Brillouin zone folding driven bound states in the continuum

Observation of Ultra-High-Q Resonators in the Ultrasound via Bound States in the Continuum

The confinement of waves in open systems represents a fundamental phenomenon extensively explored across various branches of wave physics. Recently, significant attention is directed toward bound states in the continuum (BIC), a class of modes that are trapped but do not decay in an otherwise unbounded continuum. Here, the theoretical investigation and experimental demonstration of the existence of quasi-bound states in the continuum (QBIC) for ultrasonic waves are achieved by leveraging an elastic Fabry-Pérot metasurface resonator. Several intriguing properties of the ultrasound quasi-bound states in the continuum that are robust to parameter scanning are unveiled, and experimental evidence of a remarkable Q-factor of 350 at ≈1 MHz frequency, far exceeding the state-of-the-art using a fully acoustic underwater system is presented. The findings contribute novel insights into the understanding of BIC for acoustic waves, offering a new paradigm for the design of efficient, ultra-high Q-factor ultrasound devices.

Valley edge states as bound states in the continuum

Bound states in the continuum (BICs) are spatially localized states with energy embedded in the continuum spectrum of extended states. The combination of BICs physics and nontrivial band topology theory givs rise to topological BICs, which are robust against disorders and meanwhile, the merit of conventional BICs is attracting wide attention recently. Here, we report valley edge states as topological BICs, which appear at the domain wall between two distinct valley topological phases. The robustness of such BICs is demonstrated. The simulations and experiments show great agreement. Our findings of valley related topological BICs shed light on both BICs and valley physics, and may foster innovative applications of topological acoustic devices.

High-quality quasi-bound state in the continuum enabled single-nanoparticle virus detection

Bound states in the continuum (BICs) have emerged as a powerful platform for boosting light-matter interactions because they provide an alternative way of realizing optical resonances with ultrahigh quality(Q-) factors, accompanied by extreme field confinement. In this work, we realized an optical biosensor by introducing a quasi-BIC (qBIC) supported by an elaborated all-dielectric dimer grating. Thanks to the excellent field confinement within the air gap of grating enabled by such a high-Q qBIC, the figure of merit (FOM) of a biosensor is up to 18,908.7 RIU-1. Furthermore, we demonstrated that such a high-Q grating can help push the limit of optical biosensing to the single-particle level. Our results may find exciting applications in extreme biochemical sensing like COVID-19 with ultralow concentration.

All-Dielectric Integrated Meta-Antenna Operating in 6G Terahertz Communication Window

Efficient transceivers and antennas at terahertz frequencies are leading the development of 6G terahertz communication systems. The antenna design for high-resolution terahertz spatial sensing and communication remains challenging, while emergent metallic metasurface antennas can address this issue but often suffer from low efficiency and complex manufacturing. Here, an all-dielectric integrated meta-antenna operating in 6G terahertz communication window for high-efficiency beam focusing in the sub-wavelength scale is reported. With the antenna surface functionalized by metagrating arrays with asymmetric scattering patterns, the design and optimization methods are demonstrated with a physical size constraint. The highest manipulation and diffraction efficiencies achieve 84.1% and 48.1%. The commercially accessible fabrication method with low cost and easy to implement has been demonstrated for the meta-antenna by photocuring 3D printing. A filamentous focal spot is measured as 0.86λ with a long depth of focus of 25.3λ. Its application for integrated imaging and communication has been demonstrated. The proposed technical roadmap provides a general pathway for creating high-efficiency integrated meta-antennas with great potential in high-resolution 6G terahertz spatial sensing and communication applications.

Passive trapping of biomolecules in hotspots with all-dielectric terahertz metamaterials

Electromagnetic metamaterials feature the capability of squeezing photons into hotspot regions of high intensity near-field enhancement for strong light-matter interaction, underpinning the next generation of emerging biosensors. However, randomly dispersed biomolecules around the hotspots lead to weak interactions. Here, we demonstrate an all-silicon dielectric terahertz metamaterial sensor design capable of passively trapping biomoleculars into the resonant cavities confined with powerful electric field. Specifically, multiple controllable high-quality factor resonances driven by bound states in the continuum (BIC) are realized by employing longitudinal symmetry breaking. The dielectric metamaterial sensor with nearly 15.2 experimental figure-of-merit enabling qualitative and quantitative identification of different amino acids by delivering biomolecules to the hotspots for strong light-matter interactions. It is envisioned that the presented strategy will enlighten high-performance meta-sensors design from microwaves to visible frequencies, and serve as a potential platform for microfluidic sensing, biomolecular capture, and sorting devices.

