Demonstration of a spaser-based nanolaser

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.

Beam steering of a single nanoantenna

Nanoantennas play an important role as mediators to efficiently convert free-space light into localized optical energy and vice versa. However, effective control of the beam direction of a single nanoantenna remains a great challenge. In this paper, we propose an approach to steer the beam direction of a single nanoantenna by adjusting two antenna modes with opposite phase symmetry. Our theoretical study confirmed that the combination of even- and odd-symmetric modes with a phase difference of π/2 enables effective beam steering of a single nanoantenna whose steering angle is controlled by adjusting the amplitude ratio of the two antenna modes. To implement our theory in real devices, we introduced asymmetric trapezoidal nano-slot antennas with different side air-gaps of 10 and 50 nm. The trapezoidal nanoantennas can simultaneously excite the dipole and quadrupole modes in a single nanoantenna and enables effective beam steering with an angle of greater than 35° near the resonance of the quadrupole mode. In addition, the steering angle can also be controlled by adjusting the degree of asymmetry of the trapezoidal slot structure. We believe that our beam steering method for a single nanoantenna will find many potential applications in fields such as imaging, sensing, optical communication, and quantum optics.

Engineering III-V Semiconductor Nanowires for Device Applications

III-V semiconductor nanowires offer potential new device applications because of the unique properties associated with their 1D geometry and the ability to create quantum wells and other heterostructures with a radial and an axial geometry. Here, an overview of challenges in the bottom-up approaches for nanowire synthesis using catalyst and catalyst-free methods and the growth of axial and radial heterostructures is given. The work on nanowire devices such as lasers, light emitting nanowires, and solar cells and an overview of the top-down approaches for water splitting technologies is reviewed. The authors conclude with an analysis of the research field and the future research directions.

Spaser Nanoparticles for Ultranarrow Bandwidth STED Super-Resolution Imaging

Super-resolution microscopy, as a powerful tool of seeing abundant spatial details, typically can only distinguish a few distinct targets at a time due to the spectral crosstalk between fluorophores. Spaser (i.e., surface plasmon laser) nanoprobes, which confine lasing emission into nanoscale, offer an opportunity to eliminate such obstacle. Here, realized is narrow band stimulated emission depletion (STED) nanoscopy on spaser nanoparticles by collecting the coherent spasing signals. Demonstrated are the physics concept and feasibility of erasing spaser emission by using a depletion beam to suppress the population inversion, which lays the foundation of spaser-based STED super-resolution. Thanks to the small size (47 nm) and narrow spectral linewidth (3.8 nm) of the spaser nanoparticles, a 74 nm spatial resolution in STED imaging within an acquisition bandwidth of 10 nm is finally obtained. These spaser nanoparticles, if multiplexing with different wavelengths, in principle, allow for spectral-multiplexed imaging, sensing, cytometry, and light operation of a large number of targets all at once.

Forward and Backward Switching of Nonlinear Unidirectional Emission from GaAs Nanoantennas

High-index III-V semiconductor nanoantennas have gained great attention for enhanced nonlinear light-matter interactions, in the past few years. However, the complexity of nonlinear emission profiles imposes severe constraints on practical applications, such as in optical communications and integrated optoelectronic devices. These complexities include the lack of unidirectional nonlinear emission and the severe challenges in switching between forward and backward emissions, due to the structure of the susceptibility tensor of the III-V nanoantennas. Here, we propose a solution to both issues via engineering the nonlinear tensor of the nanoantennas. The special nonlinear tensorial properties of zinc-blende material can be used to engineer the nonlinear characteristics via growing the nanoantennas along different crystalline orientations. Based on the nonlinear multipolar effect, we have designed and fabricated (110)-grown GaAs nanoantennas, with engineered tensorial properties, embedded in a transparent low-index material. Our technique provides an approach not only for unidirectional second-harmonic generation (SHG) forward or backward emission but also for switching from one to another. Importantly, switching the SHG emission directionality is obtained only by rotating the polarization of the incident light, without the need for physical variation of the antennas or the environment. This characteristic is an advantage, as compared to other nonlinear nanoantennas, including (100)- and (111)-grown III-V counterparts or silicon and germanium nanoantennas. Indeed, (110)-GaAs nanoantennas allow for engineering the nonlinear nanophotonic systems including nonlinear "Huygens metasurfaces" and offer exciting opportunities for various nonlinear nanophotonics technologies, such as nanoscale light routing and light sources, as well as multifunctional flat optical elements.

Developing a time-domain method for simulating statistical behavior of many-emitter systems in the presence of electromagnetic field

In this paper, one of the major shortcomings of the conventional numerical approaches is alleviated by introducing the probabilistic nature of molecular transitions into the framework of classical computational electrodynamics. The main aim is to develop a numerical method which is capable of capturing the statistical attributes caused by the interactions between a group of spontaneous as well as stimulated emitters and the surrounding electromagnetic field. The electromagnetic field is governed by classical Maxwell's equations, while energy is absorbed from and emitted to the (surrounding) field according to the transitions occurring for the emitters, which are governed by time-dependent probability functions. These probabilities are principally consistent with quantum mechanics. In order to validate the proposed method, it is applied to three different test cases: directionality of fluorescent emission in a corrugated single-hole gold nanodisk, spatial and temporal coherence of fluorescent emission in a hybrid photonic-plasmonic crystal, and stimulated emission of a core-shell SPASER (surface plasmon amplification by stimulated emission of radiation). The results are shown to be closely comparable to the experimental results reported in the literature.

