Engineering Directionality in Quantum Dot Shell Lasing Using Plasmonic Lattices

High-Chirality Polariton Lasing from Symmetry-Broken Plasmonic Lattices

Chiral polariton lasing is a source of circularly polarized, energy-efficient coherent emission. However, generating high optical contrast of opposite handedness is challenging because excitonic gain materials show low circular dichroism at room temperature. In addition, highly chiral lasing requires the symmetry of the optical cavities to be broken, which can affect the resonance quality and result in low-chiral purity. Here, we report how plasmonic nanoparticle lattice cavities having mismatched dimer unit cells strongly coupled to CdSe nanoplatelets can facilitate polariton lasing with low threshold fluences (8 μJ/cm2) and high chiral purity (∼0.92). This lasing threshold is at least two times lower than that of other systems at room temperature, and the chirality approaches the theoretical maximum. These room-temperature characteristics are promising for using chiral polariton lasing in a broad range of applications, from spintronics to optoelectronics to quantum information processing.

Low-threshold anisotropic polychromatic emission from monodisperse quantum dots

Colloidal quantum dots (QDs) are solution-processable semiconductor nanocrystals with favorable optoelectronic characteristics, one of which is their multi-excitonic behavior that enables broadband polychromatic light generation and amplification from monodisperse QDs. However, the practicality of this has been limited by the difficulty in achieving spatial separation and patterning of different colors as well as the high pumping intensity required to excite the multi-excitonic states. Here, we have addressed these issues by integrating monodisperse QDs in multi-excitonic states into a specially designed cavity, in which the QDs exhibit an anisotropic polychromatic emission (APE) characteristic that allows for tuning the emission from green to red by shifting the observation direction from perpendicular to lateral. Subsequently, the APE threshold under 300-ps pulsed excitation has been reduced from 32 to 21 μJ cm-2 by optimizing the cavity structure. Based on the manipulation of multi-excitonic emission and angle-dependent wavelength selectivity of the developed cavity, we have fabricated a full-color micro-pixel array with a pixel size as small as 23 μm by combining cavity-integrated monodisperse QDs and blue backlight. Furthermore, the threshold of APE under quasi-continuous-wave pumping was as low as 5 W cm-2, indicating its compatibility with commercial LEDs and/or laser diodes. Since APE arises from the multi-excitonic behavior of QDs that supports optical gain, its unprecedentedly low threshold suggests the feasibility of the diode-pumped colloidal QD laser. This work demonstrates a novel method of manipulating the QDs' optical properties beyond controlling their size, composition or structure, and reveals great potential for achieving full-color emission using monodisperse QDs.

Coupling and Energy Transfer between an Exciton in a Semiconductor Quantum Dot and a Surface Plasmon in a Metal Nanoparticle

The coupling between excitons in semiconductors or molecules and metal nanoparticles has been well-studied, primarily for nanoparticles in their ground electronic state. However, less attention has been given to exciton-nanoparticle interactions when the nanoparticle generates surface plasmons upon incident excitation. In this study, we explore the coupling and energy transfer dynamics between an exciton and the surface plasmon of a metal nanoparticle, forming a "plexciton". Significant mutual energy exchange between the exciton and the plasmon leads to unexpected effects on the exciton lifetime and coupling strength. The interaction at varying wavelengths, orientations, magnitudes, and phases was studied. Our results show that the exciton decay rate can be quenched entirely when the plasmon's energy compensates for that of the exciton radiative decay, even at separation distances of several hundred nanometers. These findings highlight the impact of surface plasmons on exciton dynamics, opening new possibilities for enhancing charge carrier dynamics in coupled systems.