Multi-mode resonance of bound states in the continuum in dielectric metasurfaces

Bound states in the continuum (BIC) represent distinct non-radiative states endowed with infinite lifetime and vanishing resonance linewidth. Introducing asymmetric perturbation to the system can convert true BICs into high quality leaky modes which is useful in many photonic applications. Previously, such perturbation and resonance of interest is only limited to a single factor. However, different perturbations by unit cell gap, geometry and rotation angle result distinctive resonance modes. The combination of two perturbation factors can excite multi-mode resonance contributed from each asymmetric factor which coexist simultaneously; thus, the number of reflectance peaks can be controlled. In addition, we have carefully analyzed the electric field variations under different perturbation factors, followed by a multipolar decomposition of resonances to reveal underlying mechanisms of distinct resonance modes. Through simulations, we find that the introduction of multiple asymmetric perturbations also influences the metasurface sensitivity in refractive index sensing and compare the performance of different resonance modes. These observations provide structural design insights for achieving high quality resonance with multiple modes and ultra-sensitive sensing.

Anomalous far-field polarization around bound states in the continuum in non-Bravais lattices

It is generally believed that at-Γ bound states in the continuum (BICs) are enclosed by a linearly polarized vortex in momentum space when the structures have mirror (σz) symmetry, in-plane inversion (I) symmetry, and time reversal symmetry (T). Here, we reveal an anomalous situation in which at-Γ BICs can be enclosed by linearly and elliptically polarized far-field even when the σz, I, and T symmetries are all maintained in non-Bravais lattices, which is radically different from previous cognition. Asymmetric, diatomic structures are designed to elaborate this intriguing phenomenon. By controlling the geometric parameters or refractive indexes of the two meta-atoms, the far-field polarization around the at-Γ BICs gradually deviates from linear polarization and approaches circular polarization. Our findings reveal that non-Bravais lattices can provide a novel platform to manipulate the far-field polarization, showing important applications in quantum entanglement, structured light, and radiation modulation.

Topological Nature of Radiation Asymmetry in Bilayer Metagratings

Manipulating radiation asymmetry of photonic structures is of particular interest in many photonic applications such as directional optical antenna, high efficiency on-chip lasers, and coherent light control. Here, we proposed a term of pseudopolarization to reveal the topological nature of radiation asymmetry in bilayer metagratings. Robust pseudopolarization vortex with an integer topological charge exists in P-symmetry metagrating, allowing for tunable directionality ranging from -1 to 1 in synthetic parameter space. When P-symmetry breaking, such vortex becomes pairs of C points due to the conservation law of charge, leading to the phase difference of radiation asymmetry from π/2 to 3π/2. Furthermore, topologically enabled coherent perfect absorption is robust with customized phase difference at will between two counterpropagating external light sources. This Letter can not only enrich the understanding of two particular topological photonic behaviors, i.e., bound state in the continuum and unidirectional guided resonance, but also provide a topological view on radiation asymmetry, opening an unexplored avenue for asymmetric light manipulation in on-chip laser, light-light switch, and quantum emitters.

Governance of Friedrich-Wintgen bound states in the continuum by tuning the internal coupling of meta-atoms

Bound state in the continuum (BIC) is a phenomenon that describes the perfect confinement of electromagnetic waves despite their resonant frequencies lying in the continuous radiative spectrum. BICs can be realized by introducing a destructive interference between distinct modes, referred to as Friedrich-Wintgen BICs (FW-BICs). Herein, we demonstrate that FW-BICs can be derived from coupled modes of individual split-ring resonators (SRR) in the terahertz band. The eigenmode results manifest that FW-BICs are in the center of the far-field polarization vortices. Quasi-BIC-I keeps an ultrahigh quality factor (Q factor) in a broad momentum range along the Γ-X direction, while the Q factor of the quasi-BIC-II drops rapidly. Our results can facilitate the design of devices with high-Q factors with extreme robustness against the incident angle.

Coupling-Based Multiple Bound States in the Continuum and Grating-Assisted Permittivity Retrieval in the Terahertz Metasurface

The terahertz (THz) metasurfaces that support bound states in the continuum (BICs) provide a promising platform for various applications due to their high Q-factor resonance. In this study, we experimentally demonstrate multiple BICs with different resonance symmetries in the THz metasurface based on mode coupling. The proposed metasurface is composed of 2 × 2 split ring resonators (SRRs) metamolecules. The SRRs of two different gap angles in the metamolecule lattice provide intrinsic resonance with different frequencies, and the coupling between them exhibits high transmission quasi-BIC resonance, which can be tuned by varying the gap angle. The arrangement of SRRs in the 2 × 2 metamolecule lattice determines the types of coupling that govern the resonance symmetry of quasi-BIC. More interestingly, the multiple quasi-BICs enabled by different couplings can be simultaneously achieved in a metasurface. Apart from tuning the gap angles, the permittivity in the vicinity of SRRs also changes the resonance frequency. Consequently, quasi-BIC can be artificially formed by deliberately constructing the permittivity difference of the dielectric environment on the SRRs. In view of this, we introduce the scheme of permittivity retrieval for the dispersive analyte, assisted by the fixed-permittivity gratings. In addition, we experimentally demonstrate the metasurface in combination with the microfluidic chip for the sensing of the glucose solution concentration. Our findings offer a possible strategy for the existing manufactured metasurface to achieve quasi-BIC resonance and provide a promising candidate for approaching the spectral analysis of the biochemical.

