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Nanolasers based on emerging dielectric cavities with deep sub‐wavelength confinement of light offer a large light‐matter coupling rate and near‐unity spontaneous emission factor, β$\beta$. These features call for reconsidering the standard approach to identifying the lasing threshold. Here, a new threshold definition is suggested, taking into account the recycling process of photons when β$\beta$ is large. This threshold reduces to the classical balance between gain and loss in the limit of macroscopic lasers, but qualitative as well as quantitative differences emerge for nanolasers. In particular, this new threshold identifies the onset of a transition regime, where the quantum statistics of the emitted light evolve into the Poissonian statistics of a coherent state. It is shown that the threshold with photon recycling consistently marks the onset of the change in the second‐order intensity correlation, g(2)(0)$g^{(2)}(0)$, toward coherent laser light, irrespective of the laser size and down to the case of a single emitter. In contrast, other threshold definitions may well predict lasing in light‐emitting diodes. An overview of different threshold definitions proposed in the literature is provided and their predictions are compared when going from macroscopic to microscopic lasers. [ABSTRACT FROM AUTHOR]

Quantum well nanolasers usually show single‐mode lasing, as gain saturation suppresses emissions in other modes. In contrast, for whispering gallery mode microdisk lasers with GaN quantum wells as active material, above threshold multimode laser emission is observed. This intriguing emission feature is manifested in the fact that several modes simultaneously show the characteristic kink in the input–output curve at the onset of lasing. A quantum theory for nanolasers is used to support the experimental finding and to analyze this behavior in the presence of gain saturation. Coupling effects between neighboring modes are identified as the origin of multimode lasing, which initiate photon exchange between modes via population pulsations similar to classical wave‐mixing effects. A reduction of this type of mode coupling with increasing mode spacing is demonstrated. The results can pave the way for multimode application of nanolasers in integrated photonic circuits. [ABSTRACT FROM AUTHOR]

Applications for an antenna structure produced with additive manufacturing technology are numerous and promising. This study focuses explicitly on utilizing micro‐laser sintering (MLS) three‐dimensional (3D) printing technology to fabricate two rectangular horn antennas identical to a standard Pasternack® horn antenna, making the fabrication straightforward. The objective is to investigate how surface roughness affects antenna performance in a frequency range of 18 to 26.5 GHz. One antenna is polished and the other is left untouched, resulting in surface roughness of 4 and 6 µm, respectively, measured using a profilometer. Then, a vector network analyzer was used to assess their reflection coefficient, transmission characteristics, and radiation pattern and determine gains of 8.83 and 9.14 dBi for the polished and unpolished antennas, respectively. The study finds that both polished and unpolished antennas perform similarly, and their characteristics closely resembling simulated and standard antennas. Finally, the performance of the MLS‐printed metallic antennas is compared with horn antennas created using other 3D printing techniques. It reveals that the MLS‐printed metallic structures exhibit better performance, indicating that the MLS technology has great potential to print 3D metallic electromagnetic designs. [ABSTRACT FROM AUTHOR]

Self‐assembled semiconductor quantum dots face challenges in terms of scalable device integration because of their random growth positions, originating from the Stranski–Krastanov growth mode. Even with existing site‐controlled growth techniques, for example, nanohole or buried stressor concepts, a further lithography and etching step with high spatial alignment requirements is necessary to accurately integrate quantum dots into the nanophotonic devices. Here, the fabrication and characterization of strain‐induced site‐controlled microcavities are reported, where site‐controlled quantum dots are positioned at the antinode of the optical mode field in a self‐aligned manner without the need of any further nano‐processing. It is shown that the cavity properties such as Q‐factor, mode volume, and mode splitting can be tailored by the geometry of the integrated buried stressor, with an opening <4 µm. The experimental results are complemented with theory calculations based on continuum elasticity. Lasing signatures, including super‐linear input‐output response and linewidth narrowing, are observed for a 3.6‐µm self‐aligned cavity with a Q‐factor of 18 000. Furthermore, the quasi‐planar site‐controlled cavities exhibit no detrimental thermal effects. This approach integrates seamlessly with the industrial‐matured manufacturing process and the buried‐stressor technique, paving the way for exceptional scalability and straightforward manufacturing of high‐β microlasers and bright quantum light sources. [ABSTRACT FROM AUTHOR]

Engineering of organic single crystal toward controllable and aligned patterns at microscale is crucial to the realization of highly integrated organic photonic devices and optoelectronics. However, precise positioning and controllable morphology of crystal structures is still challenging due to the strict conditions for crystal growth. In this paper, a solution based crystal regulation strategy with the assistance of template‐constrained growth method and femtosecond laser processing technology is developed to prepare aligned crystalline microribbon arrays. Simple solvent evaporation results in the random distribution and orientation of self‐assembled crystalline microribbons while it tends to form highly crystalline and orderly microribbon arrays assisted by the confined template microchannels. The large‐scale microribbon array can be fabricated as organic photodetectors with sensitive and fast response under 405 nm illumination. By virtue of the femtosecond laser processing technique, the microribbon arrays are precisely processed into a series of crystal subunits and each microribbon subunit can function as an individual microcavity resonator to produce stable, high‐quality organic laser. The facile solution based template‐constrained self‐assembly and femtosecond laser processing method provide novel strategies to generate highly oriented and controllable crystal structures for the potential applications in integrated organic lasers and optoelectronics. [ABSTRACT FROM AUTHOR]

