Observation of Supercavity Modes in Subwavelength Dielectric Resonators

Photonic Bound States in the Continuum in Nanostructures

Bound states in the continuum (BIC) have garnered considerable attention recently for their unique capacity to confine electromagnetic waves within an open or non-Hermitian system. Utilizing a variety of light confinement mechanisms, nanostructures can achieve ultra-high quality factors and intense field localization with BIC, offering advantages such as long-living resonance modes, adaptable light control, and enhanced light-matter interactions, paving the way for innovative developments in photonics. This review outlines novel functionality and performance enhancements by synergizing optical BIC with diverse nanostructures, delivering an in-depth analysis of BIC designs in gratings, photonic crystals, waveguides, and metasurfaces. Additionally, we showcase the latest advancements of BIC in 2D material platforms and suggest potential trajectories for future research.

Dielectric resonances of the cylindrical micro/nano cavity within epsilon-near-zero materials

The dielectric resonances of spherically symmetric micro/nano cavity in zero-index materials have been systematically studied. However, the resonance properties of other shaped dielectric cavities in zero-index materials remain unclear. Here, we theoretically investigate the electromagnetic resonances of the dielectric cavity with cylindrical symmetry in the epsilon-near-zero materials. This kind of cavity supports a set of resonances with strong light confinement, including dipole, quadrupole and higher-order modes with multiple nodes. Furthermore, there is a redshift of the resonance wavelength with an increment of its size, obeying a law as the function of diameter and height. Also, we find that the redshift will be slower for higher-order modes. Through the infinite refractive index contrast and extra degree of freedom, they should have potential application in the enhancement of light-matter interaction and multiple-functional light manipulation in the integrated optical systems.

Superscattering emerging from the physics of bound states in the continuum

We study the Mie-like scattering from an open subwavelength resonator made of a high-index dielectric material, when its parameters are tuned to the regime of interfering resonances. We uncover a novel mechanism of superscattering, closely linked to strong coupling of the resonant modes and described by the physics of bound states in the continuum (BICs). We demonstrate that the enhanced scattering occurs due to constructive interference described by the Friedrich-Wintgen mechanism of interfering resonances, allowing to push the scattering cross section of a multipole resonance beyond the currently established limit. We develop a general non-Hermitian model to describe interfering resonances of the quasi-normal modes, and study subwavelength dielectric nonspherical resonators exhibiting avoided crossing resonances associated with quasi-BIC states. We confirm our theoretical findings by a scattering experiment conducted in the microwave frequency range. Our results reveal a new strategy to boost scattering from non-Hermitian systems, suggesting important implications for metadevices.

Two Individual Super-Bound State Modes within Band Gap with Ultra-High Q Factor for Potential Sensing Applications in the Terahertz Wave Band

Bound states in the continuum (BICs) garnered significant research interest in the field of sensors due to their exceptionally high-quality factors. However, the wide-band continuum in BICs are noise to the bound states, and it is difficult to control and filter. Therefore, we constructed a top-bottom symmetric cavity containing three high permittivity rectangular columns. The cavity supports a symmetry-protected (SP) superbound state (SBS) mode and an accidental (AC) SBS mode within the bandgap. With a period size of 5 × 15, the bandgap effectively filters out the continuum, allowing only the bound states to exist. This configuration enabled us to achieve a high signal-to-noise ratio and a wide free-spectral-range. The AC SBS and the SP SBS can be converted into quasi-SBS by adjusting different parameters. Consequently, the cavity can function as a single-band sensor or a dual-band sensor. The achieved bulk sensitivity was 38 µm/RIU in terahertz wave band, and a record-high FOM reached 2.8 × 108 RIU-1. The effect of fabrication error on the performance for sensor application was also discussed, showing that the application was feasible. Moreover, for experimental realization, a 3D schematic was presented. These achievements pave the way for compact, high-sensitivity biosensing, multi-wavelength sensing, and other promising applications.

