Chiral polaritons in semiconductor perovskite metasurface enhanced by bound states in the continuum

The exploration of novel chiral optical platforms holds both fundamental and practical importances,which have shown great promise towards applications in valleytronics,chiral sensing and nanoscopic chiroptics.In this work,we combine two key concepts—chiral bound states in the continuum and exciton polaritons—to showcase a strong chiral response from polaritons.Using the finite element method,we numerically design a CsPbBr_(3)based metasurface that supports intrinsically chiral bound states in the continuum and verify the chirality by calculating the reflection spectrum and eigenpolarization mapping.We further demonstrate chirality-dependent exciton polariton angular dispersion arising from the strong coupling between the chiral BIC and excitons in CsPbBr_(3)by simulating the polariton angle-resolved absorption spectrum.Reciprocity analysis reveals that the polariton photoluminescence in different momentum space locations is selectively enhanced by chiral pumping light.Our results suggest a promising first step towards chiral polaritonics.

Enhancing the Goos-Hänchen shift based on quasi-bound states in the continuum through material asymmetric dielectric compound gratings

Quasi-bound state in the continuum(QBIC)resonance is gradually attracting attention and being applied in Goos-Hänchen(GH)shift enhancement due to its high quality(Q)factor and superior optical confinement.Currently,symmetry-protected QBIC resonance is often achieved by breaking the geometric symmetry,but few cases are achieved by breaking the material symmetry.This paper proposes a dielectric compound grating to achieve a high Q factor and high-reflection symmetry-protectede QBIC resonance based on material asymmetry.Theoretical calculations show that the symmetry-protected QBIC resonance achieved by material asymmetry can significantly increase the GH shift up to-980 times the resonance wavelength,and the maximum GH shift is located at the reflection peak with unity reflectance.This paper provides a theoretical basis for designing and fabricating high-performance GH shift tunable metasurfaces/dielectric gratings in the future.

High-Q resonances governed by the quasi-bound states in the continuum in all-dielectric metasurfaces

The realization of high-Q resonances in a silicon metasurface with various broken-symmetry blocks is reported. Theoretical analysis reveals that the sharp resonances in the metasurfaces originate from symmetry-protected bound in the continuum(BIC) and the magnetic dipole dominates these peculiar states. A smaller size of the defect in the broken-symmetry block gives rise to the resonance with a larger Q factor. Importantly, this relationship can be tuned by changing the structural parameter, resulting from the modulation of the topological configuration of BICs. Consequently, a Q factor of more than 3,000 can be easily achieved by optimizing dimensions of the nanostructure. At this sharp resonance, the intensity of the third harmonic generation signal in the patterned structure can be 368 times larger than that of the flat silicon film. The proposed strategy and underlying theory can open up new avenues to realize ultrasharp resonances, which may promote the development of the potential meta-devices for nonlinearity, lasing action, and sensing.

Hybrid bound states in the continuum in terahertz metasurfaces

Bound states in the continuum(BICs)have exhibited extraordinary properties in photonics for enhanced light-matter interactions that enable appealing applications in nonlinear optics,biosensors,and ultrafast optical switches.The most common strategy to apply BICs in a metasurface is by breaking symmetry of resonators in the uniform array that leaks the otherwise uncoupled mode to free space and exhibits an inverse quadratic relationship between quality factor(Q)and asymmetry.Here,we propose a scheme to further reduce scattering losses and improve the robustness of symmetry-protected BICs by decreasing the radiation density with a hybrid BIC lattice.We observe a significant increase of radiative Q in the hybrid lattice compared to the uniform lattice with a factor larger than 14.6.In the hybrid BIC lattice,modes are transferred toГpoint inherited from high symmetric X,Y,and M points in the Brillouin zone that reveal as multiple Fano resonances in the far field and would find applications in hyperspectral sensing.This work initiates a novel and generalized path toward reducing scattering losses and improving the robustness of BICs in terms of lattice engineering that would release the rigid requirements of fabrication accuracy and benefit applications of photonics and optoelectronic devices.

