Highly Controllable Etchless Perovskite Microlasers Based on Bound States in the Continuum

Exciton polariton condensation from bound states in the continuum at room temperature

Exciton-polaritons (polaritons) resulting from the strong exciton-photon interaction stimulates the development of novel low-threshold coherent light sources to circumvent the ever-increasing energy demands of optical communications1-3. Polaritons from bound states in the continuum (BICs) are promising for Bose-Einstein condensation owing to their theoretically infinite quality factors, which provide prolonged lifetimes and benefit the polariton accumulations4-7. However, BIC polariton condensation remains limited to cryogenic temperatures ascribed to the small exciton binding energies of conventional material platforms. Herein, we demonstrated room-temperature BIC polariton condensation in perovskite photonic crystal lattices. BIC polariton condensation was demonstrated at the vicinity of the saddle point of polariton dispersion that generates directional vortex beam emission with long-range coherence. We also explore the peculiar switching effect among the miniaturized BIC polariton modes through effective polariton-polariton scattering. Our work paves the way for the practical implementation of BIC polariton condensates for integrated photonic and topological circuits.

Bound states in the continuum in circular waveguides: toward the on-chip integration of nanofiber on silicon platform

In previously reported researches on bound state in the continuum (BIC) waveguides, almost all of them are demonstrated with top-down fabrication procedures, leading to inconvenience for post-manipulation and size tuning. Nanofibers with circular cross sections are the fundamental components to transport energy due to their intrinsic advantages of high flexibility and adjustability, which is replaceable and can be readily manipulated over size and position on the substrate. In this work, we explore the possibility of achieving on-chip integration of silica nanofiber onto a silicon-on-insulator platform. By constructing additional leakage channels in coupled nanofiber waveguides, coherently destructive interferences are successfully achieved. The heavy leakage losses from the low-index nanofiber to a high-index silicon substrate are completely eliminated with BIC, and the propagation length of the nanofiber waveguide is significantly improved.

Switchable unidirectional emissions from hydrogel gratings with integrated carbon quantum dots

Directional emission of photoluminescence despite its incoherence is an attractive technique for light-emitting fields and nanophotonics. Optical metasurfaces provide a promising route for wavefront engineering at the subwavelength scale, enabling the feasibility of unidirectional emission. However, current directional emission strategies are mostly based on static metasurfaces, and it remains a challenge to achieve unidirectional emissions tuning with high performance. Here, we demonstrate quantum dots-hydrogel integrated gratings for actively switchable unidirectional emission with simultaneously a narrow divergence angle less than 1.5° and a large diffraction angle greater than 45°. We further demonstrate that the grating efficiency alteration leads to a more than 7-fold tuning of emission intensity at diffraction order due to the variation of hydrogel morphology subject to change in ambient humidity. Our proposed switchable emission strategy can promote technologies of active light-emitting devices for radiation control and optical imaging.

Pushing Optical Virus Detection to a Single Particle through a High-Q Quasi-bound State in the Continuum in an All-dielectric Metasurface

Bound states in the continuum (BICs) have emerged as a powerful platform for boosting light-matter interactions because they provide an alternative way of realizing optical resonances with ultrahigh quality factors, accompanied by extreme field confinement. In this work, we realized an optical biosensor by harnessing a quasi-BIC (qBIC) supported by an all-dielectric metasurface with broken symmetry, whose unit cell is composed of a silicon cuboid with two asymmetric air holes. Thanks to the excellent field confinement within the air gap of a metasurface enabled by such a high-Q qBIC, the figure of merit (FOM) of the biosensor is up to 2136.35 RIU-1. Futhermore, we demonstrated that such a high-Q metasurface can push the detection limit to a few virus particles. Our results may find exciting applications in extreme biochemical sensing like COVID-19 with ultralow concentrations.

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.

Quasi-2D Dion-Jacobson phase perovskites as a promising material platform for stable and high-performance lasers

Metal halide perovskites have shown outstanding optoelectronic and nonlinear optical properties; yet, to realize wafer-scale high-performance perovskite-integrated photonics, the materials also need to have excellent ambient stability and compatibility with nanofabrication processes. In this work, we introduce Dion-Jacobson (D-J) phase perovskites for photonic device applications. By combining self-assembled monolayer-assisted film growth with thermal pressing, we obtain a series of compact and extremely smooth D-J phase perovskite thin films that exhibit excellent stability during electron-beam lithography, solvent development, and rinse. Combining spectroscopic and morphological characterizations, we further demonstrate how organic spacers can be used to fine-tune the photophysical properties and processability of the perovskite films. The distributed-feedback lasers based on the D-J phase perovskites exhibit a low lasing threshold (5.5 μJ cm-2 pumped with nanosecond laser), record high Q factor (up to 30,000), and excellent stability, with an unencapsulated device demonstrating a T90 beyond 60 hours in ambient conditions (50% relative humidity).