Feasibility of surface plasmon lasing in HgTe quantum wells with population inversion

Surface plasmon lasing in semiconductor gain media at far-infrared frequencies requires simultaneously long non-radiative recombination times and large plasmon propagation length. In this paper, we show that these conditions are realized in mercury-telluride quantum wells (HgTe QWs) near the topological transition. We derive the conditions of surface plasmon amplification in HgTe QWs with interband population inversion. To this end, we calculate the spatially-dispersive high-frequency conductivity of pumped HgTe QWs taking into account their realistic band structure, and compare the interband gain with Drude absorption and collisionless Landau damping. An extra necessary condition of plasmon lasing is revealed, namely, the non-equilibrium carrier density should be high enough to make the plasmon spectrum overlap with the frequency domain of interband excitations. The latter condition limits the processes of both stimulated and spontaneous plasmon emission at low temperatures, and should have a strong impact on the recombination kinetics of HgTe QWs at low temperatures.

Ultrastrong coupling of CdZnS/ZnS quantum dots to bonding breathing plasmons of aluminum metal-insulator-metal nanocavities in near-ultraviolet spectrum

Strong coupling originating from excitons of quantum dots and plasmons in nanocavities can be realized at room temperature due to the large electromagnetic field enhancement of plasmons, offering building blocks for quantum information systems, ultralow-power switches and lasers. However, most of the current strong coupling effects were realized by the interaction between excitons and far-field light excited bright plasmon modes in the visible range. Beyond that, there is still a lack of direct imaging of polariton modes at the nanoscale. In this work, by using cathodoluminescence, ultrastrong coupling with Rabi splitting exceeding 1 eV between bonding breathing plasmons of aluminum (Al) metal-insulator-metal (MIM) cavities and excited states of CdZnS/ZnS quantum dots was observed in the near-ultraviolet (UV) spectrum. Further, the hybridization of the QDs excitons and bonding breathing plasmonic modes is verified by deep-subwavelength images of polaritonic modes in real-space. Analytic analysis based on the coupled oscillator model and full-wave electromagnetic simulations is consistent with our experimental results. Our work not only indicates the great potential of electron excited plasmon modes for strong coupling applications, but also extends the polaritonic frequency to the UV range with Al nanocavities.

Electrical Excitation of Long-Range Surface Plasmons in PC/OLED Structure with Two Metal Nanolayers

A current-driven source of long-range surface plasmons (LRSPs) on a duplex metal nanolayer is reported. Electrical excitation of LRSPs was experimentally observed in a planar structure, where an organic light-emitting film was sandwiched between two metal nanolayers that served as electrodes. To achieve the LRSP propagation in these metal nanolayers at the interface with air, the light-emitting structure was bordered by a one-dimensional photonic crystal (PC) on the other side. The dispersion of the light emitted by such a hybrid PC/organic-light-emitting-diode structure (PC/OLED) comprising two thin metal electrodes was obtained, with a clearly identified LRSP resonance peak.

Effects of Plasmon Modes on Resonant Raman Images of a Single Molecule

Localized surface plasmons (LSPs) are excellent light sources at the nanoscale. How to precisely describe the interaction between LSPs and molecules has become an important issue. We present here a comprehensive study on the dependence of resonant Raman images on LSP modes generated by two typical nanostructures. Theoretical calculations demonstrate that the Raman images are sensitive to not only the spatial distribution but also the phase of the localized field, which should be attributed to the quantum nature of the interaction between LSP modes and molecules. We also find that the rotation of noncylindrical symmetry modes could affect the details of images, which offers an extra means to extract molecular information. Our findings extend the understanding of the LSP-matter interaction, which would be useful for the rational design of nanostructures and thus further applications of LSPs.

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.

Mid-Infrared Lasing in Lead Sulfide Subwavelength Wires on Silicon

Vapor-liquid-solid (VLS) growth of nanoscale or subwavelength scale semiconductor wires (nanowires) has been proven to be an important and effective approach to producing high-quality, substrate insensitive photonic materials with a flexible and ever-expanding coverage of wavelengths for lasing and other photonic applications. However, the materials and lasing demonstrations have so far been limited to mostly ultraviolet to visible wavelengths, with a few exceptions in the short-wavelength infrared range. A further extension to longer wavelengths (such as mid-infrared, MIR) using narrower band gap semiconductors encounters severe challenges: the ever decreasing radiative efficiency due to the Auger and other nonradiative channels with wavelengths demands extremely high material quality and significantly narrows the material choices. This situation is very unsatisfactory, given many important applications that demand materials and lasers of subwavelength scales for MIR wavelengths in an integrated platform, especially on silicon. Here we report our results on lasing demonstration in MIR (3-4 μm) based on a unique combination of high-quality material growth on a silicon substrate and the choice of an intrinsically strong MIR material in lead sulfide (PbS). Lasing is demonstrated from single wires both on the original silicon substrate and on the sapphire substrates after transferring, with sizes of lasing wires down to below half of the normalized volume (volume of wires divided by the wavelength cubed) and operating temperature up to 180 K. Such subwavelength wire lasers could be important for a wide range of MIR applications on silicon-based integrated photonic platforms, such as chemical and environmental sensing, free-space communications, and many others.

