A room temperature low-threshold ultraviolet plasmonic nanolaser

Suppression of the plasmon-quenching effect on light amplification in 20-μm-diameter plasmonic whispering gallery mode resonators fabricated from bowl-shaped organic/metal thin films

Bowl-shaped plasmonic whispering gallery mode (WGM) resonators were fabricated from a 10-nm-thick metal (Al, Ag, or Au) plasmonic layer that was covered with a 100-nm-thick 4,4'-bis(N-carbazolyl)-1,1'-biphenyl spacer layer and a 250-nm-thick 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene light-emitting layer; the layer structure was grown on a 20-μm-diameter silica microsphere. When compared with a reference structure without the plasmonic layer, the resonators, which included either Al or Ag, showed almost the same threshold excitation intensities for generation of amplified spontaneous emission (ASE). This result indicates that the ease of light amplification in the plasmonic resonators was comparable to that in the reference structure. Excitons that exist in the vicinity of metal thin films are generally easy to quench because propagating surface plasmon polaritons (SPPs) absorb the exciton energy. Therefore, the observed comparability demonstrates that the plasmonic WGM resonators overcome this quenching effect on ASE via localization of the SPPs in the vicinity of the excitons.

Circular quantum wire symmetrically loaded with a graphene strip as the plasmonic micro/nano laser: threshold conditions analysis

We study, apparently for the first time, the threshold conditions for the time-harmonic natural modes of the micro-to-nanosize plasmonic laser shaped as a circular quantum wire with a flat graphene strip, placed symmetrically inside it, in the H-polarization case. We suppose that the quantum wire is made of a nonmagnetic gain material, characterized with the aid of the "active" imaginary part of the complex refractive index. The emergence of lasers integrating plasmonic effects marks a significant trend in contemporary photonics. Here, the graphene offers a promising alternative to the noble metals as it exhibits the capacity to sustain plasmon-polariton natural surface waves across the infrared and terahertz (THz) spectra. The used innovative approach is the lasing eigenvalue problem (LEP), which is classical electromagnetic field boundary-value problem, adapted to the presence of active region. It is tailored to deliver both the mode-specific emission frequency, which is purely real at the threshold, and the value of the gain index of the active region, necessary to make the frequency real-valued. The conductivity of graphene is characterized using the quantum Kubo formalism. We reduce the LEP for the considered nanolaser to a hyper-singular integral equation for the current on the strip and discretize it by the Nystrom-type method. This method is meshless and computationally economic. After discretization, a matrix equation is obtained. The sought for mode-specific pairs {the frequency and the threshold gain index} correspond to the zeros of the matrix determinant. It should be noted that the convergence to exact LEP eigenvalues is guaranteed mathematically if the discretization order is taken progressively larger. Two families of modes are identified and studied: the modes of the quantum wire, perturbed by the presence of the graphene strip and the plasmon modes of the strip. The frequencies of all plasmon modes and the lowest mode of the quantum wire are found to be well-tuned by changing the chemical potential of graphene. Engineering analytic formulas for the plasmon-mode frequencies and thresholds are derived. We believe that the presented results can be used in the creation of single-mode tunable micro and nanolasers.

High-Q lasing via all-dielectric Bloch-surface-wave platform

Controlling the propagation and emission of light via Bloch surface waves (BSWs) has held promise in the field of on-chip nanophotonics. BSW-based optical devices are being widely investigated to develop on-chip integration systems. However, a coherent light source that is based on the stimulated emission of a BSW mode has yet to be developed. Here, we demonstrate lasers based on a guided BSW mode sustained by a gain-medium guiding structure microfabricated on the top of a BSW platform. A long-range propagation length of the BSW mode and a high-quality lasing emission of the BSW mode are achieved. The BSW lasers possess a lasing threshold of 6.7 μJ/mm2 and a very narrow linewidth reaching a full width at half maximum as small as 0.019 nm. Moreover, the proposed lasing scheme exhibits high sensitivity to environmental changes suggesting the applicability of the proposed BSW lasers in ultra-sensitive devices.

On-Chip Monolithically Integrated Ultraviolet Low-Threshold Plasmonic Metal-Semiconductor Heterojunction Nanolasers

The metal-semiconductor heterojunction is imperative for the realization of electrically driven nanolasers for chip-level platforms. Progress in developing such nanolasers has hitherto rarely been realized, however, because of their complexity in heterojunction fabrication and the need to use noble metals that are incompatible with microelectronic manufacturing. Most plasmonic nanolasers lase either above a high threshold (101 -103  MW cm-2 ) or at a cryogenic temperature, and lasing is possible only after they are removed from the substrate to avoid the large ohmic loss and the low modal reflectivity, making monolithic fabrication impossible. Here, for the first time, record-low-threshold, room-temperature ultraviolet (UV) lasing of plasmon-coupled core-shell nanowires that are directly grown on silicon is demonstrated. The naturally formed core-shell metal-semiconductor heterostructure of the nanowires leads to a 100-fold improvement in growth density over previous results. This unprecedentedly high nanowire density creates intense plasmonic resonance, which is outcoupled to the resonant Fabry-Pérot microcavity. By boosting the emission strength by a factor of 100, the hybrid photonic-plasmonic system successfully facilitates a record-low laser threshold of 12 kW cm-2 with a spontaneous emission coupling factor as high as ≈0.32 in the 340-360 nm range. Such architecture is simple and cost-competitive for future UV sources in silicon integration.

Ultrasmall InGa(As)P Dielectric and Plasmonic Nanolasers

Nanolasers have great potential for both on-chip light sources and optical barcoding particles. We demonstrate ultrasmall InGaP and InGaAsP disk lasers with diameters down to 360 nm (198 nm in height) in the red spectral range. Optically pumped, room-temperature, single-mode lasing was achieved from both disk-on-pillar and isolated particles. When isolated disks were placed on gold, plasmon polariton lasing was obtained with Purcell-enhanced stimulated emission. UV lithography and plasma ashing enabled wafer-scale fabrication of nanodisks with an intended random size variation. Silica-coated nanodisk particles generated stable subnanometer spectra from within biological cells across an 80 nm bandwidth from 635 to 715 nm.