High-Q Quasi-Bound States in the Continuum in Terahertz All-Silicon Metasurfaces

Bound states in the continuum (BIC)-based all-silicon metasurfaces have attracted widespread attention in recent years because of their high quality (Q) factors in terahertz (THz) frequencies. Here, we propose and experimentally demonstrate an all-silicon BIC metasurface consisting of an air-hole array on a Si substrate. BICs originated from low-order TE and TM guided mode resonances (GMRs) induced by (1,0) and (1,1) Rayleigh diffraction of metagratings, which were numerically investigated. The results indicate that the GMRs and their Q-factors are easily excited and manipulated by breaking the lattice symmetry through changes in the position or radius of the air-holes, while the resonance frequencies are less sensitive to these changes. The measured Q-factor of the GMRs is as high as 490. The high-Q metasurfaces have potential applications in THz modulators, biosensors, and other photonic devices.

A platform for integrated spectrometers based on solution-processable semiconductors

Acquiring real-time spectral information in point-of-care diagnosis, internet-of-thing, and other lab-on-chip applications require spectrometers with hetero-integration capability and miniaturized feature. Compared to conventional semiconductors integrated by heteroepitaxy, solution-processable semiconductors provide a much-flexible integration platform due to their solution-processability, and, therefore, more suitable for the multi-material integrated system. However, solution-processable semiconductors are usually incompatible with the micro-fabrication processes. This work proposes a facile and universal platform to fabricate integrated spectrometers with semiconductor substitutability by unprecedently involving the conjugated mode of the bound states in the continuum (conjugated-BIC) photonics. Specifically, exploiting the conjugated-BIC photonics, which remains unexplored in conventional lasing studies, renders the broadband photodiodes with ultra-narrowband detection ability, detection wavelength tunability, and on-chip integration ability while ensuring the device performance. Spectrometers based on these ultra-narrowband photodiode arrays exhibit high spectral resolution and wide/tunable spectral bandwidth. The fabrication processes are compatible with solution-processable semiconductors photodiodes like perovskites and quantum dots, which can be potentially extended to conventional semiconductors. Signals from the spectrometers directly constitute the incident spectra without being computation-intensive, latency-sensitive, and error-intolerant. As an example, the integrated spectrometers based on perovskite photodiodes are capable of realizing narrowband/broadband light reconstruction and in-situ hyperspectral imaging.

Collective excitations of a bound-in-the-continuum condensate

Spectra of low-lying elementary excitations are critical to characterize properties of bosonic quantum fluids. Usually these spectra are difficult to observe, due to low occupation of non-condensate states compared to the ground state. Recently, low-threshold Bose-Einstein condensation was realised in a symmetry-protected bound state in the continuum, at a saddle point, thanks to coupling of this electromagnetic resonance to semiconductor excitons. While it has opened the door to long-living polariton condensates, their intrinsic collective properties are still unexplored. Here we unveil the peculiar features of the Bogoliubov spectrum of excitations in this system. Thanks to the dark nature of the bound-in-the-continuum state, collective excitations lying directly above the condensate become observable in enhanced detail. We reveal interesting aspects, such as energy-flat parts of the dispersion characterized by two parallel stripes in photoluminescence pattern, pronounced linearization at non-zero momenta in one of the directions, and a strongly anisotropic velocity of sound.

Nonlinear two-photon pumped vortex lasing based on quasi-bound states in the continuum from perovskite metasurface

The experimental observation of nonlinear two-photon pumped vortex lasing from perovskite metasurfaces is demonstrated. The vortex lasing beam is based on symmetry-protected quasi-bound states in the continuum (QBICs). The topological charge is estimated to be +1 according to the simulation result. The quality factor and lasing threshold are around 1100 and 4.28 mJ/cm2, respectively. Theoretical analysis reveals that the QBIC mode originates from the magnetic dipole mode. The lasing wavelength can be experimentally designed within a broad spectral range by changing the diameter and periodicity of the metasurface. The finite array size effect of QBIC can affect the quality factor of the lasing and be used to modulate the lasing. Results shown in this study can lead to more complex vortex beam lasing from a single chip and previously unidentified ways to obtain ultrafast modulation of the QBIC lasing via the finite array size effect.

