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

Ultrahigh-Q Quasi-BICs via Precision-Controlled Asymmetry in Dielectric Metasurfaces

Achieving ultrahigh quality factor optical resonances is crucial for advancing low-threshold lasers, high-sensitivity sensors, and nonlinear photonics. While dielectric metasurfaces supporting quasi-bound states in the continuum (qBICs) show great potential, their experimental realization has been challenging due to the difficulty of precisely controlling symmetry-breaking at the nanoscale. Here, we introduce a precision-controlled symmetry-protected qBIC method using angular perturbations to precisely tune asymmetry, ensuring both high precision and reproducibility of Q-factors. In contrast to traditional SP-qBIC excitation, which relies on uncontrolled asymmetry, our method offers more accurate and consistent control by precisely tuning angular perturbations instead of structural variations. Additionally, our approach defines the lower limit of achievable Q-factors, providing a reliable lower bound. The experimental demonstration of SP-qBICs in composite nanoslit metasurfaces achieves a record-breaking Q-factor of 1.1 × 105-the highest reported for SP-qBICs to date. These findings offer a promising platform for designing ultrahigh-Q resonators for next-generation photonic-applications.

Polarization-Independent Enhancement of Third-Harmonic Generation Empowered by Doubly Degenerate Quasi-Bound States in the Continuum

Recent advancements in nonlinear nanophotonics are driven by the exploration of sharp resonances within high-index dielectric metasurfaces. In this work, we leverage doubly degenerate quasi-bound states in the continuum (quasi-BICs) to demonstrate the robust enhancement of third-harmonic generation (THG) in silicon metasurfaces. These quasi-BICs are governed by C4v symmetry and therefore can be equally excited with the pump light regardless of polarization. By tailoring the geometric parameters, we effectively control Q-factors and field confinement of quasi-BICs and thus regulate their resonantly enhanced THG process. A maximum THG conversion efficiency up to 1.03 × 10-5 is recorded under a pump intensity of 5.85 GW/cm2. Polarization-independent THG profiles are further confirmed by mapping their signals across the polarization directions. This work establishes foundational strategies for the ultracompact design of robust and high-efficiency photon upconversion systems.

Tailoring second harmonic generation in a multilayer WS2 flake via a silicon circular Bragg grating

Nonlinear emission phenomena observed in transition metal dichalcogenides (TMDCs) have significantly advanced the development of robust nonlinear optical sources within two-dimensional materials. However, the intrinsic emission characteristics of TMDCs are inherently dependent on the specific material, which constrains their tunability for practical applications. In this study, we propose a strategy for the selective enhancement and modification of second-harmonic generation (SHG) emission in a multilayer WS2 flake through the implementation of a silicon (Si)-based circular Bragg grating (CBG) structure positioned on an Au/SiO2 substrate. By selectively exciting the region of the circular Bragg grating with the grating oriented either parallel or perpendicular to the linearly polarized pump beam, we successfully achieved wavelength-tunable SHG intensity peaks at 402.5 nm and 425 nm for the respective alignments. Our experimental findings, corroborated by numerical simulations, indicate that the enhancement of SHG intensity is highly sensitive to the orientation of the grating region of the CBG. Power-dependent SHG spectra further validate the quadratic dependence of SHG, while comparative analysis with WS2 flakes on a bare Si/Au/SiO2 substrate highlights the critical role of the CBG structure in modulating SHG. This research underscores the potential of CBG-augmented TMDCs for the control of nonlinear optical emissions, suggesting promising applications in photonic devices and selective emission technologies.

