2026-09-24T02:31:06Z https://www.ideals.illinois.edu/oai-pmh

oai:www.ideals.illinois.edu:2142/125714 2026-02-03 col_2142_5131 col_2142_8888 com_2142_5130 com_2142_8887 com_2142_234

application/pdf en text Submission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2026-08-01 The student, Hao Tong, accepted the attached license on 2024-07-10 at 13:01. The student, Hao Tong, submitted this Dissertation for approval on 2024-07-10 at 13:17. This Dissertation was approved for publication on 2024-07-11 at 11:25. DSpace SAF Submission Ingestion Package generated from Vireo submission #21046 on 2025-02-04 at 21:16:56 Mechanical bound states in the continuum in phononic crystals Tong, Hao 2024-07-11 Fang, Kejie Fang, Kejie Bahl, Gaurav Dragic, Peter Zhao, Yang Bound States In The Continuum Phononic Crystal Optomechanics eng 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. 2024-08 Thesis https://hdl.handle.net/2142/125714 Copyright 2024 Hao Tong Electrical & Computer Eng Electrical & Computer Engr University of Illinois at Urbana-Champaign Ph.D. Dissertation