Spaser Research Articles

Complementary Dark and Bright Plasmonic Nanocavities with Controllable Energy Exchange for SERS Sensing

AbstractA 3D nanocavity array with each unit composed of complementarily arranged downward and upward silver nanoshells is reported. Due to the underneath planar gold film, the two differently oriented nanoshells form alternatively closed and semi‐closed nanocavities, respectively, which support dark and bright resonance modes of localized surface plasmons (LSPs). For the symmetric design of such nanocavities, mutually independent multipolar LSPs are directly excited at normal incidence, so that the near‐field coupling between them is inhibited. However, when the geometrical symmetry is broken by introducing a breach on a common sidewall of the two adjacent nanocavities, strong interaction and consequently energy exchange between them become allowed. As a result, Fano coupling between the dark and bright LSPs is clearly observed as a sharp spectroscopic resonance. The most important feature of such a coupled dark‐ and bright‐plasmon system is that the resonance modes of two different LSPs can be tuned separately, enhancing largely the flexibility of the photophysical performance and controllability. In particular, these features enable promising applications of such structures in single‐molecular detection through fluorescence enhancement, surface enhanced Raman scattering (SERS), and surface plasmon lasers. SERS measurements verify excellent performance of such nanocavity arrays in high‐sensitivity detection of low‐concentration molecules.

Gold nano-urchins for plasmonic enhancement of random lasing in a dye-doped polymer

We report our results on a plasmonic random laser with three-dimensional (3D) gold nano-urchins as scatterers distributed in rhodamine 6G dye-doped polymer film. The performance of anisotropic urchin scatterers is first studied using electromagnetic simulations for absorption/scattering cross-section and local field enhancement. This is compared to gold nanospheres of similar size. The simulation results indicate a two-fold local field enhancement, a higher scattering cross-section, and a low absorption cross-section in the 400 nm–570 nm spectral region of interest for nano-urchins. The effective scattering mean free path for urchins is calculated to be 90 µm less than nanoparticles. This suggests nano-urchins are efficient scatterers over conventional nanospheres for random lasing. A random laser is then experimentally demonstrated using gold nano-urchin scatterers, and incoherent lasing emission is observed for very low urchin number density of order ∼108 cm−3. A three-fold increase in scatterer concentration is shown to reduce the threshold energy from 0.8 mJ to 0.28 mJ per pulse. This is accompanied by a linewidth decrease from the rhodamine 6G emission bandwidth of 58 nm to up to 3 nm. Along with particle scattering, the waveguiding mechanism is identified to provide additional feedback for the lasing. This has been validated by the angular measurement of emission and by using spot pumping scheme. With low gold nano-urchin concentration, being incoherent and in film form, this plasmonic random laser could be an economical solution for speckle-free imaging applications.

Imaging of surface plasmon polaritons in low-loss highly metallic titanium nitride thin films in visible and infrared regimes.

Titanium nitride (TiN) has been identified as a promising refractory material for high temperature plasmonic applications such as surface plasmon polaritons (SPPs) waveguides, lasers and light sources, and near field optics. Such SPPs are sensitive not only to the highly metallic nature of the TiN, but also to its low loss. We have formed highly metallic, low-loss TiN thin films on MgO substrates to create SPPs with resonances between 775-825 nm. Scanning near-field optical microscopy (SNOM) allowed imaging of the SPP fringes, the accurate determination of the effective wavelength of the SPP modes, and propagation lengths greater than 10 microns. Further, we show the engineering of the band structure of the plasmonic modes in TiN in the mid-IR regime and experimentally demonstrate, for the first time, the ability of TiN to support Spoof Surface Plasmon Polaritons in the mid-IR (6 microns wavelength).

