Support Surface Plasmon Polaritons Research Articles

Novel Highly Efficient Buried Gratings for Selective Coupling of SPP Waves onto Single Interfaces

Diffraction gratings have always been used to effectively couple optical radiation within integrated waveguides. This is also valid for plasmonic structures that support Surface Plasmon Polariton (SPP) waves. Traditional gratings usually excite SPP waves at the interface where they are located or, for thin metal nanostrips, at both interfaces. But reducing the thickness of the metal layer in the presence of a grating has the handicap of increasing the tunnelling of light towards the substrate, which means higher losses and reduced coupling efficiency. In this paper, we design and optimize novel gratings buried within the metallic thin films for selective coupling of SPP waves onto individual interfaces. Compared with traditional superficial gratings, the novel buried ones demonstrate higher efficiency and much lower residual tunnelling of light through the coupling structures.

On-chip generation of single-photon circularly polarized single-mode vortex beams.

Generation of single photons carrying spin and orbital angular momenta (SAM and OAM) opens enticing perspectives for exploiting multiple degrees of freedom for high-dimensional quantum systems. However, on-chip generation of single photons encoded with single-mode SAM-OAM states has been a major challenge. Here, by using carefully designed anisotropic nanodimers fabricated atop a substrate, supporting surface plasmon polariton (SPP) propagation, and accurately positioned around a quantum emitter (QE), we enable nonradiative QE-SPP coupling and the SPP outcoupling into free-space propagating radiation featuring the designed SAM and OAM. We demonstrate on-chip room-temperature generation of well-collimated (divergence < 7.5°) circularly polarized (chirality > 0.97) single-mode vortex beams with different topological charges (𝓁 = 0, 1, and 2) and high single-photon purity, g(2)(0) < 0.15. The developed approach can straightforwardly be extended to produce multiple, differently polarized, single-mode single-photon radiation channels and enable thereby realization of high-dimensional quantum sources for advanced quantum photonic technologies.

Room-Temperature Gate Voltage Modulation of Plasmonic Nanolasers.

Stable electrical modulation of plasmonic nanolasers is achieved on a hybrid graphene-insulator-metal (GIM) platform at room temperature. To support surface plasmon polariton (SPP) resonance, a zinc oxide (ZnO) nanowire is placed on the GIM platform to create a plasmonic cavity with a compact mode volume of 2.6 × 10-2 λ3, and the graphene layer is used as a transparent electrode for electrical modulation. When a gate voltage is applied, the surface electron density of Al varied, which results in the shifting of its plasma frequency and thus affects its SPP dispersion. In particular, this variation strongly changes the internal loss of the SPP mode; thus, the lasing thresholds of the ZnO nanowire plasmonic nanolasers on the GIM platform can be modulated by the gate voltage. This study demonstrates the gate voltage modulation of ZnO nanowire plasmonic nanolasers on a GIM platform at room temperature. These nanolasers can exhibit ultrahigh modulation speed on the order of terahertz. Accordingly, plasmonic nanolasers with gate voltage modulation have high potential for plasmonic circuit applications with high operation speed and versatility.

Spontaneous emission enhancement and directional emission by an optical nanonatenna array on a metallic mirror

Optical nanoantennas support surface plasmon polariton (SPP) with a confinement of light breaking through the diffraction limit, and thereby achieving an enhancement and regulation of electromagnetic field on a deep-subwavelength scale. In this paper, a periodic array of optical nanoantennas on a metallic mirror is proposed, where the antennas are gold nanocubes forming a two-dimensional periodic array, and a single point emission source is located in the nanogap between the antenna of gold nanocube and the gold mirror. The nanogap between the antenna and mirror can support gap surface plasmon, which results in an enhanced spontaneous emission rate. Meanwhile, the periodic array of nanoantennas can support the surface lattice resonance (SLR), which can achieve directional far-field radiation perpendicular to the substrate or in a specified direction by properly designing the array period. To design the antenna that can simultaneously achieve an enhancement of spontaneous emission rate and a directional radiation of far field, the calculation of the radiation field of a single point source in a periodic structure is transformed into the calculation of the radiation fields of a set of pseudoperiodic point-source arrays by combining the array scanning method (ASM) and full-wave rigorous numerical method, thus giving the spontaneous emission rate of the emitter and the near-field distribution of the antenna. Concerning the calculation of the angular distribution of far-field radiation intensity, we start from the Maxwell’s equations and provide a rigorous formulation and proof of the reciprocity-theorem method. This proof is different from those reported in existing literature and has a more rigorous applicability for infinite-extent periodic structures or has a lower amount of computational work. Based on the reciprocity-theorem method, the antenna parameters are designed so that the enhancement factor of far-field radiation intensity reaches a maximum value of 2756 in the direction perpendicular to the substrate, and simultaneously, the enhancement factors of total spontaneous emission rate and far-field spontaneous emission rate of the point source reach 1097 and 55.50, respectively. The proposed antenna has a simple structure that is easy to design and fabricate, and the proposed design method is intuitive and easy to implement, which can be used to guide the design of high-speed, high-brightness and directional-radiation light sources.