Low-threshold surface-emitting colloidal quantum-dot circular Bragg laser array

Colloidal quantum dots (CQDs) are attractive gain media due to their wavelength-tunability and low optical gain threshold. Consequently, CQD lasers, especially the surface-emitting ones, are promising candidates for display, sensing and communication. However, it remains challenging to achieve a low-threshold surface-emitting CQD laser array with high stability and integration density. For this purpose, it is necessary to combine the improvement of CQD material and laser cavity. Here, we have developed high-quality CQD material with core/interlayer/graded shell structure to achieve a low gain threshold and high stability. Subsequently, surface-emitting lasers based on CQD-integrated circular Bragg resonator (CBR) have been achieved, wherein the near-unity mode confinement factor (Γ of 89%) and high Purcell factor of 22.7 attributed to the strong field confinement of CBR enable a low lasing threshold of 17 μJ cm-2, which is 70% lower than that (56 μJ cm-2) of CQD vertical-cavity surface-emitting laser. Benefiting from the high quality of CQD material and laser cavity, the CQD CBR laser is capable of continuous stable operation for 1000 hours (corresponding to 3.63 × 108 pulses) at room temperature. This performance is the best among solution-processed lasers composed of nanocrystals. Moreover, the miniaturized mode volume in CBR allows the integration of CQD lasers with an unprecedentedly high density above 2100 pixels per inch. Overall, the proposed low-threshold, stable and compactly integrated CQD CBR laser array would advance the development of CQD laser for practical applications.

Lasing in an assembled array of silver nanocubes

We demonstrate a surface lattice resonance (SLR)-based plasmonic nanolaser that leverages bulk production of colloidal nanoparticles and assembly on templates with single particle resolution. SLRs emerge from the hybridization of the plasmonic and photonic modes when nanoparticles are arranged into periodic arrays and this can provide feedback for stimulated emission. It has been shown that perfect arrays are not a strict prerequisite for producing lasing. Here, we propose using high-quality colloids instead. Silver colloidal nanocubes feature excellent plasmonic properties due to their single-crystal nature and low facet roughness. We use capillarity-assisted nanoparticle assembly to produce substrates featuring SLR and comprising single nanocubes. Combined with the laser dye pyrromethene-597, the nanocube array lases at 574 nm with <1.2 nm linewidth, <100 μJ cm-2 lasing threshold, and produces a beam with <1 mrad divergence, despite less-than-perfect arrangement. Such plasmonic nanolasers can be produced on a large-scale and integrated in point-of-care diagnostics, photonic integrated circuits, and optical communications applications.

High topological charge lasing in quasicrystals

Photonic modes exhibiting a polarization winding akin to a vortex possess an integer topological charge. Lasing with topological charge 1 or 2 can be realized in periodic lattices of up to six-fold rotational symmetry-higher order charges require symmetries not compatible with any two-dimensional Bravais lattice. Here, we experimentally demonstrate lasing with topological charges as high as -5, +7, -17 and +19 in quasicrystals. We discover rich ordered structures of increasing topological charges in the reciprocal space. Our quasicrystal design utilizes group theory in determining electromagnetic field nodes, where lossy plasmonic nanoparticles are positioned to maximize gain. Our results open a new path for fundamental studies of higher-order topological defects, coherent light beams of high topological charge, and realizations of omni-directional, flat-band-like lasing.

Lasing from Doubly Degenerate Bound States in the Continuum

This paper reports dual-mode vortex lasing with a topological charge of -2 via doubly degenerate bound states in the continuum. We patterned a photonic crystal slab with hexagonal TiO2 nanoparticles arranged in a hexagonal lattice to support doubly degenerate bound states in the continuum at the Γ point, and the finite array was truncated to have a square boundary. Because emission from bound states in the continuum at the Γ point is forbidden, lasing emerges at off-Γ points, where the double degeneracy is lifted, resulting in two lasing peaks close in wavelength. The coherence time of vortex lasing is about 1.4 ps under pulsed pumping with a width of 75 fs. Polarization- and angle-resolved spectral measurements reveal that the two lasing peaks show orthogonal polarization states.

