Mechanical bound states in the continuum in phononic crystals
Tong, Hao
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https://hdl.handle.net/2142/125714
Description
Title
Mechanical bound states in the continuum in phononic crystals
Author(s)
Tong, Hao
Issue Date
2024-07-11
Director of Research (if dissertation) or Advisor (if thesis)
Fang, Kejie
Doctoral Committee Chair(s)
Fang, Kejie
Committee Member(s)
Bahl, Gaurav
Dragic, Peter
Zhao, Yang
Department of Study
Electrical & Computer Eng
Discipline
Electrical & Computer Engr
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
bound states in the continuum, phononic crystal, optomechanics
Abstract
Bound states in the continuum (BICs) are unique states that are spatially confined, yet they lie within the radiation continuum where energy typically dissipates. Although BICs have been implemented in various fields, mechanical BICs in phononic crystals have not yet been realized. These mechanical BICs differ from traditional mechanical oscillators because of the higher thermal handling capability and larger mode volume. In this dissertation, we demonstrate mechanical BICs in phononic crystals, introduce mechanical BICs into optomechanics, and develop methods for improving device performance by merging mechanical BICs.
First, we introduce mechanical symmetry-protected BICs through elastic wave equations and symmetry analysis, highlighting the topological nature with the far-field polarization. We fabricate the devices in an AlN-on-oxide stack and perform an in-depth characterization of the acoustic loss mechanisms at both room and cryogenic temperatures using the piezoelectric effect.
Next, we demonstrate a new class of optomechanical crystals in two-dimensional slab-on-substrate structures, supported by mechanical BICs. We show the emergence of symmetry-induced BICs with strong optomechanical couplings, comparable to low-dimensional optomechanical crystals. By adjusting the angle between the device orientation and the material lattice, we control the optomechanical coupling strength. We then probe the mechanical modes through optical homodyne detection and identify the scattering-loss-limited quality factors resulting from fabrication variations.
Finally, we introduce mechanical accidental BICs in a high-aspect-ratio gallium arsenide phononic crystal grating. We observe the merging process of accidental BICs with symmetry-protected BICs, which results in reduced acoustic radiation losses compared to isolated BICs. This discovery opens up new possibilities for phonon trapping using BIC-based systems, with potential applications in sensing, transduction, and quantum measurements.
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