Localized ZnO Growth on a Gold Nanoantenna by Plasmon-Assisted Hydrothermal Synthesis

The excitation of localized surface plasmon resonances (LSPRs) in metal nanostructures enables subwavelength photon localization and large electric field enhancement, which can be advantageously used to strongly enhance light-matter interactions at the nanoscale. For this purpose, efficient methods for deterministically handling and arranging nanomaterials at the exact position of the localized electric field are required. In this Letter, we propose a novel method based on a hydrothermal synthesis reaction to locally and selectively synthesize zinc oxide in a plasmonic nanoantenna. We first make evident the role of LSPR for achieving efficient heating of gold nanostructures. Then, by selectively addressing one of the LSPRs of a gold antenna, we demonstrate that localized zinc oxide formation at the targeted location of the antenna can be achieved due to the nanoscale confinement of the heat production.

Materials chemistry and engineering in metal halide perovskite lasers

The invention and development of the laser have revolutionized science, technology, and industry. Metal halide perovskites are an emerging class of semiconductors holding promising potential in further advancing the laser technology. In this Review, we provide a comprehensive overview of metal halide perovskite lasers from the viewpoint of materials chemistry and engineering. After an introduction to the materials chemistry and physics of metal halide perovskites, we present diverse optical cavities for perovskite lasers. We then comprehensively discuss various perovskite lasers with particular functionalities, including tunable lasers, multicolor lasers, continuous-wave lasers, single-mode lasers, subwavelength lasers, random lasers, polariton lasers, and laser arrays. Following this a description of the strategies for improving the stability and reducing the toxicity of metal halide perovskite lasers is provided. Finally, future research directions and challenges toward practical technology applications of perovskite lasers are provided to give an outlook on this emerging field.

Localized surface plasmon resonances of a metal nanoring

Using the linear combination of atomic orbitals real-time-propagation rt-TDDFT technique and transition contribution maps, we study the optical and plasmonic features of a metal nanoring made up of sodium atoms.

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.

Interlayer exciton laser of extended spatial coherence in atomically thin heterostructures

Two-dimensional semiconductors have emerged as a new class of materials for nanophotonics owing to their strong exciton-photon interaction^1,2 and their ability to be engineered and integrated into devices³. Here we take advantage of these properties to engineer an efficient lasing medium based on direct-bandgap interlayer excitons in rotationally aligned atomically thin heterostructures⁴. Lasing is measured from a transition-metal dichalcogenide heterobilayer (WSe₂-MoSe₂) integrated in a silicon nitride grating resonator. An abrupt increase in the spatial coherence of the emission is observed across the lasing threshold. The work establishes interlayer excitons in two-dimensional heterostructures as a gain medium with spatially coherent lasing emission and potential for heterogeneous integration. With electrically tunable exciton-photon interaction strengths⁵ and long-range dipolar interactions, these interlayer excitons are promising for application as low-power, ultrafast lasers and modulators and for the study of many-body quantum phenomena⁶.

Channel competition in emitter-plasmon coupling

When an emitter is close to a plasmonic nanoantenna, besides coupling to the dipolar antenna mode, the emitter also considerably couples to a superposition of the high-order modes, referred to as a pseudomode. We comprehensively investigate the differences between the dipolar mode channel and the pseudomode channel in a representative system where a dipole emitter couples to a silver nanorod. The two channels are shown to be distinct in their mechanisms, characteristics (including chromatic dispersion and field distribution), and dependences on system parameters (including emitter-antenna distance, antenna geometry, and material loss). The study provides physical insight and reveals important design rules for controlling the competition between the two channels.

Design and Analysis of a Ag Rhombus Nanoparticle Film-Coupled Plasmonic Nanostructure

We design a coupled plasmonic nanostructure, which consists of a Ag rhombus nanoparticle positioned over a silver film, separated by a dielectric spacer layer, and perform numerical analysis by calculating the radiation loss resistance of this nanostructure as the perfect electric conductor metal based on the theory of transmission line modes. Compared with the nanocube or triangular nanodisk film-coupled plasmonic nanostructures introduced in the previous works, a stronger electric field enhancement was achieved in the Ag rhombus nanoparticle film-coupled nanostructure because of the fact that the sharp tip of the rhombus nanoparticle can generate field enhancement at a hot spot. In order to demonstrate that the sharp tip can confine the electromagnetic energies strongly, we also have calculated the Purcell factor and the far-field directivity of the quantum emitter in the vicinity of this nanostructure.

Recyclable coherent random lasers assisted by plasmonic nanoparticles in DCM-PVA thin films

Recyclable coherent random lasers assisted by plasmonic nanoparticles in DCM-PVA thin films are studied. Four DCM-PVA films with different nanoparticles are made, and the radiation characteristics of these random lasers are studied. The results show that the emission spectrum of the DCM-PVA film with Au nanoparticle of 50 nm in diameter is optimal, and its threshold is about 6.53 µJ/pulse. Underlying mechanisms are discussed in detail. Then the DCM-PVA film with Au nanoparticles of 50 nm in diameter is detached from a glass substrate and adhered to different substrates. Coherent random lasers also occur when the sample is based on different substrates. Finally, a method of making samples recyclable is proposed, and the emission spectrum of samples as a function of cycle index is studied. The results show that recyclable coherent random lasers can be realized with this method. This study provides a new way, to the best of our knowledge, to realize recyclable coherent random lasers with low-threshold.