Surface Lattice Resonance Lasers with Epitaxial InP Gain Medium

Surface lattice resonance (SLR) lasers, where the gain is supplied by a thin-film active material and the feedback comes from multiple scattering by plasmonic nanoparticles, have shown both low threshold lasing and tunability of the angular and spectral emission at room temperature. However, typically used materials such as organic dyes and QD films suffer from photodegradation, which hampers practical applications. Here, we demonstrate photostable single-mode lasing of SLR modes sustained in an epitaxial solid-state InP slab waveguide. The nanoparticle array is weakly coupled to the optical modes, which decreases the scattering losses and hence the experimental lasing threshold is as low as 94.99 ± 0.82 μJ cm2 pulse-1. The nanoparticle periodicity defines the lasing wavelength and enables tunable emission wavelengths over a 70 nm spectral range. Combining plasmonic nanoparticles with an epitaxial solid-state gain medium paves the way for large-area on-chip integrated SLR lasers for applications, including optical communication, optical computing, sensing, and LiDAR.

Large-Area Perovskite Nanocrystal Metasurfaces for Direction-Tunable Lasing

Perovskite nanocrystals (PNCs) are attractive emissive materials for developing compact lasers. However, manipulation of PNC laser directionality has been difficult, which limits their usage in photonic devices that require on-demand tunability. Here we demonstrate PNC metasurface lasers with engineered emission angles. We fabricated millimeter-scale CsPbBr3 PNC metasurfaces using an all-solution-processing technique based on soft nanoimprinting lithography. By designing band-edge photonic modes at the high-symmetry X point of the reciprocal lattice, we achieved four linearly polarized lasing beams along a polar angle of ∼30° under optical pumping. The device architecture further allows tuning of the lasing emission angles to 0° and ∼50°, respectively, by adjusting the PNC thickness to shift other high-symmetry points (Γ and M) to the PNC emission wavelength range. Our laser design strategies offer prospects for applications in directional optical antennas and detectors, 3D laser projection displays, and multichannel visible light communication.

All-Optical Control of Vortex Lasing in Silicon-Organic Lattices

This paper reports a silicon-organic hybrid lattice that can lase with vortex emission and allow all-optical control. We combine an array of amorphous silicon nanodisks with gain from dye molecules in organic solvents to generate vortex lasing from bound states in the continuum under pulsed optical pumping. Irradiating the device with an additional continuous wave green laser beam can cause optical heating in silicon and lead to negative change in the refractive index of the organic solvents; meanwhile, the green laser beam can provide additional gain. Dynamic tuning of the lasing wavelength is achieved by varying the intensity of the controlling beam. Furthermore, the vortex beam lasing can be switched to single-lobed beam lasing by moving the controlling spot to break the in-plane symmetry within the pumping spot. Our findings could shed new light on active silicon topological devices.

Symmetry-Guided Engineering of Polarization by 2D Moiré Metasurfaces

Cylindrical vector (CV) beams exhibit spatially varying polarization important in optical communication, super-resolution microscopy, and high-throughput information processing. Compared to radially or azimuthally polarized CV beams that are cylindrically symmetric, hybrid-electric (HE) beams offer increased optical tunability because of their polygonally symmetric polarizations. However, efforts to generate and isolate HE beams have relied on bulky optical assemblies or devices with complex and stringent fabrication requirements. Here, we report a moiré-based metasurface approach to engineer HE polarization states with high degrees of rotational symmetry. Importantly, polarization symmetries can be tailored based only on the reciprocal lattice of the metasurface and not the real-space patterns. Our modular method outlines important design principles for shaping light at the nanoscale.

