Quasibound States Research Articles

Metal-assisted miniaturized refractive index sensor based on quasi-bound states in the continuum

Metal-assisted miniaturized refractive index sensor based on quasi-bound states in the continuum

Quasinormal mode expansion in thin elastic plates

Elastic wave manipulation using large arrays of resonators is driving the need for advanced simulation and optimization methods. To address this we introduce and explore a robust framework for wave control: quasinormal modes (QNMs). Specifically, we consider the problem for thin elastic plates, where the Green's function formalism is well known and readily exploited to solve multiple scattering problems. By studying the associated nonlinear eigenvalue problem we derive a dispersive QNM expansion, providing a reduced-order model for efficient forced response computations which reveals physical insight into the resonant mode excitation. Furthermore, we derive eigenvalue sensitivities with respect to resonator parameters and apply a gradient-based optimization to design quasi-bound states in the continuum and position eigenfrequencies precisely in the complex plane. Scattering simulations validate our approach in structures such as graded line arrays and quasicrystals. Drawing on QNM concepts from electromagnetism we demonstrate significant advances in elastic metamaterials, highlighting their potential for tailored wave manipulation. Published by the American Physical Society 2024

Resonance Wavelength Stabilization of Quasi-Bound States in the Continuum Constructed by Symmetry Breaking and Area Compensation.

Dielectric metasurface supported symmetry-protected bound states in the continuum (SP-BIC) provide an important platform for enhancing light-matter interactions. However, the conversion of SP-BIC into quasi-BIC (QBICs) through symmetry breaking is often accompanied by a shift in the resonance wavelength. In this work, we present a generalized viewpoint aimed at achieving the wavelength stability of QBICs through symmetry breaking and area compensation (SBAC). Three SBAC schemes we propose are equal proportional compensation along both x- and y-directions and equal proportional compensation along x-direction only or y-direction only. The QBICs resonance wavelengths stabilized at about 1200, 1520, and 1434 nm are achieved in monomer, dimer, and tetramer metasurfaces, respectively. Finally, we perform an experimental demonstration in a nanohole dimer photonic crystal slab. The QBICs resonance wavelength stabilized at 1658 nm for SBAC. Our approach provides new routes for realizing QBICs with a stable wavelength and tunable Q-factor.

Thermo-optic tuning of quasi bound state in continuum in lithium niobate thin film hetero-nanograting

Lithium niobate (LN) is difficult to etch precisely due to its stable physical and chemical properties. In recent years, more and more research has focused on etchless thin film lithium niobate (TFLN). Here, a one-dimensional SiO2 nanograting structure fabricated on lithium niobate on insulator (LNOI) platform is designed in this study. This nanograting can generate one-dimensional diffraction waves. Then quasi-bound state in the continuum (q-BIC) can be introduced by aligning with the waveguide mode supported by the TFLN, which can achieve a high-quality factor and strong field enhancement, thus improving the interaction between light and matter. Furthermore, we validate the polarization characteristics of the nanograting structure, and measure the thermo-optical tuning sensitivity of the device to be 26.67 pm/K. This finding opens up potential avenues for realizing multi-dimensional tunable and dynamic photonic devices.

Chirality-controlled second-order nonlinear frequency conversion in lithium niobate film metasurfaces.

The high-quality factor resonant metasurfaces have extensive applications in enhancing nonlinear frequency conversion efficiency at the subwavelength scale. However, methods for actively modulating the frequency conversion process are limited. We design a chiral lithium niobate film metasurface and investigate the photonic spin as a new degree of freedom to dynamically control the second-order nonlinear frequency conversion, without reconfiguring the structure by using external stimuli. The chiral resonance with circular dichroism (CD) of 0.62 gives rise to a high nonlinear CD of 0.84 in second-harmonic generation efficiency. Interestingly, combining the chiral resonance and an achiral quasi-bound state in the continuum enables us to investigate the photonic-spin-controlled sum-frequency generation and the photon pair generation from the spontaneous parametric downconversion process. Owing to the ultrahigh quality factor exceeding 103 both for two resonances, the second-order nonlinear frequency conversion occurs at a wavelength region of 0.2 nm, suggesting good monochromaticity. Our work opens new, to our knowledge, avenues for practical implementation of dynamically controlled nonlinear optical devices and will find utility in holography, switchable light sources, and information processing.