van der Waals NbOCl2 Nanodisks for Enhanced Second-Harmonic Generation

Niobium oxide dichloride (NbOCl2), a van der Waals crystal, exhibits exceptional second-harmonic generation (SHG) with high conversion efficiency, making it a promising candidate for on-chip quantum light sources, photon modulators, and sensors. To increase its suitability for integrated photonic systems, it is crucial to explore strategies for further improving optical nonlinear conversion efficiency. In this article, dielectric metasurfaces are employed to enhance SHG from patterned NbOCl2 nanodisks. By precisely tuning their geometric parameters to achieve optical resonance near the excitation wavelength, a remarkable 25-fold enhancement in second-harmonic emission at 400 nm is achieved in nanodisks compared to the unpatterned thin film. The strong field enhancement within the NbOCl2 nanodisks at nanophotonic resonances contributes to the observed increase in SHG. This work offers an effective strategy for boosting SHG in van der Waals NbOCl2 thin films, paving the way for the development of efficient nonlinear optical components in integrated nanophotonics.

Trapping light in air with membrane metasurfaces for vibrational strong coupling

Optical metasurfaces can manipulate electromagnetic waves in unprecedented ways at ultra-thin engineered interfaces. Specifically, in the mid-infrared (mid-IR) region, metasurfaces have enabled numerous biochemical sensing, spectroscopy, and vibrational strong coupling (VSC) applications via enhanced light-matter interactions in resonant cavities. However, mid-IR metasurfaces are usually fabricated on solid supporting substrates, which degrade resonance quality factors (Q) and hinder efficient sample access to the near-field electromagnetic hotspots. Besides, typical IR-transparent substrate materials with low refractive indices, such as CaF2, NaCl, KBr, and ZnSe, are usually either water-soluble, expensive, or not compatible with low-cost mass manufacturing processes. Here, we present novel free-standing Si-membrane mid-IR metasurfaces with strong light-trapping capabilities in accessible air voids. We employ the Brillouin zone folding technique to excite tunable, high-Q quasi-bound states in the continuum (qBIC) resonances with our highest measured Q-factor of 722. Leveraging the strong field localizations in accessible air cavities, we demonstrate VSC with multiple quantities of PMMA molecules and the qBIC modes at various detuning frequencies. Our new approach of fabricating mid-IR metasurfaces into semiconductor membranes enables scalable manufacturing of mid-IR photonic devices and provides exciting opportunities for quantum-coherent light-matter interactions, biochemical sensing, and polaritonic chemistry.

Ultrafast Modulation of a Nonlocal Semiconductor Metasurface under Spatially Selective Optical Pumping

Time-varying optical metasurfaces have attracted significant research attention for their ability to dynamically modulate the spectral characteristics of light and achieve effects not possible in static systems. Nonlocal metasurfaces, with their spatially extended resonant modes, have the potential to further expand these capabilities by enabling spatially selective modulation. Here, we experimentally explore the application of spatially structured femtosecond laser pumping to modulate a leaky guided mode in a semiconductor metasurface. Using angle-resolved transient transmission spectroscopy, we study the dynamic response of the system across a range of wavelengths and angles of incidence, observing frequency and momentum conversion. When the pumping location is varied, this technique allows for reconstructing the temporal dynamics of the probe pulse propagating in the metasurface mode. Our results not only demonstrate a versatile toolkit for spatiotemporal light control but also provide fundamental insight into the excitation of spatially extended modes in nonlocal metasurfaces.

Experimental Demonstration of High-Efficiency Harmonic Generation in Photonic Moiré Superlattice Microcavities

Integrated photonic microcavities have demonstrated powerful enhancement of nonlinear effects, but they face a challenge in achieving critical coupling for sufficient use of incident pump power. In this work, we first experimentally demonstrate that highly efficient third-harmonic generation (THG) and detectable second-harmonic generation (SHG) can be produced from high-Q photonic moiré superlattice microcavities, where a critical coupling condition can be achieved via selecting a magic angle. Furthermore, at the magic angle of 13.17°, critical coupling is satisfied, resulting in a normalized THG conversion efficiency of 136%/W2 at a relatively low peak pump power of 6.8 MW/cm2, which is 3 orders of magnitude higher than the best results reported previously. Our work shows the power of photonic moiré superlattices in enhancing nonlinear optical performances through flexible and feasible engineering resonant modes, which can be applied in integrated frequency conversion and generation of quantum light sources.

