Spaser Research Articles

Influence of Silver Film Quality on the Threshold of Plasmonic Nanowire Lasers

In recent years, the field of plasmonics has become dependent on the ability to maximize the quality of metal films. In particular, plasmonic lasers will require a metal loss as low as possible in order to minimize their operational threshold if they are to find applications. In this article, the findings of recent studies, which report reduced thresholds in plasmonic lasers incorporating high quality metal films with both ultraflat surfaces and high crystallinity, are reassessed. In particular, the role of deposition rate and substrate temperature on silver film growth for improving the performance of hybrid plasmonic zinc oxide nanowire lasers is studied. High substrate temperatures and high deposition rates allow near‐epitaxial silver growth on mica. Even though a 2 fold reduction in metal loss for the cystalline films is demonstrated, the presented plasmonic lasers show only a modest reduction in threshold, compared to noncrystalline films. This finding is consistent with our observation that the plasmonic cavity loss is comparable to the absorption of the unexcited semiconductor. Laser threshold is thus only partially dependent on reducing metal film loss. This work increases the fundamental understanding of metal film quality in plasmonics and gives new insights into the threshold characteristics of metal‐based lasers.

Advances of plasmonic nanolasers

Semiconductor lasers are widely used for applications in biology, information storage, photonics and medical therapeutics. With the development of the emerging area of nano-optics and nanophotonics, more compact lasers attract significant interest. As the cavity size is reduced with respect to the emission wavelength, interesting physical effects in electromagnetic cavities arise. To scale down the semiconductor lasers in all three dimensions plays an important role in the development of low-dimension, low-threshold, and ultrafast coherent light sources, as well as integrated nano-optoelectronic and plasmonic circuits. In this review, the overall formalism of mode gain and confinement factor in the metal–semiconductor plasmonic lasers was introduced firstly. In addition, an updated overview of the latest developments, particularly in plasmonic nanolasers using the metal-insulator-semiconductor (MIS) configuration and another related metal-cladded semiconductor microlasers was presented. In particular, it has been experimentally demonstrated that the use of plasmonic cavities based on MIS nanostructures can indeed break the diffraction limit in three dimensions. We also present some perspectives on the challenges and development trend for the plasmonic nanolasers. This review can provide useful guide for the research of plasmonic nanolasers.

Tunable enhanced spontaneous emission in plasmonic waveguide cladded with liquid crystal and low-index metamaterial.

The control and enhancement of the spontaneous emission (SE) of emitters embedded in subwavelength structures are fundamentally interesting and of practical interest. For example, in plasmonic lasers and on-chip single photon sources, a large SE rate and the active modulation of SE over a very broad spectral band are highly desired functionalities. In this paper, we demonstrate by an explicit theoretical calculation that a plasmonic waveguide cladded with liquid crystals (LCs) and low-index metamaterials can give rise to an enhancement in the intrinsic SE rate γ0 of more than two orders of magnitude. Furthermore, by varying the refractive index of the LC cladding, thereby changing the density of states of the surface plasmons, the enhanced SE rate can be modulated over a very large range, e.g., from 131γ0 to 327γ0. In general, the modulation range increases with the anisotropy in the refractive index of the LC, while for a fixed range of modulation, the SE rate is larger with lower cladding indices. These results for active modulation and enhanced SE may find application in enabling low-threshold plasmonic nanolasers and tunable on-chip single photon sources.

Coherent Light Sources at the Nanoscale.

This review focuses on coherent light sources at the nanoscale, and specifically on lasers exploiting plasmonic cavities that can beat the diffraction limit of light. Conventional lasers exhibit coherent, intense, and directional emission with cavity sizes much larger than their operating wavelength. Plasmon lasers show ultrasmall mode confinement, support strong light-matter interactions, and represent a class of devices with extremely small sizes. We discuss the differences between plasmon lasers and traditional ones, and we highlight advances in directionality and tunability through innovative cavity designs and new materials. Challenges and future prospects are also discussed.

Red-green-blue plasmonic random laser.

A red-green-blue plasmonic random laser is achieved in a multilayer structure, which is fabricated by spin-coating three polymer solutions successively on a silica substrate. Under optical pumping, strong amplification of the polymer radiation can be observed due to the localized surface plasmon resonance of silver nanoparticles embedded in the multilayer structure. Red-green-blue random lasing is simultaneously obtained from the sample based on the enhanced scattering strength of silver nanoparticles. These results are useful for designing compact integrated random laser sources.