Design and optimization of perovskite plasmonic nano-laser for operation at room temperature

This work presents the design and optimization of a cascade nano-laser using CH3NH3PbI3 perovskite. Due to increasing threshold gain with decreasing device size and high Auger losses, the use of perovskite as the active medium in the cascade nano-laser was proposed, as the material possesses a high emission rate in the visible wavelength region, with relative ease of device fabrication. By optimizing the thickness of the perovskite, its width, and the thickness of the silica used, photonic and plasmonic modes were created, which were further considered to permit the generation of lasing, using their respective Purcell factors. The pump wavelength considered was 400 nm, with the laser emission then at 537 nm. For suitability of plasmonic lasing, a Purcell factor FP of 1.22 is reported here, with no possibility for photonic lasing due to its FP value being less than 1 in this design. However, mode-crossing effects were observed in the plasmonic mode at λ = 400 nm for two designs: at a silica thickness of 27.5 nm with perovskite thickness and width of 100 and 300 nm, respectively, and at a silica thickness of 30 nm with perovskite thickness and width of 95 and 300 nm, respectively. These mode-crossing effects can be further analyzed to use these devices in the design of potential new sensor systems, mainly for gas and chemical sensing, exploiting the refractive index sensing capability as a means to determine the concentration of the gases, or other chemicals, under study.

Low-cost biosensors based on a plasmonic random laser on fiber facet.

Low-cost and miniaturized biosensors are key factors leading to the possibility of portable and integrated biomedical system, which play an important role in clinical medicine and life sciences. Random lasers with simple structures provide opportunities for detecting biomolecules. Here, low-cost biosensors on fiber facet for label-free detecting biomolecules are demonstrated based on a plasmonic random laser. The random laser is achieved resorting to a self-assembled plasmonic scattering structure of Ag nanoparticles and polymer film on fiber facet. Refractive index sensitivity and near-surface sensitivity of the biosensor are systematically studied. Furthermore, the biosensor is used to detect IgG through specific binding to protein A, exhibiting the detecting limit of 0.68 nM. It is believed that this work may promote the applications of a plasmonic random laser bio-probe in portable or integrated medical diagnostic platforms, and provide fundamental understanding for the life science.

Numerical simulations on laser absorption enhancement in hybrid metallo-dielectric nanostructured targets for future nuclear astrophysics experiments

The linear electromagnetic interaction between innovative hybrid metallo-dielectric nanostructured targets and laser in visible and IR range is investigated through numerical simulations. The obtained results rely on the optimization of a target based on metallic nanowires (NWs) to enhance light absorption in the visible range of the electromagnetic spectrum. The NWs are grown within the ordered nanoholes of an alumina substrate, thus, forming a plasmonic lattice with triangular symmetry. The remaining volume of the nanoholes on top of the NWs is sealed with a transparent layer of aluminum oxide that is suitable to be chemically modified for containing about 25% of deuterium atoms. The study presented here is carried out within the framework of a scientific program named PLANETA (Plasmonic Laser Absorption on Nano-Engineered Targets) aiming at investigating new laser–matter interaction schemes in the ns domain and for nuclear fusion purposes, involving especially the D–D reaction.

High power edge-cum-surface emitting terahertz laser arrays phased locked by vacuum guided plasmon waves

Terahertz semiconductor quantum-cascade lasers (QCLs) are widely implemented with metallic cavities that support low-loss plasmonic optical modes at long wavelengths. However, resonant optical modes in such cavities suffer from poor radiative characteristics due to their subwavelength transverse dimensions. Consequently, single-mode terahertz QCLs with metallic cavities and large (>100 mW) output power have only been realized in the surface-emitting configuration that affords a large radiating surface. Here, we demonstrate a method to enhance radiative outcoupling from such plasmonic lasers for high-power emission in the edge-emitting (end-fire or longitudinal) direction. Single-sided plasmon waves propagating in vacuum are resonantly excited in surrounding medium of metallic cavities with the QCL semiconductor medium. The vacuum guided plasmon waves with a large wavefront phase-lock multiple metallic cavities longitudinally, which leads to intense radiation in multiple directions, including that in the longitudinal direction in a narrow single-lobed beam. The multicavity array radiates predominantly in a single spectral mode. A peak-power output of 260 mW and a slope efficiency of 303 mW/A are measured for the end-fire beam from a 3.3 THz QCL operating at 54 K in a Stirling cooler. Single-mode operation and lithographic tuning across a bandwidth of ∼150 GHz are demonstrated.

