Formation mechanism of guided resonances and bound states in the continuum in photonic crystal slabs

Efficient second- and higher-order harmonic generation from LiNbO3 metasurfaces

Lithium niobate (LiNbO3) is a material that has drawn great interest in nonlinear optics because of its large nonlinear susceptibility and wide transparency window. However, for complex nonlinear processes such as high-harmonic generation (HHG), which involves frequency conversion over a wide frequency range, it can be extremely challenging for a bulk LiNbO3 crystal to fulfill the phase-matching conditions. LiNbO3 metasurfaces with resonantly enhanced nonlinear light-matter interaction at the nanoscale may circumvent such an issue. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and HHG from a LiNbO3 metasurface enhanced by guided-mode resonance. We observe a high normalized SHG efficiency of 5.1 × 10-5 cm2 GW-1. Moreover, with the alleviated above-gap absorption of the material, we demonstrate HHG up to the 7th order with the shortest generated wavelength of 226 nm. This work may provide a pathway towards compact coherent white-light sources with frequency spanning into the deep ultraviolet region for applications in spectroscopy and imaging.

Investigations on the optical forces from three mainstream optical resonances in all-dielectric nanostructure arrays

Light can exert radiation pressure on any object it encounters, and the resulting optical force can be used to manipulate particles at the micro- or nanoscale. In this work, we present a detailed comparison through numerical simulations of the optical forces that can be exerted on polystyrene spheres of the same diameter. The spheres are placed within the confined fields of three optical resonances supported by all-dielectric nanostructure arrays, including toroidal dipole (TD), anapoles, and quasi-bound states in continuum (quasi-BIC) resonances. By elaborately designing the geometry of a slotted-disk array, three different resonances can be supported, which are verified by the multipole decomposition analysis of the scattering power spectrum. Our numerical results show that the quasi-BIC resonance can produce a larger optical gradient force, which is about three orders of magnitude higher than those generated from the other two resonances. The large contrast in the optical forces generated with these resonances is attributed to a higher electromagnetic field enhancement provided by the quasi-BIC. These results suggest that the quasi-BIC resonance is preferred when one employs all-dielectric nanostructure arrays for the trapping and manipulation of nanoparticles by optical forces. It is important to use low-power lasers to achieve efficient trapping and avoid any harmful heating effects.

Robust and non-robust bound states in the continuum in rotationally symmetric periodic waveguides

A fiber grating and a one-dimensional (1D) periodic array of spheres are examples of rotationally symmetric periodic (RSP) waveguides. It is well known that bound states in the continuum (BICs) may exist in lossless dielectric RSP waveguides. Any guided mode in an RSP waveguide is characterized by an azimuthal index m, the frequency ω, and Bloch wavenumber β. A BIC is a guided mode, but for the same m, ω and β, cylindrical waves can propagate to or from infinity in the surrounding homogeneous medium. In this paper, we investigate the robustness of nondegenerate BICs in lossless dielectric RSP waveguides. The question is whether a BIC in an RSP waveguide with a reflection symmetry along its axis z, can continue its existence when the waveguide is perturbed by small but arbitrary structural perturbations that preserve the periodicity and the reflection symmetry in z. It is shown that for m = 0 and m ≠ 0, generic BICs with only a single propagating diffraction order are robust and non-robust, respectively, and a non-robust BIC with m ≠ 0 can continue to exist if the perturbation contains one tunable parameter. The theory is established by proving the existence of a BIC in the perturbed structure mathematically, where the perturbation is small but arbitrary, and contains an extra tunable parameter for the case of m ≠ 0. The theory is validated by numerical examples for propagating BICs with m ≠ 0 and β ≠ 0 in fiber gratings and 1D arrays of circular disks.

