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Creators/Authors contains: "Fang, Kejie"

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  1. Abstract

    Mechanical bound states in the continuum (BICs) present an alternative avenue for developing high-frequency, high-Qmechanical resonators, distinct from the conventional band structure engineering method. While symmetry-protected mechanical BICs have been realized in phononic crystals, the observation of accidental mechanical BICs—whose existence is independent of mode symmetry and tunable by structural parameters—has remained elusive. This challenge is primarily attributed to the additional radiation channel introduced by the longitudinal component of elastic waves. Here, we employ a coupled wave theory to predict and experimentally demonstrate mechanical accidental BICs within a high-aspect-ratio gallium arsenide phononic crystal grating. We observe the merging process of accidental BICs with symmetry-protected BICs, resulting in reduced acoustic radiation losses compared to isolated BICs. This finding opens up new possibilities for phonon trapping using BIC-based systems, with potential applications in sensing, transduction, and quantum measurements.

     
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    Free, publicly-accessible full text available December 1, 2025
  2. Recent developments in quantum light–matter coupled systems and quantum transducers have highlighted the need for cryogenic optical measurements. In this study, we present a packaged fiber-optic coupler with a coupling efficiency of over 50% for telecom wavelength light down to the mK temperature range. Besides the high coupling efficiency, our method enables sensitive photonic device measurements that are immune to mechanical vibrations present in cryogenic setups.

     
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  3. Abstract

    Chipscale micro- and nano-optomechanical systems, hinging on the intangible radiation-pressure force, have shown their unique strength in sensing, signal transduction, and exploration of quantum physics with mechanical resonators. Optomechanical crystals, as one of the leading device platforms, enable simultaneous molding of the band structure of optical photons and microwave phonons with strong optomechanical coupling. Here, we demonstrate a new breed of optomechanical crystals in two-dimensional slab-on-substrate structures empowered by mechanical bound states in the continuum (BICs) at 8 GHz. We show symmetry-induced BIC emergence with optomechanical couplings up tog/2π≈ 2.5 MHz per unit cell, on par with low-dimensional optomechanical crystals. Our work paves the way towards exploration of photon-phonon interaction beyond suspended microcavities, which might lead to new applications of optomechanics from phonon sensing to quantum transduction.

     
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  4. Optical nonlinearity plays a pivotal role in quantum information processing using photons, from heralded single-photon sources and coherent wavelength conversion to long-sought quantum repeaters. Despite the availability of strong dipole coupling to quantum emitters, achieving strong bulk optical nonlinearity is highly desirable. Here, we realize quantum nanophotonic integrated circuits in thin-film InGaP with, to our knowledge, a record-high ratio of1.5%<#comment/>between the single-photon nonlinear coupling rate (g/2π<#comment/>=11.2MHz) and cavity-photon loss rate. We demonstrate second-harmonic generation with an efficiency of71200±<#comment/>10300%<#comment/>/Win the InGaP photonic circuit and photon-pair generation via degenerate spontaneous parametric downconversion with an ultrahigh rate exceeding 27.5 MHz/µW—an order of magnitude improvement of the state of the art—and a large coincidence-to-accidental ratio up to1.4×<#comment/>104. Our work shows InGaP as a potentially transcending platform for quantum nonlinear optics and quantum information applications.

     
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  5. Abstract

    Phonon trapping has an immense impact in many areas of science and technology, from the antennas of interferometric gravitational wave detectors to chip-scale quantum micro- and nano-mechanical oscillators. It usually relies on the mechanical suspension—an approach, while isolating selected vibrational modes, leads to serious drawbacks for interrogation of the trapped phonons, including limited heat capacity and excess noises via measurements. To circumvent these constraints, we realize a paradigm of phonon trapping using mechanical bound states in the continuum (BICs) with topological features and conducted an in-depth characterization of the mechanical losses both at room and cryogenic temperatures. Our findings of mechanical BICs combining the microwave frequency and macroscopic size unveil a unique platform for realizing mechanical oscillators in both classical and quantum regimes. The paradigm of mechanical BICs might lead to unprecedented sensing modalities for applications such as rare-event searches and the exploration of the foundations of quantum mechanics in unreached parameter spaces.

     
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  6. Integrated nonlinear photonic circuits received rapid development in recent years, providing all-optical functionalities enabled by cavity-enhanced photon-photon interaction for classical and quantum applications. A high-efficiency fiber-to-chip interface is key to these integrated photonic circuits for quantum information tasks, as photon-loss is a major source that weakens quantum protocols. Here, overcoming material and fabrication limitation of thin-film aluminum nitride by adopting a stepwise waveguiding scheme, we demonstrate low-loss adiabatic fiber-optic couplers in aluminum nitride films with a substantial thickness (∼600 nm) for optimized nonlinear photon interaction. For telecom (1550 nm) and near-visible (780 nm) transverse magnetic-polarized light, the measured insertion loss of the fiber-optic coupler is -0.97 dB and -2.6 dB, respectively. Our results will facilitate the use of aluminum nitride integrated photonic circuits as efficient quantum resources for generation of entangled photons and squeezed light on microchips.

     
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