Extraordinary optical reflection resonances and bound states in the continuum from a periodic array of thin metal plates

THz absorbers with an ultrahigh Q-factor empowered by the quasi-bound states in the continuum for sensing application

The exceptional resonances excited by symmetry-protected quasi-bound states in the continuum (QBICs) have provided significant potential in high-sensitive sensing applications. Herein, we have proposed a type of metal-insulator-metal (MIM) absorbers supported by QBIC-induced resonances, and the ideal Q-factors of QBIC-induced resonances can be enhanced up to 10⁵ in the THz regime. The coupled mode theory and the multipole scattering theory are employed to thoroughly interpret the QBIC-induced absorption mechanism. Furthermore, the refractive index sensing capacities of the as-presented absorbers have been investigated, where the maximum values of the sensing sensitivity and figure of merit (FOM) can reach up to 187 GHz per refractive index unit and 286, respectively. Therefore, it is believed that the proposed absorbers enabled by QBIC-induced resonances hold promising potential in a broad range of highly demanding sensing applications.

Enhancement of Smith-Purcell radiation from cylindrical gratings by quasi-bound states in the continuum

Smith-Purcell radiation (SPR) is an important means of generating terahertz waves, and the enhancement of SPR is an attractive topic nowadays. Inspired by the phenomenon of special SPR, where the enhancement is achieved by using a high-duty-cycle grating, we describe a new, to the best of our knowledge, but more effective approach to this challenging problem. By deriving a simple analytical solution for the SPR from an annular electron beam passing through a cylindrical metallic grating, we show that the inverse structure, a low-duty-cycle grating can exhibit rather high SPR efficiencies in the presence of quasi-bound states in the continuum (quasi-BICs). The analytical prediction is supported by particle-in-cell simulations, which show that the quasi-BICs can enhance the superradiant SPR generated by a train of electron bunches by orders of magnitude. These results present an interesting mechanism for enhancing the SPR from metallic gratings, and may find applications in terahertz free-electron lasers.

Resonance-trapped bound states in the continuum in metallic THz metasurfaces

The realization of bound states in the continuum (BICs) in optical systems has been relying mainly on symmetry breaking. In contrast, another mechanism, known as resonance-trapped (or Friedrich-Wintgen) scenario, has been reported in the limited scope of dielectric resonant inclusions or at off-Γ points. In this Letter, we demonstrate that the coupling coefficient between two coplanar metallic split-ring resonators can be tuned to satisfy the Friedrich-Wintgen BIC condition with normal terahertz (THz) incidence when metals are modeled as perfect electric conductors. Temporal coupled-mode theory is applied to validate the results. Experimentally, a BIC-induced cloaking effect has been observed, owing to the intrinsic dissipation loss of the constitutive materials. Our findings suggest an alternative strategy to construct BICs in metallic metasurfaces apart from conventional symmetry-breaking methods.

Characteristics of resonance-induced optical vortices and spatial reshaping

The spatial profile of a beam can experience complicated reshaping after interacting with a planar resonator near resonant conditions. Previously, this phenomenon was recognized as the Goos-Hänchen effect, which only partially explains the experimental observations. In this Letter, we introduce a 2D model that can fully describe the resonance-induced spatial reshaping. The model predicts several general features of the output beam profile and suggests that optical phase or polarization vortices can be generated and manipulated by an arbitrary planar resonator. We validate our theoretical results with experimental measurements using terahertz spectroscopy.

Brewster quasi bound states in the continuum in all-dielectric metasurfaces from single magnetic-dipole resonance meta-atoms

Bound states in the continuum (BICs) are ubiquitous in many areas of physics, attracting special interest for their ability to confine waves with infinite lifetimes. Metasurfaces provide a suitable platform to realize them in photonics; such BICs are remarkably robust, being however complex to tune in frequency-wavevector space. Here we propose a scheme to engineer BICs and quasi-BICs with single magnetic-dipole resonance meta-atoms. Upon changing the orientation of the magnetic-dipole resonances, we show that the resulting quasi-BICs, emerging from the symmetry-protected BIC at normal incidence, become transparent for plane-wave illumination exactly at the magnetic-dipole angle, due to a Brewster-like effect. While yielding infinite Q-factors at normal incidence (canonical BIC), these are termed Brewster quasi-BICs since a transmission channel is always allowed that slightly widens resonances at oblique incidences. This is demonstrated experimentally through reflectance measurements in the microwave regime with high-refractive-index mm-disk metasurfaces. Such Brewster-inspired configuration is a plausible scenario to achieve quasi-BICs throughout the electromagnetic spectrum inaccessible through plane-wave illumination at given angles, which could be extrapolated to other kind of waves.

Bound States in the Continuum through Environmental Design

We propose a new paradigm for realizing bound states in the continuum (BICs) by engineering the environment of a system to control the number of available radiation channels. Using this method, we demonstrate that a photonic crystal slab embedded in a photonic crystal environment can exhibit both isolated points and lines of BICs in different regions of its Brillouin zone. Finally, we demonstrate that the intersection between a line of BICs and a line of leaky resonances can yield exceptional points connected by a bulk Fermi arc. The ability to design the environment of a system opens up a broad range of experimental possibilities for realizing BICs in three-dimensional geometries, such as in 3D-printed structures and the planar grain boundaries of self-assembled systems.

Prism coupling of high-Q terahertz whispering-gallery-modes over two octaves from 0.2 THz to 1.1 THz

We report on prism coupling of high-quality (high-Q) terahertz (THz) whispering-gallery modes (WGMs) in spherical high resistivity float zone grown silicon (HRFZ-Si) resonators over two octaves from 0.2 THz to 1.1 THz. The WGMs are excited using a HRFZ-Si prism and show unprecedented quality factors of up to 2.2 × 10⁴. A detailed discussion of the phase-and mode-matching criteria of the prism coupling scheme implemented in the continuous wave THz spectroscopy system is presented. The results provide numerous opportunities for passive ultra-broadband high-Q devices operating in the THz frequency range.