Online Access: https://thesis.library.caltech.edu/4457/3/Title.pdf; https://thesis.library.caltech.edu/4457/1/front-material.pdf; https://thesis.library.caltech.edu/4457/2/Thesis-Chapters.pdf; Rokhsari Azar, Hossein (2006) High-Q Microcavities: Optomechanical Nonlinearities, Measurement Techniques and Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/CVT6-8J70. https://resolver.caltech.edu/CaltechETD:etd-11082005-204747 <https://resolver.caltech.edu/CaltechETD:etd-11082005-204747>

Optical microresonators have historically been perceived as structures that could efficiently confine optical energies. This is due to their exceedingly low losses at optical frequencies. This thesis has, for the first time, explored these structures in a starkly different frequency range. Optical microcavities like any other structure have mechanical eigenmodes or resonant modes of vibration with quality-factors representing the efficiency of energy storage at mechanical frequencies. It is shown here that micron size of these structures results in vibrations at radio frequencies (~1-100 MH), about seven orders of magnitude apart from the optical frequencies (~100 THz). Mechanical quality factors in excess of 5,000 are measured for toroidal microcavities revealing a heretofore unknown potential of these structures in storing energy at frequencies remarkably distant from their optical resonant modes. This thesis describes how radiation-pressure or the force due to impact of photons could result in exceptionally strong couplings between the mechanical and optical resonators collocated within the same device. The discovered optomechanical coupling present in toroid microcavities is shown to reach such a high level that regenerative mechanical oscillations of the cavity structure are initiated with only micro-Watts of optical power. This is the first demonstration of radiation-pressure-induced mechanical oscillations in any type of optomechanical system. Embodied within a microscale, chip-based device, this mechanism can benefit both research into macroscale quantum mechanical phenomena and improve the understanding of the mechanism within the context of Laser interferometer gravitational-wave observatory (LIGO). It also suggests that new technologies are possible that will leverage the phenomenon within photonics. Different physical functionalities are also realized in this thesis by a combination of ultra-high-Q microtoroids and extremely low-loss tapered optical fibers for efficient delivery of optical power to these structures. Using these tools an almost ideal optical band-pass filter is designed with efficiencies solely limited by intrinsic losses of the optical resonator. These intrinsic loss mechanisms are experimentally studied and differentiated by a powerful technique based on thermal nonlinearities of the microcavity material. By taking advantage of slow response times of thermal effects, an innovative pump and probe technique is also developed to unveil and measure the Kerr nonlinearity in microcavities, for the first time, at room temperature.