Polarization-Tunable Lasing from Chiral Quasi-Bound State in the Continuum in Dielectric Metasurfaces

Mid-infrared molecular sensing offers molecule-specific vibrational fingerprints, yet practical implementation with nanophotonic platforms is challenged by system complexity and strong signal damping in aqueous environments. Dielectric metasurfaces overcome ohmic losses and local heating constraints of metallic resonators and can support high-quality-factor resonances with spectral selectivity, suitable for image-based spectrometer-less sensing. However, their spatially extended near-fields typically render them susceptible to environmental absorption, preventing operation in water. We demonstrate a compact dielectric perfect-absorber metasurface combining C4-symmetric quasi-bound states in the continuum (qBICs) with a dual-gradient architecture. The C4-symmetric unit cells ensure polarization-independent resonances, enabling efficient utilization of incident light under arbitrary polarization states. The gradient architecture independently controls radiative loss and resonance wavelength, allowing distinct coupling regimes within one metasurface while achieving high absorbance (>0.8). We demonstrate poly(methyl methacrylate) sensing in air with ~20% absorbance envelope modulation under arbitrary polarization. Furthermore, we introduce a sensing configuration utilizing a 700 nm residual thin-water film that preserves qBIC (absorbance ~0.5) near the prominent water absorption peak. This enables the first demonstration of dielectric metasurface-based mid-infrared molecular sensing under a water background, achieving >30% absorbance envelope modulation. This platform extends the utility of dielectric metasurfaces to aqueous environments and supports versatile, spectrometer-less sensing schemes.

Dielectric metasurfaces can achieve strong light-matter interaction based on several types of collective (nonlocal) resonances, such as surface lattice resonances (SLRs) and quasi-bound states in the continuum (quasi-BICs). Spectral selectivity, field enhancement, and high and controllable Q-factors make these resonances appealing for technological applications in lasing, sensing, nonlinear optics, and quantum photon sources. An emerging challenge focuses on tailoring light-matter interaction via mode coupling and hybridisation between the fundamental resonances of a metasurface. While strong coupling phenomena have been demonstrated between various resonant modes, the interplay between collective resonances of different natures has not been observed to date. Here, we theoretically, numerically, and experimentally demonstrate the onset of coupling and hybridisation between symmetry-protected quasi-BICs and SLRs in a dielectric metasurface. We show the emergence of anticrossing (or Rabi splitting) in the strong coupling regime with suppression of reflection, observed under TE-polarised excitation, and the manifestation of an accidental BIC under TM-polarised illumination as a result of energy exchange between the participating collective resonances in the weak coupling regime. The first effect is accompanied by hybridised near fields of the modes. The observed coupling mechanisms can be controlled by modifying the angle of incidence, polarisation, and the surrounding environment. This foundational study on the coupling and hybridisation of collective resonances offers insights that can be leveraged for the design of metasurfaces with targeted quasi-aBIC and collective hybridised resonances. It could also open new possibilities to control the near fields associated with such resonances, with promising applications in tunable nanophotonics and light manipulation.

We use the semianalytical Cartesian multipole method to investigate the light transmission and reflection spectra of anisotropic dielectric metasurfaces, and extend the multipole decompositions method to account for cross-polarization conversion effects. We observe sharp high-Q resonances arising from a distortion of symmetry-protected bound states in the continuum in asymmetric dielectric metasurfaces, i.e., quasibound states in the continuum. In addition, by further introducing in-plane symmetric breaking perturbation, the polarization conversions of linearly polarized light can be achieved through quasibound states in the continuum. With the aid of temporal coupled-mode theory, we can obtain the limit of cross-conversion under single high-Q resonance, i.e., 0.25. Our work will help to design dielectric metasurfaces to control the polarization states of light.

Among the transitional metal dichalcogenides (TMDCs), molybdenum disulfide (MoS2) is considered an outstanding candidate for biosensing applications due to its high absorptivity and amenability to ionic current measurements. Dielectric metasurfaces have also emerged as a powerful platform for novel optical biosensing due to their low optical losses and strong near-field enhancements. Once functionalized with TMDCs, dielectric metasurfaces can also provide strong photon–exciton interactions. Here, we theoretically integrated a single layer of MoS2 into a CMOS-compatible asymmetric dielectric metasurface composed of TiO2 meta-atoms with a broken in-plane inversion symmetry on an SiO2 substrate. We numerically show that the designed MoS2-integrated metasurface can function as a high-figure-of-merit () van der Waals-based biosensor due to the support of quasi-bound states in the continuum. Moreover, owing to the critical coupling of the magnetic dipole resonances of the metasurface and the A exciton of the single layer of MoS2, one can achieve a enhanced excitonic absorption by this two-port system. Therefore, the proposed design can function as an effective biosensor and is also practical for enhanced excitonic absorption and emission applications.