Enhanced vertical second harmonic generation from layered GaSe coupled to photonic crystal circular Bragg resonators

Two-dimensional (2D) layered materials without centrosymmetry, such as GaSe, have emerged as promising novel optical materials due to large second-order nonlinear susceptibilities. However, their nonlinear responses are severely limited by the short interaction between the 2D materials and light, which should be improved by coupling them with photonic structures with strong field confinement. Here, we theoretically design photonic crystal circular Bragg gratings (CBG) based on hole gratings with a quality factor as high as Q = 8 × 103, a mode volume as small as V = 1.18 (λ/n)3, and vertical emission of light field in silicon nitride thin film platform. Experimentally, we achieved a Q value up to nearly 4 × 103, resulting in a 1,200-fold enhancement of second harmonic generation from GaSe flakes with a thickness of 50 nm coupling to the CBG structures under continuous-wave excitation. Our work endows silicon-based photonic platforms with significant second-order nonlinear effect, which is potentially applied in on-chip quantum light sources and nonlinear frequency conversion.

Tunable Intracavity Coherent Up-Conversion with Giant Nonlinearity in a Polar Fluidic Medium

The study has demonstrated a novel microcavity-based flexible photon up-conversion system using second harmonic generation (SHG) from a polar nematic fluidic medium doped with a laser dye. The idea is based on coherent light generation via stimulated emission (lasing) and simultaneous frequency doubling inside a microcavity. The polar nematic fluid equips very high even-order optical nonlinearity due to its polar symmetry and large dipole moment along the molecular long axis. At the same time, its inherent fluidic nature allows to easily functionalize the media just by doping, in the present case, with an emissive laser dye. The demonstrated system exhibits a giant nonlinear optical response to input light, while enabling spectral narrowing and multiple-signal output of up-converted light, which is not attainable through the simple SH-conversion of input light. Furthermore, the susceptibility of the liquid crystal offers dynamic modulation capabilities by an external stimulus, such as signal switching by the application of electric field or wavelength tuning through temperature variation. Such a brand-new type of simple coherent flexible up-conversion system must be promising as a new principle for easily accessible and down-scalable wavelength conversion devices.

Quasi-BICs enhanced second harmonic generation from WSe2 monolayer

Quasi-bound states in the continuum (quasi-BICs) offer unique advantages in enhancing nonlinear optical processes and advancing the development of active optical devices. Here, the tunable robust quasi-BICs resonances are experimentally achieved through the engineering of multiple-hole Si-metasurface. Notably, the quasi-BICs mode exhibits flat bands with minimal dispersion at a wide range of incident angles, as demonstrated by the angle-resolved spectroscopy measurements. Furthermore, we demonstrate a giant second-harmonic generation (SHG) enhancement by coupling a WSe2 monolayer to the quasi-BICs hosted in the metasurface. Leveraging the strong local electric field and high state density of the observed quasi-BICs, the SHG from the WSe2 monolayer can be enhanced by more than two orders of magnitude. Our work paves the way for effectively enhancing nonlinear optical processes in two dimensional (2D) materials within the framework of silicon photonics and is expected to be applied in nonlinear optical devices.

Nonlinear Optical Properties of 2D Materials and their Applications

2D materials are a subject of intense research in recent years owing to their exclusive photoelectric properties. With giant nonlinear susceptibility and perfect phase matching, 2D materials have marvelous nonlinear light-matter interactions. The nonlinear optical properties of 2D materials are of great significance to the design and analysis of applied materials and functional devices. Here, the fundamental of nonlinear optics (NLO) for 2D materials is introduced, and the methods for characterizing and measuring second-order and third-order nonlinear susceptibility of 2D materials are reviewed. Furthermore, the theoretical and experimental values of second-order susceptibility χ(2) and third-order susceptibility χ(3) are tabulated. Several applications and possible future research directions of second-harmonic generation (SHG) and third-harmonic generation (THG) for 2D materials are presented.