Luminescent supraparticles of colloidal semiconductor nanocrystals can act as microscopic lasers and are hugely attractive for biosensing, imaging, and drug delivery. However, biointerfacing these to increase functionality while retaining their main optical properties remains an unresolved challenge. Here, we propose and demonstrate red-emitting, silica-coated CdSxSe1−x/ZnS colloidal quantum dot supraparticles functionalized with a biotinylated photocleavable ligand. The success of each step of the synthesis is confirmed by scanning electron microscopy, energy dispersive X-ray and Fourier transform infrared spectroscopy, ζ-potential, and optical pumping measurements. The capture and release functionality of the supraparticle system is proven by binding to a neutravidin functionalized glass slide and subsequently cleaving off after UV-A irradiation. The biotinylated supraparticles still function as microlasers; e.g., a 9 μm diameter supraparticle has oscillating modes around 625 nm at a threshold of 58 mJ/cm2. This work is a first step toward using supraparticle lasers as enhanced labels for bionano applications. [ABSTRACT FROM AUTHOR]

The laser generation characteristics of microdisc lasers with optically coupled waveguide operating in continuous wave mode at elevated temperatures are investigated. Laser generation and waveguide effect at temperatures up to 92.5°C were demonstrated. The measured characteristic temperature of microlasers was 65 K in the range of 25–92.5°C. [ABSTRACT FROM AUTHOR]

The ability of particles to "tunnel" through potential energy barriers is a purely quantum phenomenon. A classical particle in a symmetric double-well potential, with energy below the potential barrier, will be trapped on one side of the potential well. A quantum particle, however, can sit on both sides, in either a symmetric state or an antisymmetric state. An analogous phenomenon occurs in conservative classical systems with two degrees of freedom and no potential barriers. If only the energy is conserved, the phase space will be a mixture of regular "islands" embedded in a sea of chaos. Classically, a particle sitting in one regular island cannot reach another symmetrically located regular island when the islands are separated by chaos. However, a quantum particle can sit on both regular islands, in symmetric and antisymmetric states, due to chaos-assisted tunneling. Here, we give an overview of the theory and recent experimental observations of this phenomenon. [ABSTRACT FROM AUTHOR]

The large size of lasers limits their applications in confined spaces, such as in biosensing and in vivo brain tissue imaging. In this regard, micron-sized lasers have been developed. They exhibit great potential for biological detecting, remote sensing, and depth tracking due to their small sizes, sensitive properties of their spectral fingerprints, and flexible positional modulation in the microenvironment. Lanthanide-based luminescent materials that possess long excited-state lifetime, narrow emission bandwidth, and upconversion behaviors are promising as gain mediums for novel microlasers. In addition, lanthanide-based microlasers could be generated under natural ambient conditions with pumped or continuous light sources, which significantly promotes the practical applications of microlasers. Recent progress in the design, synthesis, and biomedical applications of lanthanide-based microlasers has been outlined in this review. Lanthanide ions doped and upconverted lanthanide-based microlasers are highlighted, which exhibit advantageous structures, miniaturized dimensions, and high lasing performance. The applications of lanthanide-based microlasers are further discussed, the upconverted microlasers show great advantages for biological applications owing to their tunable excitation and emission characteristics and excellent environmental stability. Moreover, perspectives and challenges in the field of lanthanide-based microlasers are presented. [ABSTRACT FROM AUTHOR]

Photonic materials based on metal halide perovskites undergo rapid development owing to their unique optical properties and facile synthesis. Concurrently, there is also a growing interest in integrated photonics that can combine several elements on one chip. Nowadays technologies of integrated photonics are based on the traditional mask lithography combined with physical or chemical deposition methods. In this study, the possibility of facile fabrication of a simple pair of photonic elements is addressed, such as a microresonator with an outcoupling waveguide by means of inkjet printing on a glass substrate covered by a layer of polydimethylsiloxane (PDMS). The printed laser has revealed an appreciably high laser emission with a Q‐factor of 3300 and a threshold excitation fluence of 34 µJ·cm−2. The outcoupling waveguide has demonstrated the capability to transfer a reasonable part of the emitted radiation. The experimental results with a numerical simulation based on an appropriate physical model are also rationalized. Thus, the study points out a perspective for integrated photonics to be possibly implemented with this relatively cheap, flexible, and scalable fabrication method. [ABSTRACT FROM AUTHOR]

Dyes with aggregation‐induced emission (AIE) properties have gained interest due to their bright luminescence in solid‐state aggregates. While fluorescence from AIE dyes is widely exploited, relatively little is known about aggregation‐induced stimulated emission. Here, stimulated emission of tetraphenylethene (TPE)‐based organoboron AIE dyes, TPEQBN, in thin films and in microcavity lasers is investigated. Using femtosecond pump–probe spectroscopy, gain coefficients up to 230 cm−1 at 500 nm are measured. Using rate equations, concentration‐ and pump‐dependent gain dynamics as well as laser build‐up dynamics are analyzed. During laser oscillation, radiative stimulated emission allows high instantaneous quantum yield greater than 90% to be achieved. Solid‐state microspheres made of 100% AIE dyes are fabricated via microfluidic emulsion and solvent evaporation method. Coupled with high gain and high refractive index of 1.76, microspheres as small as 2 µm in diameter show lasing by nanosecond pumping with a threshold of ≈10 pJ µm–2. Polymer coated, but not bare, microspheres are internalized by live cells and generate narrowband cavity‐mode emission from within the cytoplasm. This work shows the potential of AIE dyes as laser materials. [ABSTRACT FROM AUTHOR]