Proposal for a self-excited electrically driven surface plasmon polariton generator

We propose a generator of surface plasmon polaritons (SPPs) which, unlike spasers or plasmon lasers, does not require stimulated emission in the system. Its principle of operation is based on a positive feedback which an ensemble of classical oscillating dipoles experiences from a reflective surface located in its near field. The generator design includes a nanocavity between two metal surfaces which contains metal nanoparticles in its interior. The whole structure is placed onto a prism surface that allows one to detect the generated SPPs in the Kretschmann configuration. The generation process is driven by a moderate DC voltage applied between the metal covers of the cavity. Both the generation criterion and the steady-state operation of the generator are investigated.

Dye gain gold NW array of surface plasmon polariton waveguide

Plasmon lasers can support ultrasmall mode confinement and ultrafast dynamics with device feature sizes below the diffraction limit. At present in the single visible light frequency, the optical gain method of constraint SPP on metal nanowires structure reported less. We design the gold nanowire array structure, consisting of PMMA and R6G dye molecules as gain, by 488nm pump in the middle of the nanowires position for wide range of light, use symmetry broken overcome that momentum does not match the photonic and SPP energy conversion. Theoretical analysis shows that dyes provide coherent optical feedback, resulting in nanowires face will observe laser properties of surface plasmons. Feature analysis: the incident light and pump joint strength is greater than the sum of strength which is the incident light, pump respectively. Under the effect of dye molecules gain effective, length of SPP transmission can increase 1µm. The results achieved in a single optical frequency of stimulated radiation, application of dye optical gain can achieve continuous gain effect. This is for the future development of plasma amplifier and the wavelength laser.

Plasmon lasers: coherent nanoscopic light sources.

Plasmon lasers are a new class of coherent light sources that use metals for light localization and amplification. Access to this confined light that couples to the oscillating electrons of metal enables reducing the physical size and mode volume of the laser much below the diffraction limit. The race to demonstrate new plasmon nanolasers has enabled considerable progress over the last several years regarding nanocavity design, operating temperature, pumping conditions, and material efficiency of both plasmonic nanocavity and gain medium. This article discusses some of the latest developments of coherent nanoscopic light sources, in metallic and dielectric lasers, with a specific focus on plasmon nanolasers. We highlight recent advances in plasmon lasers through plasmonic nanoparticle arrays: beam directionality, wavelength tunability, multi-modal emission, and dark and bright modes lasing. We also discuss future prospects.

Airy fiber: waveguides array coupling based light beam control method

Recently, Airy beam as a kind of non-diffracting beam, has attracted a great deal of attention due to its unique properties to have propagation-invariant intensity profile, remain transverse accelerating and exhibit self-healing features. Therefore, Airy beams have found many potential applications, such as optical micro-manipulation, imaging technology, surface plasmon polaritons and laser micromachining. Airy optical fiber as a kind of waveguide device can be applied for the Airy beam generation, carry out the exploration of new Airy fiber and expand the Airy beam application range, has important practical significance. In this paper, we give an systematical introduction from the view of the Airy beam working principle, Airy fiber structure design, Airy fiber beam generated internal mechanism, Airy beam wavelength response characteristics, and Airy fiber applications.

Large static tuning of narrow-beam terahertz plasmonic lasers operating at 78K

A new tuning mechanism is demonstrated for single-mode metal-clad plasmonic lasers, in which the refractive-index of the laser’s surrounding medium affects the resonant-cavity mode in the same vein as the refractive-index of gain medium inside the cavity. Reversible, continuous, and mode-hop-free tuning of ∼57 GHz is realized for single-mode narrow-beam terahertz plasmonic quantum-cascade lasers (QCLs), which is demonstrated at a much more practical temperature of 78 K. The tuning is based on post-process deposition/etching of a dielectric (silicon-dioxide) on a QCL chip that has already been soldered and wire-bonded onto a copper mount. This is a considerably larger tuning range compared to previously reported results for terahertz QCLs with directional far-field radiation patterns. The key enabling mechanism for tuning is a recently developed antenna-feedback scheme for plasmonic lasers, which leads to the generation of hybrid surface-plasmon-polaritons propagating outside the cavity of the laser with a large spatial extent. The effect of dielectric deposition on QCL’s characteristics is investigated in detail including that on maximum operating temperature, peak output power, and far-field radiation patterns. Single-lobed beam with low divergence (<7°) is maintained through the tuning range. The antenna-feedback scheme is ideally suited for modulation of plasmonic lasers and their sensing applications due to the sensitive dependence of spectral and radiative properties of the laser on its surrounding medium.

