Quasibound States Research Articles

Controllable perfect chiral optical response in planar metasurfaces empowered by quasi-bound states in the continuum

Chiral metasurfaces with strong chirality and high quality factors (Q-factors) have become essential components for achieving strong light-matter interactions and have a wide range of applications in chiral lasers, detectors, etc. However, current schemes primarily focus on enhancing the chiral response and Q-factor, with limited consideration of their modulability and flexibility. In this paper, we present a chiral a-Si metasurface that can support multiple symmetry-protected bound states in the continuum (BIC). The perfect extrinsic and intrinsic chiral responses (circular dichroism exceeding 0.99), with ultra-high Q-factors, are achieved by utilizing quasi-BICs induced by illumination symmetry and in-plane symmetry breaking. The circular dichroism value and the transmittance of the two circular polarization states can be arbitrarily controlled by adjusting the structural parameters. Furthermore, the feasibility of achieving dynamic modulation of chiral response is demonstrated based on the a-Si-graphene hybrid metasurface. Our research offers an approach to the design of controllable planar optical chirality, which also provides promising avenues for applications in spin-selective bio-detection, electrically tunable chiral switching, and chiral lasers.

Nonlinear Chiral Metasurfaces Based on Structured van der Waals Materials.

Nonlinear chiral photonics explores the nonlinear response of chiral structures, and it offers a pathway to novel optical functionalities not accessible through linear or achiral systems. Here we present the first application of nanostructured van der Waals materials to nonlinear chiral photonics. We demonstrate the 3 orders of magnitude enhancement of the third-harmonic generation from hBN metasurfaces driven by quasi-bound states in the continuum and accompanied by strong nonlinear circular dichroism at the resonances. This novel platform for chiral metaphotonics can be employed for achieving large circular dichroism combined with high-efficiency harmonic generation in a broad frequency range.

The exact relativistic scalar quasibound states of the dyonic Kerr–Sen black hole: quantized energy, and Hawking radiation

We consider Klein–Gordon equation in the Dyonic Kerr–Sen black hole background, which is the charged rotating axially symmetric solution of the Einstein–Maxwell–Dilaton–Axion theory of gravity. The black hole incorporates electric, magnetic, dilatonic and axionic charges and is constructed in 3+1 dimensional spacetime. We begin our investigations with the construction of the scalar field’s governing equation, i.e., the covariant Klein–Gordon equation. With the help of the ansatz of separation of variables, we successfully separate the polar part, and find the exact solution in terms of Spheroidal Harmonics, while the radial exact solution is obtained in terms of the Confluent Heun function. The quantization of the quasibound state is done by applying the polynomial condition of the Confluent Heun function that gives rise to discrete complex-valued energy levels for massive scalar fields. The real part is the scalar field relativistic quantized energy, while the imaginary part represents the quasibound states’s decay. We present all of the sixteen possible exact energy solutions for both massive and massless scalars. We also present the investigation the Hawking radiation of the Dyonic Kerr–Sen black hole’s apparent horizon, via the Sigurd–Sannan method by making use of the obtained exact scalar wave functions. The radiation distribution function, and the Hawking temperature are also obtained.

Multimodal ultra-high Q-factor C2 asymmetric all-dielectric metasurface induced by quasi-BIC states

A high Q-factor resonant cavity indicates minimal energy loss at the resonance frequency. In this paper, we propose a four-leaf clover sensor with air pillars based on the all-dielectric metasurface and discuss the C2 asymmetric induced quasi-bound states in the continuum(quasi-BIC). Firstly, we obtain dual Fano resonance by C2 symmetric configuration. Secondly, by introducing C2 asymmetric parameters, leading to a new Fano resonance peak, and the Q-factor exceeds 104. Furthermore, When the incident laser changes from TE mode to TM mode, the original Fano resonance peaks gradually disappear, and a new peak is generated near the 1273 nm. The calculated polarization sensitivity is up to 0.012 per degree. The dynamic range (DR) of the peak value of transmission spectrum with polarization is 58 dB. By calculation, we have achieved a maximum sensitivity (S) of 270 nm/RIU for an optical refractive index sensor, with a figure of merit (FOM) value of 2250 RIU-1.

