Quasi-bound States In Continuum Research Articles

High-quality factor Quasi-BIC mode via selective symmetry-breaking approach in a terahertz metasurface

This study numerically and experimentally presents a novel approach to excite bound state in the continuum (BIC) mode with a high Q-factor in the THz meta-molecule (composition of meta-atoms) system, leveraging a unique method of selective symmetry breaking in a ring-shaped metamolecule system. Unlike conventional strategies that uniformly disrupt the symmetry across all resonators to excite a quasi-BIC mode, this innovative technique targets only half of the unit cell for symmetry perturbation. This selective symmetry breaking minimizes radiative losses and enhances the Q-factor of the quasi-bound states in continuum (quasi-BIC) modes. The selective symmetry breaking is achieved in a ring-shaped metamolecule system by simple radial perturbation. The results depict a notable improvement in the Q-factor, achieving values as high as 107 in simulation, a significant enhancement compared to the uniformly symmetry-breaking approach, which exhibits Q-factors around 25.80. The experimental transmission spectrum and the near-field scanning images firmly validate the existence of the high Q BIC mode under this strategic symmetry-breaking approach. This work may open new avenues for developing advanced THz devices with promising applications in sensing, filtering, and non-linearity in the THz domain.

Efficient and Shape-Sensitive Manipulation of Nanoparticles by Quasi-Bound States in the Continuum Modes in All-Dielectric Metasurfaces.

Current optical tweezering techniques are actively employed in the manipulation of nanoparticles, e.g., biomedical cells. However, there is still huge room for improving the efficiency of manipulating multiple nanoparticles of the same composition but different shapes. In this study, we designed an array of high-index all-dielectric disk antennas, each with an asymmetric open slot for such applications. Compared with the plasmonic counterparts, this all-dielectric metasurface has no dissipation loss and, thus, circumvents the Joule heating problem of plasmonic antennas. Furthermore, the asymmetry-induced excitation of quasi-bound states in continuum (QBIC) mode with a low-power intensity (1 mW/µm2) incidence imposes an optical gradient force of -0.31 pN on 8 nm radius nanospheres, which is four orders of magnitude stronger than that provided by the Fano resonance in plasmonic antenna arrays, and three orders of magnitude stronger than that by the Mie resonance in the same metasurface without any slot, respectively. This asymmetry also leads to the generation of large optical moments. At the QBIC resonance wavelength, a value of 88.3 pN-nm will act on the nanorods to generate a rotational force along the direction within the disk surface but perpendicular to the slot. This will allow only nanospheres but prevent the nanorods from accurately entering into the slots, realizing effective sieving between the nanoparticles of the two shapes.

Quasi-bound states in continuum in double-layer silicon gratings

Bound states in the continuum (BICs) are theoretically known to possess infinite lifetimes and Q factor. However, due to the difficulties in achieving it in reality, symmetry breaking is often introduced in the structure to transform symmetrically protected BICs into quasi-BICs (q-BICs) with extremely high Q factor. Therefore, q-BICs can be utilized to enhance the Q factor of optical sensors. In this paper, we propose the design of a double-layer composite one-dimensional grating with a high Q factor. The structure consists of a double-layers silicon (Si) grating on a silicon dioxide (SiO2) substrate. By introducing a displacement in the upper-layer grating to break the symmetry, q-BICs are induced. The induced q-BICs achieve a Q factor of 2248 for transverse magnetic (TM) wave, enabling enhanced optical sensing capabilities. The proposed q-BICs sensor, exhibiting anisotropy for both TM and transverse electric wave (TE), holds great potential for narrowband polarizers and sensing applications.

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.

Zone-Folded Quasi-bound State Metasurfaces with Customized, Symmetry-Protected Energy-Momentum Relations

Energy-momentum (dispersion) relations are a foundational framework for photonic device design, but our incomplete command thereof still limits key technologies. For instance, narrowband optical combiners for augmented reality should ideally reflect with unitary efficiency selected wavelengths that encode the artificial information over a large range of oblique incident angles, yet they are conventionally severely limited in field of view by dispersion. Here, we engineer nonlocal metasurfaces to support quasi-bound states in continuum with zero first-order resonant frequency dispersion at any desired quasi-momentum by exploiting a zone-folding technique that leverages a change in the lattice family. Our platform thereby advances photonic devices by enabling unprecedented control over the energy-momentum properties of nonlocal states, rationally controlled through a perturbation scheme that produces minimal distortion to non-resonant light.