Tunable multiband circular dichroism and asymmetric transmission enabled by chiral quasi-BICs in dielectric metasurfaces covered with graphene

The realization of flexible tuning and enhanced chiral responses is vital for many applications in nanophotonics. This study proposes to manipulate the collective optical responses with heterostructures consisting of chiral dielectric metasurfaces and achiral J-aggregates. Owing to the resonance coupling between the chiral quasi-bound states in the continuum (QBICs) and the achiral exciton mode, large mode splitting and anticrossing are observed in both the transmission and circular dichroism (CD) spectra, which indicates the formation of hybrid chiral eigenmodes and the realization of the strong coupling regime. Considering that the radiative and dissipative damping of the hybrid eigenmodes depends on the coherent energy exchange, the chiral resonances can be flexibly tuned by adjusting the geometry and optical constants for the heterostructure, and the CD of the three hybrid eigenmodes approach the maximum (∼1) simultaneously when the critical coupling conditions are satisfied, which can be promising for enhanced chiral light-matter interactions.

Chiral quasi-bound states in the continuum (q-BICs) have recently emerged in metaphotonics as resonances that combine ultra-high-quality factors with near-unity circular polarization in the far field. However, these states are typically confined to the Γ-point (normal incidence) due to their symmetry-protected origins. We propose a new mechanism for realizing light-cone-proximal chiral q-BICs at large oblique angles, enabled by the divergence of the radiative density of states near the light cone. Using dielectric metasurfaces with a monoclinic lattice and broken in-plane mirror symmetry, we demonstrate that tuning the lattice angle allows for robust control of these resonances. The resulting chiral q-BICs exhibit near-unity circular dichroism in transmission and fully circularly polarized emission at angles exceeding 50° from normal. Our results establish a general route to off-normal and grazing-angle chiral q-BICs, enabling directional chiral lasing and providing a versatile platform for quantum and nonlinear photonics.

2D materials are promising candidates as nonlinear optical components for on-chip devices due to their ultrathin structure. In general, their nonlinear optical responses are inherently weak due to the short interaction thickness with light. Recently, there has been great interest in using quasi-bound states in the continuum (q-BICs) of dielectric metasurfaces, which are able to achieve remarkable optical near-field enhancement for elevating the second harmonic generation (SHG) emission from 2D materials. However, most studies focus on the design of combining bulk dielectric metasurfaces with unpatterned 2D materials, which suffer considerable radiation loss and limit near-field enhancement by high-quality q-BIC resonances. Here, we investigate the dielectric metasurface evolution from bulk silicon to monolayer molybdenum disulfide (MoS2), and discover the critical role of meta-atom thickness design on enhancing near-field effects of two q-BIC modes. We further introduce the strong-coupling of the two q-BIC modes by oblique incidence manipulation, and enhance the localized optical field on monolayer MoS2 dramatically. In the ultraviolet and visible regions, the MoS2 SHG enhancement factor of our design is 105 times higher than that of conventional bulk metasurfaces, leading to an extremely high nonlinear conversion efficiency of 5.8%. Our research will provide an important theoretical guide for the design of high-performance nonlinear devices based on 2D materials.

We design and analyze a novel all-dielectric meta-surface refractive index sensor that shows a high sensing performance based on the excitation of quasi-bound states in the continuum (quasi-BIC), which enables a high sensitivity and FOM in the midinfrared with remarkable values of 2200 nm/RIU and 7624.1, respectively.

Resonant dielectric nanostructures have achieved significant advancements in the manipulation of light at the nanoscale. Particularly, bound states in the continuum (BICs) based on dielectric metasurfaces have greatly enhanced the intensity of light–matter interaction. However, most BICs in dielectric metasurfaces are fixed in their functionality once they are made. In this study, we present the development of switchable multiple quasi-BICs by combining dielectric nanostructures with vanadium dioxide. The resulting hybrid dielectric metasurface can support three types of BICs with different multipole origins for vanadium dioxide in the insulating phase. By introducing structural asymmetry through width adjustment, one quasi-BIC with a longitudinal toroidal dipole characteristic is excited under x-polarized incidence. Further, tuning the width allows for the generation of two additional quasi-BICs with distinct electromagnetic sources under y-polarized incidence. Additionally, the hybrid dielectric metasurface also supports a high-Q transverse toroidal dipole mode. Moreover, all quasi-BICs and toroidal dipole modes can be turned off when vanadium dioxide transitions into the metallic phase. The switchable multiple quasi-BICs hold promise for applications in optical modulators, tunable harmonic generation, and biosensors.