Dual-band refractive index sensor with cascaded asymmetric resonant compound grating based on bound states in the continuum

We propose a cascaded asymmetric resonant compound grating (ARCG) for high-performance dual-band refractive index sensing. The physical mechanism of the sensor is investigated using a combination of temporal coupled-mode theory (TCMT) and ARCG eigenfrequency information, which is verified by rigorous coupled-wave analysis (RCWA). The reflection spectra can be tailored by changing the key structural parameters. And by altering the grating strip spacing, a dual-band quasi-bound state in the continuum can be achieved. The simulation results show that the highest sensitivity of the dual-band sensor is 480.1 nm/RIU, and its figure of merit is 4.01 × 10⁵. The proposed ARCG has potential application prospects for high-performance integrated sensors.

Strong resonance response with ultrahigh quality factor in grating-multilayer systems based on quasi-bound states in the continuum

Optical bound states in the continuum (BICs) exist in many photonic crystals and periodic structures with a strong resonance and ultrahigh Q factor. Such phenomena can be used in the designs of narrowband transmission filters, lasers, and sensors. In this paper, we consider the energy bands of a complex structure consisting of a grating and a multilayer substructure to obtain the position of the BIC in the structure. Hence, the higher Q factor can be obtained in the grating-multilayer structure than can be realized in the simple grating geometry. We analyze the wave propagation process in the complex structure and the change in the Q value via the use of transmission matrix theory. In addition, the reflectance spectrum is found to exhibit a series of asymmetric line-shapes with different values of the asymmetry parameter, δ, due to the interference between the two channels. One of these channels is the broadband channel, induced by the Fabry-Perot resonance, and the other channel is the narrowband channel, induced by guided mode resonance. Quasi-BICs are seen to transform into BICs as the value of δ is decreased gradually to zero. Therefore, a large Goos-Hänchen shift can be achieved as a result of the high Q factor and quasi-BIC. This work designs a complex structure with ultrahigh Q factor and strong resonance properties, which has significant implications for exploring the phenomenon of BICs.

Thickness bound for nonlocal wide-field-of-view metalenses

Metalenses-flat lenses made with optical metasurfaces-promise to enable thinner, cheaper, and better imaging systems. Achieving a sufficient angular field of view (FOV) is crucial toward that goal and requires a tailored incident-angle-dependent response. Here, we show that there is an intrinsic trade-off between achieving a desired broad-angle response and reducing the thickness of the device. Like the memory effect in disordered media, this thickness bound originates from the Fourier transform duality between space and angle. One can write down the transmission matrix describing the desired angle-dependent response, convert it to the spatial basis where its degree of nonlocality can be quantified through a lateral spreading, and determine the minimal device thickness based on such a required lateral spreading. This approach is general. When applied to wide-FOV lenses, it predicts the minimal thickness as a function of the FOV, lens diameter, and numerical aperture. The bound is tight, as some inverse-designed multi-layer metasurfaces can approach the minimal thickness we found. This work offers guidance for the design of nonlocal metasurfaces, proposes a new framework for establishing bounds, and reveals the relation between angular diversity and spatial footprint in multi-channel systems.

Double-layer symmetric gratings with bound states in the continuum for dual-band high-Q optical sensing

Herein, we theoretically demonstrate that a double-layer symmetric gratings (DLSG) resonator consisting of a low-refractive-index layer sandwiched between two high-contrast gratings (HCG) layers, can host dual-band high-quality (Q) factor resonance. We find that the artificial bound states in the continuum (BIC) and Fabry-Pérot BIC (FP-BIC) can be induced by optimizing structural parameters of DLSG. Interestingly, the artificial BIC is governed by the spacing between the two rectangular dielectric gratings, while the FP-BIC is achieved by controlling the cavity length of the structure. Further, the two types of BIC can be converted into quasi-BIC (QBIC) by either changing the spacing between adjacent gratings or changing the distance between the upper and lower gratings. The simulation results show that the dual-band high-performance sensor is achieved with the highest sensitivity of 453 nm/RIU and a maximum figure of merit (FOM) of 9808. Such dual-band high-Q resonator is expected to have promising applications in multi-wavelength sensing and nonlinear optics.