Printable Light-Emitting Metasurfaces with Enhanced Directional Photoluminescence

Nanoimprint lithography is gaining popularity as a cost-efficient way to reproduce nanostructures in large quantities. Recent advances in nanoimprinting lithography using high-index nanoparticles have demonstrated replication of photonic devices, but it is difficult to confer special properties on nanostructures beyond general metasurfaces. Here, we introduce a novel method for fabricating light-emitting metasurfaces using nanoimprinting lithography. By utilizing quantum dots embedded in resin, we successfully imprint dielectric metasurfaces that function simultaneously as both emitters and resonators. This approach to incorporating quantum dots into metasurfaces demonstrates an improvement in photoluminescence characteristics compared to the situation where quantum dots and metasurfaces are independently incorporated. Design of the metasurface is specifically tailored to support photonic modes within the emission band of quantum dots with a large enhancement of photoluminescence. This study indicates that nanoimprinting lithography has the capability to construct nanostructures using functionalized nanoparticles and could be used in various fields of nanophotonic applications.

Liquid lasing from solutions of ligand-engineered semiconductor nanocrystals

Semiconductor nanocrystals (NCs) can function as efficient gain materials with chemical versatility because of their surface ligands. Because the properties of NCs in solution are sensitive to ligand-environment interactions, local chemical changes can result in changes in the optical response. However, amplification of the optical response is technically challenging because of colloidal instability at NC concentrations needed for sufficient gain to overcome losses. This paper demonstrates liquid lasing from plasmonic lattice cavities integrated with ligand-engineered CdZnS/ZnS NCs dispersed in toluene and water. By taking advantage of calcium ion-induced aggregation of NCs in aqueous solutions, we show how lasing threshold can be used as a transduction signal for ion detection. Our work highlights how NC solutions and plasmonic lattices with open cavity architectures can serve as a biosensing platform for lab-on-chip devices.

Superior Phonon-Limited Exciton Mobility in Lead-Free Two-Dimensional Perovskites

Tin-based two-dimensional (2D) perovskites are emerging as lead-free alternatives in halide perovskite materials, yet their exciton dynamics and transport remain less understood due to defect scattering. Addressing this, we employed temperature-dependent transient photoluminescence (PL) microscopy to investigate intrinsic exciton transport in three structurally analogous Sn- and Pb-based 2D perovskites. Employing conjugated ligands, we synthesized high-quality crystals with enhanced phase stability at various temperatures. Our results revealed phonon-limited exciton transport in Sn perovskites, with diffusion constants increasing from 0.2 cm2 s-1 at room temperature to 0.6 cm2 s-1 at 40 K, and a narrowing PL line width. Notably, Sn-based perovskites exhibited greater exciton mobility than their Pb-based equivalents, which is attributed to lighter effective masses. Thermally activated optical phonon scattering was observed in Sn-based compounds but was absent in Pb-based materials. These findings, supported by molecular dynamics simulations, demonstrate that the phonon scattering mechanism in Sn-based halide perovskites can be distinct from their Pb counterparts.

Unidirectional Lasing from Mirror-Coupled Dielectric Lattices

This paper reports how a hybrid system composed of transparent dielectric lattices over a metal mirror can produce high-quality lattice resonances for unidirectional lasing. The enhanced electromagnetic fields are concentrated in the cladding of the periodic dielectric structures and away from the metal. Based on a mirror-image model, we reveal that such high-quality lattice resonances are governed by bound states in the continuum resulting from destructive interference. Using hexagonal arrays of titanium dioxide nanoparticles on a silica-coated silver mirror, we observed lattice resonances with quality factors of up to 2750 in the visible regime. With the lattice resonances as optical feedback and dye solution as the gain medium, we demonstrated unidirectional lasing under optical pumping, where the array size was down to 100 μm × 100 μm. Our scheme can be extended to other spectral regimes to simultaneously achieve strongly enhanced surface fields and high quality factors.

Far-field coupling between moiré photonic lattices

Superposing two or more periodic structures to form moiré patterns is emerging as a promising platform to confine and manipulate light. However, moiré-facilitated interactions and phenomena have been constrained to the vicinity of moiré lattices. Here we report on the observation of ultralong-range coupling between photonic lattices in bilayer and multilayer moiré architectures mediated by dark surface lattice resonances in the vertical direction. We show that two-dimensional plasmonic nanoparticle lattices enable twist-angle-controlled directional lasing emission, even when the lattices are spatially separated by distances exceeding three orders of magnitude of lattice periodicity. Our discovery of far-field interlattice coupling opens the possibility of using the out-of-plane dimension for optical manipulation on the nanoscale and microscale.

