Ultra-slow-light and dynamically quantitative optical storage modulation via quasi-BICs.

We achieve dynamically tunable dual quasi-bound states in the continuum (quasi-BICs) by implementing them in a silicon-graphene multilayer composite structure and utilize the quasi-BIC modes to achieve ultra-large group delays (velocity of light slows down 105 times), showing 2-3 orders of magnitude higher than the group delays of previous electromagnetically induced transparency modes. The double-layer graphene holds great tuning capability and leads to the dramatically reduced group delay from 1929.82 to 1.58 ps with only 100 meV. In addition, the log-linear variation rule of group delay with Fermi level (Ef) in the range of 0-10 meV is analyzed in detail, and the double-logarithmic function relationship between the group delay and quality factor (Q-factor) is theoretically verified. Finally, the quantitative modulation of the optical storage is further realized in this basis. Our research provides ideas for the reform and upgrading of slow optical devices.

Near-unity circular dichroism enabled by toroidal dipole empowered bound states in the continuum for electrical switching and sensing.

Near-unity circular dichroism (CD) with high quality (Q)-factors has wide applications in chiral lasers, modulators, detectors, etc. In this work, we firstly suggest a feasible approach to realize near-unity CD (∼0.94) with a high Q-factor (>2 × 104) supported by a toroidal dipole (TD) empowered superchiral quasi-bound states in the continuum (BICs) metasurface. Based on intensity, excellent electrical switching is achieved by adjusting the Fermi energy of the graphene on the metasurface. High refractive index sensitivity (136.2 nm/RIU) and figure of merit (1135 RIU-1) demonstrate its superior chiral sensing detection performance. Moreover, the near-unity CD displays a large robustness to the asymmetry offset. Our work paves a feasible avenue for well-designed superchiral quasi-BIC metasurfaces with high Q-factor near-unity CD for chiral applications in electrically tunable modulators, switches, sensors, etc.

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.

Dynamically electrical/thermal-tunable perfect absorber for a high-performance terahertz modulation.

We present a high-performance functional perfect absorber in a wide range of terahertz (THz) wave based on a hybrid structure of graphene and vanadium dioxide (VO2) resonators. Dynamically electrical and thermal tunable absorption is achieved due to the management on the resonant properties via the external surroundings. Multifunctional manipulations can be further realized within such absorber platform. For instance, a wide-frequency terahertz perfect absorber with the operation frequency range covering from 1.594 THz to 3.272 THz can be realized when the conductivity of VO2 is set to 100000 S/m (metal phase) and the Fermi level of graphene is 0.01 eV. The absorption can be dynamically changed from 0 to 99.98% and in verse by adjusting the conductivity of VO2. The impedance matching theory is introduced to analyze and elucidate the wideband absorption rate. In addition, the absorber can be changed from wideband absorption to dual-band absorption by adjusting the Fermi level of graphene from 0.01 eV to 0.7 eV when the conductivity of VO2 is fixed at 100000 S/m. Besides, the analysis of the chiral characteristics of the helical structure shows that the extinction cross-section has a circular dichroic response under the excitation of two different circularly polarized lights (CPL). Our study proposes approaches to manipulate the wide-band terahertz wave with multiple ways, paving the way for the development of technologies in the fields of switches, modulators, and imaging devices.

Multiple resonant modes coupling enabled strong CD response in a chiral metasurface.

The chiral structures with strong circular dichroism (CD) response and narrow linewidth are desirable in chiral sensing, circularly-polarized light detection, and polarization imaging. Here, we theoretically proposed a hybrid chiral metasurface for differential absorption of circularly polarized light. Based on the multiple resonant modes coupling effect in a two-dimensional dielectric slab, it is realizable then to achieve a nearly perfect absorption for right circularly polarized light and simultaneously reflects 90% of left circularly polarized light, suggesting the generation of strong CD of 0.886 within a narrowly spectral linewidth of 4.53 nm. The multipole analysis reveals that the electric dipole, the magnetic dipole, and the electric quadrupole make dominant contributions to chiral absorption and the high CD response in this metsurface. The excitation of guided mode resonance enhances the ability of this metasurface to absorb electric field. Moreover, the optical chirality response can be further manipulated through the geometry features. These findings pave a powerful way to realize the narrowing and strong CD platform for single-band and multiband chirality behaviors.