On-Chip Ultralow-Threshold Tunable CdSSe Nanobelt Lasers Excited by the Emission of Linked ZnO Nanowire

The integration of optical waveguide and on-chip nanolasers source has been one of the trends in photonic devices. For on-chip nanolasers, the integration of nanowires and high antidamage ability are imperative. Herein, we realized the on-chip ultralow-threshold and wavelength-tunable lasing from alloyed CdSSe nanobelt chip that is excited by the emission from linked ZnO nanowires. ZnO nanowire arrays are integrated into CdSSe nanobelt chips by the dry transfer method. A one-dimensional (1D) ZnO nanowire forms high-quality optical resonators and serves as an indirect pumping light to stimulate CdSSe nanobelt chips, and then wavelength-tunable lasing is generated with the ultralow threshold of 3.88 μW. The lasing mechanism is quite different than direct excitation by nanosecond laser pulse and indirect pumping by ZnO emission. The ZnO-CdSSe blocks provide a new solution to realize nanowire lasing from linked nanowires rather than direct laser pumping and thus avoid the light direct damage under general nanosecond laser excitation.

Substrate-mediated plasmon hybridization toward high-performance light trapping

High-performance light trapping in metamaterials and metasurfaces offers prospects for the integration of multifunctional photonic components at subwavelength scales. However, constructing these nanodevices with reduced optical losses remains an open challenge in nanophotonics. Herein, we design and fabricate aluminum-shell-dielectric gratings by integrating low-loss aluminum materials with metal-dielectric-metal designs for high-performance light trapping featuring nearly perfect light absorption with broadband and large angular tuning ranges. The mechanism governing these phenomena is identified as the occurrence of substrate-mediated plasmon hybridization that allows energy trapping and redistribution in engineered substrates. Furthermore, we strive to develop an ultrasensitive nonlinear optical method, namely, plasmon-enhanced second-harmonic generation (PESHG), to quantify the energy transfer from metal to dielectric components. Our studies may provide a mechanism for expanding the potential of aluminum-based systems in practical applications.

Ultracompact terahertz plasmonic mode division multiplexer

In this Letter, an ultracompact terahertz (THz) mode division multiplexer based on THz spoof surface plasmon polaritons (SPPs) is proposed. Compared with traditional optical multiplexing devices, the proposed mode multiplexer can be designed with a reduced footprint by exploiting more degrees of freedom in the parameters of the unit cell, namely a rectangular metallic pillar. The ultracompact mode division multiplexer can simultaneously support the propagation of four mode channels: the TM₀, TM₁, TM₂, and TM₃ modes. Then, we numerically evaluate the performance of a cascaded plasmonic mode division circuit composed of a mode multiplexer and demultiplexer. The cross talk and excess loss of the whole circuit are lower than -15 dB and 3.7 dB, respectively, for all four mode channels at a center frequency of 0.65 THz. The footprint of the whole device is about 27 × 2.3 mm and the length of each coupling region is about 2.7 mm. For the first time, to the best of our knowledge, a mode division multiplexer based on THz spoof SPPs is reported, which will form core devices for future THz on-chip multimode communication systems.

Damage protection from focused ion beam process toward nanocavity-implemented compound semiconductor nanowire lasers

A focused ion beam (FIB) can precisely mill samples and freely form any nanostructure even on surfaces with curvature, like a nanowire surface, which are difficult to implement by using conventional fabrication techniques, e.g. electron beam lithography. Thus, this tool is promising for nanofabrication; however, fabrication damage and contamination are critical issues, which deteriorate optical properties. In this work, we investigated the protective performance of Al₂O₃against the FIB process (especially by a gallium ion). Nanowires were coated with Al₂O₃as a hard mask to protect them from damage during FIB nanofabrication. To estimate the protective performance, their emission properties by photoluminescence measurement and time-resolved spectroscopy were compared with and without Al₂O₃coating conditions. From the results, we confirmed that the Al₂O₃coating protects the nanowires. In addition, the nanowires also showed lasing behavior even after FIB processing had been carried out to implement nanostructures. This indicates that their optical properties are well maintained. Thus, our study proves the usefulness of FIBs for future nanofabrication.

Nonconfinement growth of edge-curved molecular crystals for self-focused microlasers

Synthesis of single-crystalline micro/nanostructures with curved shapes is essential for developing extraordinary types of optoelectronic devices. Here, we use the strategy of liquid-phase nonconfinement growth to controllably synthesize edge-curved molecular microcrystals on a large scale. By varying the molecular substituents on linear organic conjugated molecules, it is found that the steric hindrance effect could minimize the intrinsic anisotropy of molecular stacking, allowing for the exposure of high-index crystal planes. The growth rate of high-index crystal planes can be further regulated by increasing the molecular supersaturation, which is conducive to the cogrowth of these crystal planes to form continuously curved-shape microcrystals. Assisted by nonrotationally symmetric geometry and optically smooth curvature, edge-curved microcrystals can support low-threshold lasing, and self-focusing directional emission. These results contribute to gaining an insightful understanding of the design and growth of functional molecular crystals and promoting the applications of organic active materials in integrated photonic devices and circuits.

Extraction of silver losses at cryogenic temperatures through the optical characterization of silver-coated plasmonic nanolasers

We report on the extraction of silver losses in the range 10 K-180 K by performing temperature-dependent micro-photoluminescence measurements in conjunction with numerical simulations on silver-coated nanolasers around near-infrared telecommunication wavelengths. By mapping changes in the quality factor of nanolasers into silver-loss variations, the imaginary part of silver permittivity is extracted at cryogenic temperatures. The latter is estimated to reach values an order of magnitude lower than room-temperature values. Temperature-dependent values for the thermo-optic coefficient of III-V semiconductors occupying the cavity are estimated as well. This data is missing from the literature and is crucial for precise device modeling. Our results can be useful for device designing, the theoretical validation of experimental observations as well as the evaluation of thermal effects in silver-coated nanophotonic structures.