Anomalous forward scattering of gain-assisted dielectric shell-coated metallic core spherical particles

Abstract We have investigated the scattering properties of an individual core-shell nanoparticle using the Mie theory, which can be tuned to support both electric and magnetic modes simultaneously. In general, the suppression of forward scattering can be realized by the second Kerker condition. Here, a novel mechanism has to be adopted to explain zero-forward scattering, which originates from the complex interactions between dipolar and quadrupolar modes. However, for lossy and lossless core-shell spherical nanoparticles, zero-forward scattering can never be achieved because the real parts of Mie expansion coefficients are always positive. By adding proper gain in dielectric shell, zero-forward scattering can be found at certain incident wavelengths, which means that all electric and magnetic responses in Mie scattering can be counteracted totally in the forward direction. In addition, if the absolute values of dipolar and quadrupolar terms are in the same order of magnitude, the local scattering minimum and maximum can be produced away from the forward and backward directions due to the interacting effect between the dipolar and quadrupolar terms. Furthermore, by adding suitable gain in shell, super-forward scattering can also be realized at certain incident wavelengths. We also demonstrated that anomalously weak scattering or superscattering could be obtained for the core-shell nanoparticles with suitable gain in shell. In particular, for such a choice of suitable gain in shell, we can obtain zero-forward scattering and anomalously weak scattering at the same wavelength as well as super-forward scattering at another wavelength. These features may provide new opportunities for cloaking, plasmonic lasers, optical antennas, and so on.

Amplification and Lasing of Plasmonic Modes

Plasmonic modes provide the potential for routing information with optical speeds and bandwidth at the nanoscale. The inherent tradeoff between modal energy confinement and energy dissipation, however, makes realizing this potential an extreme challenge. For decades, researchers have investigated compensating losses in plasmonic systems with active gain media. In recent years, progress has increased dramatically, as nanoscale plasmonic amplifiers and lasers have emerged. We review this progress and offer our prospective on the challenges ahead.

Lithographically Defined, Room Temperature Low Threshold Subwavelength Red-Emitting Hybrid Plasmonic Lasers.

Hybrid plasmonic lasers provide deep subwavelength optical confinement, strongly enhanced light-matter interaction and together with nanoscale footprint promise new applications in optical communication, biosensing, and photolithography. The subwavelength hybrid plasmonic lasers reported so far often use bottom-up grown nanowires, nanorods, and nanosquares, making it difficult to integrate these devices into industry-relevant high density plasmonic circuits. Here, we report the first experimental demonstration of AlGaInP based, red-emitting hybrid plasmonic lasers at room temperature using lithography based fabrication processes. Resonant cavities with deep subwavelength 2D and 3D mode confinement of λ2/56 and λ3/199, respectively, are demonstrated. A range of cavity geometries (waveguides, rings, squares, and disks) show very low lasing thresholds of 0.6-1.8 mJ/cm2 with wide gain bandwidth (610 nm-685 nm), which are attributed to the heterogeneous geometry of the gain material, the optimized etching technique, and the strong overlap of the gain material with the plasmonic modes. Most importantly, we establish the connection between mode confinements and enhanced absorption and stimulated emission, which plays critical roles in maintaining low lasing thresholds at extremely small hybrid plasmonic cavities. Our results pave the way for the further integration of dense arrays of hybrid plasmonic lasers with optical and electronic technology platforms.

Surface Lattice Resonances for Enhanced and Directional Electroluminescence at High Current Densities.

Hybrid photonic-plasmonic modes in periodic arrays of metallic nanostructures offer a promising trade-off between high-quality cavities and subdiffraction mode confinement. However, their application in electrically driven light-emitting devices is hindered by their sensitivity to the surrounding environment and to charge injecting metallic electrodes in particular. Here, we demonstrate that the planar structure of light-emitting field-effect transistor (LEFET) ensures undisturbed operation of the characteristic modes. We incorporate a square array of gold nanodisks into the charge transporting and emissive layer of a polymer LEFET in order to tailor directionality and emission efficiency via the Purcell effect and variation of the fractional local density of states in particular. Angle- and polarization-resolved spectra confirm that the enhanced electroluminescence correlates with the dispersion curves of the surface lattice resonances supported by these structures. These LEFETs reach current densities on the order of 10 kA/cm2, which may pave the way toward practical optoelectronic devices with tailored emission patterns and potentially electrically pumped plasmonic lasers.

Tailoring the energy distribution and loss of 2D plasmons

The ability to tailor the energy distribution of plasmons at the nanoscale has many applications in nanophotonics, such as designing plasmon lasers, spasers, and quantum emitters. To this end, we analytically study the energy distribution and the proper field quantization of 2D plasmons with specific examples for graphene plasmons. We find that the portion of the plasmon energy contained inside graphene (energy confinement factor) can exceed 50%, despite graphene being infinitely thin. In fact, this very high energy confinement can make it challenging to tailor the energy distribution of graphene plasmons just by modifying the surrounding dielectric environment or the geometry, such as changing the separation distance between two coupled graphene layers. However, by adopting concepts of parity-time symmetry breaking, we show that tuning the loss in one of the two coupled graphene layers can simultaneously tailor the energy confinement factor and propagation characteristics, causing the phenomenon of loss-induced plasmonic transparency.