Strong coupling between excitons and chiral quasibound states in the continuum of the bulk WS2 metasurface

In the fields of optics and photonics, the topic of light-matter interactions, particularly strong coupling effects, is a developing area of research. Exciton polariton, a hybridized state brought about by strong coupling, is a hot topic, especially from the standpoint of chiral optics. Under the incidence of right circularly polarized light and left circularly polarized (RCP and LCP) light, we investigate the strong coupling between excitons and quasibound states in the continuum (Q-BICs) resonance in a bulk WS2 metasurface. It is discovered that the Q-BICs are affected obviously on center frequencies and linewidth by structure parameters; while the exciton peaks are impacted insignificantly. Furtherly Q-BIC shows chiral enhancing features. When we take into account both Q-BIC and excitons of the metasurface simultaneously, there is a strong coupling as evidenced by the Rabi splitting up to 182 meV and the clear anti-crossing behavior in the transmittance and reflectance spectra of left and right circularly polarized light, respectively. Notably, a quasi-induced transparency window forms in the circular dichroism (CD) spectrum around the exciton band due to the non-chirality of exciton peaks. This enables the removal of excitons that are not involved in the strong coupling. The bulk WS2 chiral metasurface as a strong coupling system not only provides a way to understand the strong Light-matter interaction, but also generate a potential possibility for realizing the application of chiral optics. Additionally, it can realize the strong chiral coupling with a single self-hybridized element in the structure. Our results have potential implications in the chiral optical field and provide a chiral perspective on the study of strong photon-exciton coupling.

Strong coupling of double excitons with guided mode resonances and quasi-bound states in the continuum in heterogeneous metamaterials.

Heterogeneous metamaterials containing excitonic materials provide an ideal platform for strong exciton-photon coupling. In this Letter, we theoretically demonstrate four strong couplings in a heterogeneous metamaterial consisting of a TiO2 grating standing on a perovskite-WS2-perovskite waveguide layer by tuning the structural sizes. The quasi-bound state in the continuum (qBIC) and the guided mode resonance (GMR) both strongly coupled with the excitons of both perovskite and WS2 under oblique incident illumination, resulting in four large Rabi splittings of 177.32, 187.53, 406.25, and 435.09 meV via a reasonable combination of oscillator strengths of perovskite and WS2. Double strong coupling behaviors are also achieved when the grating period equals 222 nm with an incident light angle of 19.3°. Moreover, double ultrastrong coupling can even be realized by the GMR and qBIC respectively interacting with the exciton of WS2 when its oscillator strength reaches a certain value. Our work paves an effective avenue to realizing strong coupling and even ultrastrong coupling between multiple excitons and multiple optical modes.

Polarization-independent tetramer metastructure with multi-Fano resonances governed by quasi-bound states in the continuum

We propose what we believe to be a novel metastructure consisting of silicon nanoblock tetramer clusters and investigated theoretically and experimentally how to enhance surface sensing capabilities with polarization-independent properties through bound states in the continuum (BICs). By introducing square defects, three quasi-BIC modes are excited at the wavelengths of 1013.29 nm, 1109.57 nm, and 1310.73 nm with a modulation depth up to 100% and a Q-factor of 16618, while achieving a sensitivity of 256 nm/RIU and a figure of merit of 2519.7 RIU−1. Utilizing the finite-difference method in the time domain (FDTD), we have analyzed the electromagnetic near-field distributions, polarization properties, and robustness corresponding to resonance dips at different wavelengths. Our results reveal that the distinct electromagnetic near-field distributions excited by the quasi-BIC modes are characterized by the strong field confinement within the silica nanoblock tetramer clusters. The modes are primarily governed by magnetic quadrupole (MQ) resonance and toroidal dipole (TD) moment. Additionally, the polarization-independent properties of the metastructure are demonstrated to ensure that they can effectively respond to the incident light regardless of its polarization state, while the robustness is investigated to show the stability and reliability by shifting square defects. Moreover, the transmission spectra of the metastructure is experimentally verified by immersing the sample into CaCl2 solutions in different concentrations. This work plays a pivotal role in creating further novel and high-performance optical metastructures while demonstrating the potential applications of quasi-BIC modes in precision sensing in the future.

Harnessing Enantioselective Optical Forces by Quasibound States in the Continuum.