Evolution of optical harmonic generation near bound-states in the continuum in hybrid plasmonic-photonic structures

We investigate the nonlinear optical harmonic generation behaviors near the bound-states in the continuum (BICs) in hybrid plasmonic-photonic structures. The hybrid structures are designed to consist of a plasmonic grating covered with a nonlinear dielectric waveguide layer, which support two distinct groups of BICs, i.e. the symmetry-protected BICs and Friedrich-Wintgen BICs. The evolution of second- and third-harmonic generation (SHG and THG) near the two groups of BICs was studied. The high dependence of nonlinear response on the local field distribution and tensor components of susceptibility of nonlinear materials was determined. Especially, there exists optimized angles of incidence for efficient SHG and THG response due to the interaction of photonic and plasmonic modes. The results are important to understand the nonlinear response behaviors in hybrid plasmonic-photonic structures and to design the nonlinear photonic devices.

Recent Progress in Nanolaser Technology

Nanolasers are key elements in the implementation of optical integrated circuits owing to their low lasing thresholds, high energy efficiencies, and high modulation speeds. With the development of semiconductor wafer growth and nanofabrication techniques, various types of wavelength-scale and subwavelength-scale nanolasers have been proposed. For example, photonic crystal lasers and plasmonic lasers based on the feedback mechanisms of the photonic bandgap and surface plasmon polaritons, respectively, have been successfully demonstrated. More recently, nanolasers employing new mechanisms of light confinement, including parity-time symmetry lasers, photonic topological insulator lasers, and bound states in the continuum lasers, have been developed. Here, the operational mechanisms, optical characterizations, and practical applications of these nanolasers based on recent research results are outlined. Their scientific and engineering challenges are also discussed.

Generation and Annihilation of Topologically Protected Bound States in the Continuum and Circularly Polarized States by Symmetry Breaking

We demonstrate by breaking the C_{6} symmetry for higher-order at-Γ bound states in the continuum (BICs) with topological charge -2 in photonic crystals (i) deterministic generation of off-Γ BICs from the at-Γ BIC and (ii) a variety of pair-creation and annihilation processes of circularly polarized states with opposite topological charges and the same handedness. To explain these phenomena, we introduce the handedness-wise topological charge quantized to a half-integer. The handedness-wise charge gives a unified picture of various phenomena involving BICs and circularly polarized states.

Near-field analysis of bound states in the continuum in photonic crystal slabs

Bound states in the continuum (BICs) can be derived from a generalized waveguide condition in which the total internal reflection is substituted by coherent perfect reflection. Coherent perfect reflection can occur in the truncated photonic crystal (PhC) due to the interference of different Bloch modes. Based on the coherent reflection, BICs can be constructed by the bulk Bloch modes of PhC slabs. In contrast to the determination of BICs from the topological vortices of far-field radiation, this interpretation from coherent reflection can give the spatial field profile in detail in the near field. We show that the BICs can be characterized by the indices (or number of nodes) of their constituent Bloch modes. Moreover, all the guided resonances in addition to BICs can also be labelled by these mode indices. It is found that for the guided resonances the mode indices can change suddenly on the same frequency band. Our results may have potential applications in guided-wave optics and enhanced light-matter interaction.

All-Dielectric Active Terahertz Photonics Driven by Bound States in the Continuum

The remarkable emergence of all-dielectric meta-photonics governed by the physics of high-index dielectric materials offers a low-loss platform for efficient manipulation and subwavelength control of electromagnetic waves from microwaves to visible frequencies. Dielectric metasurfaces can focus electromagnetic waves, generate structured beams and vortices, enhance local fields for advanced sensing, and provide novel functionalities for classical and quantum technologies. Recent advances in meta-photonics are associated with the exploration of exotic electromagnetic modes called the bound states in the continuum (BICs), which offer a simple interference mechanism to achieve large quality factors (Q) through excitation of supercavity modes in dielectric nanostructures and resonant metasurfaces. Here, a BIC-driven terahertz metasurface with dynamic control of high-Q silicon supercavities that are reconfigurable at a nanosecond timescale is experimentally demonstrated. It is revealed that such supercavities enable low-power, optically induced terahertz switching and modulation of sharp resonances for potential applications in lasing, mode multiplexing, and biosensing.