Multiplexed, specific, and sensitive detection of antigens is critical for the rapid and accurate diagnosis of disease and the informed development of personalized treatment plans. Here, it is shown that polymer microsphere lasers can be used as photonic sensors to monitor and quantify direct surface binding of biomolecules via changes in the refractive index. The unique spectral signature of each individual laser can be used to find their size and effective refractive index which adds a new encoding dimension when compared to conventional fluorescent beads. Antibody‐functionalized microlasers selectively detect protein binding, as demonstrated for Immunoglobulin G and C‐reactive protein, and have the ability to resolve different stages of the multilayer surface modification. Moreover, by continuously monitoring single lasers, the possibility of real‐time monitoring of binding dynamics between antigens in solution phase and the immobilized antibodies is demonstrated. For multiplexed detection, the microlasers are employed in a flow cytometer configuration, with fast spectral detection and identification of microlasers with and without antigen binding. It is envisioned that by combining microlasers with well‐established surface modification chemistries and flow geometries, the multiplexing ability of microbead immunoassays can be strongly increased while also opening avenues for single‐cell profiling within heterogeneous cell populations. [ABSTRACT FROM AUTHOR]

The ability of particles to “tunnel” through potential energy barriers is a purely quantum phenomenon. A classical particle in a symmetric double-well potential, with energy below the potential barrier, will be trapped on one side of the potential well. A quantum particle, however, can sit on both sides, in either a symmetric state or an antisymmetric state. An analogous phenomenon occurs in conservative classical systems with two degrees of freedom and no potential barriers. If only the energy is conserved, the phase space will be a mixture of regular “islands” embedded in a sea of chaos. Classically, a particle sitting in one regular island cannot reach another symmetrically located regular island when the islands are separated by chaos. However, a quantum particle can sit on both regular islands, in symmetric and antisymmetric states, due to chaos-assisted tunneling. Here, we give an overview of the theory and recent experimental observations of this phenomenon.

Classification and recognition tasks performed on photonic hardware-based neural networks often require at least one offline computational step, such as in the increasingly popular reservoir computing paradigm. Removing this offline step can significantly improve the response time and energy efficiency of such systems. We present numerical simulations of different algorithms that utilize ultrafast photonic spiking neurons as receptive fields to allow for image recognition without an offline computing step. In particular, we discuss the merits of event, spike-time and rank-order based algorithms adapted to this system. These techniques have the potential to significantly improve the efficiency and effectiveness of optical classification systems, minimizing the number of spiking nodes required for a given task and leveraging the parallelism offered by photonic hardware.

Halide perovskite lasers based on CsPbBr3 micro‐ and nanoscale crystals have demonstrated fascinating performance owing to their low‐threshold lasing at room temperature and cost‐effective fabrication. However, chemically synthesized thin films of CsPbBr3 usually have rough polycrystalline morphology along with a large amount of crystal lattice defects and, thus, are mostly utilized for the engineering of light‐emitting devices. This obstacle prevents their usage in many photonic applications. Here, a protocol to deposit large‐grain and smooth CsPbBr3 thin films is developed. Their high quality and large scale allow to demonstrate a maximum optical gain up to 12 900 cm−1 in the spectral range of 530–540 nm, which is a record‐high value among all previously reported halide perovskites and bulk semiconductors (e.g., GaAs, GaN, etc.) at room temperature. Moreover, femtosecond laser ablation technique is employed to create high‐quality microdisc lasers on glass from these films to obtain excellent lasing characteristics. The revealed critical roles of thickness and grain size for the CsPbBr3 films with extremely high optical gain pave the way for development of low‐threshold lasers or ultimately small nanolasers, as well as to apply them for polaritonic logical elements and integrated photonic chips. [ABSTRACT FROM AUTHOR]

Organic single‐crystalline structures can effectively generate and confine photons, which can be potential building blocks for the optical circuits. While designing of organic molecules can tune the photoluminescence (PL) property, it is challenging to achieve efficient laser emission and light resonance in different forms of molecular aggregates. Herein, a dicarboxylated carbon‐bridged oligophenylenevinylene (COPV2‐(COOH)2) is newly synthesized. The THF/water solution of COPV2‐(COOH)2 exhibits green‐color PL with quantum yield >0.90 over the range of THF/water = 100/0–40/60. COPV2‐(COOH)2 forms either bent microrods (MR) or microsheets (MS) depending on the crystallization solvents. Significantly, PL resonance and lasing are only observed in MS, indicating that MS has better light confinement property than MR. MS features Q‐factor as high as 1.8 × 103 with the lasing threshold of 222 μJ cm−2. Finite‐difference time‐domain (FDTD)‐simulated electric field distributions in MR and MS validate experimental observations.