Bound states in the continuum on perfect conducting reflection gratings

Bound states can be supported on the surface of a periodically corrugated perfect conductor known as spoof surface plasmon polaritons with their dispersion curves reside below the light line.Here we show that bound states in the continuum(BICs)can also be achieved in such systems.Two types of grating structures are proposed to suppress the radiation leakage and hence generate bound states.The first one is a simple grating with broad grooves in which multiple cavity modes are accommodated.Due to the symmetry incompatibility and the destructive interaction mainly from the TM_(0)and TM_(1)modes,BICs at theΓpoint and at off-Γpoints are both realized.The second one is a dimerized grating with two grooves in each unit cell.The destructive interaction between the modes in the two grooves can suppresses the radiation and BICs at theΓpoint are observed.The Q factors of the whole bands can be further tuned by the dimerization strength effectively.This work may offer new opportunity for the applications of metallic grating in the low frequency bands.

Fundamentals and applications of photonic waveguides with bound states in the continuum

Photonic waveguides are the most fundamental element for photonic integrated circuits(PICs).Waveguide properties,such as propagation loss,modal areas,nonlinear coefficients,etc.,directly determine the functionalities and performance of PICs.Recently,the emerging waveguides with bound states in the continuum(BICs)have opened new opportunities for PICs because of their special properties in resonance and radiation.Here,we review the recent progress of PICs composed of waveguides with BICs.First,fundamentals including background physics and design rules of a BIC-based waveguide will be introduced.Next,two types of BIC-based waveguide structures,including shallowly etched dielectric and hybrid waveguides,will be presented.Lastly,the challenges and opportunities of PICs with BICs will be discussed.

Dual bound states in the continuum enhanced second harmonic generation with transition metal dichalcogenides monolayer

The emergence of two dimensional(2D)materials has opened new possibilities for exhibiting second harmonic genera-tion(SHG)at the nanoscale,due to their remarkable optical response related to stable excitons at room temperature.However,the ultimate atomic-scale interaction length with light makes the SHG of Transition Metal Dichalcogenides(TM-Ds)monolayers naturally weak.Here,we propose coupling a monolayer of TMDs with a photonic grating slab that works with doubly resonant bound states in the continuum(BIC).The BIC slabs are designed to exhibit a pair of BICs,reson-ant with both the fundamental wave(FW)and the second harmonic wave(SHW).Firstly,the spatial mode matching can be fulfilled by tilting FW's incident angle.We theoretically demonstrate that this strategy leads to more than four orders of magnitude enhancement of SHG efficiency than a sole monolayer of TMDs,under a pump light intensity of 0.1 GW/cm^(2).Moreover,we demonstrate that patterning the TMDs monolayer can further enhance the spatial overlap coefficient,which leads to an extra three orders of magnitude enhancement of SHG efficiency.These results demonstrate remarkable pos-sibilities for enhancing SHG with nonlinear 2D materials,opening many opportunities for chip-based light sources,nano-lasers,imaging,and biochemical sensing.

Bound states in the continuum in metal-dielectric photonic crystal with a birefringent defect

By using the difference of the band structure for the TE and TM waves in the metal-dielectric photonic crystals beyond the light cone and the birefringence of the anisotropic crystal,a one-dimensional photonic system is constructed to realize the bound states in the continuum(BICs).In addition to the BICs arising from the polarization incompatibility,the Friedrich-Wintgen BICs are also achieved when the leaking TM wave is eliminated due to the destructive interference of its ordinary and extraordinary wave components in the anisotropic crystal.A modified scheme favorable for practical application is also proposed.This scheme for BICs may help to suppress the radiation loss in the metal-dielectric photonic crystal systems.

Strong light–matter coupling between excitons and chiral quasi-bound states in the continuum in van der Waals metasurfaces

The study of strong coupling between photonic cavities and excitons has brought about significant advances,varying from fundamental physics to applied science.However,there are several challenges hindering its further development,including obtaining photonic modes with both low room-temperature loss and high electric field(EF)enhancements,the difficulty of precisely transferring exciton materials into the photonic cavity,and the urgent need for additional manipulation approaches.In order to overcome these challenges simultaneously,we present a theoretical strong coupling system based on the chiral metasurfaces that are built by the excitonic van der Waals material of WSe_(2)and can support the quasi-bound states in the continuum(q-BIC)mode.The q-BIC mode can sustain EF enhancements over 80 times with loss smaller than10 meV,and the strong coupling between q-BIC mode and WSe_(2)excitons can be naturally realized without material transferring.Furthermore,a large chirality beyond 0.98 can be obtained in this strong coupling system,making the circular polarization of excitation light an effective parameter to control the generation of coherent states in this metasurface system.Our results can benefit the further development of strong coupling research,shedding light onto the exploration of new quantum devices.

