Jingwei Lv

The combination of high quality factor (Q-factor) bound states in the continuum (BIC) and chiral metasurfaces has attracted much attention in the field of photonics. Here, we designed and analyzed a “sandwich” all-dielectric metasurfaces with two silicon cross-shaped cylinders distributed on the upper and lower surfaces of the silica. The transition from symmetry-protected BIC to chiral quasi-BIC is achieved by innovatively breaking both the mirror symmetry and the in-plane inversion symmetry of the structure, resulting in a transmittance-ratio circular dichroism (RCD) close to unity and a high Q-factor. In particular, three chiral QBICs (QBIC1, QBIC2, and QBIC3) are realized in the terahertz bands without increasing the complexity of the structure at each layer. Multipole decomposition and near-field analysis demonstrate that QBIC1 and QBIC2 are dominated by the toroidal dipole (TD) and magnetic quadrupole (MQ), while QBIC3 is mainly affected by the electric quadrupole (EQ) and magnetic quadrupole. In addition, the presence of positive and negative states due to the RCD values of the structure suggests a spin selectivity with different frequency bands. Theoretical assessment shows a maximum Q factor of 3.94×10 ⁴ , the maximum sensitivity (S) is 245 GHz/RIU, and the figure of merit (FOM) is 7,927 RIU ^-1 . The results reveal a novel approach for selectively modulating circularly polarized light, demonstrating significant potential in biomolecular detection, high-spectral-resolution chiral biosensors, and chemical analysis.

Metasurface transmission, where light of different wavelengths or modes is redistributed and regulated by the metasurface structure, plays a crucial role in modern photonics and nano-optics research. Herein, an in-plane symmetry-breaking approach is designed to excite quadruple transmission dips using the LiNbO₃ metasurface based on the leaked plasmonic bound states in the continuum (BIC). The influence of the difference between the major and minor diameters of the ellipse of the LiNbO₃ metasurface is accounted for, and the C_4v symmetry is maintained. According to the theoretical derivation, four quasi-BICs are obtained, with the highest quality factor (Q-factor) reaching 2.1×10⁴ and the figure of merit being 5707RIU⁻¹. The multi-level decomposition and near-field analysis of the four specific BIC modes indicate that the modes are excited by toroidal dipoles and magnetic quadrupoles rather than any related guided-mode resonances. The results reveal that the four resonances are polarization-independent, and their properties are maintained even for circularly polarized light. The results provide insights into the utilization of LiNbO₃ in advanced integrated nonlinear optics for integrated optics, biosensing, filtering, and lasers.

A highly sensitive Fabry–Pérot interferometer (FPI) is fabricated via symmetric taper fiber cleaving and centered waist-inserted assembly, a design where geometric symmetry is fundamental to the sensor’s performance. The FPI is fabricated by simple and cost-effective techniques, including fiber cleaving, splicing, and tapering. Due to the ultra-long cantilever beam with an effective length of 2.33 mm and the ultra-short Fabry–Pérot (FP) cavity with an actual length of 13.98 μm, the sensor exhibits an ultra-high strain sensitivity of 544.57 pm/με in experimental results. The sensor boasts a small temperature sensitivity of 1.02 pm/°C and a cross-temperature sensitivity of 0.0019 µε/°C in the temperature range of 25–200 °C. Furthermore, the sensor has good stability and repeatability. Owing to the symmetry-enhanced design, simple fabrication process, high strain sensitivity, as well as a stable, linearly proportional response over an extensive strain regime, the device has large potential in various sensing applications.

The use of bound states in the continuum (BICs) has emerged as an effective tool to trap light at the nanoscale and has many potential applications in photonics. Breaking the structural symmetry is regarded as an effective way to excite quasi-BICs (QBICs) and generate high-Q resonances. However, this approach may impact the resonance polarization sensitivity, consequently limiting its practicality in multi-wavelength polarization-dependent applications. Furthermore, the introduction of different types of structural perturbations into the design to form BICs has yet to be explored in depth. In this study, we present an optical sensor consisting of an L-shaped metasurface that supports three quasi-BIC modes in the terahertz band, where specific displacements, collective perturbations, or both occur. Furthermore, we analyze the field distributions in detail and combine them with multipolar decomposition to reveal the underlying mechanisms of the different resonant modes. Multiple asymmetric perturbations are found to affect the sensitivity of the metasurface in refractive index sensing, thus allowing for a comparison of different resonant modes. The quasi-BIC mode can attain a Q-factor of 1067.6, a sensitivity (S) of 300 GHz/RIU, and a figure of merit (FOM) of 5367.8 RIU⁻¹ for vertical light incidence. These three quasi-BIC modes are polarization-independent, and their properties are maintained even for circularly polarized light. The results reveal a novel design strategy for metasurface-based sensors with promising application potential in biosensing, filtering, and lasers.

