Toroidal dipole bound states in the continuum based on hybridization of surface lattice resonances.

Obtaining a high quality factor (Q factor) in applications based on metasurfaces is crucial for improving device performance. Therefore, bound states in the continuum (BICs) with ultra-high Q factors are expected to have many exciting applications in photonics. Breaking the structure symmetry has been viewed as an effective way of exciting quasi-bound states in the continuum (QBICs) and generating high-Q resonances. Among these, one exciting strategy is based on the hybridization of surface lattice resonances (SLRs). In this study, we investigated for the first time the Toroidal dipole bound states in the continuum (TD-BICs) based on the hybridization of Mie surface lattice resonances (SLRs) in an array. The unit cell of metasurface is made of a silicon nanorods dimer. The Q factor of QBICs can be precisely adjusted by changing the position of two nanorods, while the resonance wavelength remains quite stable against the change of position. Simultaneously, the far-field radiation and near-field distribution of the resonance are discussed. The results indicate that the toroidal dipole dominates this type of QBIC. Our results indicate that this quasi-BIC can be tuned by adjusting the size of the nanorods or the lattice period. Meanwhile, through the study of the shape variation, we found that this quasi-BIC exhibits excellent robustness, whether in the case of two symmetric or asymmetric nanoscale structures. This will also provide large fabrication tolerance for the fabrication of devices. Our research results will improve the mode analysis of surface lattice resonance hybridization, and may find promising applications in enhancing light-matter interaction, such as lasing, sensing, strong-coupling, and nonlinear harmonic generation.

Fabrication and Characterization of Visible to Near-Infrared Photodetector Based on Multilayer Graphene/Mg2Si/Si Heterojunction.

In this investigation, p-Mg2Si/n-Si heterojunction photodetector (PD) is fabricated by magnetron sputtering and low vacuum annealing in the absence of argon or nitrogen atmosphere. Multilayer Graphene (MLG)/Mg2Si/Si heterojunction PD is first fabricated by transferring MLG to Mg2Si/Si heterojunction substrate using the suspended self-help transfer MLG method. After characterizing the phase composition, morphology and detection properties of Mg2Si/Si and MLG/Mg2Si/Si heterojunction PDs, the successful fabrication of the Mg2Si/Si and MLG/Mg2Si/Si heterojunction PDs are confirmed and some detection capabilities are realized. Compared with the Mg2Si/Si heterojunction PD, the light absorption and the ability to effectively separate and transfer photogenerated carriers of MLG/Mg2Si/Si heterojunction PD are improved. The responsivity, external quantum efficiency (EQE), noise equivalent power (NEP), detectivity (D*), on/off ratio and other detection properties are enhanced. The peak responsivity and EQE of the MLG/Mg2Si/Si heterojunction PD are 23.7 mA/W and 2.75%, respectively, which are better than the previous 1-10 mA/W and 2.3%. The results illustrate that the fabrication technology of introducing MLG to regulate the detection properties of the Mg2Si/Si heterojunction PD is feasible. In addition, this study reveals the potential of MLG to enhance the detection properties of optoelectronic devices, broadens the application prospect of the Mg2Si/Si-based heterojunction PDs and provides a direction for the regulation of optoelectronic devices.