Active optical modulation of quasi-BICs in Si-VO2 hybrid metasurfaces

Active dual quasi-BICs in a dielectric metasurface with VO2 for slow light and optical modulation

We investigate the temperature tunable dual quasi-bound states in the continuum (qBICs) in a silicon/vanadium dioxide (Si/VO2) hybrid metasurface with Q-factor being as large as 9.3 × 106 and 2.8 × 107 by breaking the in-plane C2 symmetry. The far-field scattering of multipoles and near-field distributions confirm that the toroidal dipole and magnetic quadrupole dominate the dual qBICs resonance. The high performance of slow light with ultralarge group index exceeding 5.6 × 105 and the inverse quadratic law between the group index and asymmetric parameter are achieved. By temperature tuning of the VO2 thin film at the sub-10 K scale, a modulation depth of 90% and the ON/OFF ratio exceeding 12.8 dB are obtained. The proposed temperature tunable dual qBICs have potential applications in the fields of tunable slow light, temperature switches, and sensors.

Dynamically tunable multi-band quasi-permittivity-asymmetric BIC in the GST225 metasurface

Bound states in the continuum (BICs) on metasurfaces have garnered significant interest for their ultrahigh Q-factor potential in sensing applications. Reconfigurability and multi-band resonance are highly desirable for sensing systems. In this work, we introduce a metasurface comprising four nanocubes with different permittivity asymmetries, which can be dynamically adjusted using Ge2Sb2Te5 (GST225), a phase-change material, in the near-infrared (NIR) region. Additionally, a simulation for a liquid molecule sensor based on the metasurface shows a sensitivity of 1017 nm/RIU. This research introduces a novel, to the best of our knowledge, approach for designing multi-band, dynamically tunable quasi-BIC metasurfaces, which are good candidates for tunable, high-sensitivity biochemical sensing and nonlinear optics applications.

Switchable Vanadium Dioxide Metasurface for Terahertz Ultra-Broadband Absorption and Reflective Polarization Conversion

Based on the unique insulator-metal phase transition property of vanadium dioxide (VO2), we propose an integrated metasurface with a switchable mechanism between ultra-broadband absorption and polarization conversion, operating in the terahertz (THz) frequency range. The designed metasurface device is constructed using a stacked structure composed of VO2 quadruple rings, a dielectric layer, copper stripes, VO2 film, a dielectric layer, and a copper reflection layer. Our numerical simulations demonstrate that our proposed design, at high temperatures (above 358 K), exhibits an ultra-broadband absorption ranging from 4.95 to 18.39 THz, maintaining an absorptivity greater than 90%, and achieves a relative absorption bandwidth of up to 115%, significantly exceeding previous research records. At room temperature (298 K), leveraging VO2's insulating state, our proposed structure transitions into an effective polarization converter, without any alteration to its geometry. It enables efficient conversion between orthogonal linear polarizations across 3.51 to 10.26 THz, with cross-polarized reflection exceeding 90% and a polarization conversion ratio over 97%. More importantly, its relative bandwidth reaches up to 98%. These features highlight its wide-angle, extensive bandwidth, and high-efficiency advantages for both switching functionalities. Such an ultra-broadband convertible design offers potential applications in optical switching, temperature dependent optical sensors, and other tunable THz devices in various fields.

Enhanced Tunability of Dual-Band Chiral Metasurface in the Mid-Infrared Range via Slotted Nanocircuit Design

Multi-band circular dichroism (CD) response and tunability on the chiral metasurface are crucial for this device's applications in sensing and detection. This work proposes a dual-band CD Au-CaF2-Au dimer elliptical metasurface absorber, where chiroptical sensing is realized by breaking the geometric symmetry between two ellipses. The proposed metasurface can achieve high CD values of 0.8 and -0.74 for the dual-band within the 3-5 μm region, and the CD values can be manipulated by independently adjusting the geometric parameters of the metasurface. Furthermore, a slotted nanocircuit is introduced onto the metasurface to enhance its tunability by manipulating the geometry parameter in the design process, and the related mechanism is explained using an equivalent circuit model. The simulation of the sensing model revealed that the slotted nanocircuit enhances the sensor's tunability and significantly improves its bandwidth and sensitivity, achieving peak enhancements at approximately 753 nm and 1311 nm/RIU, respectively. Due to the strong dual-band positive (and negative) responses of the CD values, flexible wavelength tunability, and nonlinear sensitivity enhancement, this design provides a new approach for the development and application of mid-infrared chiroptical devices.