Mid-IR Surface Plasmon Polaritons in CdZnO thin films on GaAs

Metal-oxides are known to be ideal alternatives to noble metals for mid-IR plasmonics, but they are typically grown on substrates as sapphire or glass, which cannot be used as active materials in electronic devices. We demonstrate here that thin Cd(Zn)O films can be integrated on GaAs, one of the dominant optoelectronic material platforms in the mid-IR. Cd(Zn)O is likely the best plasmonic material in this region of the spectrum, and we show that when deposited on GaAs it yields polycrystalline films that support surface plasmon polaritons (SPPs). As a result of the preserved out-of-plane orientation of the crystal grains, the plasma damping in these layers is very moderately degraded with respect to monocrystalline layers on sapphire, yielding an SPP quality factor comparable to other typical TCOs like AZO, GZO or ITO, but with the added functionality of being integrated on GaAs. Hence, these results open the door for the development of future hybrid Cd(Zn)O/GaAs technology, where Cd(Zn)O can be used to amplify the electric field in underlying devices, e.g. photodetectors.

Water Diffusion Effects at Gold–Graphene Interfaces Supporting Surface Plasmon Polaritons

Water Diffusion Effects at Gold–Graphene Interfaces Supporting Surface Plasmon Polaritons

Label-free detection of single nanoparticles with disordered nanoisland surface plasmon sensor

We report sensing of single nanoparticles using disordered metallic nanoisland substrates supporting surface plasmon polaritons (SPPs). Speckle patterns arising from leakage radiation of elastically scattered SPPs provide a unique fingerprint of the scattering microstructure at the sensor surface. Experimental measurements of the speckle decorrelation are presented and shown to enable detection of sorption of individual gold nanoparticles and polystyrene beads. Our approach is verified through bright-field and fluorescence imaging of particles adhering to the nanoisland substrate.

Microscopic analysis of the energy, momentum, and spin distributions in a surface plasmon-polariton wave

We analyze the electromagnetic field near a plane interface between a conductive and a dielectric media, under conditions supporting surface plasmon-polariton (SPP) propagation. The conductive medium is described by the hydrodynamic electron-gas model that enables a consistent analysis of the field-induced variations of the electron density and velocity at the interface and its nearest vicinity. The distributions of electromagnetic dynamical characteristics: energy, energy flow, spin and momentum are calculated analytically and illustrated numerically, employing silver-vacuum interface as an example. A set of the “field” and material contributions to the energy, spin and momentum are explicitly identified and classified with respect to their physical origins and properties, and the orbital (canonical) and spin (Belinfante) momentum constituents are separately examined. In this context, a procedure for the spin-orbital momentum decomposition in the presence of free charges is proposed and substantiated. The microscopic results agree with the known phenomenological data but additionally show specific nanoscale structures in the near-interface behavior of the SPP energy and momentum, which can be deliberately created, controlled and used in nanotechnology applications.

Infrared Plasmonic Sensing with Anisotropic Two-Dimensional Material Borophene.

Borophene, a new member of the two-dimensional material family, has been found to support surface plasmon polaritons in visible and infrared regimes, which can be integrated into various optoelectronic and nanophotonic devices. To further explore the potential plasmonic applications of borophene, we propose an infrared plasmonic sensor based on the borophene ribbon array. The nanostructured borophene can support localized surface plasmon resonances, which can sense the local refractive index of the environment via spectral response. By analytical and numerical calculation, we investigate the influences of geometric as well as material parameters on the sensing performance of the proposed sensor in detail. The results show how to tune and optimize the sensitivity and figure of merit of the proposed structure and reveal that the borophene sensor possesses comparable sensing performance with conventional plasmonic sensors. This work provides the route to design a borophene plasmonic sensor with high performance and can be applied in next-generation point-of-care diagnostic devices.