Flexible Plasmonics Using Aluminum and Copper Epitaxial Films on Mica

We demonstrate here the growth of aluminum (Al), copper (Cu), gold (Au), and silver (Ag) epitaxial films on two-dimensional, layered muscovite mica (Mica) substrates via van der Waals (vdW) heteroepitaxy with controllable film thicknesses from a few to hundreds of nanometers. In this approach, the mica thin sheet acts as a flexible and transparent substrate for vdW heteroepitaxy, which allows for large-area formation of atomically smooth, single-crystalline, and ultrathin plasmonic metals without the issue of film dewetting. The high-quality plasmonic metal films grown on mica enable us to design and fabricate well-controlled Al and Cu plasmonic nanostructures with tunable surface plasmon resonances ranging from visible to the near-infrared spectral region. Using these films, two kinds of plasmonic device applications are reported, including (1) plasmonic sensors with high effective index sensitivities based on surface plasmon interferometers fabricated on the Al/Mica film and (2) Cu/Mica nanoslit arrays for plasmonic color filters in the visible and near-infrared regions. Furthermore, we show that the responses of plasmonic nanostructures fabricated on the Mica substrates remain unaltered under large substrate bending conditions. Therefore, the metal-on-mica vdW heteroepitaxy platform is suitable for flexible plasmonics based on their bendable properties.

Nonlinear light amplification via 3D plasmonic nanocavities

Plasmonic nanocavities offer prospects for the amplification of inherently weak nonlinear responses at subwavelength scales. However, constructing these nanocavities with tunable modal volumes and reduced optical losses remains an open challenge in the development of nonlinear nanophotonics. Herein, we design and fabricate three-dimensional (3D) metal-dielectric-metal (MDM) plasmonic nanocavities that are capable of amplifying second-harmonic lights by up to three orders of magnitude with respect to dielectric-metal counterparts. In combination with experimental estimations of quantitative contributions of constituent parts in proposed 3D MDM designs, we further theoretically disclose the mechanism governing this signal amplification. We discover that this phenomenon can be attributed to the plasmon hybridization of both dipolar plasmon resonances and gap cavity resonances, such that an energy exchange channel can be attained and helps expand modal volumes while maintaining strong field localizations. Our results may advance the understanding of efficient nonlinear harmonic generations in 3D plasmonic nanostructures.

Stabilizing nanolasers via polarization lifetime tuning

We investigate the emission dynamics of mutually coupled nanolasers and predict ways to optimize their stability, i.e., maximize their locking range. We find that tuning the cavity lifetime to the same order of magnitude as the dephasing time of the microscopic polarization yields optimal operation conditions, which allow for wider tuning ranges than usually observed in conventional semiconductor lasers. The lasers are modeled by Maxwell-Bloch type class-C equations. For our analysis, we analytically determine the steady state solutions, analyze the symmetries of the system and numerically characterize the emission dynamics via the underlying bifurcation structure. The polarization lifetime is found to be a crucial parameter, which impacts the observed dynamics in the parameter space spanned by frequency detuning, coupling strength and coupling phase.

Submicrometer perovskite plasmonic lasers at room temperature

Plasmonic lasers attracted interest for their ability to generate coherent light in mode volume smaller than the diffraction limit of photonic lasers. While nanoscale devices in one or two dimensions were demonstrated, it has been difficult to achieve plasmonic lasing with submicrometer cavities in all three dimensions. Here, we demonstrate submicrometer-sized, plasmonic lasers using cesium-lead-bromide perovskite (CsPbBr3) crystals, as small as 0.58 μm by 0.56 μm by 0.32 μm (cuboid) and 0.79 μm by 0.66 μm by 0.18 μm (plate), on polymer-coated gold substrates at room temperature. Our experimental and simulation data obtained from more than 100 plasmonic and photonic devices showed that enhanced optical gain by the Purcell effect, large spontaneous emission factor, and high group index are key elements to efficient plasmonic lasing. The results shed light on the three-dimensional miniaturization of plasmonic lasers.

Stable organic self-assembled microwire lasers for chemical vapor sensing

Organic microlasers hold great potentials in fabricating on-chip sensors for integrated photonic circuits due to their chemical versatility and reactivity. However, chemical vapor detection is still challenging for organic microlaser sensors, as it requires not only optical gain and self-assembly capability, but also rapid response to stimuli and long-term stability under high excitation power. In this work, a new laser dye 4,7-bis(9-octyl-7-(4-(octyloxy)phenyl)-9H-carbazol-2-yl)benzo[c][1,2,5]thiadiazole (BPCBT) is designed and synthesized, which self-assembles into microwires showing strong intramolecular charge transfer (ICT) photoluminescence with >80% quantum efficiency. It enables the lasing from BPCBT microwires under a low threshold of 16 μJ·mm^-2·pulse^-1 with significantly improved stability over conventional organic microlasers. The stimulated emission amplifies the fluorescence change in the BPCBT microwires under chemical vapors including various acid, acetone, and ethanol vapors, indicating high sensitivity and high selectivity of organic microlaser sensors desirable for compact sensor arrays in integrated photonics.

The Design and Research of a New Hybrid Surface Plasmonic Waveguide Nanolaser

Using the hybrid plasmonic waveguide (HPW) principle as a basis, a new planar symmetric Ag-dielectric-SiO₂ hybrid waveguide structure is designed and applied to nanolasers. First, the effects on the electric field distribution and the characteristic parameters of the waveguide structure of changes in the material, the nanometer radius, and the dielectric layer thickness were studied in detail using the finite element method with COMSOL Multiphysics software. The effects of two different dielectric materials on the HPW were studied. It was found that the waveguide performance could be improved effectively and the mode propagation loss was reduced when graphene was used as the dielectric, with the minimum effective propagation loss reaching 0.025. Second, the gain threshold and the quality factor of a nanolaser based on the proposed hybrid waveguide structure were analyzed. The results showed that the nanolaser has a lasing threshold of 1.76 μm^-1 and a quality factor of 109 when using the graphene dielectric. A low-loss, low-threshold laser was realized, and the mode field was constrained by deep sub-wavelength light confinement. This structure has broad future application prospects in the integrated optics field and provides ideas for the development of subminiature photonic devices and high-density integrated circuits.

Improving Aluminum Ultraviolet Plasmonic Activity through a 1 nm ta-C Film

Aluminum (Al) can actively support plasmonic response in the ultraviolet (UV) range compared to noble metals (e.g., Au, Ag) and thus has broad applications including UV sensing, displays, and photovoltaics. High-quality Al films with no oxidation are essential and critical in these applications. However, Al is very prone to fast oxidation in air, which critically depends on the fabrication process. Here, we report that by leveraging the in situ sputter etching and sputter deposition of a 1 nm tetrahedral amorphous carbon (ta-C) film on the Al nanostructures, Al plasmonic activity can be improved. The prior sputter etching process greatly reduces the oxidized layer of the Al films, and the subsequent sputter deposition of ta-C keeps Al oxidation-free. The ta-C film outperforms the naturally passivated Al₂O₃ layer on the Al film because the ta-C film has a denser structure, higher permittivity, and better biocompatibility. Therefore, it can effectively improve the plasmonic response of Al and be beneficial to molecule sensing, which is proved in our experiments and is also verified in simulations. Our results can enable the various applications based on plasmon resonance in the UV range.