Bound States in the Continuum through Environmental Design

We propose a new paradigm for realizing bound states in the continuum (BICs) by engineering the environment of a system to control the number of available radiation channels. Using this method, we demonstrate that a photonic crystal slab embedded in a photonic crystal environment can exhibit both isolated points and lines of BICs in different regions of its Brillouin zone. Finally, we demonstrate that the intersection between a line of BICs and a line of leaky resonances can yield exceptional points connected by a bulk Fermi arc. The ability to design the environment of a system opens up a broad range of experimental possibilities for realizing BICs in three-dimensional geometries, such as in 3D-printed structures and the planar grain boundaries of self-assembled systems.

Nonlinear response from optical bound states in the continuum

We consider nonlinear effects in scattering of light by a periodic structure supporting optical bound states in the continuum. In the spectral vicinity of the bound states the scattered electromagnetic field is resonantly enhanced triggering optical bistability. Using coupled mode approach we derive a nonlinear equation for the amplitude of the resonant mode associated with the bound state. We show that such an equation for the isolated resonance can be easily solved yielding bistable solutions which are in quantitative agreement with the full-wave solutions of Maxwell’s equations. The coupled mode approach allowed us to cast the the problem into the form of a driven nonlinear oscillator and analyze the onset of bistability under variation of the incident wave. The results presented drastically simplify the analysis nonlinear Maxwell’s equations and, thus, can be instrumental in engineering optical response via bound states in the continuum.

Characterization of microresonator-geometry-deformation for cavity optomechanics

We have studied the effect of geometry deformation on the mechanical frequencies and quality factors for different modes in the Whispering Gallery Mode (WGM) microresonators, that is unavoidable in the practical fabrication. The subsidence of the sphere and a more general condition with fewer symmetries and complex deformation of eccentricity, subsidence, and offset are first modeled in this paper, which could tune the mechanical frequency in a much wider spectral range than the pillar-diameter-induced perturbation. we also show that the mechanical quality factors for the non-whispering-gallery mechanical mode could be increased in the order of 4 magnitudes at a specific subsidence, and form a mechanical bound state in the continuum (BIC) which is induced by the symmetry breaking and reveals new mechanisms to confine radiation. A much broader BIC window width with higher mechanical quality factor could be achieved, which is of great importance in both fundamental research and scientific applications.

Formation of Bound States in the Continuum in Hybrid Plasmonic-Photonic Systems

A bound state in the continuum (BIC) is a localized state of an open structure with access to radiation channels, yet it remains highly confined with, in theory, an infinite lifetime and quality factor. There have been many realizations of such exceptional states in dielectric systems without loss. However, realizing BICs in lossy systems such as those in plasmonics remains a challenge. In this Letter, we explore the possibility of realizing BICs in a hybrid plasmonic-photonic structure consisting of a plasmonic grating coupled to a dielectric optical waveguide with diverging radiative quality factors. The plasmonic-photonic system supports two distinct groups of BICs: symmetry-protected BICs at the Γ point and off-Γ Friedrich-Wintgen BICs. The photonic waveguide modes are strongly coupled to the gap plasmons in the grating, leading to an avoided crossing behavior with a high value of Rabi splitting of 150 meV. Moreover, we show that the strong coupling significantly alters the band diagram of the hybrid system, revealing opportunities for supporting stopped light at an off-Γ wide angular span.