Class-C semiconductor lasers with time-delayed optical feedback

We perform a linear stability analysis and numerical bifurcation diagrams of a class-C laser with time-delayed optical feedback. We employ a rate equation system based on the Maxwell-Bloch equations, and study the influence of the dephasing time on the laser dynamics. We find a stabilizing effect of an intermediate dephasing time, i.e. when moving from a class-B to a class-C laser. At long dephasing times, a destabilization of the laser solution occurs by a feedback-induced unlocking of Rabi oscillations at the second laser threshold. We predict an optimum resistance to time-delayed optical feedback for dephasing times close to the photon cavity lifetime. This article is part of the theme issue 'Nonlinear dynamics of delay systems'.

Quantizing single-molecule surface-enhanced Raman scattering with DNA origami metamolecules

Tailored metal nanoclusters have been actively developed to manipulate light at the subwavelength scale for nanophotonic applications. Nevertheless, precise arrangement of molecules in a hot spot with fixed numbers and positions remains challenging. Here, we show that DNA origami metamolecules with Fano resonances (DMFR) can precisely localize single dye molecules and produce quantified surface-enhanced Raman scattering (SERS) responses. To enable tailored plasmonic permutations, we develop a general and programmable method for anchoring a set of large gold nanoparticles (L-AuNPs) on prescribed n-tuple docking sites of super-origami DNA frameworks. A tetrameric nanocluster with four spatially organized 80-nm L-AuNPs exhibits peak-and-dip Fano characteristics. The drastic enhancement at the wavelength of the Fano minimum allows the collection of prominent SERS spectrum for even a single dye molecule. We expect that DMFR provides physical insights into single-molecule SERS and opens new opportunities for developing plasmonic nanodevices for ultrasensitive sensing, nanocircuits, and nanophotonic lasers.

Electromagnetic energy in multilayered spherical particles

We obtain exact analytic expressions for (i) the electromagnetic energy radial density within and outside a multilayered sphere and (ii) the total electromagnetic energy stored within its core and each of its shells. Explicit expressions for the special cases of lossless core and shell are also provided. The general solution is based on the compact recursive transfer-matrix method, and its validity includes also magnetic media. The theory is illustrated on examples of electric field enhancement within various metallo-dielectric silica-gold multilayered spheres. The user-friendly MATLAB code, which includes the theoretical treatment, is available as a supplement to the paper.

Lightning-Rod Effect of Plasmonic Field Enhancement on Hydrogen-Absorbing Transition Metals

The plasmonic enhancement of electromagnetic field energy density at the sharp tips of nanoparticles or nanoscale surface roughnesses of hydrogen-absorbing transition metals, Pd, Ti, and Ni, is quantitatively investigated. A large degree of energy focusing is observed for these transition metals in the microwave region, even surpassing the enhancement for noble metals according to the conditions. Pd, for instance, exhibits peak field enhancement factors of 6000 and 2 × 108 in air for morphological aspect ratios of 10 and 100, respectively. Metal surfaces possibly contain such degrees of nano- or micro-scale native random roughnesses, and, therefore, the field enhancement effect may have been unknowingly produced in existing electrical and optical systems. In addition, for future devices under development, particularly in hydrogen-related applications, it is desirable to design and optimize the systems, including the choice of materials, structures, and operating conditions, by accounting for the plasmonic local energy enhancement effect around the metal surfaces.

Photoswitchable Spasers with a Plasmonic Core and Photoswitchable Fluorescent Proteins

Photoswitchable fluorescent proteins (PFPs) that can change fluorescence color upon excitation have revolutionized many applications of light such as tracking protein movement, super-resolution imaging, identification of circulating cells, and optical data storage. Nevertheless, the relatively weak fluorescence of PFPs limits their applications in biomedical imaging due to strong tissue autofluorecence background. Conversely, plasmonic nanolasers, also called spasers, have demonstrated potential to generate super-bright stimulated emissions even inside single cells. Nevertheless, the development of photoswitchable spasers that can shift their stimulated emission color in response to light is challenging. Here, we introduce the novel concept of spasers using a PFP layer as the active medium surrounding a plasmonic core. The proof of principle was demonstrated by synthesizing a multilayer nanostructure on the surface of a spherical gold core, with a non-absorbing thin polymer shell and the PFP Dendra2 dispersed in the matrix of a biodegradable polymer. We have demonstrated photoswitching of spontaneous and stimulated emission in these spasers below and above the spasing threshold, respectively, at different spectral ranges. The plasmonic core of the spasers serves also as a photothermal (and potentially photoacoustic) contrast agent, allowing for photothermal imaging of the spasers. These results suggest that multimodal photoswitchable spasers could extend the traditional applications of spasers and PFPs in laser spectroscopy, multicolor cytometry, and theranostics with the potential to track, identify, and kill abnormal cells in circulation.