Surface roughness and substrate induced symmetry-breaking: influence on the plasmonic properties of aluminum nanostructure arrays

The surface topography is known to play an important role on the near- and far-field optical properties of metallic nanoparticles. In particular, aluminum (Al) nanoparticles are commonly fabricated through evaporation techniques, therefore exhibiting elevated surface roughness additionally to their native oxide layer. In this study, the mode-dependent influence of surface roughness on the plasmonic properties sustained by Al nanodisks (NDs) is first numerically investigated using a realistic model taking into account the thin native oxide layer. Due to the symmetry-breaking induced by the supporting dielectric substrate to Al ND, it appears that the roughness affects differently the substrate-induced out-of-plane quadrupolar mode (below 300 nm) and the in-plane dipolar mode sustained by the Al ND. By increasing the top surface roughness of the Al ND, the substrate-induced quadrupolar mode is significantly damped especially in the ultraviolet regime, while the dipolar resonance is broadened and redshifted. The explanation of these effects relies in the decoherence and dissipation of the collective electronic oscillations as a result of the top surface roughness to the different near-field distribution of the out-of-plane quadrupolar mode and in-plane dipolar mode. Moreover, the influences of the diameter of Al ND, dielectric substrate with different refractive index, and the oxidation of Al ND on these two modes are also investigated. Particularly, the quadrupolar mode disappears with surface roughness and oxidation, explaining why this mode is very weak and sometimes barely visible on evaporated Al nanostructures reported in the literature. Finally, these results are experimentally confirmed by characterizing the optical properties of periodic Al ND arrays.

Nanowire Waveguides and Lasers: Advances and Opportunities in Photonic Circuits

Due to their single-crystalline structures, comparatively large aspect ratios, tight optical confinement and smooth surfaces, nanowires have increasingly attracted research interests for both fundamental studies and technological applications in on-chip photonic devices. This class of nanostructures typically have cross-sections of 2~200 nm and lengths upwards of several micrometers, allowing for the bridging of the nanoscopic and macroscopic world. In particular, the lasing behaviors can be established from a nanowire resonator with positive feedback via end-facet reflection, making the nanowire a promising candidate in the next generation of optoelectronics. Consequently, versatile nanowire-based devices ranging from nanoscale coherent lasers, optical sensors, waveguides, optical switching, and photonic networks have been proposed and experimentally demonstrated in the past decade. In this article, significant progresses in the nanowire fabrication, lasers, circuits, and devices are reviewed. First, we focus on the achievements of nanowire synthesis and introduce the basics of nanowire optics. Following the cavity configurations and mode categories, then the different light sources consisting of nanowires are presented. Next, we review the recent progress and current status of functional nanowire devices. Finally, we offer our perspective of nanowires regarding their challenges and future opportunities in photonic circuits.

Tunable metamaterial-based silicon waveguide

A tunable metamaterial (MM)-based silicon (Si) waveguide is presented that is composed of an MM nanodisk array on a Si-on insulator substrate. A significant modulation efficiency of transmission intensity could be realized by elevating individually or simultaneously the column number of MM nanodisks. For a convenient description, an MM-based Si waveguide with one, two, three, four, and five columns of MM nanodisks are denoted as MM-1, MM-2, MM-3, MM-4, and MM-5, respectively. Transmission intensity of MM-based Si waveguides could be switched between on and off states by driving different columns of MM nanodisks on the Si waveguide surface. Transmission intensities could be attenuated from 100% to 56%, 24%, 6%, 1%, and 0% for MM-1, MM-2, MM-3, MM-4, and MM-5, respectively, at the wavelength of 1.525 µm. Furthermore, the MM-5 device is exposed to an ambient environment with different refraction indices. It exhibits a linear relationship of resonance dips and refraction indexes. The proposed design of the MM-based Si waveguide provides potential possibilities in an optical switch, variable optical attenuator, and sensor applications.

Design and Simulation of Low-Threshold Miniaturized Single-Mode Nanowire Lasers Combined with a Photonic Crystal Microcavity and Asymmetric Distributed-Bragg-Reflector Mirrors

A low-threshold miniaturized single-mode nanowire laser operating at telecommunication wavelengths was proposed and simulated. The device was constructed by combining a single InGaAs nanowire with a photonic crystal microcavity and asymmetric distributed-Bragg-reflector mirrors. The mode characteristics and threshold properties were calculated using the three-dimensional finite-different time-domain method. Due to the effective subwavelength confinement and strong optical feedback, provided by the photonic crystal microcavity, and distributed-Bragg-reflector mirrors, respectively, the confinement factor, end-facet reflectivity, and quality factor significantly improved. A lowest threshold of ~80 cm-1 and ultra-small cut-off radius of ~40 nm are obtained, reduced by 67%, and 70%, respectively, compared with a traditional nanowire laser. In addition, due to the photonic band gap effect, single-mode lasing is achieved with a high side-mode suppression ratio of >12 dB. By placing several identical nanowires in the photonic crystal with different lattice constants, an on-chip laser array is realized, which is promising in wavelength division multiplexing applications. This work may pave the way for the development of low-threshold miniaturized nanolasers and low-consumption high-density photonic integrated circuits.

Plasmonic Nanolasers in On-Chip Light Sources: Prospects and Challenges

The plasmonic nanolaser is a class of lasers with the physical dimensions free from the optical diffraction limit. In the past decade, progress in performance, applications, and mechanisms of plasmonic nanolasers has increased dramatically. We review this advance and offer our prospectives on the remaining challenges ahead, concentrating on the integration with nanochips. In particular, we focus on the qualifications for electrical pumping, energy consumption, and ultrafast modulation. At last, we evaluate the strategies for on-chip source construction design and further threshold reduction to achieve a long-term room-temperature electrically pumped plasmonic nanolaser, the ultimate goal toward practical applications.