Inverse design of quasi-bound states in the continuum metasurface for the polarization independent enhancement of Goos-H?nchen shift

Bound states in the continuum(BIC)have been widely researched and applied in optics due to their unique electromagnetic response.However,there are still difficulties in predicting and customizing BIC spectra.To address this issue,we design an efficient combined neural network for highly accurate prediction of quasi-bound states in the continuum(q-BIC)spectrum,as well as for the inverse design of the polarization independent enhancement of the Goos-H?nchen(GH)shift.Firstly,we propose a C_(4)symmetric metasurface for achieving q-BIC spectrum and providing the condition of enhanced GH shift.By employing a combined neural network,the intensity,position,shape,and phase of q-BIC spectrum with ultra-narrow resonance can be accurately predicted and on-demand customized,even under a small dataset.Besides,we develop a screening algorithm for the q-BIC spectrum to quickly realize the polarization independent enhancement of GH shift.As an application,an ultra-high sensitivity refractive index sensor has been proposed,whose sensitivity can reach 2.31×10~7μm/RIU for TM polarization and 1.03×10~6μm/RIU for TE polarization.Therefore,this work brings new solutions for quick prediction of q-BIC spectrum and the development of flexible polarization photonic devices.

Interface state-based bound states in continuum and below-continuum-resonance modes with high-Q factors in the rotational periodic system

We have introduced a new approach to calculate the orbital angular momentum(OAM)of bound states in continuum(BICs)and below-continuum-resonance(BCR)modes in the rotational periodic system nested inside and outside by transforming the Bloch wave number from the translational periodic system.We extensively classify and study these BICs and BCR modes,which exhibit high-quality(high-Q)factors,in different regions relative to the interface of the system.These BICs and BCR modes with a high-Q factor have been studied in detail based on distinctive structural parameters and scattering theory.The outcomes of this research break the periodic limitation of interface state-based BICs,and realize more and higher symmetry interface state-based BICs and BCR modes.Moreover,we can control the region where light is captured by adjusting the frequency,and show that the Q factor of BICs is more closely related to the ordinal number of rings and the rotational symmetry number of the system.

All-dielectric terahertz metasurface governed by bound states in the continuum with high-Q factor

The method of terahertz(THz)resonance with a high-quality(high-Q)factor offers a vital physical mechanism for metasurface sensors and other high-Q factor applications.However,it is challenging to excite the resonance with a high-Q factor in metasurfaces with proper sensitivity as well as figure of merit(FOM)values.Here,an all-dielectric metasurface composed of two asymmetrical rectangular blocks is suggested.Quartz and silicon are the materials applied for the substrate and cuboids respectively.The distinct resonance governed by bound states in the continuum(BIC)is excited by forming an asymmetric cluster by a novel hybrid method of cutting and moving the cuboids.The investigation focuses on analyzing the transmission spectra of the metasurface under different variations in structural parameters and the loss of silicon refractive index.When the proposed defective metasurface serves as a transmittance sensor,it shows a Q factor of 1.08×10^(4)and achieves an FOM up to 4.8×10^(6),which is obtained under the asymmetric parameter equalling 1μm.Simultaneously,the proposed defective metasurface is sensitive to small changes in refractive index.When the thickness of the analyte is 180μm,the sensitivity reaches a maximum value of 578 GHz/RIU.Hence,the proposed defective metasurface exhibits an extensive number of possible applications in the filters,biomedical diagnosis,security screening,and so on.