Superbound state in photonic bandgap and its application to generate complete tunable SBS-EIT, SBS-EIR and SBS-Fano

Bound states in continua (BICs) have high-quality factors that may approach infinity. However, the wide-band continua in BICs are noise to the bound states, limiting their applications. Therefore, this study designed fully controlled superbound state (SBS) modes in the bandgap with ultra-high-quality factors approaching infinity. The operating mechanism of the SBS is based on the interference of the fields of two phase-opposite dipole sources. Quasi-SBSs can be obtained by breaking the cavity symmetry. The SBSs can also be used to produce high-Q Fano resonance and electromagnetically-induced-reflection-like modes. The line shapes and the quality factor values of these modes could be controlled separately. Our findings provide useful guidelines for the design and manufacture of compact and high-performance sensors, nonlinear effects, and optical switches.

Broadband generation of quasi bound-state-in-continuum modes using subwavelength truncated cone resonators

To allow a high quality factor (Q-factor) to a sub-wavelength dielectric resonator, quasi-bound states in the continuum (Q-BICs) have gained much interest. However, the Q-BIC resonance condition is too sensitive to the geometry of the resonator, and its practical broadband generation on a single-wafer platform has been limited. Here we present that, employing the base angle as a structural degree of freedom, the truncated nano-cone resonator supports the Q-BIC resonance with a high Q-factor of >150 over a wide wavelength range of >100 nm. We expect our approach will boost the utilization of the Q-BIC resonance for various applications requiring broadband spectral tuning.

Resonators with tailored optical path by cascaded-mode conversions

Optical resonators enable the generation, manipulation, and storage of electromagnetic waves. The physics underlying their operation is determined by the interference of electromagnetic waves, giving rise to the resonance spectrum. This mechanism causes the limitations and trade-offs of resonator design, such as the fixed relationship between free spectral range, modal linewidth, and the resonator's refractive index and size. Here, we introduce a new class of optical resonators, generating resonances by designing the optical path through transverse mode coupling in a cascaded process created by mode-converting mirrors. The generalized round-trip phase condition leads to resonator characteristics that are markedly different from Fabry-Perot resonators and can be tailored over a wide range. We confirm the existence of these modes experimentally in an integrated waveguide cavity with mode converters coupling transverse modes into one supermode. We also demonstrate a transverse mode-independent transmission and show that its engineered spectral properties agree with theoretical predictions.

Coherent full polarization control based on bound states in the continuum

Bound states in the continuum (BICs) are resonant modes of open structures that do not suffer damping, despite being compatible with radiation in terms of their momentum. They have been raising significant attention for their intriguing topological features, and their opportunities in photonics to enhance light-matter interactions. In parallel, the coherent excitation of optical devices through the tailored interference of multiple beams has been explored as a way to enhance the degree of real-time control over their response. Here, we leverage the combination of these phenomena, and exploit the topological features of BICs in the presence of multiple input beams to enable full polarization control on the entire Poincaré sphere in a photonic crystal slab only supporting a symmetry-protected BIC, experimentally demonstrating highly efficient polarization conversion controlled in real time through the superposition of coherent excitations. Our findings open exciting opportunities for a variety of photonic and quantum optics applications, benefitting from extreme wave interactions and topological features around BICs combined with optical control through coherent interference of multiple excitations.

Guiding-mode-assisted double-BICs in an all-dielectric metasurface

The electromagnetically induced transparency (EIT) effect realized in a metasurface is potential for slow light applications for its extreme dispersion variation in the transparency window. Herein, we propose an all-dielectric metasurface to generate a double resonance-trapped quasi bound states in the continuum (BICs) in the form of EIT or Fano resonance through selectively exciting the guiding modes with the grating. The group delay of the EIT is effectively improved up to 2113 ps attributing to the ultrahigh Q-factor resonance carried by the resonance-trapped quasi-BIC. The coupled harmonic oscillator model and a full multipole decomposition are utilized to analyze the physical mechanism of EIT-based quasi-BIC. In addition, the BIC based on Fano and EIT resonance can simultaneously exist at different wavelengths. These findings provide a new feasible platform for slow light devices in the near-infrared region.