Enantioselective optical forces have garnered significant attention, because they provide a noninvasive means to separate chiral objects. A promising approach to enhance enantioselective optical forces is spatially overlapping and boosting electric and magnetic fields to create giant superchiral fields. Here, we utilize metasurfaces composed of asymmetric silicon dimers that support two distinct quasibound states in the continuum (quasi BICs). By precisely engineering these quasi BICs, we achieve nearly perfect spatial overlap of electric and magnetic fields near their anticrossing point, resulting in a remarkable 10^{4}-fold enhancement of the superchiral field. Consequently, the enantioselective optical force exerting on a single molecule exhibits a substantial increase, with magnitude up to pN/mW μm^{2}. Furthermore, by encircling the anticrossing point, we can switch the handedness of the superchiral field and the enantioselective optical force. Last, we analyze the dynamics of quasi-BIC-assisted chiral separation, highlighting its potential applications in chiral sensing and sorting, circular dichroism spectroscopy, and pharmacology.

Metamaterial-induced-transparency engineering through quasi-bound states in the continuum by using dielectric cross-shaped trimers

This study presents a novel approach to activate a narrowband transparency line within a reflecting broadband window in all-dielectric metasurfaces, in analogy to the electromagnetically-induced transparency effect, by means of a quasi-bound state in the continuum (qBIC). We demonstrate that the resonance overlapping of a bright mode and a qBIC-based nearly-dark mode with distinct Q-factor can be fully governed by a silicon trimer-based unit cell with broken-inversion-symmetry cross shape, thus providing the required response under normal incidence of a linearly-polarized light. Our analysis that is derived from the far-field multipolar decomposition and near-field electromagnetic distributions uncovers the main contributions of different multipoles on the qBIC resonance, with governing magnetic dipole and electric quadrupole terms supplied by distinct parts of the dielectric “molecule”. The findings extracted from this research open up new avenues for the development of polarization-dependent technologies, with particular interest in its capabilities for sensing and biosensing.

Tailoring directional chiral emission from molecules coupled to extrinsic chiral quasi-bound states in the continuum

Tailoring directional chiral emission from molecules coupled to extrinsic chiral quasi-bound states in the continuum

Magneto-optical metasurfaces for high-Q perfect absorption with quasi-bound states in the continuum.

We present a novel, to the best of our knowledge, magneto-optical (MO) metasurface composed of a bismuth iron garnet (BIG) nanocube array, designed to achieve near-perfect absorption through quasi-bound states in the continuum (QBICs). This metasurface supports a stable QBIC mode induced by MO-induced permittivity terms that break the symmetry of the permittivity tensors, corresponding to a longitudinal electric dipole (ED) mode. By integrating graphene to introduce material loss, the absorption reaches 99.6% at a wavelength of 1512.3 nm with a Q factor of 9440, despite monolayer graphene's inherent absorption being only 2.3%. The inherent transverse ED background mode, with high reflection and low Q, helps decrease the radiative loss of the QBIC mode, allowing the structure to surpass the 50% absorption limit. This approach offers a simplified pathway for designing high-Q metasurface perfect absorbers, with potential applications in optical switches and modulators.

Mie-resonant metaphotonics

Mie-resonant metaphotonics is a rapidly developing field that employs the physics of Mie resonances to control light at the nanoscale. Mie resonances are excited in high-refractive-index transparent nanoparticles and voids created in dielectric media, and they can be used to achieve a wide range of optical effects, including enhanced light–matter interaction, nonlinear optical effects, and topological photonics. Here, we review the recent advances in Mie-resonant metaphotonics, with a focus on the physics of Mie resonances and their applications in metaphotonics and metasurfaces. Through a comprehensive multipolar analysis, we demonstrate the complex interplay of electric and magnetic multipoles that govern their interaction with light. Recent advances have unveiled a diverse spectrum of scattering phenomena that can be achieved within precisely engineered structures. Within this framework, we review the underlying mechanics of the first and second Kerker conditions and describe the intricate mechanisms guiding these nanostructures’ light-scattering properties. Moreover, we cover intriguing phenomena such as the anapole and bound or quasi-bound states in the continuum. Of profound interest are the numerous practical applications that result from these revelations. Ultrafast processes, the emergence of nanolasers, and advancements in magneto-optic devices represent just a fraction of the transformative applications.