Ultrahigh-Q Lead Halide Perovskite Microlasers

Lead halide perovskites have been promising platforms for micro- and nanolasers. However, the fragile nature of perovskites poses an extreme challenge to engineering a cavity boundary and achieving high-quality (Q) modes, severely hindering their practical applications. Here, we combine an etchless bound state in the continuum (BIC) and a chemically synthesized single-crystalline CsPbBr₃ microplate to demonstrate on-chip integrated perovskite microlasers with ultrahigh Q factors. By pattering polymer microdisks on CsPbBr₃ microplates, we show that record high-Q BIC modes can be formed by destructive interference between different in-plane radiation from whispering gallery modes. Consequently, a record high Q-factor of 1.04 × 10⁵ was achieved in our experiment. The high repeatability and high controllability of such ultrahigh Q BIC microlasers have also been experimentally confirmed. This research provides a new paradigm for perovskite nanophotonics.

Ultra-low threshold continuous-wave quantum dot mini-BIC lasers

Highly compact lasers with ultra-low threshold and single-mode continuous wave (CW) operation have been a long sought-after component for photonic integrated circuits (PICs). Photonic bound states in the continuum (BICs), due to their excellent ability of trapping light and enhancing light-matter interaction, have been investigated in lasing configurations combining various BIC cavities and optical gain materials. However, the realization of BIC laser with a highly compact size and an ultra-low CW threshold has remained elusive. We demonstrate room temperature CW BIC lasers in the 1310 nm O-band wavelength range, by fabricating a miniaturized BIC cavity in an InAs/GaAs epitaxial quantum dot (QD) gain membrane. By enabling effective trapping of both light and carriers in all three dimensions, ultra-low threshold of 12 μW (0.052 kW cm^-2) is achieved at room temperature. Single-mode lasing is also realized in cavities as small as only 5 × 5 unit cells (~2.5 × 2.5 μm² cavity size) with a mode volume of 1.16(λ/n)³. The maximum operation temperature reaches 70 °C with a characteristic temperature of T₀ ~93.9 K. With its advantages in terms of a small footprint, ultra-low power consumption, and adaptability for integration, the mini-BIC lasers offer a perspective light source for future PICs aimed at high-capacity optical communications, sensing and quantum information.

Monolithically integrated photonic crystal surface emitters on silicon with a vortex beam by using bound states in the continuum

Optical resonant cavities with high quality factor (Q-factor) are widely used in science and technology for their capabilities of strong confinement of light and enhanced light-matter interaction. The 2D photonic crystal structure with bound states in the continuum (BICs) is a novel concept for resonators with ultra-compact device size, which can be used to generate surface emitting vortex beams based on symmetry-protected BICs at the Γ point. Here, to the best of our knowledge, we demonstrate the first photonic crystal surface emitter with a vortex beam by using BICs monolithically grown on CMOS-compatible silicon substrate. The fabricated quantum-dot BICs-based surface emitter operates at 1.3 µm under room temperature (RT) with a low continuous wave (CW) optically pumped condition. We also reveal the BIC's amplified spontaneous emission with the property of a polarization vortex beam, which is promising to provide a novel degree of freedom in classical and quantum realms.

A High Quality-Factor Optical Modulator with Hybrid Graphene-Dielectric Metasurface Based on the Quasi-Bound States in the Continuum

In this paper, we combine the dielectric metasurface with monolayer graphene to realize a high quality(Q)-factor quasi-BIC-based optical modulator, and the corresponding modulation performances are investigated by using the finite-difference time-domain (FDTD) method, which can be well fitting by the Fano formula based on the temporal couple-mode theory. The results demonstrate that bound states in the continuum (BIC) will turn into the quasi-BIC with high Q-factor by breaking the symmetry of every unit of the metasurface. Meanwhile, the amplitude and bandwidth of transmission based on the quasi-BIC mode can be efficiently adjusted by changing the Fermi energy (E_F) of monolayer graphene, and the maximum difference in transmission up to 0.92 is achieved. Moreover, we also discuss the influence of the asymmetry degree to further investigate the modulation effect of graphene on the quasi-BIC mode.