High-Q terahertz chirality enhancement using elliptical hole metasurface

Circular dichroism (CD) technology has garnered significant attention due to its extensive applications in ultra-sensitive biosensing and spin-selective optical frequency conversion. However, existing terahertz chiral structures are constrained by linewidth, which limits their effectiveness in narrowband signal processing. In this study, we propose the notion of quasi-bound states in the continuum (quasi-BIC) within a planar elliptical hole all-silicon terahertz metasurface that exhibits broken mirror symmetry. This approach achieves a CD value as high as 0.97, with a linewidth below 0.5 GHz and a Quality (Q)-factor reaching up to 107 in the 1.3 THz to 1.55 THz band, thereby enabling ultra-narrowband terahertz chirality. This method significantly enhances the Q-factor of optical resonant systems, reduces linewidth, and achieves strong CD while addressing the trade-off between high Q-factor and high CD observed in existing structures. The theoretical foundations for achieving ultra-narrow linewidth are established through band structure calculations and far-field polarization analysis. Additionally, the Q-factor of quasi-BIC can be flexibly optimized through parameter tuning, rendering it more practical than perfect BIC in real-world applications. This study presents a novel solution for terahertz narrowband chirality and optical filters, potentially advancing technologies in related fields.© 2024 Optica Publishing Group under the terms of the Optica Publishing Group Open Access Publishing Agreement.

Formation of quasi-bound states in the continuum in a single deformed microcavity

Bound states in the continuum (BICs) hold significant promise in manipulating electromagnetic fields and reducing losses in optical structures, leading to advancements in fundamental research and practical applications. Despite their observation in various optical systems, the behavior of BIC in whispering-gallery-modes (WGMs) optical microcavities, essential components of photonic integrated chips, has yet to be thoroughly explored. In this study, we propose and experimentally identify a robust mechanism for generating quasi-BIC in a single deformed microcavity. By introducing boundary deformations, we construct stable unidirectional radiation channels as leaking continuum shared by different resonant modes and experimentally verify their external strong mode coupling. This results in drastically suppressed leaking loss of one originally long-lived resonance, manifested as more than a threefold enhancement of its quality (Q) factor, while the other short-lived resonance becomes more lossy, demonstrating the formation of Friedrich–Wintgen quasi-BICs as corroborated by the theoretical model and experimental data. This research will provide a practical approach to enhance the Q-factor of optical microcavities, opening up potential applications in the area of deformed microcavities, nonlinear optics, quantum optics, and integrated photonics.

High-Q triple-mode quasi-bound states in the continuum in an asymmetric dielectric metamaterial

Bound states in the continuum (BICs) in metamaterials have attracted much attention due to their high efficiency in field enhancing. In this paper, we numerically demonstrated the triple-mode symmetry-protected BICs (SP-BICs) in an all-dielectric microtube metamaterial. By moving the hollows transversely and longitudinally in microtubes, three SP-BICs are transformed into high quality-factor (Q-factor) quasi-BICs (q-BICs), which are supported by asymmetric magnetic toroidal dipole electric hexapolar resonances respectively. The maximum Q-factor of q-BICs enabled by asymmetric magnetic toroidal dipole reaches 67647. This high Q-factor triple-mode q-BICs in all-dielectric microtube metamaterial has great potential in the applications of sensing, laser generation, and nonlinear optics.

Quasi-bound states in the continuum with stable dual-resonance wavelengths in mid-infrared metasurface for ultrasensitive refractive index sensing and molecular vibrations of strong coupling

In this paper, a dual-resonances mid-infrared all-dielectric metasurface sensor based on asymmetric cross dimer, which is driven by quasi-bound states in the continuum (QBIC), is proposed and investigated. The metasurface sensor maintains the total permittivity constant when the asymmetric parameter is adjusted, thereby ensuring the stability of the QBIC resonance wavelengths, which exhibit Q-factors of 6351 and 13561, respectively. The multiple decompositions and electromagnetic field distributions reveal that the toroidal dipole is the dominant component of the dual-resonance modes. The sensitivities to the refractive index are 3559 nm/RIU and 1146 nm/RIU, with corresponding figures of merit of 4449 RIU−1 and 2453 RIU−1, respectively. Further numerical simulations have demonstrated a strong coupling phenomenon between the QBIC and the molecular vibrations of polymethyl methacrylate (PMMA), resulting in a significant enhancement of the infrared absorption signal. With the 50-nm-thick PMMA layer, the enhancement of molecular signal is 90.614%.

