Free-space cavity optomechanical systems on a chip with III-V heterostructures

Abstract: Cavity optomechanics examines the mutual interaction between light and mechanical motion for controlling mechanical resonators down to the quantum regime. A major challenge in the field of cavity optomechanics remains accessing a strong interaction between the light field and mechanics on the level of single quanta. Optomechanical systems with small mode volumes show considerable enhancement in the interaction strength. However, in a majority of these systems the increase in the interaction strength comes at the cost of additional optical losses. Therefore, one cannot exploit the novel capabilities of such systems often. This thesis is about the development of a monolithic cavity optomechanical platform using III-V materials which demonstrates a pathway to combine a free-space optical cavity with an integrated mechanical system. To this end, we showcase the design, fabrication and characterization of optomechanical microresonators in AlGaAs/InGaP heterostructures. We demonstrate the enhancement of the out-of-plane reflectivity by reflectance engineering using photonic crystals. We utilize the features of III-V heterostructures by realizing monolithic fully-suspended micromechanical resonator arrays with sub-µm gap in GaAs. This would enable the possibility of enhancing the optomechanical interaction using the concept of multi-element optomechanics. We explore integrated cavity optomechanical systems formed by two photonic crystals reflectors and by a photonic crystal reflector with an integrated distributed Bragg reflector mirror. Furthermore, we propose the use of highly-frequency dependent photonic crystal reflectors in the optomechanical system for realizing photonic bound states in a continuum, which decouple the otherwise coupled cavity loss rates and coupling strength. The quality factor of the mechanical resonator can be increased by using tensile-strained InGaP which is compatible with AlGaAs heterostructures growth. We determine the material properties of InGaP relevant for mechanical resonators. We demonstrate quality factors of 10^7 in trampoline resonators in InGaP at room temperature. The quality factor is pressure limited and can be enhanced using strain engineering. Free-space integrated multi-element cavity optomechanical systems in III-V heterostructures have the potential to enter the quantum optomechanics regime at room temperature.