Composite Structure Based on Gold-Nanoparticle Layer and HMM for Surface-Enhanced Raman Spectroscopy Analysis.

Hyperbolic metamaterials (HMMs), supporting surface plasmon polaritons (SPPs), and highly confined bulk plasmon polaritons (BPPs) possess promising potential for application as surface-enhanced Raman scattering (SERS) substrates. In the present study, a composite SERS substrate based on a multilayer HMM and gold-nanoparticle (Au-NP) layer was fabricated. A strong electromagnetic field was generated at the nanogaps of the Au NPs under the coupling between localized surface plasmon resonance (LSPR) and a BPP. Additionally, a simulation of the composite structure was assessed using COMSOL; the results complied with those achieved through experiments: the SERS performance was enhanced, while the enhancing rate was downregulated, with the extension of the HMM periods. Furthermore, this structure exhibited high detection performance. During the experiments, rhodamine 6G (R6G) and malachite green (MG) acted as the probe molecules, and the limits of detection of the SERS substrate reached 10−10 and 10−8 M for R6G and MG, respectively. Moreover, the composite structure demonstrated prominent reproducibility and stability. The mentioned promising results reveal that the composite structure could have extensive applications, such as in biosensors and food safety inspection.

Terahertz surface plasmon propagation in a near field coupled asymmetric resonators based planar waveguide

In this article, we investigate terahertz surface plasmons propagation in a planar waveguide comprising asymmetric resonators placed in close proximity. The waveguide is designed to support surface plasmon polaritons at two distinct terahertz frequencies, ω 1 and ω 2, which are near to each other. This is accomplished by carefully designing the unit cell comprising of two resonators with slightly different sizes. The resonators in the form of rectangular apertures are placed along the transverse direction in the near field regime. We observe an absorption window surrounded by two transmission resonances which are coupled to each other. As the resonators are moved apart, we observe a switching from the coupled to an uncoupled resonance state. The absorption window can be tuned by changing the resonance frequency of the resonators, which depends upon its structural parameters. We have employed a theoretical model to understand the coupling mechanism between the resonators and the dispersive behavior resulting in the absorption window. The proposed study is significant in the construction of planar terahertz components such as slow lights systems, buffers, etc, where strongly dispersive mediums play a vital role.

Tunable far-infrared hyperbolic metamaterial based on graphene-polar dielectric

Abstract In this work, we propose a tunable hyperbolic graphene-polar dielectric metamaterial in the far-infrared range. Here, we employ LiF as a strongly dispersive photonic material which can support surface plasmon polariton within its polaritonic gap. The proposed metamaterial has been analyzed using effective medium approximation (EMA) and transfer matrix method (TMM). It is found that the characterization of the hyperbolic behavior and reflection coefficient of the proposed metamaterial can be effectively controlled by Fermi energy, number of graphene layers and thickness of LiF. According to the results, the hyperbolic bandwidth broadening is possible by increasing Fermi energy, number of graphene layers and decreasing LiF thickness. Furthermore, we have shown fully controllable reflective filtering property of the proposed metamaterial using EMA and TMM. It can be clearly visualized that for thick LiF, effective medium approximation results in complete validation in calculation of the reflection coefficient. This is demonstrating by comparison between the reflection coefficient of the proposed metamaterial calculated using EMA and TMM. We believe that the proposed graphene-polar dielectric metamaterial offers a potential strategy for fine tunning the hybridized surface plasmon phonon polariton in the far-infrared region towards high performance, tunable and active metamaterials.

Laser-printed hollow nanostructures for nonlinear plasmonics

Ultrafast deposition of laser energy to a thin substrate-supported Au film enables formation of unique 3D surface morphologies, hollow nanobumps, possessing prospective linear optical properties useful for advanced plasmonic biosensors and nanoscale light sources. Here, we study the nonlinear optical response of such nanostructures by measuring second harmonic generation (SHG) from the individual nanobumps and their ordered arrays supporting local- and lattice-type plasmons at near-infrared frequencies. The results reveal the maximal SHG yield for the isolated nanobump correlated with its geometry-defined electromagnetic near-field enhancement at fundamental frequency. Additional (up to 110-fold) SHG enhancement with respect to the signal from a smooth Au film is achieved via a positive feedback provided through proper arrangement of the resonant nanobumps into the array supporting surface plasmon polariton resonant excitation near the fundamental frequency. Our results substantiate the laser-printed nanobumps as an inexpensive and flexible platform for nanoscale infrared-to-visible light conversion that can be applied for nonlinear plasmonics.