Rotation-Induced Plasmonic Chiral Quasi-Bound States in the Continuum

Rotation-Induced Plasmonic Chiral Quasi-Bound States in the Continuum

High-sensitivity multi-gas detection using dual-ridge metasurface emitters with polarization-distinguishable emission spectra

The metasurface thermal emitter offers an energy-efficient, compact, and sensitive solution as a radiation source for non-contact gas detection, enabling the “molecular fingerprint” technique to be widely applied, from medical diagnostics to environmental monitoring. However, most narrowband emitters are designed for a single target gas, hindering the miniaturization of multi-gas detection systems. In this work, a one-dimensional dual-ridge grating emitter is employed, achieving dual-band and tri-band polarization-distinguishable emission spectra through the excitation of Fabry-Perot (FP) resonances and quasi-bound states in the continuum (qBICs). These emission spectra can be readily matched to multiple non-overlapping absorption peaks of gases such as CH4, CO2, CO, NO, and NH3 within the 3–6 µm range, thereby reducing the impact of mixed gases on measurements. Compared to conventional metal-dielectric-metal structures, the use of a single metal layer results in lower material losses, enabling higher Q-factors and more pronounced directional radiation intensity variations. Furthermore, adjusting the asymmetry to modulate the qBIC-excited absorption peaks does not affect the Q-factor of the FP resonance absorption, thus achieving high-sensitivity multi-band gas detection. This work provides a promising approach for the miniaturization and integration of multi-gas channel detection, facilitating more accurate and sensitive sensing strategies.

Quasi-bound state in the continuum enhancing background limited infrared detectivity

Quasi-bound states in the continuum (quasi-BIC) have tunable high Q factors and thus have many potential applications in optoelectronics. Here, we propose to construct a quasi-BIC with a perturbated elliptical-cylinder photonic crystal slab, and then develop a new way based on the quasi-BIC to address the background noise problem of infrared detectors. By integrating the photonic crystal slab with a quantum well infrared photodetector (QWIP), and by pushing the quasi-BIC system with absorption to the critical coupling state, an ultra-narrow photoresponse band (Q factor equal to 1150) with a high peak quantum efficiency (over 60%) is achieved in the long-wavelength infrared range. The critical coupling state is realized by matching the radiation Q factor, which is mainly controlled by the quasi-BIC, to the absorption Q factor, which is mainly controlled by the doping of the quantum wells (QWs). This device rejects most background radiation and thus significantly suppresses the background noise, resulting in a background limited specific detectivity (DBLIP∗) 11.85 times that of a conventional ideal photoconductor. A detailed formula is employed for calculation of DBLIP∗, incorporating the wavelength and incident angle dependence of the quantum efficiency. The ultra-narrow photoresponse band with a high peak responsivity can be tuned over the photosensitive wavelength range of the QWs by simply modifying the in-plane structural parameters. This work opens a new avenue to greatly enhance the specific detectivity for infrared detectors based on the joint effect of quasi-BIC and critical coupling.

Terahertz laser-fabricated all-polymer hole arrays with high-quality quasi-bound states in the continuum

Abstract Dielectric metasurfaces promise to realize ultrahigh-quality (Q) resonances due to their ultralow material absorption. Most of them are silicon-based metasurfaces, requiring complex fabricated steps and thus suffering high costs. Laser etching processing has simple steps accompanied by low time consumption and exemplary processing efficiency. Here, an all-polymer metasurface based on hole arrays fabricated by laser processing has been proposed and investigated. Such metasurfaces achieve sharp quasi-bound states in the continuum (quasi-BICs) via breaking structural symmetry, form two annular circulation electric fields in different directions, and thus allow strong coupling between holes. Owing to the low refractive index of polymer, the calculated Q-factor reaches 9555 while the diameter discrepancy is 4 μm. Simulated results proved that the Q-factors of quasi-BICs can be further improved by reducing the film thickness and refractive indices of materials, which can be predicted by the fitting equation. Also, the fields in holes can be enhanced by reducing the film refractive index. These results in simulations and experiments provide an alternative method for designing high-Q resonators in terahertz regions.

Chiral quasi-bound states in the continuum for refractive-index sensing in metasurfaces

Chiral quasi-bound states in the continuum for refractive-index sensing in metasurfaces

Tunable lithium niobate metasurfaces for phase-only modulation based on quasi-bound states in the continuum

Tunable lithium niobate metasurfaces for phase-only modulation based on quasi-bound states in the continuum

Graphene nanoribbon resonant tunneling diode with dual connection between contacts

In this paper, we numerically study the transport properties of a resonant tunneling diode (RTD) based on graphene nanoribbon (GNR) with an H-type antidote between the contacts. The structure may also be thought of as having two parallel (W-shape) parts connecting the contacts, each having a wider channel region sandwiched between two narrower barrier regions. The energy at which quasi-bound states occur in each part depends on the dimensional parameters of the respective portion in the structure. We study how the transmission through quasi-bound states is influenced by the edge states on the contacts and dimensional parameters such as barrier length and also by the ambient temperature. The results are compared with those of an RTD with a single part connecting the contacts. Transmission peaks at different energies are observed for an RTD with asymmetrical lower and upper parts between the contacts. This is then utilized for the creation of two negative differential resistance (NDR) peaks. For numerical computation, the non-equilibrium Green’s function formalism (NEGF) based on the nearest neighbor tight-binding model is employed.