A Low-Threshold Miniaturized Plasmonic Nanowire Laser with High-Reflectivity Metal Mirrors

A reflectivity-enhanced hybrid plasmonic GaAs/AlGaAs core-shell nanowire laser is proposed and studied by 3D finite-difference time-domain simulations. The results demonstrate that by introducing thin metal mirrors at both ends, the end facet reflectivity of nanowire is increased by 30–140%, resulting in a much stronger optical feedback. Due to the enhanced interaction between the surface charge oscillation and light, the electric field intensity inside the dielectric gap layer increases, resulting in a much lower threshold gain. For a small diameter in the range of 100–150 nm, the threshold gain is significantly reduced to 60–80% that of nanowire without mirrors. Moreover, as the mode energy is mainly concentrated in the gap between the nanowire and metal substrate, the output power maintains >60% that of nanowire without mirrors in the diameter range of 100–150 nm. The low-threshold miniaturized plasmonic nanowire laser with simple processing technology is promising for low-consumption ultra-compact optoelectronic integrated circuits and on-chip communications.

Perovskite Quantum Dot Lasing in a Gap-Plasmon Nanocavity with Ultralow Threshold

Lead halide perovskite materials have recently received considerable attention for achieving an economic and tunable laser owing to their solution-processable feature and promising optical properties. However, most reported perovskite-based lasers operate with a large lasing-mode volume, resulting in a high lasing threshold due to the inefficient coupling between the optical gain medium and cavity. Here, we demonstrate a continuous-wave nanolasing from a single lead halide perovskite (CsPbBr₃) quantum dot (PQD) in a plasmonic gap-mode nanocavity with an ultralow threshold of 1.9 Wcm^-2 under 120 K. The calculated ultrasmall mode volume (∼0.002 λ³) with a z-polarized dipole and the significantly large Purcell enhancement at the corner of the nanocavity inside the gap dramatically enhance the light-matter interaction in the nanocavity, thus facilitating lasing. The demonstration of PQD nanolasing with an ultralow-threshold provides an approach for realizing on-chip electrically driven lasing and integration into on-chip plasmonic circuitry for ultrafast optical communication and quantum information processing.

Aluminum Nanocrystals Grow into Distinct Branched Aluminum Nanowire Morphologies

Plasmonic nanowires (NWs) have generated great interest in their applications in nanophotonics and nanotechnology. Here we report the synthesis of Al nanocrystals (NCs) with controlled morphologies that range from nanospheres to branched NW and NW bundles. This is accomplished by catalyzing the pyrolysis of triisobutyl aluminum (TIBA) with Tebbe's reagent, a titanium(III) catalyst with two cyclopentadienyl ligands. The ratio of TIBA to Tebbe's reagent is critical in determining the morphology of the resulting Al NC. The branched Al NWs grow in their ⟨100⟩ directions and are formed by oriented attachment of isotropic Al NCs on their {100} facets. Branched NWs are strongly absorptive from the UV to the mid-IR, with longitudinal dipolar, higher-order, and transverse plasmons, all contributing to their broadband response. This rapid Al NW synthesis enables the expanded use of Al for plasmonic and nanophotonic applications in the ultraviolet, visible, and infrared regions of the spectrum.

ZnO Nanowires on Single-Crystalline Aluminum Film Coupled with an Insulating WO3 Interlayer Manifesting Low Threshold SPP Laser Operation

ZnO nanowire-based surface plasmon polariton (SPP) nanolasers with metal-insulator-semiconductor hierarchical nanostructures have emerged as potential candidates for integrated photonic applications. In the present study, we demonstrated an SPP nanolaser consisting of ZnO nanowires coupled with a single-crystalline aluminum (Al) film and a WO3 dielectric interlayer. High-quality ZnO nanowires were prepared using a vapor phase transport and condensation deposition process via catalyzed growth. Subsequently, prepared ZnO nanowires were transferred onto a single-crystalline Al film grown by molecular beam epitaxy (MBE). Meanwhile, a WO3 dielectric interlayer was deposited between the ZnO nanowires and Al film, via e-beam technique, to prevent the optical loss from dominating the metallic region. The metal-oxide-semiconductor (MOS) structured SPP laser, with an optimal WO3 insulating layer thickness of 3.6 nm, demonstrated an ultra-low threshold laser operation (lasing threshold of 0.79 MW cm-2). This threshold value was nearly eight times lower than that previously reported in similar ZnO/Al2O3/Al plasmonic lasers, which were ≈2.4 and ≈3 times suppressed compared to the SPP laser, with WO3 insulating layer thicknesses of 5 nm and 8 nm, respectively. Such suppression of the lasing threshold is attributed to the WO3 insulating layer, which mediated the strong confinement of the optical field in the subwavelength regime.

Ultrafast plasmonic lasing from a metal/semiconductor interface

To date, plasmonic nanowire lasers mostly adopt hybrid plasmonic waveguides, while there is a lack of study in terms of the confinement effect and the corresponding ultrafast dynamics of non-hybridized plasmonic lasers. Here, we report ultrafast plasmonic nanowire lasers composed of a single CH3NH3PbBr3 nanowire on a silver film without any insulating layer at room temperature. The non-hybridized plasmonic nanowire lasers exhibit ultrafast lasing dynamics with around 1.9 ps decay rate and 1 ps peak response time. Such values are among the best ones ever reported. Interestingly, the threshold of the non-hybridized plasmonic nanowire lasers is in the same order as that of their hybrid counterparts. The low threshold is due to the ultra-flat single-crystal silver films and high-quality single-crystal perovskite nanowires. The non-hybridized plasmonic lasing in CH3NH3PbBr3 nanowires originates from the stimulated emission of an electron-hole plasma based on our experiments. This work deepens the understanding of non-hybridized plasmonic lasers and paves the way to design electric pump plasmonic lasers by getting rid of insulating layers.

Conformable Nanowire-in-Nanofiber Hybrids for Low-Threshold Optical Gain in the Ultraviolet

The miniaturization of diagnostic devices that exploit optical detection schemes requires the design of light sources combining small size, high performance for effective excitation of chromophores, and mechanical flexibility for easy coupling to components with complex and nonplanar shapes. Here, ZnO nanowire-in-fiber hybrids with internal architectural order are introduced, exhibiting a combination of polarized stimulated emission, low propagation losses of light modes, and structural flexibility. Ultrafast transient absorption experiments on the electrospun material show optical gain which gives rise to amplified spontaneous emission with a threshold lower than the value found in films. These systems are highly flexible and can conveniently conform to curved surfaces, which makes them appealing active elements for various device platforms, such as bendable lasers, optical networks, and sensors, as well as for application in bioimaging, photo-cross-linking, and optogenetics.