Highly efficient nonlinear optical emission from a subwavelength crystalline silicon cuboid mediated by supercavity mode

The low quantum efficiency of silicon (Si) has been a long-standing challenge for scientists. Although improvement of quantum efficiency has been achieved in porous Si or Si quantum dots, highly efficient Si-based light sources prepared by using the current fabrication technooloy of Si chips are still being pursued. Here, we proposed a strategy, which exploits the intrinsic excitation of carriers at high temperatures, to modify the carrier dynamics in Si nanoparticles. We designed a Si/SiO₂ cuboid supporting a quasi-bound state in the continuum (quasi-BIC) and demonstrated the injection of dense electron-hole plasma via two-photon-induced absorption by resonantly exciting the quasi-BIC with femtosecond laser pulses. We observed a significant improvement in quantum efficiency by six orders of magnitude to ~13%, which is manifested in the ultra-bright hot electron luminescence emitted from the Si/SiO₂ cuboid. We revealed that femtosecond laser light with transverse electric polarization (i.e., the electric field perpendicular to the length of a Si/SiO₂ cuboid) is more efficient for generating hot electron luminescence in Si/SiO₂ cuboids as compared with that of transverse magnetic polarization (i.e., the magnetic field perpendicular to the length of a Si/SiO₂ cuboid). Our findings pave the way for realizing on-chip nanoscale Si light sources for photonic integrated circuits and open a new avenue for manipulating the luminescence properties of semiconductors with indirect bandgaps.

Enhancing the efficiency of quantum emitters is essential for exploring new functionalities. Here the authors show Si cuboids that sustain bound states in the continuum enable the injection of dense electron-hole plasma and provide high quantum efficiency.

Plexcitonic Quasi-Bound States in the Continuum

Enhancing light-matter interactions is fundamental to the advancement of nanophotonics and optoelectronics. Yet, light diffraction on dielectric platforms and energy loss on plasmonic metallic systems present an undesirable trade-off between coherent energy exchange and incoherent energy damping. Through judicious structural design, both light confinement and energy loss issues could be potentially and simultaneously addressed by creating bound states in the continuum (BICs) where light is ideally decoupled from the radiative continuum. Herein, the authors present a general framework based on the two-coupled resonances to first conceptualize and then numerically demonstrate a type of quasi-BICs that can be achieved through the interference between two bare resonance modes and is characterized by the considerably narrowed spectral line shape even on lossy metallic nanostructures. The ubiquity of the proposed framework further allows the paradigm to be extended for the realization of plexcitonic quasi-BICs on the same metallic systems. Owing to the topological nature, both plasmonic and plexcitonic quasi-BICs display strong mode robustness against parameters variation, thereby providing an attractive platform to unlock the potential of the coupled plasmon-exciton systems for manipulation of the photophysical properties of condensed phases.

From Fano to Quasi-BIC Resonances in Individual Dielectric Nanoantennas

Sharp optical resonances in high-index dielectric nanostructures have recently attracted significant attention for their promising applications in nanophotonics. Fano resonances, as well as resonances associated with bound states in the continuum (BIC), have independently shown a great potential for applications in nanoscale lasers, sensors, and nonlinear optical devices. Here, we demonstrate experimentally a close connection between Fano and quasi-BIC resonances excited in individual dielectric nanoantennas. We analyze systematically the resonant response of AlGaAs nanoantennas pumped with a structured light in the near-infrared range. We trace a variation of the scattering spectrum that fully agrees with an analytical expression governed by a Fano parameter and observe directly a transition to a quasi-BIC resonance. Our results suggest a unified approach toward the analysis of sharp resonances in subwavelength nanostructures originating from strong coupling of optical modes that can provide high energy localization for enhanced light-matter interactions.

Mie Resonance Engineering in Two Disks

Recently the recipes to achieve the high-Q subwavelength resonances in an isolated dielectric disk have been reported based on avoided crossing (anticrossing) of the TE resonances under variation of the aspect ratio of the disk. In a silicon disk that recipe gives an enhancement of the Q factor by one order of magnitude. In the present paper we present the approach based on engineering of the spherical Mie resonances with high orbital index in two coaxial disks by two-fold avoided crossing of the resonant modes of the disks. At the first step we select the resonant modes of single disk which are degenerate because of the opposite symmetry. Approaching of the second disk removes this degeneracy because of interaction between the disks. As a result at certain distances we realize the hybridized anti-bonding resonant modes whose morphology becomes close to the spherical Mie resonant mode with high orbital index. Respectively the Q factor of the anti-bonding resonant mode can be enhanced by three orders of magnitude compared to the case of single disk.