Hybrid Plasmonic-Ferroelectric Architectures for Lasing and SHG Processes at the Nanoscale

Coherent light sources providing sub-wavelength confined modes are in ever more demand to face new challenges in a variety of disciplines. Scalability and cost-effective production of these systems are also highly desired. The use of ferroelectrics in functional optical platforms, on which plasmonic arrangements can be formed, is revealed as a simple and powerful method to develop coherent light sources with improved and novel functionalities at the nanoscale. Two types of sources with sub-diffraction spatial confinement and improved performances are presented: i) plasmon-assisted solid-state nanolasers based on the interaction between metallic nanostructures and optically active rare earth doped ferroelectric crystals and ii) nonlinear radiation sources based on quadratic frequency mixing processes that are enhanced by means of localized surface plasmon (LSP) resonances. The mechanisms responsible for the intensification of the radiation-matter interaction processes by LSP resonances are discussed in each case. The challenges, potential applications, and future perspectives of the field are highlighted.

Lasing action in low-resistance nanolasers based on tunnel junctions

We experimentally demonstrate the lasing action of a new nanolaser design with a tunnel junction. By using a heavily doped tunnel junction for hole injection, we can replace the p-type contact material of a conventional nanolaser diode with a low-resistance n-type contact layer. This leads to a significant reduction of the device resistance and lowers the threshold voltage from 5 V to around 0.95 V at 77 K. The lasing behavior is verified by the light output versus the injection current (L-I) characterization and second-order coherence function measurements. Because of less Joule heating during current injection, the nanolaser can be operated at temperatures as high as 180 K under CW pumping. The incorporation of heavily doped tunnel junctions may pave the way for other nanoscale cavity design for improved heat management.

Inducing lasing in organic materials with low optical gain by three-dimensional plasmonic nanocavity arrays

Lasing in organic media with very low gain has been pursued for a long time in optoelectronics. Here, we experimentally demonstrate that plasmonic lasing in the visible regime at room temperature can be achieved by hybridizing active media of very low optical gain such as ionic liquid and polymethylmethacrylate with three-dimensional (3D) plasmonic metamaterials. The 3D nanostructure consists of a double-layer N-shaped silver wire-hole array with strongly coupled multiple hot spots densely packed in each unit cell. These hot spots overlap perfectly with the gain media, allowing efficient gain-plasmon coupling in subwavelength volumes. The periodic arrangement of hot spots, as the metal and dielectric are distributed in an alternate manner along both transverse and vertical directions, results in ultrastrong suppression of scattering losses. In addition, the lasing characteristics, including threshold, intensity and polarization can be controlled by the lattice constant and geometry of metamaterials. Such a plasmonic nanolaser proves to be of low threshold and low gain requirement, providing an essential step towards easy-processing organic based optoelectronics.

Fourier transform plasmon resonance spectrometer using nanoslit-nanowire pair

In this paper, we present a nanoscale Fourier transform spectrometer using a plasmonic interferometer consisting of a tilt subwavelength slit-nanowire pair on a metallic surface fabricated by the focused ion beam microfabrication technique. The incident broadband light strongly couples with the surface plasmons on the gold surface, and thus, surface plasmon polaritons (SPPs) are generated. The launched SPPs interfere with the incident light and generate high contrast interference fringes in the nanoslit. The transmitted SPPs through the metal nanoslit can decouple into free space and are collected by an objective in the far field. The spectroscopic information of the incidence light is obtained by fast Fourier transform of the fringe pattern of the SPPs. In our design, there is no need for a bulky dispersive spectrometer or dispersive optical elements. The dimension of the spectrometer is around 200 μm length. Our design is based on inherent coherence of the SPP waves propagating through the subwavelength metal nanoslit structures etched into an opaque gold film.

Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience

The research field of liquid crystals and their applications is recently changing from being largely focused on display applications and optical shutter elements in various fields, to quite novel and diverse applications in the area of nanotechnology and nanoscience. Functional nanoparticles have recently been used to a significant extent to modify the physical properties of liquid crystals by the addition of ferroelectric and magnetic particles of different shapes, such as arbitrary and spherical, rods, wires and discs. Also, particles influencing optical properties are increasingly popular, such as quantum dots, plasmonic, semiconductors and metamaterials. The self-organization of liquid crystals is exploited to order templates and orient nanoparticles. Similarly, nanoparticles such as rods, nanotubes and graphene oxide are shown to form lyotropic liquid crystal phases in the presence of isotropic host solvents. These effects lead to a wealth of novel applications, many of which will be reviewed in this publication.

A Self-Consistent Quantum Field Theory for Random Lasing

The spatial formation of coherent random laser modes in strongly scattering disordered random media is a central feature in the understanding of the physics of random lasers. We derive a quantum field theoretical method for random lasing in disordered samples of complex amplifying Mie resonators which is able to provide self-consistently and free of any fit parameter the full set of transport characteristics at and above the laser phase transition. The coherence length and the correlation volume respectively is derived as an experimentally measurable scale of the phase transition at the laser threshold. We find that the process of stimulated emission in extended disordered arrangements of active Mie resonators is ultimately connected to time-reversal symmetric multiple scattering in the sense of photonic transport while the diffusion coefficient is finite. A power law is found for the random laser mode diameters in stationary state with increasing pump intensity.

Lasing in Ni Nanodisk Arrays

We report on lasing at visible wavelengths in arrays of ferromagnetic Ni nanodisks overlaid with an organic gain medium. We demonstrate that by placing an organic gain material within the mode volume of the plasmonic nanoparticles both the radiative and, in particular, the high ohmic losses of Ni nanodisk resonances can be compensated. Under increasing pump fluence, the systems exhibit a transition from lattice-modified spontaneous emission to lasing, the latter being characterized by highly directional and sub-nanometer line width emission. By breaking the symmetry of the array, we observe tunable multimode lasing at two wavelengths corresponding to the particle periodicity along the two principal directions of the lattice. Our results are relevant for loss-compensated magnetoplasmonic devices and topological photonics.