Chromatic Dispersion Manipulation Based on Metalenses

Metasurfaces are 2D metamaterials composed of subwavelength nanoantennas according to specific design. They have been utilized to precisely manipulate various parameters of light fields, such as phase, polarization, amplitude, etc., showing promising functionalities. Among all meta-devices, the metalens can be considered as the most basic and important application, given its significant advantage in integration and miniaturization compared with traditional lenses. However, the resonant dispersion of each nanoantenna in a metalens and the intrinsic chromatic dispersion of planar devices and optical materials result in a large chromatic aberration in metalenses that severely reduces the quality of their focusing and imaging. Consequently, how to effectively suppress or manipulate the chromatic aberration of metalenses has attracted worldwide attention in the last few years, leading to variety of excellent achievements promoting the development of this field. Herein, recent progress in chromatic dispersion control based on metalenses is reviewed.

Planar Growth, Integration, and Applications of Semiconducting Nanowires

Silicon and other inorganic semiconductor nanowires (NWs) have been extensively investigated in the last two decades for constructing high-performance nanoelectronics, sensors, and optoelectronics. For many of these applications, these tiny building blocks have to be integrated into the existing planar electronic platform, where precise location, orientation, and layout controls are indispensable. In the advent of More-than-Moore's era, there are also emerging demands for a programmable growth engineering of the geometry, composition, and line-shape of NWs on planar or out-of-plane 3D sidewall surfaces. Here, the critical technologies established for synthesis, transferring, and assembly of NWs upon planar surface are examined; then, the recent progress of in-plane growth of horizontal NWs directly upon crystalline or patterned substrates, constrained by using nanochannels, an epitaxial interface, or amorphous thin film precursors is discussed. Finally, the unique capabilities of planar growth of NWs in achieving precise guided growth control, programmable geometry, composition, and line-shape engineering are reviewed, followed by their latest device applications in building high-performance field-effect transistors, photodetectors, stretchable electronics, and 3D stacked-channel integration.

Quantum Leap from Gold and Silver to Aluminum Nanoplasmonics for Enhanced Biomedical Applications

Nanotechnology has been used in many biosensing and medical applications, in the form of noble metal (gold and silver) nanoparticles and nanostructured substrates. However, the translational clinical and industrial applications still need improvements of the efficiency, selectivity, cost, toxicity, reproducibility, and morphological control at the nanoscale level. In this review, we highlight the recent progress that has been made in the replacement of expensive gold and silver metals with the less expensive aluminum. In addition to low cost, other advantages of the aluminum plasmonic nanostructures include a broad spectral range from deep UV to near IR, providing additional signal enhancement and treatment mechanisms. New synergistic treatments of bacterial infections, cancer, and coronaviruses are envisioned. Coupling with gain media and quantum optical effects improve the performance of the aluminum nanostructures beyond gold and silver.

Ten years of spasers and plasmonic nanolasers

Ten years ago, three teams experimentally demonstrated the first spasers, or plasmonic nanolasers, after the spaser concept was first proposed theoretically in 2003. An overview of the significant progress achieved over the last 10 years is presented here, together with the original context of and motivations for this research. After a general introduction, we first summarize the fundamental properties of spasers and discuss the major motivations that led to the first demonstrations of spasers and nanolasers. This is followed by an overview of crucial technological progress, including lasing threshold reduction, dynamic modulation, room-temperature operation, electrical injection, the control and improvement of spasers, the array operation of spasers, and selected applications of single-particle spasers. Research prospects are presented in relation to several directions of development, including further miniaturization, the relationship with Bose-Einstein condensation, novel spaser-based interconnects, and other features of spasers and plasmonic lasers that have yet to be realized or challenges that are still to be overcome.

Hybrid Electro-Optical Pumping of Active Plasmonic Nanostructures

Surface plasmon polaritons (SPPs) offer a unique opportunity to overcome the diffraction limit of light. However, this opportunity comes at the cost of the strong absorption of the SPP field in a metal, which unavoidably limits the SPP propagation length to a few tens of micrometers in nanostructures with deep-subwavelength mode confinement. The only possibility to avoid the propagation losses is to compensate for them by optical gain in the adjacent active medium. Different approaches for surface plasmon amplification by stimulated emission of radiation have been proposed based on either optical or electrical pumping. However, each has its own disadvantages caused by the selected type of pumping scheme. Here, we study, for the first time, hybrid electro-optical pumping of active plasmonic waveguide structures, and by using comprehensive self-consistent numerical simulations, demonstrate that this hybrid approach can outperform both pure electrical pumping and pure optical pumping. The SPP modal gain is higher than under pure optical pumping, while one can precisely and locally adjust it by tuning the electric current, which allows the reduction of amplification noise and provides additional functionalities. We believe that our findings lay a solid foundation for the development of a new generation of active plasmonic devices and stimulate further research in this area.

Topological Spaser

We theoretically introduce a topological spaser, which consists of a hexagonal array of plasmonic metal nanoshells containing an achiral gain medium in their cores. Such a spaser can generate two mutually time-reversed chiral surface plasmon modes in the K and K^{'} valleys, which carry the opposite topological charges, ±1, and are described by a two-dimensional E^{'} representation of the D_{3h} point symmetry group. Due to the mode competition, this spaser exhibits a bistability: only one of these two modes generates, which is a spontaneous symmetry breaking. Such a spaser can be used for an ultrafast all-optical memory and information processing, and biomedical detection and sensing with chirality resolution.

Electron-Beam-Driven III-Nitride Plasmonic Nanolasers in the Deep-UV and Visible Region

Plasmonic nanolasers based on wide bandgap semiconductors are presently attracting immense research interests due to the breaking in light diffraction limit and subwavelength mode operation with fast dynamics. However, these plasmonic nanolasers have so far been mostly realized in the visible light ranges, or most are still under optical excitation pumping. In this work, III-nitride-based plasmonic nanolasers emitting from the green to the deep-ultraviolet (UV) region by energetic electron beam injection are reported, and a threshold as low as 8 kW cm^-2 is achieved. A fast decay time as short as 123 ps is collected, indicating a strong coupling between excitons and surface plasmon. Both the spatial and temporal coherences are observed, which provide a solid evidence for exciton-plasmon coupled polariton lasing. Consequently, the achievements in III-nitride-based plasmonic nanolaser devices represent a significant step toward practical applications for biological technology, computing systems, and on-chip optical communication.