Giant Enhancement and Directional Second Harmonic Emission from Monolayer WS2 on Silicon Substrate via Fabry-Pérot Micro-Cavity

Two-dimensional transition metal dichalcogenides (TMDs) possess large second-order optical nonlinearity, making them ideal candidates for miniaturized on-chip frequency conversion devices, all-optical interconnection, and optoelectronic integration components. However, limited by subnanometer thickness, the monolayer TMD exhibits low second harmonic generation (SHG) conversion efficiency (<0.1%) and poor directionality, which hinders their practical applications. Herein, we proposed a Fabry-Pérot (F-P) cavity formed by coupling an atomically thin WS₂ film with a silicon hole matrix to enhance the SH emission. A maximum enhancement (∼1580 times) is achieved by tuning the excitation wavelength to be resonant with the microcavity modes. The giant enhancement is attributed to the strong electric field enhancement in the F-P cavity and the oscillator strength enhancement of excitons from suspended WS₂. Moreover, directional SH emission (divergence angle ∼5°) is obtained benefiting from the resonance of the F-P microcavity. Our research results can provide a practical sketch to develop both high-efficiency and directional nonlinear optical devices for silicon-based on-chip integration optics.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Multifunctional and Transformative Metaphotonics with Emerging Materials

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Directional Lasing from Nanopatterned Halide Perovskite Nanowire

Halide perovskite nanowire-based lasers have become a powerful tool for modern nanophotonics, being deeply subwavelength in cross-section and demonstrating low-threshold lasing within the whole visible spectral range owing to the huge gain of material even at room temperature. However, their emission directivity remains poorly controlled because of the efficient outcoupling of radiation through their subwavelength facets working as pointlike light sources. Here, we achieve directional lasing from a single perovskite CsPbBr₃ nanowire by imprinting a nanograting on its surface, which provides stimulated emission outcoupling to its vertical direction with a divergence angle around 2°. The nanopatterning is carried out by the high-throughput laser ablation method, which preserves the luminescent properties of the material that is typically deteriorated after processing via conventional lithographic approaches. Moreover, nanopatterning of the perovskite nanowire is found to decrease the number of the lasing modes with a 2-fold increase of the quality factor of the remaining modes.

Light-matter interactions enhanced by quasi-bound states in the continuum in a graphene-dielectric metasurface

In this paper, we propose a graphene-dielectric metasurface to enhance the light-matter interactions in graphene. The dielectric metasurface consists of periodically arranged silicon split rings placed on the silica substrate, which supports a symmetry-protected bound state in the continuum (BIC). When perturbation is introduced into the system to break symmetry, the BIC will transform into the quasi-BIC with high quality (Q)-factor. As the graphene layer is integrated with the dielectric metasurface, the absorption of graphene can be enhanced by the quasi-BIC resonance and a bandwidth-tunable absorber can be achieved by optimizing the Fermi energy of graphene and the asymmetry parameter of the metasurface to satisfy the critical coupling condition. By varying the Fermi energy of graphene, the quasi-BIC resonances can be effectively modulated and the max transmission intensity difference is up to 81% and a smaller asymmetry parameter will lead to better modulation performance. Our results may provide theoretical support for the design of absorber and modulator based on the quasi-BIC.

Giant Enhancement of Continuous Wave Second Harmonic Generation from Few-Layer GaSe Coupled to High-Q Quasi Bound States in the Continuum

Two-dimensional (2D) layered materials such as GaSe recently have emerged as novel nonlinear optical materials with exceptional properties. Although exhibiting large nonlinear susceptibilities, the nonlinear responses of 2D materials are generally limited by the short interaction lengths with light, thus further enhancement via resonant photonic nanostructures is highly desired for building high-efficiency nonlinear devices. Here, we demonstrate a giant second-harmonic generation (SHG) enhancement by coupling 2D GaSe flakes to silicon metasurfaces supporting quasi-bound states in the continuum (quasi-BICs) under continuous-wave (CW) operation. Taking advantage of both high-quality factors and large mode areas of quasi-BICs, SHG from a GaSe flake is uniformly enhanced by nearly 4 orders of magnitude, which is promising for high-power coherent light sources. Our work provides an effective approach for enhancing nonlinear optical processes in 2D materials within the framework of silicon photonics, which also brings second-order nonlinearity associated with 2D materials to silicon photonic devices.

Circularly Polarized Emission from Organic-Inorganic Hybrid Perovskites via Chiral Fano Resonances

Organic-inorganic hybrid perovskites hold great potential for various optoelectronic devices with exceptional properties. Although the direct generation of circularly polarized emission from perovskites would enable various compact devices, achieving a large degree of circular polarization (DCP) at room temperature still remains challenging. Herein, we demonstrate that DCP can be strongly enhanced at the narrow mode position of chiral Fano resonances. In our design, a perovskite film is spin-coated on a symmetry-broken structure with a relatively large feature size. A large DCP of more than 0.5 is achieved at room temperature without the direct patterning of the perovskite layer. Reciprocity calculation reveals that chiral field enhancement enables the emission of opposite helicity to couple into counter-propagating slab modes and leads to a large DCP. Our design is very general and scalable. Our work may lead to circularly polarized light sources based on various perovskite materials.