Second harmonic generation enhancement based on quasi-BICs in centrosymmetric materials

In centrosymmetric optical materials, the second-order nonlinear polarization of the bulk electric dipolar contribution is zero. More effective utilization of the contribution of the surface term is one of the key methods to efficiently obtain second-order nonlinear responses on these materials. Herein, a design of densely packed slotted nanopillar arrays based on quasi-bound states in the continuum (quasi-BICs) is proposed. The quasi-BICs are analyzed by using the finite element method as an example of silicon and the second harmonic generation (SHG) process is simulated. In the structure, normal-incidence linearly polarized light excites magnetic dipole-like quasi-BICs with a high quality factor which effectively promotes light-matter interactions. Increasing the nanopillar radius or decreasing the lattice constant within a certain range can cause the distribution of quality factors in k-space of the ky direction to contract toward the Γ point, which leads to a quasi-BIC with higher quality factors at the Γ point. By conjunctively adjusting the nanopillar radius and lattice constant or changing the slot azimuth, the resonance wavelength can be adjusted over a wide range (about several hundred nanometers) or finely (within about one nanometer) while maintaining high quality factors. When the symmetry perturbation introduced by the slot is small, it is calculated that the SHG conversion efficiency is about 10−6∼10−5 at an incident light power density of 1 MW/m2, and the SHG power is about 107∼108 times enhancement compared with the structure without slots. As the slot width decreases, higher SHG conversion efficiency with more significant SHG enhancement can be achieved at a specific slot length. The results provide new insights into the modulation of the resonant wavelength and quality factor of quasi-BICs, as well as the control of second-order nonlinear effects in centrosymmetric materials.

Active tuning of near-infrared interaction of single-walled carbon nanotubes with quasibound states in the continuum in all-dielectric metasurfaces

Active tuning of near-infrared interaction of single-walled carbon nanotubes with quasibound states in the continuum in all-dielectric metasurfaces

Design of tunable selective light-absorbing metasurfaces driven by intrinsically chiral quasi-bound states in the continuum

Chiral metasurfaces with high quality factors (Q-factors) and strong circular dichroism (CD) are excellent platforms for studying chiral optical response. Here, a design is proposed of an intrinsic chiral silicon metasurface driven by bound states in the continuum (BIC), with ultra-high Q-factor (Q = 3722) and chirality response close to the unit CD (CD > 0.99). By breaking the in-plane and out-of-plane symmetry of the structure, the intrinsic chirality based on BIC can be precisely controlled. In addition to studying intrinsic chirality, we have also achieved extrinsic chirality by obliquely incident circularly polarized light without introducing out-of-plane asymmetry. Moreover, we introduce graphene into the intrinsically chiral metasurface to form a graphene-Si hybridized metasurface. Selective absorption of intensity-controlled right-handed/left-handed circularly polarized light (RCP/LCP) was achieved by actively tuning the Fermi level and out-of-plane tilt angle of the graphene structure based on coupled-mode theory. Our research provides another insight into the application of intrinsic optical chirality, which is expected to be widely used in the fields of optical filters, polarization detectors, and chiral imaging.

Acoustic quasibound states and tachyonic instabilities from binary Bose-Einstein condensates

Acoustic quasibound states and tachyonic instabilities from binary Bose-Einstein condensates

Second Harmonic Generation from exciton-polaritons: Strong coupling between monolayer WS[formula omitted] and multilayer WSe[formula omitted] metasurfaces Quasibound states in the continuum

The second harmonic generation (SHG) from exciton-polaritons, produced by the strong coupling between quantum emitters and cavity photons, can provide a unique insight for controlling nonlinear responses on the nanoscale. Achieving SHG generally depends on phase matching, limited by nonlinear materials and nanostructure fabrication. Here, we demonstrate a novel way to obtain tunable SHG from van der Waals metasurfaces composed of nanostructured multilayer WSe2 strong coupling with monolayer WS2. By breaking symmetry via varying the metasurface unit tilted angle and period, quasibound state in the continuum (q-BIC) mode resonances energy is tuned across the monolayer WS2 exciton resonance energy, enabling anticrossing behavior of strong coupling in both reflection spectrum and SHG spectrum, which means that SHG was successfully observed by strong coupling. Furthermore, the SHG spectrum can be tuned by changing the armchair direction of the monolayer WS2 and pump polarization direction. Moreover, the mechanism of exciton-polaritonic SHG is investigated by a coupled nonlinear oscillator model, which reveals the SHG origin from exciton-polaritons, providing a new rule for fine-tuning the SHG. Our work provides a new strategy and additional freedom in the nonlinear light-matter interaction and coherent control of the SHG in integrated photonic devices.