Active control of an electromagnetically induced transparency analogue in a coupled dual bound states in the continuum system integrated with graphene

Electronically induced transparency (EIT) is a coherent optical phenomenon that induces interference within atoms, allowing certain specific frequencies of light to pass through atomic media without being absorbed. However, EIT systems face challenges related to narrow transparency windows and precise control of slow light. We propose an interference structure based on a coupled dual bound states in the continuum (BIC) system to emulate the EIT-like effect. By integrating quasi-BIC (bright mode) with BIC (dark mode), our design successfully achieves an EIT-like effect in a narrow bright mode with a full width at half maximum (FWHM) of less than 1 nm. Its notable features are the bright mode's wide tunability achieved through structural parameter adjustment and a significant group delay of up to 14.43 ps. Additionally, integrating graphene into the BIC structure introduced a form of active tunability akin to the EIT-like effect. We numerically calculate the coupling structure, and its intrinsic mechanism is analyzed. Analysis based on coupled-mode theory confirms that this active modulation primarily stems from changes in the BIC structure's loss. Due to its special frequency selectivity and insensitivity to the polarization of the light source, this narrow-band EIT-like structure is particularly suitable for high-precision optical sensing and spectroscopy. The significant group delay of this structure enhances the interaction between light and matter, improving the accuracy and efficiency of optical signal control and data transmission, opening up new avenues for slow light applications and making significant progress in the development of active tunable optical switches and modulators.

High-Sensitivity Optical Sensors Empowered by Quasi-Bound States in the Continuum in a Hybrid Metal-Dielectric Metasurface

Enhancing light-matter interaction is a key requisite in the realm of optical sensors. Bound states in the continuum (BICs), possessing high quality factors (Q factors), have shown great advantages in sensing applications. Recent theories elucidate the ability of BICs with hybrid metal-dielectric architectures to achieve high Q factors and high sensitivities. However, the experimental validation of the sensing performance in such hybrid systems remains equivocal. In this study, we propose two symmetry-protected quasi-BIC modes in a metal-dielectric metasurface. Our results demonstrate that, under the normal incidence of light, the quasi-BIC mode dominated by dielectric can achieve a high Q factor of 412 and a sensing performance with a high bulk sensitivity of 492.7 nm/RIU (refractive index unit) and a figure of merit (FOM) of 266.3 RIU-1, while the quasi-BIC mode dominated by metal exhibits a stronger surface affinity in the biotin-streptavidin bioassay. These findings offer a promising approach for implementing metasurface-based sensors, representing a paradigm for high-sensitivity biosensing platforms.

Efficient polarization-insensitive quasi-BIC modulation by VO2 thin films

Bound states in the continuum (BIC) offer great design freedom for realizing high-quality factor metasurfaces. By deliberately disrupting the inherent symmetries, BIC can degenerate into quasi-BIC exhibiting sharp spectra with strong light confinement. This transformation has been exploited to develop cutting-edge sensors and modulators. However, most proposed quasi-BICs in metasurfaces are composed of unit cells with Cs symmetry that may experience performance degradation due to polarization deviation, posing challenges in practical applications. Addressing this critical issue, our research introduces an innovative approach by incorporating metasurfaces with C4v unit cell symmetry to eliminate polarization response sensitivity. Vanadium Dioxide (VO2) is a phase-change material with a relatively low transition temperature and reversibility. Here, we theoretically investigate the polarization-insensitive quasi-BIC modulation in Si-VO2 hybrid metasurfaces. By introducing defects into metasurfaces with Cs, C4, and C4v symmetries, we enable the emergence of quasi-BICs characterized by strong Fano resonance in their transmission spectra. Via numerically calculating the multipole decomposition, distinct dominant multipoles for different quasi-BICs are identified. A comprehensive investigation into the polarization responses of these structures under varying directions of linearly polarized light reveals the superior polarization-independent characteristics of metasurfaces with C4 and C4v symmetries, a feature that ensures the maintenance of maximum resonance peaks irrespective of polarization direction. Utilizing the polarization-insensitive quasi-BIC, we thus designed two different Si-VO2 hybrid metasurfaces with C4v symmetry. Each configuration presents complementary benefits, leveraging the VO2 phase transition's loss change to facilitate efficient modulation. Our quantitative calculation indicates notable achievements in modulation depth, with a maximum relative modulation depth reaching up to 342%. For the first time, our research demonstrates efficient modulation using polarization-insensitive quasi-BICs in designed Si-VO2 hybrid metasurfaces, achieving identical polarization responses for quasi-BIC-based applications. Our work paves the way for designing polarization-independent quasi-BICs in metasurfaces and marks a notable advancement in the field of tunable integrated devices.