A coherent optical frequency comb with a gigahertz line spacing is important in diverse fields, such as calibration of astronomical spectrographs and wavelength division multiplexing optical communications. In this paper, we realize octave-spanning 10-GHz optical combs using a directly-modulated 1.55 μm microlaser as a seeding source. A 6.4 ps transform-limited pulse is successfully generated from the directly-modulated laser output through optimal chirp compensation in a fiber. Moreover, after amplifying by an EDFA, the optical pulse width is further reduced to 140 fs by combing the self-phase modulation effect and the anomalous group velocity dispersion in commercial fibers. Finally, a coherent optical comb spanning from 900-2400 nm is produced in a home-made fluorotellurite fiber using the 140-fs pulse boosted to a few Watts as a seeding source. The experimental results demonstrate that optical combs generated from the directly-modulated microlaser have the potential for a fully stabilized frequency comb. [ABSTRACT FROM AUTHOR]

Low‐dimensional perovskite structures are ideal materials for nano/microlasers owing to their excellent optical gain properties and high emission efficiency. But it is still a challenge to achieve single‐mode and tunable lasing behaviors in micro‐scale perovskite, especially showing ultralow thresholds in tens of µJ cm−2. In this work, CsPbBr3/Ag hemispheroids have been synthesized in a designed chemical vapor deposition (CVD) process. Benefiting from the whispering gallery resonance mode and plasmon‐enhanced light–matter interactions in well‐confined hybrid cavities, the CsPbBr3/Ag plasmonic microlasers exhibit low pump thresholds down to 35–51 µJ cm−2. Interestingly, single‐mode and two‐mode lasing behaviors are determined by the morphology of plasmonic Ag nanostructures, and lasing modes could be selectively tuned by increasing the pump fluence accompanying with dynamic competitions of photonic and plasmonic modes. Simulations of electromagnetic field have been plotted to show near‐field resonant modes for a comprehensive understanding of plasmon‐assisted lasing. These results not only provide a solution to fabricate low‐threshold plasmonic microlasers, but also advocate the prospect of all‐inorganic perovskite nanostructures as promising candidates for on‐chip integrated light sources. [ABSTRACT FROM AUTHOR]

Halide perovskite micro‐ and nanolasers have become a novel platform for efficient generation of coherent emission in the whole visible range. However, the laser emission outcoupling from the perovskite microcrystals faces a number of problems related to large divergence angle, light‐soaking by a substrate, and high losses in the case of coupling with nanowaveguides. Here the perovskite nano‐ and microlasers are proposed to be integrated with a photonic crystal supporting Bloch surface waves (BSWs), which are coupled directly from the perovskite lasers with the efficiency of over 16%. The BSWs exciting at 535‐nm wavelength show high in‐plane directivity, down to 9∘${}^{\circ }$, and long‐propagation length, up to 50 μ$\umu$m. Moreover, a pronounced beam steering effect for the generated laser emission is demonstrated by varying the pumping laser beam position, which provides an additional degree of freedom for in‐plane control of the lasing mode outcoupling. [ABSTRACT FROM AUTHOR]

Single‐crystalline micro‐/nanostructures based on organic stimulus‐responsive materials have attracted wide interests for their unique functional roles in various photonic applications including optical wave guiding, optical vapor sensing, and miniaturized lasing. Yet one imminent challenge is to pattern micro‐/nanostructured organic 1D arrays with controlled geometry, precise alignment, and pure crystallographic orientation owing to the uncontrollable dewetting dynamics in the solution processes. Herein, a smart assembly method is employed to regulate a confined crystallization of organic molecules. Sensitive, stable, and reproducible optical 1D‐array vapor sensors can detect the alkaline and acidic vapors based on the proton transfer process. As‐fabricated vapor sensors based on organic 1D arrays also selectively identify four similar amine vapors. Meanwhile, based on these 1D microstructure arrays, high‐performing Fabry–Pérot resonators with deep‐red laser emission, a low lasing threshold of 0.31 µJ, and high quality factor Q (≈2243) are realized. Owing to these multiple functions, organic microwire arrays not only provide intrinsic insight into optical vapor sensing but also offer guidance for the development of miniaturized lasers with specific functionalities, which show considerable potential in multifunctional photonic integrated circuits. [ABSTRACT FROM AUTHOR]

Spin‐controlled lasers are highly interesting photonic devices and have been shown to provide ultrafast polarization dynamics in excess of 200 GHz. In contrast to conventional semiconductor lasers their temporal properties are not limited by the intensity dynamics, but are governed primarily by the interaction of the spin dynamics with the birefringent mode splitting that determines the polarization oscillation frequency. Another class of modern semiconductor lasers are high‐β emitters, which benefit from enhanced light–matter interaction due to strong mode confinement in low‐mode‐volume microcavities. In such structures, the emission properties can be tailored by the resonator geometry to realize for instance bimodal emission behavior in slightly elliptical micropillar cavities. This attractive feature is utilized to demonstrate and explore spin‐lasing effects in bimodal high‐β quantum dot micropillar lasers. The studied microlasers with a β‐factor of 4% show spin‐laser effects with experimental polarization oscillation frequencies up to 15 GHz and predicted frequencies up to about 100 GHz, which are controlled by the ellipticity of the resonator. These results reveal appealing prospects for very compact, ultrafast, and energy‐efficient spin‐lasers and can pave the way for future purely electrically injected spin‐lasers enabled by short injection path lengths. [ABSTRACT FROM AUTHOR]

[Display omitted] Core-shell architecture enables to impart unique customized properties to microparticles, through the proper selection of composition and aggregation state of the inner and outer materials. Here, the synthesis of microparticles with a chiral dielectric core and a metallic shell of gold nanoparticles is demonstrated. The chiral core is obtained by UV induced polymerization of the self-organized droplets of a cholesteric reactive mesogen in a chloroauric acid aqueous solution. Gold nanoparticles precipitation contemporarily occurs upon UV irradiation, covering the microparticles surface. Electron microscopy and optical spectroscopy investigations give evidence that the degree of coverage of the core by gold nanoparticles, with size less than 100 nm, depends on the chloroauric acid concentration, while their aggregation is influenced by the polymeric surface morphology. The optical properties of the chiral microparticles are modified by the gold shell. Specifically, gold coating of dye doped chiral microparticles, working as Bragg onion resonators, clearly improves the stability of omnidirectional microlasers. The proposed strategy, due to the flexibility of the chiral material and of the method, opens a route toward fabrication of microdevices with wide control over light manipulation, in term of intensity, polarization, generation. [ABSTRACT FROM AUTHOR]