Dual resonance based on quasi-bound states in continuum in the all-dielectric terahertz metasurface and its application in sensing

In recent years, the concept of bound states in continuum (BIC) in metasurfaces has become a topic of interest. BIC involves the physical mechanics of wave optics, metamaterial sensors and other high-quality factor (Q-factor) applications. In this paper, the basic concept of BIC is introduced, followed by a brief description of the progression of the research regarding the application of BIC in terahertz (THz) metasurfaces. Subsequently, an all-dielectric metasurface consisting of tetramer clusters with dual terahertz resonances is proposed. The materials used for the substrate and the tetramer cluster are quartz and silicon, respectively. Quasi-BIC works by varying the cylinder radius on the metasurface structure. The transmission spectra are analyzed under different structural and material parameters with control variables. When the metasurface is applied as a refractive-index sensor, the Q-factor is 2303.77 and the figure of merit (FOM) is 7022.90, which are calculated using the asymmetric parameter k = 0.8 μm. Additionally, when the incident wave is x- or y-polarized, resonance peaks with the same frequencies can be obtained, and the overall trend of the transmission spectrum remains the same.

Tunable dual quasi-bound states in continuum and electromagnetically induced transparency enabled by the broken material symmetry in all-dielectric compound gratings.

Dual quasi-bound states in continuum (quasi-BICs) enabled by the broken geometric symmetry offer an effective way to design high-quality photonic devices, yet challenged by tunable functionalities. Here we employ the material asymmetry originating from the tunable material property of phase-change materials to design quasi-BICs in all-dielectric compound gratings. We find the even and odd quasi-BICs are modulated by the geometric and material asymmetries, respectively, and this effect is ensured by two different types of structural symmetries in the compound structure. Particularly, tunable electromagnetically induced transparency (EIT) can be achieved by modulating the material asymmetry. Furthermore, we systematically design the compound gratings consisting of the phase-change material of Sb2Se3 to demonstrate tunable dual quasi-BICs and EITs. Analytical calculations and numerical simulations are performed to verify these findings. Our work provides a promising way to enhance the flexibility of realizing quasi-BICs, which may boost tunable applications in nanodevices assisted by quasi-BICs.

Compact optically controlling the emission chirality of microlasers in single subwavelength particles supporting quasi-bound states in the continuum

On-chip light sources play significant roles in the implementation of highly integrated photonic circuits. So far, the size of miniaturized semiconductor lasers is mainly limited to a few microns. Although the use of plasmonic structures based on metals can effectively reduce the volumes, there are limitations in practical applications attributed to the high ohmic loss of the metal. On the other hand, an all-dielectric structures with high refractive index can avoid the loss problem of the metal structure, which provides a new way for the realization of high-performance, miniaturized and highly integrated photonic circuits. In this work, we numerically demonstrate that in all-dielectric structures, supercavity modes, also known as quasi-bound states in continuum, with high quality factor are achievable and hence can be employed in a microlaser to reduce its footprint. This proposed structure has low radiation loss and the chirality of the laser wavefront can be easily controlled optically. Moreover, due to the existence of the quasi-BIC, these merits can be achieved with a single dielectric pillar. Therefore, our work provides a new approach to realize an ultra-small laser with orbital angular momentum.

Ultra-high Q-factor toroidal dipole resonance and magnetic dipole quasi-bound state in the continuum in an all-dielectric hollow metasurface

Fano resonance with high Q-factor plays a pivotal role in the field of optoelectronic applications, especially refractive index sensing. However, ultra-high Q-factor for metallic metasurfaces has been a challenge due to their great ohmic losses. Herein, we propose and numerically analyze an all-dielectric hollow metasurface in the near infrared region which consists of silicon cylinders with two asymmetric rectangular hollows. A sharp Fano resonance with modulation depth close to 100% excited by quasi-bound state in continuum, whose Q-factor can reach 8428 when = 40 nm. With the Cartesian multipole decomposition technique, the two excited Fano resonances can be characterized by toroidal dipole response and magnetic dipole (MD) response, respectively. It is worth noting that the Q-factor of MD mode can reach 17106. Moreover, the dependence of the transmission spectra on different geometric parameters is investigated as well. Due to their narrow linewidths and strong near-field confinement, the proposed structure can be applied to a refractive index sensor, yielding a maximum sensitivity (S) of 160 nm RIU−1 and a maximum figure of merit of 575 RIU−1. It is believed that the proposed structure can offer an excellent prospect for biomedical, agriculture, and chemical sensing applications.

Improved refractive-index sensing performance in medium contrast gratings by asymmetry engineering

Silicon nitride (Si3N4) subwavelength medium contrast gratings (MCGs) directly integrated with CMOS photodetectors are a promising option for on-chip label-free biosensing. The narrow spectral features required for sensing are often realized in Si3N4 nanostructures by weakly corrugated gratings which limit design flexibility. We numerically investigate the optical properties of asymmetry-engineered MCG gratings and predict the formation of ultra-sharp spectral features via the excitation of quasi-bound states in continuum (QBIC) resonances. Systematic investigation of the design parameter space shows that sharp spectral features are obtained for a wide range of parameters without requiring ultrathin grating profiles. Transmission-mode refractive index sensing simulations for bulk and surface sensing, considering both wavelength-shift and intensity-shift modalities, indicate performance gains using these structures.