Out-of-equilibrium electron dynamics of silver driven by ultrafast electromagnetic fields - a novel hydrodynamical approach

We investigate the ultrafast nonlinear response of silver upon excitation by infrared electromagnetic radiation pulses with a duration of a few femtoseconds. By adopting the Landau weak coupling approach to account for electron-electron and electron-phonon collisions, we solve the Boltzmann equation through the method of moments obtaining a novel set of hydrodynamical equations describing the ultrafast nonlinear dynamics of electrons in silver. While the novel hydrodynamical model that was obtained reduces to the Drude model for small intensities of the driving field, it predicts that absorption saturates for large but experimentally attainable peak intensities of the order of GW cm-2. Our results are important for absorption mitigation in plasmonic devices, with potential impact for low-loss plasmonic waveguides and interconnects.

Sharp selective scattering of red, green, and blue light achieved via gain material's loss compensation

Frequency-selective scattering of light can be achieved by metallic nanoparticle's localized surface plasmon resonance (LSPR). And this property may find an application in a transparent projection screen: ideally, specially designed metallic nanoparticles dispersed in a transparent matrix only selectively scatter red, green and blue light and transmit the visible light of other colors. However, optical absorption and surface dispersion of a metallic nanoparticle, whose size is comparable or smaller than mean free path of electrons in the constituent material, degenerate the desired performance by broadening the resonance peak width (i.e., decreasing frequency-selectivity) and decreasing light scattering intensity. In this work, it is shown that the problem can be solved by introducing gain material. Numerical simulations are performed on nanostructures based on silver (Ag), gold (Au) or aluminum (Al) with or without gain material, to examine the effect of gain material and to search for suitable structures for sharp selective scattering of red, green and blue light. And it is found that introducing gain material greatly improves performance of the structures based on Ag or Au except the structures based on Al. The most suitable structures for sharp selective scattering of red, green and blue light are, respectively, found to be the core-shell structures of silica/Au (core/shell), silica/Ag and Ag/silica, all with gain material.

Design of anapole mode electromagnetic field enhancement structures for biosensing applications

The design of an all-dielectric nanoantenna based on nonradiating "anapole" modes is studied for biosensing applications in an aqueous environment, using FDTD electromagnetic simulation. The strictly confined electromagnetic field within a circular or rectangular opening at the center of a cylindrical silicon disk produces a single point electromagnetic hotspot with up to 6.5x enhancement of |E|, for the 630-650 nm wavelength range, and we can increase the value up to 25x by coupling additional electromagnetic energy from an underlying PEC-backed substrate. We characterize the effects of the substrate design and slot dimensions on the field enhancement magnitude, for devices operating in a water medium.

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.

Scattering characteristics of an exciton-plasmon nanohybrid made by coupling a monolayer graphene nanoflake to a carbon nanotube

A hybrid nanostructure where a graphene nanoflake (GNF) is optically coupled to a carbon nanotube (CNT) could potentially possess enhanced sensing capabilities compared to the individual constituents whilst inheriting their high biocompatibility, favourable electrical, mechanical and spectroscopic properties. Therefore, in this paper, we investigate the scattering characteristics of an all-carbon exciton-plasmon nanohybrid which was made by coupling an elliptical GNF resonator to a semiconducting CNT gain element. We analytically model the nanohybrid as an open quantum system using cavity quantum electrodynamics. We derive analytical expressions for the dipole moment operator and the dipole response field of the GNF and characterize the Rayleigh scattering spectrum of the nanohybrid. These analytical expressions are valid for any arbitrary ellipsoidal nanoresonator coupled to a quantum emitter. Furthermore, we perform a detailed numerical analysis, the results of which indicate that the GNF-CNT nanohybrid exhibits enhanced and versatile scattering capabilities compared to the individual constituents. We show that the spectral signatures of the nanohybrid are highly tunable using a multitude of system parameters such as Fermi energy of the GNF, component dimensions, GNF-CNT separation distance and the permittivity of the submerging medium. We finally demonstrate the prospect of using the proposed nanohybrid to reconstruct the permittivity profile of a tumour. The high biocompatibility and high sensitivity to the dielectric properties of the environment make the proposed GNF-CNT nanohybrid an ideal candidate for such biosensing applications.

In-plane coherent control of plasmon resonances for plasmonic switching and encoding

Considerable attention has been paid recently to coherent control of plasmon resonances in metadevices for potential applications in all-optical light-with-light signal modulation and image processing. Previous reports based on out-of-plane coherent control of plasmon resonances were established by modulating the position of a metadevice in standing waves. Here we show that destructive and constructive absorption can be realized in metallic nano-antennas through in-plane coherent control of plasmon resonances, which is determined by the distribution rule of electrical-field components of nano-antennas. We provide proof-of-principle demonstrations of plasmonic switching effects in a gold nanodisk monomer and dimer, and propose a plasmonic encoding strategy in a gold nanodisk chain. In-plane coherent control of plasmon resonances may open a new avenue toward promising applications in optical spectral enhancement, imaging, nanolasing, and optical communication in nanocircuits.