Fluorescent microbeads are widely used for applications in life sciences and medical diagnosis. The spectral contrast and sharpness of photoluminescence are critical in the utilities of microbeads for imaging and multiplexing. Here, microbeads capable of generating single‐peak laser emission with a sub‐nanometer linewidth are demonstrated. The microbeads are made of quantum dots that are tightly packed and crosslinked via ligand exchange for high optical gain and refractive index as well as material stability. Bright single‐mode lasing with no photobleaching is achieved with particle diameters as small as 1.5 µm in the air. Sub‐nm lasing emission is maintained even inside high‐index surroundings, such as organic solvents and biological tissues. Feasibility of intracellular tagging and multi‐color imaging in vivo is demonstrated. [ABSTRACT FROM AUTHOR]

Conjugated polymers have emerged as ideal organic laser materials for the excellent optoelectrical properties and facile processability. During a typical lasing process, resonator configurations with specific geometry are essential to provide optical feedback and then amplified light. Herein, we summarized the geometry and working mechanism of several typical resonator configurations formed with conjugated polymers. Meanwhile, recent advances in fabrication techniques and lasing performance are also discussed to provide new ideas for the design and optimization of microcavity geometries. Followed by the advances of practical applications in fields of laser sensing, bioimaging, and laser illumination/display, we make a summary of the existing bottlenecks and future perspectives of electrically driven organic lasers toward laser display and illumination.

The miniaturization of random lasers to the micrometer scale is challenging but fundamental for the integration of lasers with photonic integrated circuits and biological tissues. Herein, it is demonstrated that random lasers with a diameter from 30 to 160 μm can be achieved by using a simple emulsion process and selective chemical etching. These tiny random laser sources are made of either dye‐doped polyvinyl alcohol (PVA) or bovine serum albumin (BSA) and they are in the form of microporous spheres with monodisperse pores of 1.28 μm in diameter. Clear lasing action is observed when the microporous spheres are optically excited with powers larger than the lasing threshold, which is 154 μJ mm−2 for a 75 μm diameter PVA microporous sphere. The lasing wavelength redshifts 10 nm when the PVA microsphere diameter increases from 34 to 160 μm. For BSA microspheres, the lasing threshold is around 55 μJ mm−2 for a 70 μm diameter sphere and 104 μJ mm−2 for a 35 μm diameter sphere. The simple fabrication process reported allows for detail studies of morphology and size, important for fundamental studies of light–matter interaction in complex media, and applications in photonic integrated circuits, photonic barcoding, and optical biosensing.

Micro‐ and nanolasers are emerging optoelectronic components with many properties still to be explored and understood. On the one hand, they make it possible to address fundamental physical questions in the border area between classical physics and quantum physics, on the other hand, they open up new application perspectives in many areas of photonics. This progress report provides an overview of the exciting developments from conventional semiconductor lasers toward nanoscale lasers, whose function relies on increased light–matter interaction in low‐mode‐volume resonators and unconventional gain concepts. The latest advances in the physical understanding of light emission from high‐β lasers, in which a large part of the spontaneous emission is coupled into the laser mode, are highlighted. In the limit of β = 1, this leads to thresholdless lasing and it is shown that quantum optical characterization is required to fully explore the underlying emission processes. In addition, emerging nanolaser concepts based on Fano resonators, topological photonics, and 2D materials are presented. Open questions, future prospects, and application scenarios of high‐β lasers in integrated photonics, quantum nanophotonics, and neuromorphic computing are discussed. [ABSTRACT FROM AUTHOR]

Microlasers, relying on the strong coupling between active particles and optical microcavity, exhibit fundamental differences from conventional lasers, such as multi-threshold/thresholdless behavior and nonclassical photon emission. As light sources, microlasers possess extensive applications in precision measurement, quantum information processing, and biochemical sensing. Here we propose a whispering-gallery-mode microlaser scheme, where ultracold alkaline-earth metal atoms, i.e., gain medium, are tightly confined in a two-color evanescent lattice that is in the ring shape and formed around a microsphere. To suppress the influence of the lattice-induced ac Stark shift on the moderately-narrow-linewidth laser transition, the red-detuned trapping beams operate at a magic wavelength while the wavelength of the blue-detuned trapping beam is set close to the other magic wavelength. The tiny mode volume and high quality factor of the microsphere ensure the strong atom-microcavity coupling in the bad-cavity regime. As a result, both saturation photon and critical atom numbers, which characterize the laser performance, are substantially reduced below unity. We explore the lasing action of the coupled system by using the Monte Carlo approach. Our scheme may be potentially generalized to the microlasers based on the forbidden clock transitions, holding the prospect for microscale active optical clocks in precision measurement and frequency metrology. [ABSTRACT FROM AUTHOR]