Directional Amplified Photoluminescence through Large-Area Perovskite-Based Metasurfaces

Perovskite nanocrystals are high-performance, solution-processed materials with a high photoluminescence quantum yield. Due to these exceptional properties, perovskites can serve as building blocks for metasurfaces and are of broad interest for photonic applications. Here, we use a simple grating configuration to direct and amplify the perovskite nanocrystals' original omnidirectional emission. Thus far, controlling these radiation properties was only possible over small areas and at a high expense, including the risks of material degradation. Using a soft lithographic printing process, we can now reliably structure perovskite nanocrystals from the organic solution into light-emitting metasurfaces with high contrast on a large area. We demonstrate the 13-fold amplified directional radiation with an angle-resolved Fourier spectroscopy, which is the highest observed amplification factor for the perovskite-based metasurfaces. Our self-assembly process allows for scalable fabrication of gratings with predefined periodicities and tunable optical properties. We further show the influence of solution concentration on structural geometry. By increasing the perovskite concentration 10-fold, we can produce waveguide structures with a grating coupler in one printing process. We analyze our approach with numerical modeling, considering the physiochemical properties to obtain the desired geometry. This strategy makes the tunable radiative properties of such perovskite-based metasurfaces usable for nonlinear light-emitting devices and directional light sources.

Planar Optical Cavities Hybridized with Low-Dimensional Light-Emitting Materials

Low-dimensional light-emitting materials have been actively investigated due to their unprecedented optical and optoelectronic properties that are not observed in their bulk forms. However, the emission from low-dimensional light-emitting materials is generally weak and difficult to use in nanophotonic devices without being amplified and engineered by optical cavities. Along with studies on various planar optical cavities over the last decade, the physics of cavity-emitter interactions as well as various integration methods are investigated deeply. These integrations not only enhance the light-matter interaction of the emitters, but also provide opportunities for realizing nanophotonic devices based on the new physics allowed by low-dimensional emitters. In this review, the fundamentals, strengths and weaknesses of various planar optical resonators are first provided. Then, commonly used low-dimensional light-emitting materials such as 0D emitters (quantum dots and upconversion nanoparticles) and 2D emitters (transition-metal dichalcogenide and hexagonal boron nitride) are discussed. The integration of these emitters and cavities and the expect interplay between them are explained in the following chapters. Finally, a comprehensive discussion and outlook of nanoscale cavity-emitter integrated systems is provided.

Ligand Engineering and Recrystallization of Perovskite Quantum-Dot Thin Film for Low-Threshold Plasmonic Lattice Laser

Solution-process perovskite quantum dots (QDs) are promising materials to be utilized in photovoltaics and photonics with their superior optical properties. Advancements in top-down nanofabrication for perovskite are thus important for practical photonic and plasmonic devices. However, different from the chemically synthesized nano/micro-structures that show high quality and low surface roughness, the perovskite QD thin film prepared by spin-coating or the drop-casting process shows a large roughness and inhomogeneity. Low-roughness and low-optical loss perovskite QD thin film is highly desired for photonic and optoelectronic devices. Here, this work presents a pressure-assisted ligand engineering/recrystallization process for high-quality and well-thickness controlled CsPbBr₃ QD film and demonstrates a low-threshold and single-mode plasmonic lattice laser. A recrystallization process is proposed to prepare the QD film with a low roughness (RMS = 1.3 nm) and small thickness (100 nm). Due to the low scattering loss and strong interaction between gain media and plasmonic nanoparticles, a low lasing threshold of 16.9 µJ cm^-2 is achieved. It is believed that this work is not only important to the plasmonic laser field but also provides a promising and general nanofabrication method of solution-processed QDs for various photonic and plasmonic devices.