Improved refractive-index sensing performance in medium contrast gratings by asymmetry engineering

Silicon nitride (Si3N4) subwavelength medium contrast gratings (MCGs) directly integrated with CMOS photodetectors are a promising option for on-chip label-free biosensing. The narrow spectral features required for sensing are often realized in Si3N4 nanostructures by weakly corrugated gratings which limit design flexibility. We numerically investigate the optical properties of asymmetry-engineered MCG gratings and predict the formation of ultra-sharp spectral features via the excitation of quasi-bound states in continuum (QBIC) resonances. Systematic investigation of the design parameter space shows that sharp spectral features are obtained for a wide range of parameters without requiring ultrathin grating profiles. Transmission-mode refractive index sensing simulations for bulk and surface sensing, considering both wavelength-shift and intensity-shift modalities, indicate performance gains using these structures.

Giant Goos-Hänchen shift induced by bounded states in optical PT-symmetric bilayer structures.

Goos-Hänchen (GH) effect is a fundamental phenomenon in optics. Here we demonstrate theoretically that the surface modes at Parity-time (PT) symmetric interfaces, can induce a giant GH shift at a specific incident angle. It is found that the amplitude of the GH shift can be tuned by adjusting the thickness of the bilayer, and as the thickness grows, its maximum value can go to infinity in theory. The physical mechanism behind this interesting feature is that the surface modes at PT interfaces are quasi-bound states in continuum (BICs), which lead to rapid variation in the phase of the scattered waves. Our work enriches the previous studies about GH effect in PT bilayer structures and provides a way in turn to explore the BICs in non-Hermitian photonic systems.

Bound states, scattering states, and resonant states inPT-symmetric open quantum systems

We study a simple open quantum system with a PT-symmetric defect potential as a prototype to illustrate general features of PT-symmetric open quantum systems; however, the potential could be mimicked by a number of recent PT experiments. One key feature is the resonance in continuum (RIC), which appears in both the discrete spectrum and scattering spectrum. The RIC forms a standing wave extending throughout the spatial extent of the system, representing a resonance between the open environment and the central PT-symmetric potential. We illustrate that as one deforms the system parameters, the RIC may exit the continuum by splitting into a bound state and a virtual bound state at the band edge, a process that should be experimentally observable. We also study the exceptional points (EPs) at which two eigenvalues coalesce; we categorize these as either EP2As, at which two real-valued solutions coalesce before becoming complex-valued, or EP2Bs, for which the two solutions are complex on either side of the EP. The EP2As are associated with PT-symmetry breaking; we argue that these are more stable against parameter perturbation than the EP2Bs. We also study complex-valued solutions of the discrete spectrum for which the wave function is nevertheless spatially localized, something not allowed in traditional open quantum systems; we illustrate that these may form quasi-bound states in continuum (QBICs) under some circumstances. We also study the scattering properties of the system, including states that support invisible propagation and some general features of perfect transmission states. We finally construct scattering states that satisfy PT-symmetric boundary conditions; while these states do not conserve the traditional probability current, we introduce the PT-current that is preserved. The perfect transmission states appear as a special case of the PT-symmetric scattering states.

Two-channel quantum wire with an adatom impurity: Role of the van Hove singularity in the quasibound state in continuum, decay rate amplification, and the Fano effect

We provide detailed analysis of the complex eigenenergy spectrum for a two-channel quantum wire with an attached adatom impurity. The study is based on our previous work [Phys. Rev. Lett. 99, 210404 (2007)], in which we presented the quasibound states in continuum (or QBIC states). These are resonant states with very long lifetime that form as a result of two overlapping continuous energy bands, one of which, at least, has a divergent van Hove singularity at the band edge. We provide analysis of the full energy spectrum for all solutions, including the QBIC states, and obtain an expansion for the complex eigenvalue of the QBIC state. We show that it has a small decay rate of the order ${g}^{6}$, where $g$ is the coupling constant of the adatom impurity. As a result of this expansion, we find that this state is a nonanalytic effect resulting from the van Hove singularity; it cannot be predicted from the ordinary perturbation analysis that relies on Fermi's golden rule. We also show that the QBIC state emerges as a direct result of the destabilization of the stable state that often exists on the outside edge of a band due to the divergence. As another result of the van Hove singularity, it has been previously reported that the decay rate of an unstable state is amplified in the vicinity of the band edge such that it is proportional to ${g}^{4/3}$. This again results from a breakdown of the Fermi rule. Here we explicitly show how the system behaves in the crossover region between the ${g}^{4/3}$ region and the Fermi region. Finally, we calculate the local density of states near the adatom. We are able to demonstrate that the interference between two unstable states with a very large decay rate and one unstable state with a small decay rate results in a characteristic asymmetric Fano profile. This effect leads to the best chance of detecting the QBIC by scanning tunneling microscopy probe.