From weak to strong coupling: quasi-BIC metasurfaces for mid-infrared light-matter interactions

Thanks to their giant, yet tunable, Q-factor resonances, all-dielectric metasurfaces supporting the quasi-bound states in the continuum (q-BIC) resonances are well-suited to provide a promising platform for quantum-coherent light-matter interactions. Yet, the strong coupling regime, characterized by the hybrid light-matter states - polaritons, has not yet been fully explored in the mid-infrared regime. This paper investigates the parameter space of vibrational strong coupling (VSC) between material and metasurface cavities supporting q-BIC resonances in the mid-infrared spectral range. We outline the effects of transition dipole strength, damping rate, and the number of molecules coupled to a single cavity, as well as the cavity damping rates, to understand their respective impacts on VSC. By tuning the Q-factor of the metasurface and material parameters, a new transition light-matter coupling zone is introduced, bridging the gap between weak and strong coupling, where polaritons form but their linewidths prohibit their spectral identification. The study further identifies the effects of cavity linewidth on polariton peak separability in strongly coupled systems, highlighting that the cavities with smaller nonradiative losses and narrower linewidths facilitate better polariton separability. Moreover, we found that matching cavity and material loss, satisfying the critical strong coupling condition, enhances the coupling strength between cavity and material. Overall, these findings can guide the design of photonic cavities suited for VSC experiments, contributing to the burgeoning fields of polaritonic chemistry, light-mediated modulation of chemical reactivity, and highly sensitive molecular spectroscopy.

Extraordinary Enhancement of Nonlinear Optical Interaction in NbOBr2 Microcavities

2D materials are burgeoning as promising candidates for investigating nonlinear optical effects due to high nonlinear susceptibilities, broadband optical response, and tunable nonlinearity. However, most 2D materials suffer from poor nonlinear conversion efficiencies, resulting from reduced light-matter interactions and lack of phase matching at atomic thicknesses. Herein, a new 2D nonlinear material, niobium oxide dibromide (NbOBr2) is reported, featuring strong and anisotropic optical nonlinearities with scalable nonlinear intensity. Furthermore, Fabry-Pérot (F-P) microcavities are constructed by coupling NbOBr2 with air holes in silicon. Remarkable enhancement factors of ≈630 times in second harmonic generation (SHG) and 210 times in third harmonic generation (THG) are achieved on cavity at the resonance wavelength of 1500 nm. Notably, the cavity enhancement effect exhibits strong anisotropic feature tunable with pump wavelength, owing to the robust optical birefringence of NbOBr2. The ratio of the enhancement factor along the b- and c-axis of NbOBr2 reaches 2.43 and 5.27 for SHG and THG at 1500 nm pump, respectively, which leads to an extraordinarily high SHG anisotropic ratio of 17.82 and a 10° rotation of THG polarization. The research presents a feasible and practical strategy for developing high-efficiency and low-power-pumped on-chip nonlinear optical devices with tunable anisotropy.

Observation of tunable accidental bound state in the continuum in silicon nanodisk array

We experimentally demonstrate the tuning of accidental bound states in the continuum (A-BICs) in silicon nanodisk arrays. The A-BIC emerges of the destructive interference of multipoles, which are the dominating out-of-plane electric dipole and in-plane magnetic dipole, and weak electric quadrupole and magnetic quadrupole. We further show that the spectral and angular position of the A-BIC can be conveniently tuned by varying the nanodisk size or the lattice period. Remarkably, the angular position can be tuned even to 0°, suggesting an interesting transition of the A-BIC from an off-Γ-BIC to an at-Γ-BIC. Our work provides a new strategy for light trapping with high quality factors, and the obtained tunable A-BICs can find potential applications in low-threshold lasing, enhanced nonlinear optics, and optical sensing.