Loss-Driven Topological Transitions in Lasing

We experimentally observe lasing in a hexamer plasmonic lattice and find that, when tuning the scale of the unit cell, the polarization properties of the emission change. By a theoretical analysis, we identify the lasing modes as quasi-bound-states in continuum of topological charges of zero, one, or two. A T-matrix simulation of the structure reveals that the mode quality (Q) factors depend on the scale of the unit cell, with highest-Q modes favored by lasing. The system thus shows a loss-driven transition between lasing in modes of trivial and high-order topological charge.

Theoretical study of extremely narrow plasmonic surface lattice resonances observed by MIM nanogratings under normal incidence in asymmetric environments

Nanoarray structures can support plasmonic surface lattice resonances (SLRs) with extremely narrow linewidths and huge electric field enhancement features, which are attractive applications in nanolasers, biochemical sensors, and nonlinear optics. However, current nanoarray structures located in an asymmetric dielectric environment with a refractive index contrast of 1.00/1.52 of the superstrate/substrate excite much poorer SLRs under normal incidence, which largely limits their application range. In this work, we report extremely narrow SLRs supported by one-dimensional metal-insulator-metal nanograting in asymmetric dielectric environments. The simulation results show that an SLRs with linewidth of 3.26 nm and quality factor of 233.2 can be excited under normal incidence. This high-quality SLRs is attributed to the interference formation between the out-of-plane dipole resonance mode and the out-of-plane quadrupole resonance mode. We also show that the resonance wavelength and quality factor can be tuned by changing the structure geometry and period, and we calculate the normal incidence SLRs quality factor to be up to 248 in 1.33/1.52 and 250 in 1.45/1.52. We expect the SLRs of this work to find potential applications in asymmetric dielectric environments.

Lasing Action from Quasi-Propagating Modes

Band edges at the high symmetry points in reciprocal space of periodic structures hold special interest in materials engineering for their high density of states. In optical metamaterials, standing waves found at these points have facilitated lasing, bound-states-in-the-continuum, and Bose-Einstein condensation. However, because high symmetry points by definition are localized, properties associated with them are limited to specific energies and wavevectors. Conversely, quasi-propagating modes along the high symmetry directions are predicted to enable similar phenomena over a continuum of energies and wavevectors. Here, quasi-propagating modes in 2D nanoparticle lattices are shown to support lasing action over a continuous range of wavelengths and symmetry-determined directions from a single device. Using lead halide perovskite nanocrystal films as gain materials, lasing is achieved from waveguide-surface lattice resonance (W-SLR) modes that can be decomposed into propagating waves along high symmetry directions, and standing waves in the orthogonal direction that provide optical feedback. The characteristics of the lasing beams are analyzed using an analytical 3D model that describes diffracted light in 2D lattices. Demonstrations of lasing across different wavelengths and lattice designs highlight how quasi-propagating modes offer possibilities to engineer chromatic multibeam emission important in hyperspectral 3D sensing, high-bandwidth Li-Fi communication, and laser projection displays.

Light-Matter Interactions in Hybrid Material Metasurfaces

This Review focuses on the integration of plasmonic and dielectric metasurfaces with emissive or stimuli-responsive materials for manipulating light-matter interactions at the nanoscale. Metasurfaces, engineered planar structures with rationally designed building blocks, can change the local phase and intensity of electromagnetic waves at the subwavelength unit level and offers more degrees of freedom to control the flow of light. A combination of metasurfaces and nanoscale emitters facilitates access to weak and strong coupling regimes for enhanced photoluminescence, nanoscale lasing, controlled quantum emission, and formation of exciton-polaritons. In addition to emissive materials, functional materials that respond to external stimuli can be combined with metasurfaces to engineer tunable nanophotonic devices. Emerging metasurface designs including surface-functionalized, chemically tunable, and multilayer hybrid metasurfaces open prospects for diverse applications, including photocatalysis, sensing, displays, and quantum information.