Lead halide perovskite (LHP) is a promising material for various optoelectronic applications. Surface coating on particles is a common strategy to improve their functionality and environmental stability, but LHP is not amenable to most coating chemistries because of its intrinsic weakness against polar solvents. Here, a novel method of synthesizing LHP microlasers in a super‐saturated polar solvent using sonochemistry and applying various functional coatings on individual microlasers in situ is described. Cesium lead bromine perovskite (CsPbBr3) microcrystals, capped with organic poly‐norepinephrine (pNE) layers, are synthesized. The catechol group of pNE coordinates to bromine‐deficient lead atoms, forming a defect‐passivating and diffusion‐blocking shell. The pNE layer enhances the material lifetime of CsPbBr3 in water by 2000‐fold, enabling bright luminescence and lasing from single microcrystals in water. Furthermore, the pNE shell permits biofunctionalization with proteins, small molecules, and lipid bilayers. Luminescence from CsPbBr3 microcrystals is sustained in water over 1 h and observed in live cells. The functionalization method may enable new applications of LHP laser particles in water‐rich environments. [ABSTRACT FROM AUTHOR]

Microlasers in near-degenerate supermodes lay the cornerstone for studies of non-Hermitian physics, novel light sources, and advanced sensors. Recent experiments of the stimulated scattering in supermode microcavities reported beating phenomena, interpreted as dual-mode lasing, which, however, contradicts their single-mode nature due to the clamped pump field. Here, we investigate the supermode Raman laser in a whispering-gallery microcavity and demonstrate experimentally its single-mode lasing behavior with a side-mode suppression ratio (SMSR) up to 37 dB, despite the emergence of near-degenerate supermodes by the backscattering between counterpropagating waves. Moreover, the beating signal is recognized as the transient interference during the switching process between the two supermode lasers. Self-injection is exploited to manipulate the lasing supermodes, where the SMSR is further improved by 15 dB and the laser linewidth is below 100 Hz. [ABSTRACT FROM AUTHOR]

Spatial filtering is an important mechanism to improve the spatial quality of laser beams. Typically, a confocal arrangement of lenses with a diaphragm in the focal plane is used for intracavity spatial filtering. Such conventional filtering requires access to the far‐field domain. In microlasers, however, conventional filtering is impossible due to the lack of space in microresonators to access the far‐field. Therefore, a novel concept for more compact and efficient spatial filtering is necessary. In this study, a conceptually novel mechanism of spatial filtering in the near‐field domain is proposed and demonstrated, by a nanostructured multilayer coating—a 2D photonic crystal structure with a periodic index modulation along the longitudinal and transverse directions to the beam propagation. The structure is built on a nanomodulated substrate, to provide the transverse periodicity. The physical vapor deposition is used to provide self‐repeating modulation in the longitudinal direction. A 5 µm thick photonic multilayer structure composed of nanostructured multiple layers of alternating high‐ and low‐index materials providing spatial filtering in the near‐infrared frequencies with 2° low angle passband is experimentally demonstrated. The proposed photonic structure can be considered as an ideal component for intracavity spatial filtering in microlasers. [ABSTRACT FROM AUTHOR]

The nonlinear scaling of complexity with the increased number of components in integrated photonics is a major obstacle impeding large-scale, phase-locked laser arrays. Here, we develop a higher-dimensional supersymmetry formalism for precise mode control and nonlinear power scaling. Our supersymmetric microlaser arrays feature phase-locked coherence and synchronization of all of the evanescently coupled microring lasers—collectively oscillating in the fundamental transverse supermode—which enables high-radiance, small-divergence, and single-frequency laser emission with a two-orders-of-magnitude enhancement in energy density. We also demonstrate the feasibility of structuring high-radiance vortex laser beams, which enhance the laser performance by taking full advantage of spatial degrees of freedom of light. Our approach provides a route for designing large-scale integrated photonic systems in both classical and quantum regimes. [ABSTRACT FROM AUTHOR]

Background: The term "plasmonic" describes the relationship between electromagnetic fields and metallic nanostructures. Plasmon-based sensors have been used innovatively to accomplish different biomedical tasks, including detection of cancer. Plasmonic sensors also have been used in biochip applications and biosensors and have the potential to be implemented as implantable point-of-care devices. Many devices and methods discussed in the literature are based on surface plasmon resonance (SPR) and localized SPR (LSPR). However, the mathematical background can be overwhelming for researchers at times. Objective: This review article discusses the theory of SPR, simplifying the underlying physics and bypassing many equations of SPR and LSPR. Moreover, we introduce and discuss the hybrid whispering gallery mode (WGM) sensing theory and its applications. Methods: A literature search in ScienceDirect was performed using keywords such as "surface plasmon resonance," "localized plasmon resonance," and "whispering gallery mode/plasmonic." The search results retrieved many articles, among which we selected only those that presented a simple explanation of the SPR phenomena with prominent biomedical examples. Results: SPR, LSPR, tilted fiber Bragg grating, and hybrid WGM phenomena were explained and examples on biosensing applications were provided. Conclusions: This minireview presents an overview of biosensor applications in the field of biomedicine and is intended for researchers interested in starting to work in this field. The review presents the fundamental notions of plasmonic sensors and hybrid WGM sensors, thereby allowing one to get familiar with the terminology and underlying complex formulations of linear and nonlinear optics. [ABSTRACT FROM AUTHOR]