Nonreciprocal toroidal dipole resonance and one-way quasi-bound state in the continuum

Bound states in the continuum (BICs) provide an alternative way of trapping light at nanoscale. Although the last 10 years have witnessed tremendous progress on BICs from fundamentals to applications, nonreciprocal BICs and their potential applications have not been fully exploited yet. In this study, we demonstrated a one-way quasi-BIC by leveraging an all-dielectric magneto-optical (MO) metasurface. We show that the key point for achieving a one-way quasi-BIC is to excite a magnetization-induced leaky resonance. Here we adopt the longitudinal toroidal dipole (TD) resonance characterized by a vortex distribution of head-to-tail magnetic dipoles parallel to the plane of the MO metasurface. We show that, by breaking the time-reversal symmetry, at critical conditions, the TD resonance can be enhanced in the forward channel and perfectly canceled in the time-reversed channel, resulting in a one-way quasi-BIC. The demonstrated phenomena hold significant promise for practical applications such as magnetic field optical sensing, nonreciprocal optical switching, isolation, and modulation.

Directive giant upconversion by supercritical bound states in the continuum

Photonic bound states in the continuum (BICs), embedded in the spectrum of free-space waves1,2 with diverging radiative quality factor, are topologically non-trivial dark modes in open-cavity resonators that have enabled important advances in photonics3,4. However, it is particularly challenging to achieve maximum near-field enhancement, as this requires matching radiative and non-radiative losses. Here we propose the concept of supercritical coupling, drawing inspiration from electromagnetically induced transparency in near-field coupled resonances close to the Friedrich-Wintgen condition2. Supercritical coupling occurs when the near-field coupling between dark and bright modes compensates for the negligible direct far-field coupling with the dark mode. This enables a quasi-BIC field to reach maximum enhancement imposed by non-radiative loss, even when the radiative quality factor is divergent. Our experimental design consists of a photonic-crystal nanoslab covered with upconversion nanoparticles. Near-field coupling is finely tuned at the nanostructure edge, in which a coherent upconversion luminescence enhanced by eight orders of magnitude is observed. The emission shows negligible divergence, narrow width at the microscale and controllable directivity through input focusing and polarization. This approach is relevant to various physical processes, with potential applications for light-source development, energy harvesting and photochemical catalysis.

Inverse design of all-dielectric metasurfaces with accidental bound states in the continuum

Metasurfaces with bound states in the continuum (BICs) have proven to be a powerful platform for drastically enhancing light-matter interactions, improving biosensing, and precisely manipulating near- and far-fields. However, engineering metasurfaces to provide an on-demand spectral and angular position for a BIC remains a prime challenge. A conventional solution involves a fine adjustment of geometrical parameters, requiring multiple time-consuming calculations. In this work, to circumvent such tedious processes, we develop a physics-inspired, inverse design method on all-dielectric metasurfaces for an on-demand spectral and angular position of a BIC. Our suggested method predicts the core-shell particles that constitute the unit cell of the metasurface, while considering practical limitations on geometry and available materials. Our method is based on a smart combination of a semi-analytical solution, for predicting the required dipolar Mie coefficients of the meta-atom, and a machine learning algorithm, for finding a practical design of the meta-atom that provides these Mie coefficients. Although our approach is exemplified in designing a metasurface sustaining a BIC, it can, also, be applied to many more objective functions. With that, we pave the way toward a general framework for the inverse design of metasurfaces in specific and nanophotonic structures in general.