Plasmon-induced dual-wavelength operation in a Yb3+ laser

Expanding the functionalities of plasmon-assisted lasers is essential for emergent applications in nanoscience and nanotechnology. Here, we report on a novel ability of plasmonic structures to induce dual-wavelength lasing in the near-infrared region in a Yb³⁺ solid-state laser. By means of the effects of disordered plasmonic networks deposited on the surface of a Yb³⁺-doped nonlinear RTP crystal, room-temperature dual-wavelength lasing, with a frequency difference between the lines in the THz range, is realized. The dual-wavelength laser is produced by the simultaneous activation of two lasing channels, namely, an electronic- and a phonon-terminated laser transition. The latter is enabled by the out-of-plane field components that are generated by the plasmonic structures, which excite specific Raman modes. Additionally, multiline radiation at three different wavelengths is demonstrated in the visible spectral region via two self-frequency conversion processes, which occur in the vicinities of the plasmonic structures. The results demonstrate the potential of plasmonic nanostructures for inducing drastic modifications in the operational mode of a solid-state laser and hold promise for applications in a variety of fields, including multiplexing, precise spectroscopies, and THz radiation generation via a simple and cost-effective procedure.

Electromagnetic analysis of the lasing thresholds of hybrid plasmon modes of a silver tube nanolaser with active core and active shell

Results from the electromagnetic modeling of the threshold conditions of hybrid plasmon modes of a laser based on a silver nanotube with an active core and covered with an active shell are presented. We study the modes of such a nanolaser that have their emission wavelengths in the visible-light range. Our analysis uses the mathematically grounded approach called the lasing eigenvalue problem (LEP) for the set of the Maxwell equations and the boundary and radiation conditions. As we study the modes exactly at the threshold, there is no need to invoke nonlinear and quantum models of lasing. Instead, we consider a laser as an open plasmonic resonator equipped with an active region. This allows us to assume that at threshold the natural-mode frequency is real-valued, according to the situation where the losses, in the metal and for the radiation, are exactly balanced with the gain in the active region. Then the emission wavelength and the associated threshold gain can be viewed as parts of two-component eigenvalues, each corresponding to a certain mode. In the configuration considered, potentially there are three types of modes that can lase: the hybrid localized surface plasmon (HLSP) modes of the metal tube, the core modes, and the shell modes. The latter two types can be kept off the visible range in thin enough configurations. Keeping this in mind, we focus on the HLSP modes and study how their threshold gain values change with variations in the geometrical parameters of the nanotube, the core, and the shell. It is found that essentially a single-mode laser can be designed on the difference-type HLSP mode of the azimuth order m = 1, shining in the orange or red spectral region. Furthermore, the threshold values of gain for similar HLSP modes of order m = 2 and 3 can be several times lower, with emission in the violet or blue parts of the spectrum.

Strong and weak couplings in molecular vibration-plasmon hybrid structures

Molecular vibration-plasmon couplings in a hybrid structure, which are composed of a silver grating filled with polymethyl methacrylate (PMMA) molecules (SG-PMMA), have been investigated theoretically. It is found that the interaction between the vibrational transitions and plasmons can transform from weak coupling into strong coupling by reducing the distance between the elements. When the space between grating elements is large, the localized surface plasmon resonance (LSP) of the silver elements greatly enhances the absorption of the PMMA molecules. As the gap between elements becomes small, the plasmonic nanocavity (NC) mode emerges and couples strongly with the molecular vibrational mode of PMMA. The strong coupling results in two new hybridized modes and the Rabi splitting energy is about 15 meV. Our work provides an effective way to alter the coupling strength of the molecular vibration-plasmon hybrid system and may be beneficial to the further biochemical and biophysical applications.

Singular properties generated by finite periodic PT-symmetric optical waveguide network

In this work we investigate the extraordinary characteristics of one-dimensional (1D) finite periodic parity-time (PT) symmetric network. On the basis of the transfer matrix method, three simple expressions are analytically obtained for transmission, left reflection and right reflection coefficients. For this periodic structure, we provide new criteria for the PT-symmetry breaking transition in terms of the elements of the transfer matrix and the scattering matrix. These criteria indicate that the exceptional points are related only to the cell structure, but not to the cell number. Utilizing these criteria and expressions, the relationships between the transmittances (reflectances) and the cell number are considered in detail. Furthermore, the conditions for ultrastrong transmission are analytically derived. We also show how a PT-symmetric network can become unidirectionally and bidirectionally transparent at specific frequencies. The conditions and related properties of unidirectional and bidirectional transparencies are also examined. Finally, we find that the finite periodic PT-symmetric network with certain cell number can be viewed as a unidirectionally invisible structure at the exceptional points. Our work may pave the way for designing a diversefamily of optical structures and networks with new properties and functionalities.