Electromagnetic heating-assisted metasurface for stably tunable, fast-responding chiroptics

Herein, a graphene-dielectric metasurface with the function of stably tunable and fast responding on the chiroptics is theoretically investigated and numerically demonstrated. Via utilizing the intrinsic thermo-optical effect of the silicon, the circular dichroism (CD) peak position can be linearly scaled with a spectral sensitivity of up to 0.06 nm/K by artificially adjusting the temperature. Moreover, a perfectly adjusting manipulation with a wavelength shift of full width at half maximum for the resonant spectrum and the simultaneously maintained CD values can be realized by a slight temperature variation of ∼0.8 K. Additionally, we take a graphene layer as the heating source to actually demonstrate the ultra-fast thermal generation. Applying an input voltage of 2 V to the graphene with only 10 µs can rapidly increase the metasurface temperature of up to 550 K. Such performances hold the platform with wide applications in functional chiroptics and optoelectronics.

Near-perfect quantitatively tunable Q factors of quasi-bound states in the continuum via material-based thermal-optic perturbations

We successfully achieved high-Q dual-band quasi-bound states in the continuum (BICs) by introducing geometrical perturbations and thermally induced material perturbations into silicon half-disk nanodimers. Importantly, it is found that the Q factor obtained from the thermally induced material perturbations fits better with the inverse quadratic function of the asymmetry relation than that of the geometrical-perturbations-based system. Notably, we demonstrated that changes occurring at the sub-K scale can enable the simultaneous realization of the full width at half maximum offset distance for quasi-BICs and a maximum contrast ratio exceeding 44 dB. Our research provides novel, to the best of our knowledge, insights for potential applications in nano-lasers, temperature sensors, and infrared imaging.

Quasi-symmetry-protected BICs in a double-notched silicon nanodisk metasurface

Bound states in the continuum (BICs) hold great promise in enhancing light-matter interaction as they have an infinite Q-factor. To date, the symmetry-protected BIC (SP-BIC) is one of the most intensively studied BICs because it is easily found in a dielectric metasurface satisfying certain group symmetry. To convert SP-BICs into quasi-BICs (QBICs), structural symmetry shall be broken so that external excitation can access them. Usually, the unit cell's asymmetry is created by removing or adding parts of dielectric nanostructures. The QBICs are usually excited only by s-polarized or p-polarized light because of the symmetry-breaking of the structure. In this work, we investigate the excited QBIC properties by introducing double notches on the edges of highly symmetrical silicon nanodisks. The QBIC shares the same optical response under the s-polarized and p-polarized light. The effect of polarization on coupling efficiency between the QBIC mode and incident light is studied, and the highest coupling efficiency occurs at a polarization angle of 135 ^∘, which corresponds to the radiative channel. Moreover, the near-field distribution and multipole decomposition confirm that the QBIC is dominated by the magnetic dipole along the z direction. It is noted that the QBIC covers a wide spectrum region. Finally, we present an experimental confirmation; the measured spectrum shows a sharp Fano resonance with a Q-factor of 260. Our results suggest promising applications in enhancing light-matter interaction, such as lasing, sensing, and nonlinear harmonic generation.

Trapped mode control in metasurfaces composed of particles with the form birefringence property

Progress in developing advanced photonic devices relies on introducing new materials, discovered physical principles, and optimal designs when constructing their components. Optical systems operating on the principles of excitation of extremely high-quality factor trapped modes (also known as the bound states in the continuum, BICs) are of great interest since they allow the implementation of laser and sensor devices with outstanding characteristics. In this paper, we discuss how one can utilize the anisotropic properties of novel materials (transition metal dichalcogenides, TMDs), particularly, the bulk molybdenum disulfide (MoS₂), to realize the excitation of trapped modes in dielectric metasurfaces. The bulk MoS₂ is a thin-film structure in which the light wave behaves the same way as that in the uniaxial anisotropic material with the form birefringence property. Our metasurface is composed of an array of disk-shaped nanoparticles (resonators) made of the MoS₂ material under the assumption that the anisotropy axis of MoS₂ can be tilted to the rotation axis of the disks. We perform a detailed analysis of eigenwaves and scattering properties of such anisotropic resonators as well as the spectral features of the metasurface revealing dependence of the excitation conditions of the trapped mode on the anisotropy axis orientation of the MoS₂ material used.