Twisted lattice nanocavity with theoretical quality factor exceeding 200 billion

Simultaneous localization of light to extreme spatial and spectral scales is of high importance for testing fundamental physics and various applications. However, there is a longstanding trade-off between localizing a light field in space and in frequency. Here we discover a new class of twisted lattice nanocavities based on mode locking in momentum space. The twisted lattice nanocavity hosts a strongly localized light field in a 0.048 λ3 mode volume with a quality factor exceeding 2.9 × 1011 (∼250 μs photon lifetime), which presents a record high figure of merit of light localization among all reported optical cavities. Based on the discovery, we have demonstrated silicon-based twisted lattice nanocavities with quality factor over 1 million. Our result provides a powerful platform to study light-matter interaction in extreme conditions for tests of fundamental physics and applications in nanolasing, ultrasensing, nonlinear optics, optomechanics and quantum-optical devices.

Enhancement of spontaneous emission from CdSe/ZnS quantum dots through silicon nitride photonic crystal cavity based on miniaturized bound states in the continuum

Colloidal CdSe/ZnS quantum dots (QDs) exhibit excellent optical properties for wide potential applications in light-emitting diodes, solar concentrators, and single-photon sources. However, the ultra-thin films with low concentration of QDs still encounter inefficient photoluminescence (PL) and poor directionality of radiation, which need to be enhanced using nanophotonics device designs. Here we design and experimentally demonstrate an on-substrate silicon nitride (SiN) photonic crystal (PhC) microcavity encapsulated by a layer of PMMA hosting CdSe/ZnS QDs. The miniaturized bound states in the continuum (BIC) supported by our structures, provide high-Q resonant modes with highly-directional emission patterns. Experimental results show that the BIC mode in the microcavity has a Q-factor up to 7000 owing to the symmetric refractive index distribution along the Z-direction, rendering 8.5-fold enhancement of PL intensity and 8.4-fold acceleration of radiative emission rate. Our work provides a practical way for constructing efficient on-chip surface-emitting light sources on silicon-based integrated photonic devices.

Observation of multiple bulk bound states in the continuum modes in a photonic crystal cavity

Obtaining bound states in the continuum (BICs) in photonic crystals gives rise to the realization of resonances with high quality factors for lasing and nonlinear applications. For BIC cavities in finite-size photonic crystals, the bulk resonance band turns into discrete modes with different mode profiles and radiation patterns. Here, photonic-crystal BIC cavities encircled by the photonic bandgap of lateral heterostructures are designed. The mirror-like photonic bandgap exhibits strong side leakage suppression to confine the mode profile in the designed cavity. Multiple bulk quantized modes are observed both in simulation and experiment. After exciting the BIC cavity at different positions, different resonance peaks are observed. The physical origin of the dependence between the resonance peak and the illuminating position is explained by analyzing the mode profile distribution and further verified by numerical simulations. Our findings have potential applications regarding the mode selectivity in BIC devices to manipulate the lasing mode in photonic-crystal surface-emitting lasers or the radiation pattern in nonlinear optics.

Evolutionary design of two-dimensional material Fabry-Perot structures for enhanced second harmonic generation

The integration of two-dimensional (2D) materials with resonant photonic structures is seen as a promising direction for enhancing its nonlinear optical response. The design of such heterogeneous resonant structures has often relied on multi-parameter sweeps to determine the optimized dimensions of resonant optical structure that results in good resonance characteristics, often in the absence of the 2D material. Such an approach is computationally intensive and may not necessarily result in efficient generation or collection of nonlinear signals from the designed structure. Here, we report hybrid-genetic optimization (HGA) based design and experimental demonstration of second harmonic generation (SHG) enhancement from Fabry-Perot structures of single and double multilayer gallium selenide (GaSe) flakes with bottom silicon dioxide, and index matched polymethyl methacrylate spacer/encapsulation layers. HGA technique utilized here speeds up the multilayer cavity design by 8.8 and 89-times for the single and double GaSe structures when compared to the full parameter-sweep, with measured SHG enhancement of 128- and 400-times, respectively, when compared to a reference sample composed of GaSe layer of optimized thickness on 300 nm silicon dioxide layer. SHG conversion efficiencies obtained from the HGA structures are 1-2 orders of magnitude higher than previous reports on 2D material integrated resonant metasurfaces or Bragg cavities.

