Multimode And Silicon Nitride Membrane Cavity Optomechanics Based On Whispering-Gallery Cavities

Whispering gallery mode(WGM)cavities have ultrahigh quality factors and small mode volume,which could greatly enhance the light-matter interaction,making them ad-vantageous as platforms for cavity optomechanics.The optomechanical systems based on WGM cavities have been widely employed for processing of optical signals,ground state cooling and highly sensitive sensors.Different with the two-mode optomechanical systems,in which one optical mode couples to one mechanical mode,multimode op-tomechanics could offer more than two active degrees of freedom to control.Also the multimode systems can study the optomechanical interference processes between differ-ent modes,thus they show a rich variety of physical phenomena,such as stationary entan-glement and exceptional points.This thesis is mainly focused on the research and appli-cations of the multimode cavity optomechanical systems based on the whispering-gallery cavity.To study the multimode optomechanical system of WGM mode coupling to silicon nitride,we design and simulate the silicon nitride nanomechanical oscillators and couple them with the WGM microsphere in experiment.The main works are as follows:1.We theoretically study the multiple-pathway interference effect in a single micro-cavity optomechanical system,in which we can switch back and forth between optome-chanically induced transparency and absorption.Benefiting by the phenomenon that two optical modes indirectly couple to each other mediated by the tapered fiber,the three-pathway interference could induce an absorption dip(transparent window)within the transparent window(absorption dip).While we can switch back and forth between trans-parency window and absorption dip with the four-pathway interference with both optical modes coupling to the mechanical mode.2.We theoretically study the Brillouin cavity optomechanics in coupled WGM mi-crocavities and the application for highly sensitive sensing.The passive optical micro-cavity with the Brillouin optomechanics couples with an active microcavity,which can enhance the optical spring effect.Also the optomechanical induced dynamical backaction is sensitive to the perturbation of optical mode.We apply the system to WGM sensing ap-plications.We show that by monitoring the shift of the mechanical frequency,the optical frequency shift sensing can be achieved and the detection sensitivity is greatly enhanced.3.Design for silicon nitride nanomechanical oscillator arrays.We simulate the prop-erties of the mechanical oscillator through the finite element method,and study the effect of geometrical parameters on the coupling between different mechanical oscillators.We also design the arrays with the phononic bandgap structure to dilute intrinsic dissipation,increasing the quality factor higher than 10~7.4.Near-field coupling between WGM microsphere and silicon nitride mechanical os-cillator.We study the fabrication process for silicon nitride nanomechanical oscillator,and fabricate the oscillators both with and without phononic bandgap structure.We measure both dispersive coupling and dissipative coupling,and show the feasibility of coupling the microcavity with nanomechanical oscillator arrays with detection of optomechanical dynamical backaction.