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Bound states in the continuum (BICs) spectrum are localized states in the continuum domain with an infinite lifetime, zero radiation, and an infinite quality factor (Q-factor). The structural symmetry is typically broken to transform BICs into quasi-BICs with ultrahigh Q-factors, which are used in fields such as optical switches, nonlinear optics, and sensing. Herein, we present an all-dielectric metasurface fabricated by depositing Si on the SiO2 substrate and etching an "8"-shape hole in the middle of each unit. By breaking the symmetry of the structure, the BIC is transformed into a quasi-BIC mode, which can realize triple Fano resonances with a high Q-factor. The effects of different structural parameters on the Fano resonances are investigated by varying the structural parameters. The physical mechanisms of the three Fano resonances are elucidated by combining the electromagnetic field and multipole decomposition of scattered power. Calculation results show that the maximum Q-factor reaches 4.7 × 104, the maximum figure-of-merit (FOM) value reaches 675, and the modulation depths of the three resonances are approximately 100%. The metasurface exhibits a high Q-factor, FOM value, and modulation depth, thus rendering it suitable for application to high-performance biosensors, nonlinear optics, and optical switches. [ABSTRACT FROM AUTHOR]

The applications of terahertz (THz) technology can be greatly extended using non-diffractive beams with unique field distributions and non-diffractive transmission characteristics. Here, we design and experimentally demonstrate a set of dual non-diffractive THz beam generators based on an all-dielectric metasurface. Two kinds of non-diffractive beams with dramatically opposite focusing properties, Bessel beam and abruptly autofocusing (AAF) beam, are considered. A Bessel beam with long-distance non-diffractive characteristics and an AAF beam with low energy during transmission and abruptly increased energy near the focus are generated for x- and y-polarized incident waves, respectively. These two kinds of beams are characterized and the results agree well with simulations. In addition, we show numerically that these two kinds of beams can also carry orbital angular momentum by further imposing proper angular phases in the design. We believe that these metasurface-based beam generators have great potential use in THz imaging, communications, non-destructive evaluation, and many other fields. [ABSTRACT FROM AUTHOR]

Conventional all-dielectric metasurfaces show remarkable properties including high efficiency and tunability of the optical response. However, extreme narrow bandwidth is a limitation that reduce their application in the photonic sensor devices. In this work, an efficient hybrid silicon-graphene metasurface is numerically proposed and designed. Through the sandwiched graphene layer, the structure shows unique quarter-wave properties, tunable through the dimensions of silicon, the Fermi energy of graphene, and an external gate voltage. Dynamic tuning is achieved by reversing the gate voltage: circular polarization state is switched between the right- and the left-handed states by reversing the gate voltage. A 95% polarization conversion ratio and a 96% ellipticity ratio are obtained while converting linearly polarized light into circularly polarized light in the near infrared. Additionally, by integrating graphene with silicon, the Q-factor and the trapped magnetic modes in the silicon are effectively modulated. The structure is compact and has an ultrathin design thickness of ∼0.1 λ, in the telecommunication wavelength. The above properties are essential for integration into photonic sensing devices and for compatibility with the CMOS devices. [ABSTRACT FROM AUTHOR]