Ultranarrow and multiband perfect absorbers based on dual quasi-bound states in the continuum in near infrared

High quality factor (Q-factor) and multiband perfect absorbers are highly desired in light-harvesting efficiency related optical devices, yet they are always limited by conventional mechanisms like electromagnetically induced transparency, Tamm or plasmonic resonance. In this paper, through constructing the strong coupling between two guided-mode resonances in subwavelength gratings, we propose an ultranarrow and multiband perfect absorber based the dual-band quasi-bound states in the continuum (quasi-BIC) design in near infrared. Benefiting from its angle-dependent characteristic, the device can be straightforwardly utilized for light-switched lamps and optical sensors. Q-factor 8337 and the modulation depth 98.1% are realized while the insertion loss is only 0.07 dB. Compared with its conventional counterparts, the quasi-BIC based absorber enables an excellent hexa-frequency and ultranarrow asynchronous switch function. The quatary-band perfect absorption below 1 nm bandwidth can be applied to the optical sensor to detect four light signals with weak signal strength. This work provides a theoretical basis for multifunctional absorbers with high Q-factor, high modulation depth, and low insertion loss.

Asymmetric dumbbell dimers simultaneously supporting quasi-bound states in continuum and anapole modes for terahertz biosensing

Abstract Multi-resonant metasurfaces are of great significance in the applications of multi-band nanophotonics. Here, we propose a novel metasurface design scheme for simultaneously supporting quasi-bound states in continuum (QBIC) and other resonant modes, in which QBIC resonance is generated by mirror or rotational symmetry breaking in oligomers while other resonant modes can be simultaneously excited by rationally designing the shapes of meta-atoms within oligomers. As an example, the simultaneous excitation of QBIC and anapole modes are demonstrated in a dimer metasurface composed of asymmetric dumbbell-shaped apertures. Based on the far-field multipole decomposition and near-field electromagnetic field distributions, the origin mechanisms of QBIC and anapole mode are elucidated. The symmetry breaking of dumbbell-shaped dimer results in QBIC. Within a certain asymmetric variation range, the contributions of toroidal dipole moment and electric dipole moment with approximately equal magnitudes remain dominant, which allows the anapole mode to always present. The effectiveness of the proposed design scheme is further confirmed by the experimental results identical with the evolutions of numerical simulation. In terahertz biosensing experiments, the anapole mode exhibits a higher sensitivity of 271.3 GHz (nmol/μl)−1, whereas the QBIC can achieve a lower detection limit of 0.015 nmol/μl and expands the detection range by almost an order of magnitude. Our findings are beneficial to designing multi-resonant metasurfaces with different resonance modes and promote the corresponding applications in the fields of biosensing, lasers, filtering, and nonlinearity.

WITHDRAWN: Fano-modulated chiral metasurface for near-infrared sensing via bound states in the continuum

Here, we report polarization-sensitive chiral metasurface with broken in-plane inverse symmetry to exploit symmetry-protected quasi-bound states in the continuum (quasi-BIC) for highly sensitive and sharply resonant surface interactions enabling refractive index sensing in near infrared region. Chirality is induced by rotating silicon nanofins placed on top of dielectric spacer with a gold backplate, creating a chiral arrangement that results in a strong chiroptical response. Our chiral metasurface offers controllable spectral linewidth and chiral selectivity in dual-wavelength bands where each band can be excited simultaneously or independently by controlling meta-atom geometry offering multispectral index sensing. We report a novel phenomenon of controlling interplay of chirality between two closely positioned wavelength bands to tailor symmetry breaking for a specific resonant mode (chiral) while preserving symmetry for other mode (achiral). Additionally, we report the phase-sensitive evolution of Fano resonance from pure reflectance dip when controlled by dielectric spacer height, and further demonstrated phase-insensitive control of Fano resonance amplitude. We divided Fano resonance into distinct spectral peak and dip to improve light manipulation within the metasurface. Our proposed sensor demonstrates sensitivity and a quality factor of about 435 nm/RIU and 3888, respectively. Furthermore, we compared different phenomenon (chiral selectivity, quasi-BIC, Fano) and sensing parameter values for different metasurface configurations (including single nanofin and absence of a spacer) and observed that the configuration without a spacer achieved the highest sensitivity of approximately 640 nm/RIU.