Metal‐organic frameworks (MOFs) have recently emerged as appealing platforms to construct microlasers owing to their compelling characters combining the excellent stability of inorganic materials and processable characters of organic materials. However, MOF microstructures developed thus far are generally composed of multiple edge boundaries due to their crystalline nature, which consequently raises significant scattering losses that are detrimental to lasing performance. In this work, we propose a strategy to overcome the above drawback by designing spherically shaped MOFs microcavities. Such spherical MOF microstructures are constructed by amorphizing MOFs with a topological distortion network through introducing flexible building blocks into the growth environment. With an ultra‐smooth surface and excellent circular boundaries, the acquired spherical microcavities possess a Q factor as high as ≈104 and can provide sufficient feedback for high‐quality single‐mode lasing oscillations. We hope that these results will pave an avenue for the construction of new types of flexible MOF‐based photonic components. [ABSTRACT FROM AUTHOR]

MOF‐based one‐dimensional materials have received increasing attention in the nanophotonics field, but it is still difficult in the flexible shape evolution of MOF micro/nanocrystals for desired optical functionalities due to the susceptible solvothermal growth process. Herein, we report on the well‐controlled shape evolution of pure‐MOF microcrystals with optical waveguide and lasing performances based on a bottom‐up and top‐down synergistic method. The MOF microcrystals from solvothermal synthesis (bottom‐up) enable the evolution from microrods via microtubes to nanowires through a chelating agent‐assisted etching process (top‐down). The three types of MOF 1D‐microstructures with high crystallinity and smooth surfaces all exhibit efficient optical waveguide performance. Furthermore, MOF nanowire with lowest propagation loss served as low‐threshold pure‐MOF nanolasers with Fabry–Pérot resonance. These results advance the fundamental understanding on the controlled MOF evolution mechanism, and offer a valuable route for the development of pure‐MOF‐based photonic components with desired functionalities. [ABSTRACT FROM AUTHOR]

Abstract Combined dry–wet transient materials and devices are introduced, which are based on water‐dissolvable dye‐doped polymers layered onto nonpolar cyclic hydrocarbon sublimating substrates. Light‐emitting heterostructures showing amplified spontaneous emission are obtained on transient elements and used as illumination sources for speckle‐free, full‐field imaging, and transient optical labels are realized that incorporate QR‐codes with stably encoded information. The transient behavior is also studied at the microscopic scale, highlighting the real‐time evolution of material domains in the sublimating compound. Finally, the exhausted components are fully soluble in water thus being naturally degradable. This technology opens new and versatile routes for environmental sensing, storage conditions monitoring, and organic photonics.

Semiconductor heterostructures hold the promise for developing novel integrated devices with on‐demand photonic and optoelectronic properties. Herein, the perovskite lateral heterostructures in forms of microplates and microwires are fabricated by precise control of two‐step atmospheric pressure chemical vapor deposition course and the unique optical properties are demonstrated for the first time. The comprehensive spectroscopic and elemental mapping characterizations reveal the core‐shell‐like configuration of the individual heterostructure, which features the fascinating energy routing nature. Taking advantage of the effective photon recycling and the energy transfer processes, the record‐low threshold (≈3.5 µJ cm−2) lasing and the indirect pumping induced lasers from the single micro‐heterostructure are demonstrated. Moreover, the distinctive optical waveguides with propagation length independent output spectra from the microwire heterostructure are realized. These findings represent a significant advance in developing novel and functional optoelectronic devices by engineering the perovskite heterostructure. [ABSTRACT FROM AUTHOR]

Lasers based on biological materials are attracting an increasing interest in view of their use in integrated and transient photonics. Deoxyribonucleic acid (DNA) as optical biopolymer in combination with highly emissive dyes has been reported to have excellent potential in this respect. However, achieving miniaturized lasing systems based on solid‐state DNA shaped in different geometries to confine and enhance emission is still a challenge, and the physicochemical mechanisms originating fluorescence enhancement are not fully understood. Herein, a class of wavelength‐tunable lasers based on DNA nanofibers is demonstrated, for which optical properties are highly controlled through the system morphology. A synergistic effect is highlighted at the basis of lasing action. Through a quantum chemical investigation, it is shown that the interaction of DNA with the encapsulated dye leads to hindered twisting and suppressed channels for the nonradiative decay. This is combined with effective waveguiding, optical gain, and tailored mode confinement to promote morphologically controlled lasing in DNA‐based nanofibers. The results establish design rules for the development of bright and tunable nanolasers and optical networks based on DNA nanostructures. [ABSTRACT FROM AUTHOR]

Combined dry–wet transient materials and devices are introduced, which are based on water‐dissolvable dye‐doped polymers layered onto nonpolar cyclic hydrocarbon sublimating substrates. Light‐emitting heterostructures showing amplified spontaneous emission are obtained on transient elements and used as illumination sources for speckle‐free, full‐field imaging, and transient optical labels are realized that incorporate QR‐codes with stably encoded information. The transient behavior is also studied at the microscopic scale, highlighting the real‐time evolution of material domains in the sublimating compound. Finally, the exhausted components are fully soluble in water thus being naturally degradable. This technology opens new and versatile routes for environmental sensing, storage conditions monitoring, and organic photonics. [ABSTRACT FROM AUTHOR]

We analytically investigate the performance of whispering gallery mode (WGM) spherical microlasers for gas sensing based on intra-cavity absorption. Using a simple model we estimate the response of a compact optical gas sensor based on these microlasers. We use detection of carbon dioxide by an Er:ZBLAN mid-IR microlaser with a relative intensity noise (RIN) of - 110 dB/Hz as an example, and show that using a commercial mid-IR photodetector, the limit of detection (LOD) for the resulting sensing system is about 5 ppm. We also investigate the impact of using a fiber optical amplifier on the LOD for different values of RIN. The numerical calculations and the presented approach can be applied to analyze the performance of any intracavity sensor based on WGM microlaser. [ABSTRACT FROM AUTHOR]