Single layer graphene band hybridization with silver nanoplates: Interplay between doping and plasmonic enhancement

In this paper, we report single layer graphene (SLG) hybridized with silver nanoplates, in which nanoplates act as either a charge doping or a field enhancement source for the SLG Raman spectrum. Surprisingly, the stiffening of both G and 2D peaks of more than 10 cm−1 was observed with no plasmonic enhancement of peaks, indicating that p-doping from nanoplates on SLG is occurring. Such observation is explained in terms of the contact separation distance between the graphene and the silver nanoplates being enough (∼4 Å) to cause a Fermi level shift in graphene to allow p-doping. When nanoplates were modified in shape with laser irradiation by either photothermal plasmon printing or laser induced ablation, the charge doping was lifted and the strong plasmonic enhancement of Raman signals was observed, indicating that the separation distance is increased. Further, when the nanoplates are oxidized, the two effects on the Raman bands of SLG are turned off, returning the Raman signals back to the original SLG state.

Theoretical study of plasmonic lasing in junctions with many molecules

We calculate the quantum state of the plasmon field excited by an ensemble of molecular emitters, which are driven by exchange of electrons with metallic nano-particle electrodes. Assuming identical emitters that are coupled collectively to the plasmon mode but are otherwise subject to independent relaxation channels, we show that symmetry constraints on the total system density matrix imply a drastic reduction in the numerical complexity. For $N_{\text{m}}$ three-level molecules we may thus represent the density matrix by a number of terms scaling as $(N_{\rm m}+8)!/(8!N_{\rm m}!)$ instead of $9^{N_{\text{m}}}$, and this allows exact simulations of up to $N_{\text{m}}=10$ molecules. Our simulations demonstrate that many emitters compensate strong plasmon damping and lead to the population of high plasmon number states and a narrowed linewidth of the plasmon field. For large $N_{\text{m}}$, our exact results are reproduced by an approximate approach based on the plasmon reduced density matrix. With this approach, we have extended the simulations to more than $50$ molecules and shown that the plasmon number state population follows a Poisson-like distribution. An alternative approach based on nonlinear rate equations for the molecular state populations and the mean plasmon number also reproduce the main lasing characteristics of the system.

Terahertz plasmonic laser radiating in an ultra-narrow beam

Plasmonic lasers (spasers) generate coherent surface-plasmon-polaritons (SPPs) and could be realized at subwavelength dimensions in metallic cavities for applications in nanoscale optics. Plasmonic cavities are also utilized for terahertz quantum-cascade lasers (QCLs), which are the brightest available solid-state sources of terahertz radiation. A long standing challenge for spasers is their poor coupling to the far-field radiation. Unlike conventional lasers that could produce directional beams, spasers have highly divergent radiation patterns due to their subwavelength apertures. Here, we theoretically and experimentally demonstrate a new technique for implementing distributed-feedback (DFB) that is distinct from any other previously utilized DFB schemes for semiconductor lasers. The so-termed antenna-feedback scheme leads to single-mode operation in plasmonic lasers, couples the resonant SPP mode to a highly directional far-field radiation pattern, and integrates hybrid SPPs in surrounding medium into the operation of the DFB lasers. Experimentally, the antenna-feedback method, which does not require the phase matching to a well-defined effective index, is implemented for terahertz QCLs, and single-mode terahertz QCLs with beam divergence as small as 4 x 4 degree are demonstrated, which is the narrowest beam reported for any terahertz QCL to-date. Moreover, in contrast to negligible radiative-field in conventional photonic band-edge lasers, in which the periodicity follows the integer multiple of half-wavelength inside active medium, antenna-feedback breaks this integer-limit for the first time and enhances the radiative-field of lasing mode. The antenna-feedback scheme is generally applicable to any plasmonic laser with a Fabry-Perot cavity irrespective of its operating wavelength, and could bring plasmonic lasers closer to practical applications.

Plasmonic random lasing in polymer fiber

A random fiber laser is achieved based on the plasmonic feedback mechanism, which is constructed by first siphoning the polymer solution doped with silver nanoparticles into a 300-μm capillary tube and then evaporating the solvent. Strong amplification of the radiation can be obtained by employing the variable gain region, the fiber waveguide scheme and three-dimensional plasmonic feedback provided by the silver nanoparticles. Low-threshold directional random lasing is observed in the polymer fiber. This simple and straightforward approach facilitates the investigation of plasmonic random fiber lasers.

Wide-aperture planar lasers

Wide-aperture planar lasers (vertical-cavity surface emitting lasers, distributed feedback lasers, 2D spaser arrays, and plasmon lasers on stopped-light mode) are reviewed. Structure, parameters (directional pattern and amplitude modulation frequency), and physical principles of such lasers are discussed.