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.

Polariton Dynamics in Two-Dimensional Ruddlesden-Popper Perovskites Strongly Coupled with Plasmonic Lattices

Strong coupling between light and matter can produce hybrid eigenstates known as exciton-polaritons. Although polariton dynamics are important photophysical properties, the relaxation pathways of polaritons in different coupling regimes have seen limited attention. This paper reports the dynamics of hybridized states from 2D Ruddlesden-Popper perovskites coupled to plasmonic nanoparticle lattices. The open cavity architecture of Al lattices enables the coupling strength to be modulated by varying either the lead halide perovskite film thickness or the superstrate refractive index. Both experiments and finite-difference time-domain simulations of the optical dispersion diagrams showed avoided crossings that are a signature of strong coupling. Our analytical model also elucidated the correlation between the exciton/plasmon mixing ratio and polariton coupling strength. Using fs-transient absorption spectroscopy, we found that both the upper and lower polaritons have shorter lifetimes than the excitons and that polaritons can show faster excited-state dynamics when they have access to additional energy transfer channels.

Quasi-BIC Mode Lasing in a Quadrumer Plasmonic Lattice

Plasmonic lattices of metal nanoparticles have emerged as an effective platform for strong light-matter coupling, lasing, and Bose-Einstein condensation. However, the full potential of complex unit cell structures has not been exploited. On the other hand, bound states in continuum (BICs) have attracted attention, as they provide topologically protected optical modes with diverging quality factors. Here, we show that quadrumer nanoparticle lattices enable lasing in a quasi-BIC mode with a highly out-of-plane character. By combining theory with polarization-resolved measurements of the emission, we show that the lasing mode has a topological charge. Our analysis reveals that the mode is primarily polarized out-of-plane as a result of the quadrumer structure. The quality factors of the out-of-plane BIC modes of the quadrumer array can be exceedingly high. Our results unveil the power of complex multiparticle unit cells in creating topologically protected high-Q modes in periodic nanostructures.

Strong coupling between monolayer quantum emitter WS2 and degenerate/non-degenerate surface lattice resonances

Strong light-matter coupling manifested by Rabi splitting has drawn considerable interest owing to its fundamental significance for impressive interaction enhancement in the fields of ultrafast active plasmonic devices and quantum information. In this paper, we investigate the coherent optical properties of a plasmonic system consisting of periodic metal nanoparticle arrays covered by a WS₂ thin film of atomic layer thickness. The coupling factor, energy splitting, and temporal dynamics of this coherent coupling phenomenon are quantitatively revealed by finite-difference time-domain (FDTD) simulation and a full quantum mechanical model proves that the exciton behavior of the fermionic quantum emitter WS₂ is carefully modulated by bosonic surface lattice resonances. This work may pave the way for coherent modulation of polariton and plasmon devices and can potentially open up diverse exciting possibilities like nanoscale light sources, single-photon emitters, and all-optical transistors.

Topological Control of 2D Perovskite Emission in the Strong Coupling Regime

Momentum space topology can be exploited to manipulate radiation in real space. Here we demonstrate topological control of 2D perovskite emission in the strong coupling regime via polaritonic bound states in the continuum (BICs). Topological polarization singularities (polarization vortices and circularly polarized eigenstates) are observed at room temperature by measuring the Stokes parameters of photoluminescence in momentum space. Particularly, in symmetry-broken structures, a very large degree of circular polarization (DCP) of ∼0.835 is achieved in the perovskite emission, which is the largest in perovskite materials to our knowledge. In the strong coupling regime, lower polariton modes shift to the low-loss spectral region, resulting in strong emission enhancement and large DCP. Our reciprocity analysis reveals that DCP is limited by material absorption at the emission wavelength. Polaritonic BICs based on 2D perovskite materials combine unique topological features with exceptional material properties and may become a promising platform for active nanophotonic devices.