2D materials integrated with metallic nanostructures: fundamentals and optoelectronic applications

Abstract Due to their novel electronic and optical properties, atomically thin layered two-dimensional (2D) materials are becoming promising to realize novel functional optoelectronic devices including photodetectors, modulators, and lasers. However, light–matter interactions in 2D materials are often weak because of the atomic-scale thickness, thus limiting the performances of these devices. Metallic nanostructures supporting surface plasmon polaritons show strong ability to concentrate light within subwavelength region, opening thereby new avenues for strengthening the light–matter interactions and miniaturizing the devices. This review starts to present how to use metallic nanostructures to enhance light–matter interactions in 2D materials, mainly focusing on photoluminescence, Raman scattering, and nonlinearities of 2D materials. In addition, an overview of ultraconfined acoustic-like plasmons in hybrid graphene–metal structures is given, discussing the nonlocal response and quantum mechanical features of the graphene plasmons and metals. Then, the review summarizes the latest development of 2D material–based optoelectronic devices integrated with plasmonic nanostructures. Both off-chip and on-chip devices including modulators and photodetectors are discussed. The potentials of hybrid 2D materials plasmonic optoelectronic devices are finally summarized, giving the future research directions for applications in optical interconnects and optical communications.

Colloidal Bimetallic Nanorings for Strong Plasmon Exciton Coupling

Nobel-metal nanostructures strongly localize and manipulate light at nanoscale dimension by supporting surface plasmon polaritons. In fact, the optical properties of the nobel-metal nanostructures ...

Enhanced subwavelength coupling and nano-focusing with optical fiber-plasmonic hybrid probe.

Metallic nanowires supporting surface plasmon polaritons can localize optical fields at nanoscale tapered ends for near-field imaging. Radially polarized light through the optical fiber or free space efficiently excites the converging radial plasmons at the apex of a sharp tip. However, such radial vector mode excitation through optical fiber requires precise polarization control and strongly polarization maintaining state in optical fiber, thus inducing a complexity for optical applications. In this paper, we propose a photonic-plasmonic probe that uses the linearly polarized source to excite the nanoscale plasmonic hotspot. The linearly polarized fiber mode is converted to radial surface plasmon polaritons (SPPs) through asymmetric coupling at the base of the metallic nano-tip. The radial SPP's then propagate along the half ellipsoid tip and are focused at the tip apex giving rise to the nanoscale concentration of optical energy. The probe can be implemented with near-field imaging techniques such as tip-enhanced Raman microscopy to obtain topographic and chemical spectroscopic information in atomic resolution for studying light-matter interaction at the nanoscale.

Plasmonic Metamaterials for Nanochemistry and Sensing.