Optical bound states in the continuum in periodic structures:mechanisms,effects,and applications

Optical bound states in the continuum(BICs)have recently stimulated a research boom,accompanied by demonstrations of abundant exotic phenomena and applications.With ultrahigh quality(Q)factors,optical BICs have powerful abilities to trap light in optical structures from the continuum of propagation waves in free space.Besides the high Q factors enabled by the confined properties,many hidden topological characteristics were discovered in optical BICs.Especially in periodic structures with well-defined wave vectors,optical BICs were discovered to carry topological charges in momentum space,underlying many unique physical properties.Both high Q factors and topological vortex configurations in momentum space enabled by BICs bring new degrees of freedom to modulate light.BICs have enabled many novel discoveries in light-matter interactions and spin-orbit interactions of light,and BIC applications in lasing and sensing have also been well explored with many advantages.In this paper,we review recent developments of optical BICs in periodic structures,including the physical mechanisms of BICs,explored effects enabled by BICs,and applications of BICs.In the outlook part,we provide a perspective on future developments for BICs.

Enhanced sum-frequency generation from etchless lithium niobate empowered by dual quasi-bound states in the continuum

The miniaturization of nonlinear light sources is central to the integrated photonic platform,driving a quest for high-efficiency frequency generation and mixing at the nanoscale.In this quest,the high-quality(Q)resonant dielectric nanostructures hold great promise,as they enhance nonlinear effects through the resonantly local electromagnetic fields overlapping the chosen nonlinear materials.Here,we propose a method for the enhanced sum-frequency generation(SFG)from etcheless lithium niobate(LiNbO_(3))by utilizing the dual quasi-bound states in the continuum(quasi-BICs)in a one-dimensional resonant grating waveguide structure.Two high-Q guided mode resonances corresponding to the dual quasi-BICs are respectively excited by two near-infrared input beams,generating a strong visible SFG signal with a remarkably high conversion efficiency of 3.66×10^(-2)(five orders of magnitude higher than that of LiNbO_(3)films of the same thickness)and a small full-width at half-maximum less than 0.2 nm.The SFG efficiency can be tuned via adjusting the grating geometry parameter or choosing the input beam polarization combination.Furthermore,the generated SFG signal can be maintained at a fixed wavelength without the appreciable loss of efficiency by selectively exciting the angle-dependent quasi-BICs,even if the wavelengths of input beams are tuned within a broad spectral range.Our results provide a simple but robust paradigm of high-efficiency frequency conversion on an easy-fabricated platform,which may find applications in nonlinear light sources and quantum photonics.

Bound states in the continuum and Fano resonances in the strong mode coupling regime

The study of resonant dielectric nanostructures with a high refractive index is a new research direction in the nanoscale optics and metamaterial-inspired nanophotonics.Because of the unique optically induced electric and magnetic Mie resonances,high-index nanoscale structures are expected to complement or even replace different plasmonic components in a range of potential applications.We study a strong coupling between modes of a single subwavelength high-index dielectric resonator and analyze the mode transformation and Fano resonances when the resonator’s aspect ratio varies.We demonstrate that strong mode coupling results in resonances with high-quality factors,which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich–Wintgen scenario of destructive interference.We explain the physics of these states in terms of multipole decomposition,and show that their appearance is accompanied by a drastic change in the far-field radiation pattern.We reveal a fundamental link between the formation of the high-quality resonances and peculiarities of the Fano parameter in the scattering cross-section spectra.Our theoretical findings are confirmed by microwave experiments for the scattering of high-index cylindrical resonators with a tunable aspect ratio.The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high-quality factors helps to extend substantially functionalities of all-dielectric nanophotonics,which opens horizons for active and passive nanoscale metadevices.

Displacement-mediated bound states in the continuum in all-dielectric superlattice metasurfaces

Bound states in the continuum(BICs)are localized states coexisting with extended waves inside the continuous spectrum range,which have infinite lifetimes without any radiation.To extract high-Q quasi-BIC resonances from the symmetry-protected BIC for practical applications,symmetry-breaking approaches are usually exploited,either by slightly breaking the excitation field symmetry or structure symmetry.Here,we introduce an all-dielectric superlattice metasurface that can symmetrycompatibly convert BIC states into high-Q quasi-BIC modes based on the guidedmode resonance coupling by relative displacement tuning.The metasurface is composed of a superlattice of multiple nanobeams,supporting both magnetic mode and toroidal mode with large tunability.Both modes can interact with the incident continuum by mediating the displacement between nanobeams,which empowers dual asymmetric Fano resonances with high Q-factors.The bandwidth of the toroidal mode under y-polarized incidences and that of the magnetic mode under x-polarized incidences can be readily tuned by the local displacement between nanobeams in each unit cell.Such displacement-mediated BIC resonance is promising for various applications such as bio-molecule sensing and low threshold lasing.