Bound State in the Continuum in Nanoantenna-Coupled Slab Waveguide Enables Low-Threshold Quantum-Dot Lasing

Colloidal quantum dots (CQDs) are a promising gain material for solution-processed, wavelength-tunable lasers, with potential application in displays, communications, and biomedical devices. In this work, we combine a CQD film with an array of nanoantennas, made of titanium dioxide cylinders, to achieve lasing via bound states in the continuum (BICs). Here, the BICs are symmetry-protected cavity modes with giant quality factors, arising from slab waveguide modes in the planar CQD film, coupled to the periodic nanoantenna array. We engineer the thickness of the CQD film and size of the nanoantennas to achieve a BIC with good spatial and spectral overlap with the CQDs, based on a second-order transverse-electric (TE)-polarized waveguide mode. We obtain room-temperature lasing with a low threshold of approximately 11 kW/cm² (peak intensity) under 5-ns-pulsed optical excitation. This work sheds light on the optical modes in solution-processed, distributed-feedback lasers and highlights BICs as effective, versatile, surface-emitting lasing modes.

Identification of Brillouin Zones by In-Plane Lasing from Light-Cone Surface Lattice Resonances

Because of translational symmetry, electromagnetic fields confined within 2D periodic optical structures can be represented within the first Brillouin zone (BZ). In contrast, the wavevectors of scattered electromagnetic fields outside the lattice are constrained by the 3D light cone, the free-photon dispersion in the surrounding medium. Here, we report that light-cone surface lattice resonances (SLRs) from plasmonic nanoparticle lattices can be used to observe the radiated electromagnetic fields from extended BZ edges. Our coupled dipole radiation theory reveals how lattice geometry and induced surface plasmon dipole orientation affect angular distributions of the radiated fields. Using dye molecules as local dipole emitters to excite the light-cone SLR modes, we experimentally identified high-order BZ edges by directional, in-plane lasing emission. These results provide insight into nanolaser architectures that can emit at multiple wavelengths and in-plane directions simply by rotating the nanocavity lattice.

Recent Progress in Nanolaser Technology

Nanolasers are key elements in the implementation of optical integrated circuits owing to their low lasing thresholds, high energy efficiencies, and high modulation speeds. With the development of semiconductor wafer growth and nanofabrication techniques, various types of wavelength-scale and subwavelength-scale nanolasers have been proposed. For example, photonic crystal lasers and plasmonic lasers based on the feedback mechanisms of the photonic bandgap and surface plasmon polaritons, respectively, have been successfully demonstrated. More recently, nanolasers employing new mechanisms of light confinement, including parity-time symmetry lasers, photonic topological insulator lasers, and bound states in the continuum lasers, have been developed. Here, the operational mechanisms, optical characterizations, and practical applications of these nanolasers based on recent research results are outlined. Their scientific and engineering challenges are also discussed.

Room-Temperature Lasing in Colloidal Nanoplatelets via Mie-Resonant Bound States in the Continuum

Solid-state room-temperature lasing with tunability in a wide range of wavelengths is desirable for many applications. To achieve this, besides an efficient gain material with a tunable emission wavelength, a high quality-factor optical cavity is essential. Here, we combine a film of colloidal CdSe/CdZnS core-shell nanoplatelets with square arrays of nanocylinders made of titanium dioxide to achieve optically pumped lasing at visible wavelengths and room temperature. The all-dielectric arrays support bound states in the continuum (BICs), which result from lattice-mediated Mie resonances and boast infinite quality factors in theory. In particular, we demonstrate lasing from a BIC that originates from out-of-plane magnetic dipoles oscillating in phase. By adjusting the diameter of the cylinders, we tune the lasing wavelength across the gain bandwidth of the nanoplatelets. The spectral tunability of both the cavity resonance and nanoplatelet gain, together with efficient light confinement in BICs, promises low-threshold lasing with wide selectivity in wavelengths.