All-Dielectric Surface-Enhanced Infrared Absorption-Based Gas Sensor Using Guided Resonance

The surface-enhanced infrared absorption (SEIRA) technique has been focusing on the metallic resonator structures for decades, exploring different approaches to enhance sensitivity. Although the high enhancement is achieved, the dissipative loss and strong heating are the intrinsic drawbacks of metals. Recently, the dielectric platform has emerged as a promising alternative. In this work, we report a guided resonance-based all-dielectric photonic crystal slab as the platform for SEIRA. The guided resonance-induced enhancement in the effective path length and electric field, together with gas enrichment polymer coating, leads to a detection limit of 20 ppm in carbon dioxide (CO₂) sensing. This work explores the feasibility to apply low loss all-dielectric structures as a surface enhancement method in the transmission mode.

Bound states in the continuum on periodic structures: perturbation theory and robustness

On periodic structures, a bound state in the continuum (BIC) is a standing or propagating Bloch wave with a frequency in the radiation continuum. Some BICs (e.g., antisymmetric standing waves) are symmetry protected, since they have incompatible symmetry with outgoing waves in the radiation channels. The propagating BICs do not have this symmetry mismatch, but they still crucially depend on the symmetry of the structure. In this Letter, a perturbation theory is developed for propagating BICs on two-dimensional periodic structures. The Letter shows that these BICs are robust against structural perturbations that preserve the symmetry, indicating that these BICs, in fact, are implicitly protected by symmetry.

Hyperbolic-polaritons-enabled dark-field lens for sensitive detection

Sensitive detection of features in a nanostructure may sometimes be puzzled in the presence of significant background noise. In this regard, background suppression and super-resolution are substantively important for detecting weakly scattering nanoscale features. Here, we present a lens design, termed hyperbolic-polaritons-enabled dark-field lens (HPEDL), which has the ability to accomplish straightforward sensitive detection. This HPEDL structure consists of type I and type II hyperbolic media that support high-k field waves via hyperbolic polaritons (HPs). We show that the cone-like characteristics of the HPs could be manipulated while the influence of the low-k field waves would be removed. Numerical simulations demonstrate that this proposed structure can successfully realize straightforward sensitive detection by modifying its thickness under the phase compensation condition. Besides, the minimum resolvable length and angular-dependent performance for sensitive detection are also demonstrated by simulations. Remarkably, these findings are very promising for propelling nanophotonics technologies and constitute a further important step towards practical applications of optical microscopy.

Light enhancement by quasi-bound states in the continuum in dielectric arrays

The article reports on light enhancement by structural resonances in linear periodic arrays of identical dielectric elements. As the basic elements both subwavelength spheres and rods with circular cross section have been considered. In either case it has been demonstrated numerically that high-Q structural resonant modes originated from bound states in the continuum enable near-field amplitude enhancement by factor of 10-25 in the red-to-near infrared range in lossy silicon. The asymptotic behavior of the Q-factor with the number of elements in the array is explained theoretically by analyzing quasi-bound states propagation bands.

Tunability and Sensing Properties of Plasmonic/1D Photonic Crystal

Gold/one-dimensional photonic crystal (Au/1D-PC) is fabricated and applied for sensitive sensing of glucose and different chemical molecules of various refractive indices. The Au layer thickness is optimized to produce surface plasmon resonance (SPR) at the right edge of the photonic band gap (PBG). As the Au deposition time increased to 60 sec, the PBG width is increased from 46 to 86 nm in correlation with the behavior of the SPR. The selectivity of the optimized Au/1D-PC sensor is tested upon the increase of the environmental refractive index of the detected molecules. The resonance wavelength and the PBG edges increased linearly and the transmitted intensity increased nonlinearly as the environment refractive index increased. The SPR splits to two modes during the detection of chloroform molecules based on the localized capacitive coupling of Au particles. Also, this structure shows high sensitivity at different glucose concentrations. The PBG and SPR are shifted to longer wavelengths, and PBG width is decreased linearly with a rate of 16.04 Å/(μg/mm³) as the glucose concentration increased. The proposed structure merits; operation at room temperature, compact size, and easy fabrication; suggest that the proposed structure can be efficiently used for the biomedical and chemical application.