The orbital angular momentum (OAM) intrinsically carried by vortex light beams holds a promise for multidimensional high-capacity data multiplexing, meeting the ever-increasing demands for information. Development of a dynamically tunable OAM light source is a critical step in the realization of OAM modulation and multiplexing. By harnessing the properties of total momentum conservation, spin-orbit interaction, and optical non-Hermitian symmetry breaking, we demonstrate an OAM-tunable vortex microlaser, providing chiral light states of variable topological charges at a single telecommunication wavelength. The scheme of the non–Hermitian-controlled chiral light emission at room temperature can be further scaled up for simultaneous multivortex emissions in a flexible manner. Our work provides a route for the development of the next generation of multidimensional OAM-spin-wavelength division multiplexing technology. [ABSTRACT FROM AUTHOR]

The development of classical and quantum information–processing technology calls for on-chip integrated sources of structured light. Although integrated vortex microlasers have been previously demonstrated, they remain static and possess relatively high lasing thresholds, making them unsuitable for high-speed optical communication and computing. We introduce perovskite-based vortex microlasers and demonstrate their application to ultrafast all-optical switching at room temperature. By exploiting both mode symmetry and far-field properties, we reveal that the vortex beam lasing can be switched to linearly polarized beam lasing, or vice versa, with switching times of 1 to 1.5 picoseconds and energy consumption that is orders of magnitude lower than in previously demonstrated all-optical switching. Our results provide an approach that breaks the long-standing trade-off between low energy consumption and high-speed nanophotonics, introducing vortex microlasers that are switchable at terahertz frequencies. [ABSTRACT FROM AUTHOR]

Acoustic levitation has become a crucial technique for contactless manipulation in several fields, particularly in biological applications. However, its application in the photonics field remains largely unexplored. In this study, we implement an affordable and innovative phased-array levitator that enables stable trapping in the air of micrometer dye-doped droplets, thereby enabling the creation of microlasers. For the first time, this paper presents a detailed performance of the levitated microlaser cavity, supported by theoretical analysis concerning the hybrid technology based on the combination of whispering-gallery modes and acoustic fields. The pressure field distribution inside the acoustic cavity is numerically solved and qualitatively matched with the schlieren deflectometry technique. The optical lasing features of the levitated microlasers are highly comparable with those devices based On-a-Chip registering maximum Q-factors of ∼ 1 0 5 , and minimum lasing thresholds ∼ 150 nJ cm−2. The emission comb is explained as a sum of multiple individual-supported whispering-gallery modes. The use of novel touchless micrometric lasers, produced with an acoustic levitator brings new technological opportunities based on photonic-acoustic technological platforms. [Display omitted] • Contactless laser made with a 3D-printed confronted phased array acoustic levitator. • Whispering Gallery Mode laser emission is produced on levitated dye-doped droplets. • The microcavities' performance is comparable with On-a-Chip-based laser technology. [ABSTRACT FROM AUTHOR]

Among the efforts to improve the performances of microlasers, optimization of the gain properties and cavity parameters of these lasers has attracted significant attention recently. Distributed feedback lasers, as one of the most promising candidate technologies for electrically pumped microlasers, can be combined with dual-gratings. This combination provides additional freedom for the design of the laser cavity. Here, a holographic dual-grating is designed to improve the distributed feedback laser performance. The holographic dual-grating laser consists of a colloidal quantum dot film with two parallel gratings, comprising first-order (210 nm) and second-order (420 nm) gratings that can be fabricated easily using a combination of spin coating and interference lithography. The feedback and the output from the cavity are controlled using the first-order grating and the second-order grating, respectively. Through careful design and analysis of the dual-grating, a balance is achieved between the feedback and the cavity output such that the lasing threshold based on the dual-grating is nearly half the threshold of conventional distributed feedback lasers. Additionally, the holographic dual-grating laser shows a high level of stability because of the high stability of the colloidal quantum dots against photobleaching.

Liquid crystal microdroplets have received considerable attention over recent years owing to their potential applications in chemical, biomedical sensing and lasing. We report experimental studies on whispering gallery mode lasing from dye-doped ferroelectric liquid crystal microdroplets suspended in a low refractive index and highly transparent perfluoropolymer at ambient temperature. We show that the lasing threshold pump energy of ferroelectric microdroplets is much lower than that of the nematic and cholesteric microdroplets. With the increasing electric field, the linewidth increases, while the lasing intensity decreases and eventually switches off beyond a particular field. Since the switching response time is fast (≈350 μs), ferroelectric liquid crystal based microlasers are useful for applications as electrically switchable sources in miniaturised devices and in soft photonic circuits. [ABSTRACT FROM AUTHOR]

Dye-doped polymer microdroplet resonators were fabricated by injecting dye solutions into a low refractive index UV curable polymer. The self-assembled microdroplets had the various sizes ranging from 5 to 200 μm. Pumped by a pulse laser, whispering-gallery-mode (WGM) lasing emission from the microresonators was achieved. Experimental results show that the microresonators had the advantages of high-Q factors, low lasing thresholds, and good long-term stabilities. The injection method can provide a promising approach for fabricating the high-Q WGM microresonators, which had some potential applications in the fields of tunable microlasers and on-chip biosensing system.