Exploiting Zone-Folding Induced Quasi-Bound Modes to Achieve Highly Coherent Thermal Emissions

Thermal emissions with high coherence, although not as high as those of lasers, still play a crucial role in many practical applications. In this work, by exploiting the geometric perturbation-induced optical lattice tripling and the associated Brillion zone folding effect, we propose and investigate thermal emissions in the mid-infrared with simultaneous high temporal and spatial coherence. In contrast with the case of period-doubling perturbation in our previous work, the steeper part of the guided mode dispersion band will be folded to the high-symmetry Γ point in the ternary grating. In this case, a specific emission wavelength corresponds to only a very small range of wavevectors. Consequently, apart from the high temporal coherence characterized by an experimental bandwidth around 30 nm, the achieved thermal emissions also feature ultrahigh spatial coherence. Calculations show that at the thermal emission wavelengths in the mid-infrared, the spatial coherence length can easily reach up to mm scale.

Photonic Bound States in the Continuum in Nanostructures

Bound states in the continuum (BIC) have garnered considerable attention recently for their unique capacity to confine electromagnetic waves within an open or non-Hermitian system. Utilizing a variety of light confinement mechanisms, nanostructures can achieve ultra-high quality factors and intense field localization with BIC, offering advantages such as long-living resonance modes, adaptable light control, and enhanced light-matter interactions, paving the way for innovative developments in photonics. This review outlines novel functionality and performance enhancements by synergizing optical BIC with diverse nanostructures, delivering an in-depth analysis of BIC designs in gratings, photonic crystals, waveguides, and metasurfaces. Additionally, we showcase the latest advancements of BIC in 2D material platforms and suggest potential trajectories for future research.

Multi-topological state via the Brillouin zone overlap for nonlinear frequency conversion

Multiband topological edge states (TESs) or topological corner states (TCSs) in photonic crystals provide effective ways to manipulate the nonlinear frequency conversions. However, the deliberate design and the limited number of multibands lead to the difficulty of experimental realization of the topological nonlinear frequency conversion or higher harmonic generation. Here, we propose an effective method to achieve multiple TESs and TCSs by combining the Brillouin zones of multiple different systems. It is shown that the spectra of the subsystems disperse into different energy levels due to the inter-system hopping. Based on this approach, we construct a topological photonic crystal based on the Brillouin zone overlapped SSH model, which enables the overlapped TCSs to participate in nonlinear frequency conversion. Our scheme can provide a significant way to realize the topological nonlinear frequency conversion with double resonances or multiple resonances.

High-Q Quasi-Bound States in the Continuum in Terahertz All-Silicon Metasurfaces

Bound states in the continuum (BIC)-based all-silicon metasurfaces have attracted widespread attention in recent years because of their high quality (Q) factors in terahertz (THz) frequencies. Here, we propose and experimentally demonstrate an all-silicon BIC metasurface consisting of an air-hole array on a Si substrate. BICs originated from low-order TE and TM guided mode resonances (GMRs) induced by (1,0) and (1,1) Rayleigh diffraction of metagratings, which were numerically investigated. The results indicate that the GMRs and their Q-factors are easily excited and manipulated by breaking the lattice symmetry through changes in the position or radius of the air-holes, while the resonance frequencies are less sensitive to these changes. The measured Q-factor of the GMRs is as high as 490. The high-Q metasurfaces have potential applications in THz modulators, biosensors, and other photonic devices.

Infrared bound states in the continuum: random forest method

In this Letter, we consider optical bound states in the continuum (BICs) in the infrared range supported by an all-dielectric metasurface in the form of subwavelength dielectric grating. We apply the random forest machine learning method to predict the frequency of the BICs as dependent on the optical and geometric parameters of the metasurface. It is found that the machine learning approach outperforms the standard least square method at the size of the dataset of ≈4000 specimens. It is shown that the random forest approach can be applied for predicting the subband in the infrared spectrum into which the BIC falls. The important feature parameters that affect the BIC wavelength are identified.