Promising ultra-short channel transistors based on OM2S (M = Ga, In) monolayers for high performance and low power consumption

It is hoped that two-dimensional (2D) semiconductors overcome the short channel effect and continue Moore's law. However, 2D material-based ultra-short channel devices still face the challenge of simultaneously achieving high-performance (HP) and low-power (LP) consumption. Here, we theoretically designed monolayer OM₂S (M = Ga, In)-based metal-oxide-semiconductor field-effect transistors (MOSFETs), considering the gate length from 1 to 5 nm, doping concentration and underlap structure. We found that in HP (LP) applications, the on-state current exceeds 1000 (500) μA μm^-1 under a 1 nm (2 nm) gate length, surpassing the needs of the International Technology Roadmap for Semiconductors (ITRS) in 2028. The subthreshold swing is close to the Boltzmann tyranny (60 mV dec^-1) even as the gate length shrinks to 2 nm. The energy-delay product is two orders lower than 1.02 × 10^-28 J s μm^-1, indicating extraordinary high-speed manipulation and low-energy expending. Therefore, monolayer OM₂S has great application in ultra-short scale devices with HP and LP consumption, and can be taken as a candidate to extend Moore's Law.

3D Dirac semimetals supported tunable terahertz BIC metamaterials

Based on the 3D Dirac semimetals (DSM) supported tilted double elliptical resonators, the tunable propagation properties of quasi-bound in continuum (BIC) resonance have been investigated in the THz regime, including the effects of rotation angles, DSM Fermi level, and the configuration of resonators. The results manifest that by altering the rotation angle of elliptical resonator, an obvious sharp BIC transmission dip is observed with the Q-factor of more than 60. The DSM Fermi level affects the BIC resonance significantly, a sharp resonant dip is observed if Fermi level is larger than 0.05 eV, resulting from the contributions of reflection and absorption. If Fermi level changes in the range of 0.01-0.15 eV, the amplitude and frequency modulation depths are 92.75 and 44.99%, respectively. Additionally, with the modified configurations of elliptical resonators, e.g. inserting a dielectric hole into the elliptical resonator, another transmission dip resonance is excited and indicates a red shift with the increase of the permittivity of the dielectric filling material. The results are very helpful to understand the mechanisms of DSM plasmonic structures and develop novel tunable THz devices, such as modulators, filters, and sensors in the future.

Dual-band bound states in the continuum based on hybridization of surface lattice resonances

We propose and experimentally demonstrate a novel strategy to achieve dual-band symmetry-protected bound states in the continuum (BICs) in silicon metasurfaces. This strategy is based on the hybridization of Mie surface lattice resonances (SLRs) in periodic silicon bipartite nanodisk arrays, of which the central nanodisk displaced from the center of the unit cell. We show that dual-band electric quadrupole and magnetic dipole BICs can be supported in such a system, and transfer to quasi-BICs with ultrahigh measured quality factors up to 1240 at the Γ point. Taking advantage of the SLR characteristics, we show that the spectral separation and the quality factors of these two quasi-BICs can be conveniently tuned by varying the nanodisk diameter or the lattice period. Making use of these dual-band quasi-BICs, we numerically obtain bulk sensitivities above 480 nm/RIU and high figures of merit up to 1200. We also show that when the central nanodisk is not displaced but has different diameter, the silicon bipartite nanodisk array supports an electric dipole BIC that was referred to as subradiant SLR in the literature. Our work provides a new approach for realizing and tuning dual-band BICs, and the obtained ultrahigh-Q quasi-BICs can find potential applications in nonlinear optics, multimodal lasing, and optical sensing.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Robust enhancement of high-harmonic generation from all-dielectric metasurfaces enabled by polarization-insensitive bound states in the continuum