Ultralow-threshold, continuous-wave upconverting lasing from subwavelength plasmons

Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in vivo cellular imaging, solid-state lighting and fast three-dimensional sensing in smartphones^1-3. Continuous-wave lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions^4,5. Plasmonic nanocavities integrated with gain can generate coherent light at subwavelength scales^6-9, beyond the diffraction limit that constrains mode volumes in dielectric cavities such as semiconducting nanowires^10,11. However, insufficient gain with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or low-temperature operation^6-9,12-15. Here, we show continuous-wave upconverting lasing at room temperature with record-low thresholds and high photostability from subwavelength plasmons. We achieve selective, single-mode lasing from Yb³⁺/Er³⁺-co-doped upconverting nanoparticles conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode and greater than wavelength λ/20 field confinement in the vertical dimension. The intense electromagnetic near-fields localized in the vicinity of the nanopillars result in a threshold of 70 W cm^-2, orders of magnitude lower than other small lasers. Our plasmon-nanoarray upconverting lasers provide directional, ultra-stable output at visible frequencies under near-infrared pumping, even after six hours of constant operation, which offers prospects in previously unrealizable applications of coherent nanoscale light.

Tunable surface plasmon polaritons and ultrafast dynamics in 2D nanohole arrays

High-quality and unique surface plasmon resonance (SPR) with a narrow linewidth and controllable resonance energy plays a key role in wide applications including ultrahigh-resolution spectroscopy, on-chip sensing, optical modulation, and solar cell technology. In this work, the response of surface plasmon polariton (SPP) modes in Au nanohole arrays has been effectively tuned by properly adjusting the sample orientation without changing the geometrical parameters, and a very narrow linewidth down to 8 nm is achieved via the strong interference of two (0, -1) and (-1, 0) SPP modes in the Γ-M direction under transverse magnetic polarization. These results have been validated excellently by finite-element-method numerical simulations. More importantly, we have quantitatively investigated the contribution of conduction-band electron distribution to the SPP intensity of the array within a 20 ps timescale with ultrahigh sensitivity by utilizing home-built femtosecond transient absorption spectroscopy, and observed the minimum SPP intensity at ∼700 fs following excitation with a 0.2 μJ pulse. This study may help enhance the understanding toward the intrinsic micromechanism of SPR, thus offering opportunities for potential applications in strong coupling and new-style optical wave manipulations.

Raman characterization of single-crystalline Ga0.96Mn0.04As:Zn nanowires realized by ion-implantation

Recent progress in the realization of magnetic GaAs nanowires (NWs) doped with Mn has attracted a lot of attention due to their potential application in spintronics. In this work, we present a detailed Raman investigation of the structural properties of Zn doped GaAs (GaAs:Zn) and Mn-implanted GaAs:Zn (Ga_0.96Mn_0.04As:Zn) NWs. A significant broadening and redshift of the optical TO and LO phonon modes are observed for these NWs compared to as-grown undoped wires, which is attributed to strain induced by the Zn/Mn doping and to the presence of implantation-related defects. Moreover, the LO phonon modes are strongly damped, which is interpreted in terms of a strong LO phonon-plasmon coupling, induced by the free hole concentration. Moreover, we report on two new interesting Raman phonon modes (191 and 252 cm^-1) observed in Mn ion-implanted NWs, which we attribute to E_g (TO) and A_1g (LO) vibrational modes in a sheet layer of crystalline arsenic present on the surface of the NWs. This conclusion is supported by fitting the observed Raman shifts for the SO phonon modes to a theoretical dispersion function for a GaAs NW capped with a dielectric shell.

Deep subwavelength confinement and threshold engineering in a coupled nanorods based spaser

In recent years, extensive efforts have been made for design and fabrication of low threshold spasers or plasmonic nanolasers at a deep subwavelength scale. Plasmonic nanolasers with coupled-nanorods structure can realize this purpose due to energy concentration in nano size volumes and effective amplification mechanisms. In this study, a group of structures based on metallic and CdS coupled nanorods are designed and analyzed using the finite element method (FEM). By changing the lateral adjacent surfaces of the metal and semiconductor nanorods through utilizing regular polygons as the cross sections of the nanorods, different characteristics of the plasmonic nanolaser are investigated. Simulation results show that the mode area normalized by the diffraction limit area is as low as 0.0062 in the structures based on hexagonal metallic core with circular semiconductor nanorods while structures based on circular Ag core with hexagonal CdS nanorods can provide a low threshold gain as 1.310 μm^-1. Also, it is shown that if ZnO be used as the semiconductor gain material instead of CdS, a normalized mode area of almost one tenth can be attained in a structure with dodecagonal metallic core and circular ZnO nanorods.

Ultraviolet luminescence enhancement of planar wide bandgap semiconductor film by a hybrid microsphere cavity/dual metallic nanoparticles sandwich structure

Here we report a novel hybrid structure composing of microsphere array (MA), Al nanoparticles (Al-NPs), ZnO thin film (luminescence layer), Au nanoparticles (Au-NPs), and substrate (sapphire) for ultraviolet (UV) luminescence enhancement of planar wide bandgap semiconductor film. The plasmonic sandwich structure of Al-NPs/ZnO/Au-NPs boosts the hot electron state density in the conduction band by electron trapping from deep-defect level of ZnO and localized surface plasmon resonances (LSPRs) coupling around dual metallic NPs, enhancing UV emission and suppressing visible emission efficiently. The dielectric microsphere array capping on the plasmonic sandwich structure further increases UV emission intensity via photonic nanojets, optical whispering-gallery modes (WGMs), and directional antenna effect, by which the interaction between photon and exciton is strengthened. The contribution of microsphere cavity coupling with LSPRs to UV luminescence enhancement is therefore revealed. The maximum enhancement ratio of up to two orders of magnitude (~250-fold) is achieved by the optimized 5.06-μm-diameter-MA/Al-NPs/ZnO/Au-NPs/sapphire structure and the UV emission is highly directional with a divergent angle of ~5°. The present work provides a simple and easily-prepared structure incorporating optical WGMs and LSPRs to manipulate UV luminescence of planar wide-bandgap semiconductors for potential optoelectronic applications.

