Tailoring the Optomechanical Coupling in Multimode Photonic Crystal Resonators

Abstract: In this thesis, I describe the design, fabrication and characterization of a quasi-twodimensional photonic crystal multi-mode optomechanical resonator formed from thin film silicon. This cavity optomechanical device consists of a dielectric membrane patterned with a triangular lattice of holes, in which three line defect nanoscale slots creating a so-called zipper-style double nanobeam [1] in between photonic crystal slabs. The coupled fundamental optical modes localized in the slots can be designed to form three high-Q optical resonances that can achieve large couplings to the mechanical modes of the zipper beam.

In general, cavity optomechanics examines the interaction between light and the mechanical motion of nano to macro scale oscillators. The coupling is typically mediated by generalized radiation pressure forces. Cavity optomechanics adresses various fundamental and technological challenges in physics and engineering such as force sensing, wavelength conversion, generation of quantum states of motion, squeezing of light and mechanical motion, state transfer between light and mechanics and gravitational wave detection. Recently, a particular class of devices based on photonic crystals have shown their ability to confine optical and mechanical modes to exceptionally small mode volumes, thus boosting heir coupling strength while maintaining high quality factors. These so-called optomechanical-crystals hold great promise towards the realization of strong optomechanical coupling, which remains a key challenge in the field. Recently, some theoretical works proposed multi-mode optomechanical systems to achieve a large ratio between the optomechanical coupling g0 and cavity decay rate . Motivated by this theoretical prediction [2] and by recent achievements in the development of membrane membrane-in-the-middle setup on an optomechanical crystal basis [3], the goal of this work is to push the idea even further by introducing more optical and mechanical modes to approach regimes of larger optomechanical coupling strengths.

The first chapter of this thesis introduces the cavity optomechanics. The following chapter describes the realization of cavity optomechanics employing photonic crystals. In addition, the computational design and numerical modeling tools to obtain a photonic crystal optomechanical cavity are introduced. The third chapter is an indepth analysis of the triple- slotted photonic crystal optomechanical cavity design.

This chapter details the optimization of the multimode optomechanical cavity and the impact of the geometry on the coupling strength. The fourth chapter presents the nanofabrication process used for these devices. The following chapter gives an overview of the device characterization. Lastly, the results and future perspectives for this work are summarized.