Improved third-order nonlinear effect in graphene based on bound states in the continuum

The scattering matrix theory has been developed to calculate the third-order nonlinear effect in sphere-grapheneslab structures. By designing structural parameters, we have demonstrated that the incident electromagnetic wave can be well confined in the graphene in these structures due to the formation of a bound state in the continuum(BIC) of radiation modes. Based on such a bound state, third-harmonic(TH) generation and four-wave mixing(FWM) have been studied. It is found that the efficiency of TH generation in monolayer graphene can be enhanced about 7 orders of magnitude. It is interesting that we can design structure parameters to make all beams(the pump beam, probe beam, and generated FWM signal) be BICs at the same time. In such a case, the efficiency of FWM in monolayer graphene can be enhanced about 9 orders of magnitude. Both the TH and FWM signals are sensitive to the wavelength, and possess high Q factors, which exhibit very good monochromaticity. By taking suitable BICs, the selective generation of TH and FWM signals for S-and P-polarized waves can also be realized,which is beneficial for the design of optical devices.

Enhanced Fano resonance for high-sensitivity sensing based on bound states in the continuum

Although previously reported terahertz absorbers can achieve high-sensitivity refractive index sensing,the resonant peak is too broad,which leads to a low figure of merit[FOM].Transmissive sensors based on bound states in the continuum[BIC]can achieve high FOM,but they have some limitations in high sensitivity.Herein,we propose a periodic triple parallel metal bars structure to obtain high quality,a strong field,and multiple hot spots by the Friedrich-Wintgen BIC.Numerical results show the sensitivity and FOM can reach 1877 GHz/RIU and 665,respectively.Compared to the previously reported transmissive sensors based on BIC,the sensitivity has been greatly improved.

Unconventional bound states in the continuum from metamaterial-induced electron acoustic waves

Photonic bound states in the continuum(BICs)are spatially localized modes with infinitely long lifetimes,which exist within a radiation continuum at discrete energy levels.These states have been explored in various systems,including photonic and phononic crystal slabs,metasurfaces,waveguides,and integrated circuits.Robustness and availability of the BICs are important aspects for fully taming the BICs toward practical applications.Here,we propose a generic mechanism to realize BICs that exist by first principles free of fine parameter tuning based on non-Maxwellian double-net metamaterials(DNMs).An ideal warm hydrodynamic double plasma(HDP)fluid model provides a homogenized description of DNMs and explains the robustness of the BICs found herein.In the HDP model,these are standing wave formations made of electron acoustic waves(EAWs),which are pure charge oscillations with vanishing electromagnetic fields.EAW BICs have various advantages,such as(i)frequency-comb-like collection of BICs free from normal resonances;(ii)robustness to symmetry-breaking perturbations and formation of quasi-BICs with an ultrahigh Q-factor even if subject to disorder;and(iii)giving rise to subwavelength microcavity resonators hosting quasi-BIC modes with an ultrahigh Q-factor.

Observation of miniaturized bound states in the continuum with ultra-high quality factors

Light trapping is a constant pursuit in photonics because of its importance in science and technology.Many mechanisms have been explored,including the use of mirrors made of materials or structures that forbid outgoing waves,and bound states in the continuum that are mirror-less but based on topology.Here we report a compound method,combining lateral mirrors and bound states in the continuum in a cooperative way,to achieve a class of on-chip optical cavities that have high quality factors and small modal volumes.Specifically,light is trapped in the transverse direction by the photonic band gap of the lateral hetero-structure and confined in the vertical direction by the constellation of multiple bound states in the continuum.As a result,unlike most bound states in the continuum found in photonic crystal slabs that are de-localized Bloch modes,we achieve light-trapping in all three dimensions and experimentally demonstrate quality factors as high as Q=1.09×10^(6)and modal volumes as low as V=17.74(λ_(0)/n)^(3)in the telecommunication regime.We further prove the robustness of our method through the statistical study of multiple fabricated devices.Our work provides a new method of light trapping,which can find potential applications in photonic integration,nonlinear optics and quantum computing.