Low threshold room-temperature UV surface plasmon polariton lasers with ZnO nanowires on single-crystal aluminum films with Al2O3 interlayers

ZnO is one of the most promising optical gain media and allows lasing in ZnO nanowires at room temperature. Plasmonic lasers are potentially useful in applications in biosensing, photonic circuits, and high-capacity signal processing. In this work, we combine ZnO nanowires and single-crystalline aluminum films to fabricate Fabry–Perot type surface plasmon polariton (SPP) lasers to overcome the diffraction limit of conventional optics. High quality ZnO nanowires were synthesized by a vapor phase transport process via catalyzed growth. The ZnO nanowires were placed on a single-crystalline Al film grown by molecular beam epitaxy with an interlayer Al₂O₃ deposited by atomic layer deposition. The plasmonic laser is of metal-oxide-semiconductor (MOS) structure, compatible with silicon device processing. An optimal thickness of atomic layer deposited Al₂O₃ layer can lead to a low lasing threshold, 6.27 MW cm⁻², which is 3 times and 12 times lower than that of previous reports for ZnO/Al and Zno/Al₂O₃/Al plasmonic lasers, respectively, owing to low materials loss. Both the thickness and quality of insulating layers were found to critically influence the lasing threshold of the SPP nanolasers in the subwavelength regime. The simulation results also manifest the importance of the quality of the dielectric interlayer.

Ultralow threshold room-temperature UV surface plasmon polariton lasers using ZnO nanowires on single-crystal aluminum films with Al₂O₃ interlayers.

Surface Plasmon Nanolaser: Principle, Structure, Characteristics and Applications

Photonic devices are becoming more and more miniaturized and highly integrated with the advancement of micro-nano technology and the rapid development of integrated optics. Traditional semiconductor lasers have diffraction limit due to the feedback from the optical system, and their cavity length is more than half of the emission wavelength, so it is difficult to achieve miniaturization. Nanolasers based on surface plasmons can break through the diffraction limit and achieve deep sub-wavelength or even nano-scale laser emission. The improvement of modern nanomaterial preparation processes and the gradual maturity of micro-nano machining technology have also provided technical conditions for the development of sub-wavelength and nano-scale lasers. This paper describes the basic principles of surface plasmons and nano-resonators. The structure and characteristics of several kinds of plasmonic nanolasers are discussed. Finally, the paper looks forward to the application and development trend of nanolasers.

Dual-wavelength lasing from organic dye encapsulated metal–organic framework microcrystals

Efficient and stable dual-wavelength microlasers are obtained by simultaneously incorporating two kinds of organic dyes into metal–organic framework microcrystals.

On-Chip Monolithically Fabricated Plasmonic-Waveguide Nanolaser

Plasmonic-waveguide lasers, which exhibit subdiffraction limit lasing and light propagation, are promising for the next-generation of nanophotonic devices in computation, communication, and biosensing. Plasmonic lasers supporting waveguide modes are often based on nanowires grown with bottom-up techniques that need to be transferred and aligned for use in optical circuits. Here, we demonstrate a monolithically fabricated ZnO/Al plasmonic-waveguide nanolaser compatible with the fabrication requirements of on-chip circuits. The nanolaser is designed with a plasmonic metal layer on the top of the laser cavity only, providing highly efficient energy transfer between photons, excitons, and plasmons, and achieving lasing in the ultraviolet region up to 330 K with a low threshold intensity (0.20 mJ/cm² at room temperature). This work demonstrates the realization of a plasmonic-waveguide nanolaser without the need for transfer and positioning steps, which is the key for on-chip integration of nanophotonic devices.

Tailoring the Energy Levels and Cavity Structures toward Organic Cocrystal Microlasers

Organic cocrystals with unique energy-level structures are potentially a new class of materials for the development of versatile solid-state lasers. However, till now, the stimulated emission in cocrystal materials remains a big challenge possibly because of the nonradiative charge-transfer (CT) transitions. Here, for the first time, we report organic cocrystal microlasers constructed by simultaneously tailoring the energy levels and cavity structures based on the intermolecular halogen-bonding interactions. The intermolecular interactions triggered different self-assembly processes, resulting in distinct types of high-quality resonant microcavities. More importantly, the halogen-bonding interactions alleviated intermolecular CT and thus brought about a favorable four-level energy structure for the population inversion and tunable lasing in the cocrystals.

High Performance Plasmonic Nanolasers with External Quantum Efficiency Exceeding 10

Plasmonic nanolasers break the diffraction limit for an optical oscillator, which brings new capabilities for various applications ranging from on-chip optical interconnector to biomedical sensing and imaging. However, the inevitably accompanied metallic absorption loss could convert the input power to heat rather than radiations, leading to undesired low external quantum efficiency and device degradation. To date, direct characterization of quantum efficiency of plasmonic nanolasers is still a forbidden task due to its near-field surface plasmon emissions, divergent emission profile, and the limited emission power. Here, we develop a method to characterize the external quantum efficiency of plasmonic nanolasers by synergizing experimental measurement and theoretical calculation. With systematical device optimization, we demonstrate high performance plasmonic nanolasers with external quantum efficiency exceeding 10% at room temperature. This work fills in a missing yet essential piece of key metrics of plasmonic nanolasers. The demonstrated high external quantum efficiency of plasmonic nanolasers not only clarifies the long-standing debate, but also endorses the exploration of them in various practical applications such as near-field spectroscopy and sensing, integrated optical interconnects, solid-state lighting, and free-space optical communication.

Lasing threshold of thresholdless and non-thresholdless metal-semiconductor nanolasers

The recently developed plasmonic and photonic metal-semiconductor nanolasers feature unique properties, such as ultra-small mode volume and footprint, high Purcell factor, and ultra-fast modulation. However, it is often difficult to recognize when the transition to lasing occurs, while the most important feature of laser radiation, i.e., coherence, is available only above the lasing threshold. Here we systematically study the second-order coherence properties of metal-semiconductor nanolasers at both low- and high-pump rates. We find the lasing threshold using a clear coherence definition and derive a simple expression for the threshold pump current (optical pump power), which can be applied to most thresholdless and non-thresholdless metal-semiconductor nanolasers.