The emerging all-dielectric platform exhibits high-quality (Q) resonances governed by the physics of bound states in the continuum (BIC) that drives highly efficient nonlinear optical processes. Here we demonstrate the robust enhancement of third-(THG) and fifth-harmonic generation (FHG) from all-dielectric metasurfaces composed of four silicon nanodisks. Through the symmetry breaking, the genuine BIC transforms into the high-Q quasi-BIC resonance with tight field confinement for record high THG efficiency of 3.9 × 10^-4 W^-2 and FHG efficiency of 4.8 × 10^-10 W^-4 using a moderate pump intensity of 1 GW/cm². Moreover, the quasi-BIC and the resonantly enhanced harmonics exhibit polarization-insensitive characteristics due to the special C₄ arrangement of meta-atoms. Our results suggest the way for smart design of efficient and robust nonlinear nanophotonic devices.

Quasi-BIC-Based High-Q Perfect Absorber with Decoupled Resonant Wavelength and Q Factor

The Q factor in a quasi-BIC-based optical device can approach infinity and has therefore been attracting the attention of many researchers in recent years. However, this mode is barely applied to absorbers since it mainly tunes the radiative loss. The resonant wavelength of quasi-BICs normally couples with the Q factor, and it is difficult to independently tune one of them while maintaining the other, which weakens the flexibility of tuning. In this work, a quasi-BIC-based high-Q perfect absorber with some unique features is proposed. It shows a decoupled relationship between the resonant wavelength and the Q factor such that these two properties can be independently tuned by changing different structure parameters. In addition, both radiative and resistive losses are tunable. An easy method is proposed to design a perfect absorber with different resonant wavelengths and different Q factors, and a near-infrared perfect absorber with a Q factor as high as 5.13 × 105 is designed. This work proposes a method to tune the quasi-BIC mode, thereby introducing a new paradigm for the design of a high-Q perfect absorber.

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.

Third- and Second-Harmonic Generation in All-Dielectric Nanostructures: A Mini Review

Frequency conversion such as harmonic generation is a fundamental physical process in nonlinear optics. The conventional nonlinear optical systems suffer from bulky size and cumbersome phase-matching conditions due to the inherently weak nonlinear response of natural materials. Aiming at the manipulation of nonlinear frequency conversion at the nanoscale with favorable conversion efficiencies, recent research has shifted toward the integration of nonlinear functionality into nanophotonics. Compared with plasmonic nanostructures showing high dissipative losses and thermal heating, all-dielectric nanostructures have demonstrated many excellent properties, including low loss, high damage threshold, and controllable resonant electric and magnetic optical nonlinearity. In this review, we cover the recent advances in nonlinear nanophotonics, with special emphasis on third- and second-harmonic generation from all-dielectric nanoantennas and metasurfaces. We discuss the main theoretical concepts, the design principles, and the functionalities of third- and second-harmonic generation processes from dielectric nanostructures and provide an outlook on the future directions and developments of this research field.

Flatband mode in photonic moiré superlattice for boosting second-harmonic generation with monolayer van der Waals crystals

We theoretically investigate boosting second-harmonic generation (SHG) of monolayer van der Waals crystals by employing flatband modes hosted by photonic moiré superlattices. Such a system with high quality factor and a monolayer crystal accommodated on the top of it, provides a unique opportunity to enhance and manipulate SHG emission. We show that employing a doubly resonant diagram on such a moiré superlattice system not only boosts the SHG, but also tunes the directional emission of the second-harmonic wave. Moreover, we demonstrate that a structured beam illumination could further boost SHG, with the phase structure retrieved through a two-beam second-harmonic interference configuration. These results suggest the flatband modes in moiré superlattice as a promising platform for boosting SHG with monolayer van der Waals crystals, offering new possibilities for developing compact nonlinear photonic devices.