Dual-Wavelength Lasing in Quantum-Dot Plasmonic Lattice Lasers

Arrays of metallic particles patterned on a substrate have emerged as a promising design for on-chip plasmonic lasers. In past examples of such devices, the periodic particles provided feedback at a single resonance wavelength, and organic dye molecules were used as the gain material. Here, we introduce a flexible template-based fabrication method that allows a broader design space for Ag particle-array lasers. Instead of dye molecules, we integrate colloidal quantum dots (QDs), which offer better photostability and wavelength tunability. Our fabrication approach also allows us to easily adjust the refractive index of the substrate and the QD-film thickness. Exploiting these capabilities, we demonstrate not only single-wavelength lasing but dual-wavelength lasing via two distinct strategies. First, by using particle arrays with rectangular lattice symmetries, we obtain feedback from two orthogonal directions. The two output wavelengths from this laser can be selected individually using a linear polarizer. Second, by adjusting the QD-film thickness, we use higher-order transverse waveguide modes in the QD film to obtain dual-wavelength lasing at normal and off-normal angles from a symmetric square array. We thus show that our approach offers various design possibilities to tune the laser output.

Laser Raman scattering by graphene plasmons

A new scheme of Raman scattering of a laser by THz graphene plasmons in the presence of both space charge and electromagnetic surface plasmons is proposed. Plasmon–laser coupling arises through the two dimensional response of electrons in graphene. For optical frequency laser scattering by THz plasmons, phase matching conditions are satisfied for near normal incidence and the Raman scattered wave also propagates normal to the graphene surface. The amplitude of the electric field of the Raman scattered wave scales linearly with the plasmon density fluctuation level and laser amplitude and inversely with the electron effective mass. Raman scattering would provide in situ diagnostics of the amplitude and frequency of the THz wave.

Lasing from Finite Plasmonic Nanoparticle Lattices

Small lasers can generate coherent light for integrated photonics, in vivo cellular imaging, and solid-state lighting. Unlike conventional lasers, plasmonic lasers can generate coherent light at su...

Subharmonic Temporal Response and Spontaneous Breaking of Discrete Time Symmetry in Plasmonic Distributed Feedback Laser

Distributed feedback (DFB) lasers exploiting plasmonic structures as cavities are basic elements for various applications from sensorics to optoelectronics. Herein, the time response of a plasmonic DFB laser in the small‐signal modulation regime is studied. It is shown that the temporal period of the plasmonic DFB laser response can be equal to an integral multiple of the period of the pumping modulation, i.e., the temporal response can be subharmonic. This behavior of systems with time‐dependent parameters is a manifestation of spontaneous breaking of the discrete time symmetry. The appearance of subharmonic frequencies in the plasmonic DFB laser response is a consequence of synchronization of the beat frequency of the modes of plasmonic structures. Synchronization occurs when this beat frequency is nearly equal to the modulation frequency divided by an integer. It is shown that the ratio of the response period to the pump modulation period is determined by the size of the pumped region in the active medium, and the modulation frequency for the plasmonic DFB laser in the small‐signal modulation regime can reach several terahertz. These facts pave the way for the creation of tunable optoelectronic devices with a controlled temporal response.

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.

Resonant energy transfer and light scattering enhancement of plasmonic random lasers embedded with silver nanoplates.