Quasi-bound states in the continuum provide an effective and observable way to improve metasurface performance, usually with an ultra-high-quality factor. Dielectric metasurfaces dependent on Mie resonances have the characteristic of significantly low loss, and the polarization can be affected by the parameter tuning of the structure. Based on the theory of quasi-bound states in the continuum, we propose and simulate a bifunctional resonant metasurface, whose periodic unit structure consists of four antiparallel and symmetrical amorphous silicon columns embedded in a poly(methyl methacrylate) layer. The metasurface can exhibit an extreme Huygens’ regime in the case of an incident plane wave with linear polarization, while exhibiting chirality in the case of incident circular polarized light. Our structure provides ideas for promoting the multifunctional development of flat optical devices, as well as presenting potential in polarization-dependent fields.

Metasurfaces are a class of two-dimensional artificial resonators, creating new opportunities for strong light-matter interactions. One type of nonradiative optical metasurface that enables substantial light concentration is based on quasi-Bound States in the Continuum (quasi-BIC). Here we report the design and fabrication of a quasi-BIC dielectric metasurface that serves as an optical frequency antenna for photocatalysis. By depositing Ni nanoparticle reactors onto the metasurface, we create an antenna-reactor photocatalyst, where the virtually lossless metasurface funnels light to drive a chemical reaction. This quasi-BIC-Ni antenna-reactor drives H2 dissociation under resonant illumination, showing strong polarization, wavelength, and optical power dependencies. Both E-field-induced electronic and photothermal heating effects drive the reaction, supported by load-dependent reactivity studies and our theoretical model. This study unlocks new opportunities for photocatalysis that employ dielectric metasurfaces for light harvesting in an antenna-reactor format.

Chiral molecules cannot be superposed with their own mirror image. This yields two enantiomers of opposite handedness that are made out of the same building blocks, but interact differently with their environment. Hence, chiral sensing is of utmost importance for biology, chemistry, and life sciences. However, the impact of the handedness and chirality is very weak in most sensing schemes. Nevertheless, it has been demonstrated recently that chirality may result in strong coupling between resonant states with high quality factors. This is achieved by spectrally overlapping two quasibound states in the continuum in a periodic array of nanostructures. We demonstrate that this requires neither quasibound states in the continuum nor periodic arrays, which is exemplified for three achiral systems: a sphere with equal permittivity and permeability, a single core-shell structure, and a dielectric metasurface. For such achiral systems, we have shown previously that isolated resonant states exhibit a quadratic energy shift in the Pasteur parameter. However, for quasidegenerate states, we observe, using the rigorous resonant-state expansion and full-wave simulations, a linear energy shift and linear splitting in the presence of a chiral medium or molecule. Thus, the splitting is more sensitive to low concentrations of chiral molecules, which paves the way for novel chiral sensing schemes. Published by the American Physical Society 2024

It was reported previously that the quality factor of a symmetry-protected quasi-BIC mode increases as the degree of structure asymmetry is reduced. In this work, we propose and investigate an alternative approach to increase the quality factor of a quasi-BIC mode without reducing the degree of asymmetry. Specifically, we calculate the quality factor of the quasi-BIC mode of a double-gap dielectric split-ring metasurface for different split angles. It is found that the quality factor increases exponentially with the increase of the split angles while the degree of asymmetry of the structure is constant. To explain the phenomena, multipole moment decomposition of the local electromagnetic field is conducted to calculate the change of major multipole moments versus the split angles. It is revealed that the double-gap split-ring array structure stores more energy in the higher order multipoles, and the rate of radiation energy loss stays constant when the two splitting angles increase simultaneously. Additionally, the enhancement of third harmonic generation is investigated in the double-gap split-ring metasurface structure.

Metasurfaces with high quality factor resonances can be fabricated by breaking the symmetry of the constituent resonators, thereby converting symmetry-protected dark states to quasi-bound states in the continuum. As the quality factor is inversely proportional to the degree of the asymmetry, high quality factor metasurfaces typically comprise fine-tuned resonators requiring state-of-the-art lithographic technologies to fabricate. In this study, a method is proposed to create quasi-bound states in the continuum in dielectric metasurfaces without high-resolution lithography. The results show that quasi-bound states in the continuum can be formed by introducing a non-vertical sidewall in an array of cube resonators, such that the resonator symmetry breaks in an out-of-plane direction. The proposed method enables fabrication by controlling the dry etching angle without extensive use of high-resolution lithography. Using full-wave electromagnetic simulations and multipole analyses, high quality factor resonances are observed in the proposed metasurface structure, the connections of which to the quasi-bound states in the continuum are clarified. Our work enables fabrication of high quality factor metasurfaces with low cost and high controllability, and is expected to greatly benefit the field of enhanced light-matter interactions in nanophotonics.