Plasmonic nanostructures were initially developed for sensing and nanophotonic applications but, recently, have shown great promise in chemistry, optoelectronics, and nonlinear optics. While smooth plasmonic films, supporting surface plasmon polaritons, and individual nanostructures, featuring localized surface plasmons, are easy to fabricate and use, the assemblies of nanostructures in optical antennas and metamaterials provide many additional advantages related to the engineering of the mode structure (and thus, optical resonances in the given spectral range), field enhancement, and local density of optical states required to control electronic and photonic interactions. Focusing on two of the many applications of plasmonic metamaterials, in this Account, we review our work on the sensing and nanochemistry applications of metamaterials based on the assemblies of plasmonic nanorods under optical, as well as electronic interrogation. Sensors are widely employed in modern technology for the detection of events or changes in their local environment. Compared to their electronic counterparts, optical sensors offer a combination of high sensitivity, fast response, immunity to electromagnetic interference, and provide additional options for signal retrieval, such as optical intensity, spectrum, phase, and polarization. Owing to the ability to confine and enhance electromagnetic fields on subwavelength scales, plasmonics has been attracting increasing attention for the development of optical sensors with advantages including both nanometer-scale spatial resolution and single-molecule sensitivity. Inherent hot-electron generation in plasmonic nanostructures under illumination or during electron tunneling in the electrically biased nanostructures provides further opportunities for sensing and stimulation of chemical reactions, which would otherwise not be energetically possible. We first provide a brief introduction to a metamaterial sensing platform based on arrays of strongly coupled plasmonic nanorods. Several prototypical sensing examples based on this versatile metamaterial platform are presented. Record-high refractive index sensitivity of gold nanorod arrays in biosensing based on the functionalization of the nanorod surface for selective absorption arises because of the modification of the electromagnetic coupling between the nanorods in the array. The capabilities of nanorod metamaterials for ultrasound and hydrogen sensing were demonstrated by precision coating of the nanorods with functional materials to create core-shell nanostructures. The extension of this metamaterial platform to nanotube and nanocavity arrays, and metaparticles provides additional flexibility and removes restrictions on the illumination configurations for the optical interrogation. We then discuss a nanochemical platform based on the electrically driven metamaterials to stimulate and detect chemical reactions in the tunnel junctions constructed with the nanorods by exploiting elastic tunneling for the activation of chemical reactions via generated hot-electrons and inelastic tunneling for the excitation of plasmons facilitating optical monitoring of the process. This represents a new paradigm merging electronics, plasmonics, photonics and chemistry at the nanoscale, and creates opportunities for a variety of practical applications, such as hot-electron-driven nanoreactors and high-sensitivity sensors, as well as nanoscale light sources and modulators. With a combination of merits, such as the ability to simultaneously support both localized and propagating modes, nanoporous texture, rapid and facile functionalization, and low cost and scalability, plasmonic nanorod metamaterials provide an attractive and versatile platform for the development of optical sensors and nanochemical platforms using hot-electrons with high performance for applications in fundamental research and chemical and pharmaceutical industries.

Low loss semi‐MIM hybrid plasmonic waveguide with high electric field confinement

AbstractAmong different types of plasmonic waveguides that support surface‐plasmon‐polariton (SPP) mode, metal‐insulator‐metal (MIM) waveguides have the best electric field confinement in expense of high propagation loss. On the other hand, hybrid plasmonic waveguides (HPWGs) are represented as low loss plasmonic waveguides with moderate confinement. In this work, a new design of HPWG is proposed with higher electric field confinement compare with conventional HPWG by adding lossy spacer layer to the designed HPWG. This modification causes the proposed design be more similar to metal‐insulator‐metal (MIM) waveguides but with less propagation loss. The proposed design is called semi‐MIM HPWG and its structure is studied analytically and numerically. Dimensions of the proposed semi‐MIM HPWG is optimized to work at 193.5 THz and the proper stepped taper transition is designed to feed the waveguide by photonic feed lines.

Electrically Tunable Exciton-Plasmon Coupling in a WSe2 Monolayer Embedded in a Plasmonic Crystal Cavity.

We realize a new electroplasmonic switch based upon electrically tunable exciton-plasmon interactions. The device consists of a hexagonal boron nitride (hBN)-encapsulated tungsten diselenide (WSe2) monolayer on top of a single-crystalline silver substrate. The ultrasmooth silver substrate serves a dual role as the medium to support surface plasmon polaritons (SPPs) and the bottom gate electrode to tune the WSe2 exciton energy and brightness through electrostatic doping. To enhance the exciton-plasmon coupling, we implement a plasmonic crystal cavity on top of the hBN/WSe2/hBN/Ag heterostructure with a quality factor reaching 550. The tight confinement of the SPPs in the plasmonic cavity enables strong coupling between excitons and SPPs when the WSe2 exciton absorption is resonant with the cavity mode, leading to a vacuum Rabi splitting of up to 18 meV. This strong coupling can also be switched off with the application of a modest gate voltage that increases the doping density in the monolayer. This demonstration paves the way for new plasmonic modulators and a general device architecture to enhance light-matter interactions between SPPs and various embedded emitters.

High radiation efficiency of coupled plasmonic graphene-based THz patch antenna utilizing strip slot ground plane removal

This paper proposes an enhancement radiation efficiency method of graphene-based patch antenna using partially strip slot ground plane removal. Graphene as a one-atom thickness (2D) supports surface plasmon polariton (SPP) of considerably decreasing wave length in comparing to the free space one. Based on this fact yields efficiency of graphene antennas is degraded. The effect of strip line ground plane removal reduces the losses due to dielectric substrate and surface waves. The proposed design has reached up to 90% at both radiation and total efficiency. Also due to using graphene, it can provide tunable conductivity derived from variable parameters. The graphene-based patch THz antenna is modeled to achieve the optimum parameter values.