Relativistic artificial molecule of two coupled graphene quantum dots at tunable distances

In a molecule formed by two atoms, energy difference between bonding and antibonding orbitals depends on distance between the two atoms. However, exploring molecular orbitals of two natural atoms with tunable distance has remained an outstanding experimental challenge. Graphene quantum dots can be viewed as relativistic artificial atoms, thus offering a unique platform to study molecular physics. Here, through scanning tunneling microscope, we create and directly visualize the formation process of relativistic artificial molecules based on two coupled graphene quantum dots with tunable distance. Our study indicates that energy difference between the bonding and antibonding orbitals of the lowest quasibound state increases linearly with inverse distance between the two graphene quantum dots due to the relativistic nature of the artificial molecule. For quasibound states with higher orbital momenta, the coupling between these states leads to half-energy spacing of the confined states because the length of the molecular-like orbit is approximately twice that of the atomic-like orbit. Evolution from ring-like whispering-gallery modes in the artificial atoms to figure-eight orbitals in the artificial molecules is directly imaged. The ability to resolve the coupling and orbitals of the relativistic artificial molecule at the nanoscale level yields insights into the behavior of quantum-relativistic matter.

A metasurface for linear-to-circular polarization conversion and sensing based on quasi-BIC

This work presents a multifunctional metastructure (MS) which realizes linear to circular polarization conversion and sensing function based on quasi-bound states in the continuum (quasi-BIC). MS is made of silicon dioxide as substrate, and silicon as surface material, by etching cross holes and square holes on it to form a 2 × 2 structure, through the transmission of terahertz (THz) band, to form an ultrahigh quality factor (Q-factor), and realize the conversion of linearly polarized waves to circularly polarized ones. At 178.190 THz, it achieves a Q value of 2969, and in the range 178.193 TH to 178.200 THz, the axial ratio (AR) is less than 3 dB and the insertion loss is less than 0.0001. In addition, by changing the permittivity of the surrounding environment, the minimum of the output wave will produce a good linear frequency shift. Using this feature, the given device can also be used as a dielectric constant sensor to detect air quality. The device has a sensing sensitivity (S) of 6.415 THz RIU−1 and a figure of merit (FOM) of 106.9. The parameters (H, w2, L2, g2), incidence angle (θ) and the polarization angle (φ) are discussed. The effects of different parameters on the Q-factor and AR were analyzed, which helped to select the optimal parameters. The design can also be used in communication and biosensing.

Giant magneto-optical Kerr effects governed by the quasi-bound states in the continuum

The magneto-optical Kerr effect (MOKE), as one of the magneto-optical effects, exhibits polarization change upon reflection that can be used to explore the internal information of magnetic materials with broad applications in modern information technology. However, typically, MOKE is quite weak due to the lower magneto-optical interaction. To tremendously enhance the MOKE, quasi-bound states in the continuum in a one-dimensional Ce- doped Y3Fe5O12 (CeYIG) film photonic crystal slabs (PCS) are proposed to improve the magneto-optical interaction in this work. A giant enhancement in the rotation angle and ellipticity of the longitudinal MOKE, which is about 93.4 and 136.8 times stronger than a pure uniform CeYIG, can be realized. Almost circularly polarized reflected beams with different chiralities are obtained with the CeYIG film. By tuning the geometric parameters of the PCS and the applied external magnetic field, dynamic control of polarization states of the reflected beams with different wavelengths can be realized. This magneto-optical metasurface provides a convenient way for the implementation of magneto-optical devices such as information memory devices, sensors, polarizers, and chiral devices.

General features and contact modeling of $N$-body isolated resonances near threshold

NN-body non efimovian bound or quasi-bound states for particles with short range interactions are considered in arbitrary dimensions. The different resonance regimes near the threshold are depicted by using a generalization of the effective range approximation. This two-parameter description can be used in various contexts from ultracold to hadronic physics. The universal character of these states makes it possible a formulation in terms of a contact model. The singularity at the contact imposes the introduction of a modified scalar product to solve the normalization catastrophe and to restore the self-adjoint character of the model. An equivalence with the standard scalar product used for realistic finite range models is derived.