The resonant energy transfer enhancement from a plasmonic random laser (PRL) has been investigated by means of a dye-covered PVA film with embedded silver nanoplates (DC-PVA/AgNPs). Different sizes and morphologies of AgNPs were adopted to shift the localized surface plasmon resonance (LSPR) and intensify recurrent light scattering between the AgNPs. For better overlap between surface plasmon resonance and the photoluminescence of fluorescent molecules with appropriately-sized silver nanoprisms, the slope efficiency of the PRL was greatly enhanced and the lasing threshold was obviously reduced. In addition, the photon lifetime for the DC-PVA/AgNPs film reveals an apparent decline around 1.39 ns owing to better coupling with LSPR. The stronger light scattering of samples with bigger-sized silver nanoprisms has been demonstrated by coherent back scattering measurements, which reveals a smaller transport mean free path around 3.3 μm. With α-stable analysis, it has been successfully demonstrated that the tail exponent α can be regarded as an identifier of the threshold of random lasing.

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.

Design and simulation of a germanium multiple quantum well metal strip nanocavity plasmon laser

In order to achieve electrically pumped plasmon nano lasers, several structures, materials and methods, have been proposed recently. However, there is still a long way to find out a reliable appropriate on-chip plasmon source for commercial plasmonic integrated circuits. In this paper, a new integrated nanocavity plasmon laser is proposed, analyzed and simulated for 1550 nm free-space wavelength. Due to its significant field confinement resulted by the metal strip structure and strong interaction of plasmonic modes with the germanium quantum wells this structure has a remarkable output performance. Purcell factor of this nanocavity is about 291. Using semi-classical rate equations in combination with finite difference time domain (FDTD) cavity mode analysis, the output performance measures are estimated and confirmed with respect to various physical models and simulation tools. Simulation results for this structure which has 0.073 µm2 area show a 2.8 µW output power with 10 µA injection current and about 4.16 mW output power with the threshold pump current of 27 mA, while maintaining its performance in a wide spectral bandwidth about 1.46 THz. It also can be electrically modulated by the pump current up to 5.7 GHz.

Footprint of plexcitonic states in low-power green–blue plasmonic random laser

Green–blue plasmonic random laser is attained by two-dimensional plexcitonic structure. The main gain plexcitonic media contained two-dimensional periodic arrays of gold nanowires which is covered by dye layer. Due to the change in the strength of exciton and plasmon coupling in these plexcitonic gain structures, different close loop, and thus random lasing must be takes place. For this purpose, we fabricate six samples with different plexcitonic power and pumped fabricated two-dimensional nanostructures by green nanosecond pulsed laser. Our results show efficient coherent random lasing due to the plexcitonic nanostructure in the blue, because two-photon absorption and also green part of the visible spectral region considering its applicability in the design and fabrication of compact and miniaturized random laser sources.

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.

Semiconductor nanowire plasmonic lasers

Abstract Semiconductor nanowires (NW) hold great promise for micro/nanolasers owing to their naturally formed resonant microcavity, tightly confined electromagnetic field, and outstanding capability of integration with planar waveguide for on-chip optoelectronic applications. However, constrained by the optical diffraction limit, the dimension of semiconductor lasers cannot be smaller than half the optical wavelength in free space, typically several hundreds of nanometers. Semiconductor NW plasmonic lasers provide a solution to break this limitation and realize deep sub-wavelength light sources. In this review, we summarize the advances of semiconductor NW plasmonic lasers since their first demonstration in 2009. First of all, we briefly look into the fabrication and physical/chemical properties of semiconductor NWs. Next, we discuss the fundamentals of surface plasmons as well as the recent progress in semiconductor NW plasmonic lasers from the aspects of multicolor realization, threshold reduction, ultrafast modulation, and electrically driven operations, along with their applications in sensing and integrated optics. Finally, we provide insights into bright perspectives and remaining challenges.

Lasing under ultralow pumping.

Ultralow-threshold plasmonic lasers under continuous-wave pumping at room temperature have been created using lattice plasmonic cavities integrated with gain material consisting of upconverting nanoparticles.

Plasmonic laser with distributed feedback

We have demonstrated a low-threshold surface-emitting plasmonic laser radiating two symmetrical beams (at 30° from the normal to the sample) characterized by a narrow (≤1 nm) spectral width, and explained its performance in terms of the distributed feedback mechanism. The results of our study provide an extra degree of freedom to the plasmonic laser design.