Release-free silicon optomechanical devices with Bound-state In the Continuum
Liu, Shengyan
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Permalink
https://hdl.handle.net/2142/120455
Description
Title
Release-free silicon optomechanical devices with Bound-state In the Continuum
Author(s)
Liu, Shengyan
Issue Date
2023-05-05
Director of Research (if dissertation) or Advisor (if thesis)
Fang, Kejie
Department of Study
Electrical & Computer Eng
Discipline
Electrical & Computer Engr
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
M.S.
Degree Level
Thesis
Keyword(s)
Optomechanics
phononic crystal
Bound-state In the Continuum
Abstract
Cavity optomechanics is the study of the interaction between photons and mechanical oscillators within an optical cavity. Among different physical implementations, optomechanical crystals based on micro- and nano-fabrication techniques have shown promising performance due to their high mechanical frequency, high optical and mechanical quality factors, and good integration capability. However, most optomechanical crystals are released from the substrate to isolate them with the surrounding environment for good phonon confinement, which leads to poor thermal conductivity. As a result, photon-absorption-induced phonons can accumulate in the device for a long time and degrade the system's quantum operation due to the raised noise level. In this thesis, we present a detailed investigation of the mechanical Bound-state In the Continuum (mBIC) in phononic crystals, and develop a kind of non-suspended optomechanical crystal devices based on this concept.
We first derive the master equation of phononic crystals, study the eigenmodes of the system using the point groups, and identify the existence of symmetry-protected mBICs. We point out that mechanical BICs are polarization singularities of the transverse components on the nodal line of the longitudinal component. We further derive the scaling rule of the quality factors near the symmetry-protected mechanical BICs.
Next, we examine the coupling between mechanical BIC and optical modes. Using symmetry as a powerful tool, we derive the condition for non-zero coupling between mechanical BIC and optical modes. We also derive the relationship between the optomechanical coupling of finite-sized optomechanical crystals and that of the unit cell.
Finally, we develop optomechanical devices with mechanical BICs on the silicon-on-insulator (SOI) platform. Because silicon is an anisotropic crystal, we can control the behavior of the device by controlling its orientation. We study the coupling between the optical band-edge mode and mechanical BICs at room temperature and measure the unit-cell optomechanical coupling to be 2.5 MHz and the highest frequency of the mechanical mode to be up to 8 GHz, which can be compared with existing released optomechanical crystals. We also discuss the reasons for the lower mechanical quality factors of the devices at room temperature and propose possible solutions.
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