A silicon-nanostructured array coating on silicon film (SAS film) is designed based on the plasmonic optical tweezer and demonstrated for optical trapping and manipulation of nanospheres with negligible impact on the local thermal conditions. The electric field enhancement, optical force, and trapping potential of the SAS film are investigated by the finite element method. The trapping position is affected by the incident light wavelength, structure of the nanoarray, and refractive index of the nanospheres. The presence of four energy wells around the nanoarray suggests that it is possible to trap multiple nanoparticles. Moreover, the circularly polarized light, Gaussian beam, and silicon nanoarray facilitate the trapping of nanoparticles. This study showcases the potential of SAS film as optical tweezers to capture nanoparticles for the development of nanophotonic devices.

A nanoantenna with Fano response is designed with plasmonic oligomers as a refractive index sensor to enhance surface-enhanced Raman scattering (SERS) in the visible light spectrum. The scattered radiation and field-enhanced interactions of the outer gallium phosphide (GaP) nanoring assembled with an inner heptamer of silver with Fano response are investigated systematically using the finite element method. The characteristics of Fano resonance are found to depend on the size, shape and nature of the materials in the hybrid nanoantenna. The confined electromagnetic field produces a single-point electromagnetic hotspot with up to 159.59 V/m. The sensitivity obtained from the wavelength shift and variation in the scattering cross-section (SCS) shows a maximum value of 550 nm/RIU. The results validate the design concept and demonstrate near-field enhancement, enabling the design of high-performance nanoantennas with enhanced optical sensing and SERS properties.

A multifunctional biosensor composed of a dual-channel photonic crystal fiber (PCF) based on localized surface plasmon resonance (LSPR) is presented to measure dynamic changes in the magnetic field, temperature, and analyte refractive index at mid-infrared wavelengths. The finite-element method (FEM) is used to model and determine the sensing properties of the sensor. The flat dual-channel surface is coated with a gold film, and two nanowires are put on the fan-shaped openings to create directional resonance coupling to detect the analyte refractive index and temperature. By utilizing that the refractive index (RI) of the filled magnetic fluid (MF) is sensitive to the external magnetic field and temperature, a sensor with multi-physical detection functions is obtained. For refractive indexes ranging from 1.47 to 1.52, the maximum sensitivity is as high as 31,000 nm/RIU, with a resolution of 3.22 × 10⁻⁶ RIU. The maximum sensitivities for the magnetic field and temperature are 1970 pm/Oe and −5500 pm/°C, respectively.

High refractive index dielectric nanostructures represent a new frontier in nanophotonics, and the unique semiconductor characteristics of dielectric systems make it possible to enhance electric fields by exploiting this fundamental physical phenomenon. In this work, the scattered radiation spectral features and field-enhanced interactions of gallium phosphide disks with forked slits at anapole modes are investigated systematically by numerical and multipole decomposition analyses. Additional enhancement of the electric field is achieved by opening the forked slits to create high-intensity hot spots inside the disk, and nearby molecules can access these hot spots directly. The results reveal a novel approach for near-field engineering such as electric field localization, nonlinear optics, and optical detection.

An optical nanoantenna consisting of a Au-dielectric core-shell nanocube dimer with switchable directionality was designed and described. Our theoretical model and numerical simulation showed that switching between forward and backward directions can be achieved with balanced gain and loss, using a single element by changing the coefficient κ in the core, which can be defined by the relative phase of the polarizability. The optical response indicated a remarkable dependence on the coefficient κ in the core as well as frequency. The location of the electric field enhancement was specified by the different coefficient κ and, furthermore, the chained optical nanoantenna and coupled electric dipole emitted to the optical nanoantenna played significant roles in unidirectional scattering. This simple method to calculate the feasibility of unidirectional and switchable scattering provides an effective strategy to explore the functionalities of nanophotonic devices.