Topological Luminophor Y2O3:Eu3++Ag with High Electroluminescence Performance

Improving luminescent intensity is a significant technical requirement and scientific problem for the luminescent performance of fluorophor materials through the ages. The process control and luminescence performance still limit the developments of luminescent intensity even though it can be improved partly by covering or magnetron sputtering of precious metals on the surface of the fluorophore materials. On the basis of the improvement of luminescence center radiative transition rate by surface plasma resonance and Y₂O₃:Eu³⁺ microsheet phosphors, a fundamental model for topological luminophor Y₂O₃:Eu³⁺+Ag was designed through referencing the concepts of topological materials to enhance luminescent performance by composite luminescence, which is composed of Eu³⁺ centric electroluminescence and surface plasma-enhanced photoluminescence by Ag. The topological luminophor Y₂O₃:Eu³⁺+Ag was successfully synthesized with an asymmetric-discrete Ag nanocrystal topological structure on the surface just via illumination. Experimental results suggest that the luminescence performance of topological luminophor Y₂O₃:Eu³⁺+Ag increased by about 300% compared to that of Y₂O₃:Eu³⁺ phosphors in the same conditions. The design of a topological luminophor provides a new approach to further improve the luminescent intensity of phosphors.

Study of silica-based intrinsically emitting nanoparticles produced by an excimer laser

We report an experimental study demonstrating the feasibility to produce both pure and Ge-doped silica nanoparticles (size ranging from tens up to hundreds of nanometers) using nanosecond pulsed KrF laser ablation of bulk glass. In particular, pure silica nanoparticles were produced using a laser pulse energy of 400 mJ on pure silica, whereas Ge-doped nanoparticles were obtained using 33 and 165 mJ per pulse on germanosilicate glass. The difference in the required energy is attributed to the Ge doping, which modifies the optical properties of the silica by facilitating energy absorption processes such as multiphoton absorption or by introducing absorbing point defects. Defect generation in bulk pure silica before nanoparticle production starts is also suggested by our results. Regarding the Ge-doped samples, scanning electron microscopy (SEM) and cathodoluminescence (CL) investigations revealed a good correspondence between the morphology of the generated particles and their emission signal due to the germanium lone pair center (GLPC), regardless of the energy per pulse used for their production. This suggests a reasonable homogeneity of the emission features of the samples. Similarly, energy dispersive X-ray spectroscopy (EDX) data showed that the O, Ge and Si signals qualitatively correspond to the particle morphology, suggesting a generally uniform chemical composition of the Ge-doped samples. No significant CL signal could be detected in pure silica nanoparticles, evidencing the positive impact of Ge for the development of intrinsically emitting nanoparticles. Transmission electron microscope (TEM) data suggested that the Ge-doped silica nanoparticles are amorphous. SEM and TEM data evidenced that the produced nanoparticles tend to be slightly more spherical in shape for a higher energy per pulse. Scanning transmission electron microscope (STEM) data have shown that, regardless of size and applied energy per pulse, in each nanoparticle, some inhomogeneity is present in the form of brighter (i.e., more dense) features of a few nanometers.

Applications of nanolasers

Nanolasers generate coherent light at the nanoscale. In the past decade, they have attracted intense interest, because they are more compact, faster and more power-efficient than conventional lasers. Thanks to these capabilities, nanolasers are now an emergent tool for a variety of practical applications. In this Review, we explain the intrinsic merits of nanolasers and assess recent progress on their applications, particularly for optical interconnects, near-field spectroscopy and sensing, optical probing for biological systems and far-field beam synthesis through near-field eigenmode engineering. We highlight the scientific and engineering challenges that remain for forging nanolasers into powerful tools for nanoscience and nanotechnology.

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.

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.

Free electrons excited SPASER

Surface plasmon amplification by stimulated emission of radiation (SPASER) is discovered and used for realizing lasers at nanometer scale. The conventional gain media that are applied in SPASER are solid materials, such as organic dye or semiconductor, which limits the frequency range of SPASER. The free electrons could be considered as a kind of gain medium for emitting radiation. Here, we investigate theoretically the SPASER, which is excited by free electrons. We also demonstrate the tunable, deep-ultraviolet, and ultracompact laser numerically by having free electrons interact with surface plasmon polariton mode supported on metal surface. The output power density could reach about 30 W/μm² and the wavelength in deep ultraviolet could be widely tuned by varying the electron energy. This work offers a way of realizing integrated free electron laser in the ultraviolet frequency region.

Low Threshold Plasmonic Nanolaser Based on Graphene

A hybrid plasmonic nanolaser based on nanowire/air slot/semicircular graphene and metal wire structure was designed. In this structure, the waveguides in the nanowires and the graphene-metal interface are coupled to form a hybrid plasma mode, which effectively reduces the metal loss. The mode and strong coupling of the laser are analyzed by using the finite-element method. Its electric field distribution, propagation loss, normalized mode area, quality factor, and lasing threshold are studied with the different geometric model. Simulation results reveal that the performance of the laser using this structure can be optimized by adjusting the model parameters. Under the optimal parameters, the effective propagation loss is only 0.0096, and the lasing threshold can be as low as 0.14 μm−1. This structure can achieve deep sub-wavelength confinement and low-loss transmission, and provides technical support for the miniaturization and integration of nano-devices.

Lasing threshold of the bound states in the continuum in the plasmonic lattices

Distributed feedback plasmonic laser based on the periodic array of holes in the silver half-space and covered with gain medium is considered. Square, hexagonal and several rectangular lattices are studied. It is demonstrated that the bound states in the continuum provide substantially lower threshold than radiating modes. Moreover, it is shown that while the hole size increases the lasing threshold of some modes decreases. Among the studied types of lattices, lasing in the hexagonal lattice requires the lowest material gain of only 18 cm^-1. Our results pave the way to the development of the efficient low-threshold distributed feedback plasmonic lasers.