Propagation lengths and group velocities of plasmons in chemically synthesized gold and silver nanowires

Electrodynamic Interference and Induced Polarization in Nanoparticle-Based Optical Matter Arrays

Optical matter (OM) arrays are self-organizing, ordered arrangements of nanometer- to micrometer-size particles, where interparticle forces are mediated by incident and scattered coherent light. The structures that form and their dynamics depend on the properties (e.g., material, size) of the constituent particles, as well as the incident and scattered light. While significant progress has been made toward understanding how the OM arrays are affected by the phase, polarization, and intensity profile of the incident light, the polarization induced in the particles and the light scattered by OM arrays have received less attention. In this paper, we establish the roles of electrodynamic interference, many-body coupling, and induced-polarization concomitant with the coherent light scattered by OM arrays. Experiments and simulations together demonstrate that the spatial profile and directionality of coherent light scattered by OM arrays in the far field are primarily influenced by interference, while electrodynamic coupling (interactions) and the associated polarization induced in the nanoparticle constituents have a quantitative wavelength-dependent effect on the total amount of light scattered by the arrays. Furthermore, the electrodynamic coupling in silver nanoparticle OM arrays is significantly enhanced by constructive interference and increases superextensively with the number of particles in the array. Particle size, and hence polarizability, also has a significant effect on the strength of the coupling. Finally, we simulate larger hexagonal OM arrays of Ag nanoparticles to demonstrate that the electrodynamic coupling and scattering enhancement observed in small OM arrays develop into surface lattice resonances observed in the infinite array limit. Our work provides insights for designing OM arrays to tune many-body forces and the coherent light that they scatter.

Single-Molecule Fluorescence Probes Interactions between Photoactive Protein-Silver Nanowire Conjugate and Monolayer Graphene

In this work, we apply single-molecule fluorescence microscopy and spectroscopy to probe plasmon-enhanced fluorescence and Förster resonance energy transfer in a nanoscale assemblies. The structure where the interplay between these two processes was present consists of photoactive proteins conjugated with silver nanowires and deposited on a monolayer graphene. By comparing the results of continuous-wave and time-resolved fluorescence microscopy acquired for this structure with those obtained for the reference samples, where proteins were coupled with either a graphene monolayer or silver nanowires, we find clear indications of the interplay between plasmonic enhancement and the energy transfer to graphene. Namely, fluorescence intensities calculated for the structure, where proteins were coupled to graphene only, are less than for the structure playing the central role in this study, containing both silver nanowires and graphene. Conversely, decay times extracted for the latter are shorter compared to a protein-silver nanowire conjugate, pointing towards emergence of the energy transfer. Overall, the results show that monitoring the optical properties of single emitters in a precisely designed hybrid nanostructure provides an elegant way to probe even complex combination of interactions at the nanoscale.

Modulation of Cathodoluminescence by Surface Plasmons in Silver Nanowires

The waveguide modes in chemically-grown silver nanowires on silicon nitride substrates are observed using spectrally- and spatially-resolved cathodoluminescence (CL) excited by high-energy electrons in a scanning electron microscope. The presence of a long-range, travelling surface plasmon mode modulates the coupling efficiency of the incident electron energy into the nanowires, which is observed as oscillations in the measured CL with the point of excitation by the focused electron beam. The experimental data are modeled using the theory of surface plasmon polariton modes in cylindrical metal waveguides, enabling the complex mode wavenumbers and excitation strength of the long-range surface plasmon mode to be extracted. The experiments yield insight into the energy transfer mechanisms between fast electrons and coherent oscillations in surface charge density in metal nanowires and the relative amplitudes of the radiative processes excited in the wire by the electron.

Near-field manipulation of Tamm plasmon polaritons

Tamm plasmon polaritons (TPPs) arise from electromagnetic resonant phenomena which appear at the interface between a metallic film and a distributed Bragg reflector. They differ from surface plasmon polaritons (SPPs), since TPPs possess both cavity mode properties and surface plasmon characteristics. In this paper, the propagation properties of TPPs are carefully investigated. With the aid of nanoantenna couplers, polarization-controlled TPP waves can propagate directionally. By combining nanoantenna couplers with Fresnel zone plates, asymmetric double focusing of TPP wave is observed. Moreover, radial unidirectional coupling of the TPP wave can be achieved when the nanoantenna couplers are arranged along a circular or a spiral shape, which shows superior focusing ability compared to a single circular or spiral groove since the electric field intensity at the focal point is 4 times larger. In comparison with SPPs, TPPs possess higher excitation efficiency and lower propagation loss. The numerical investigation shows that TPP waves have great potential in integrated photonics and on-chip devices.

A physics-based machine learning approach for modeling the complex reflection coefficients of metal nanowires

Metal nanowires are attractive building blocks for next-generation plasmonic devices with high performance and compact footprint. The complex reflection coefficients of the plasmonic waveguides are crucial for estimation of the resonating, lasing, or sensing performance. By incorporating physics-guided objective functions and constraints, we propose a simple approach to convert the specific reflection problem of nanowires to a universal regression problem. Our approach is able to efficiently and reliably determine both the reflectivity and reflection phase of the metal nanowires with arbitrary geometry parameters, working environments, and terminal shapes, merging the merits of the physics-based modeling and the data-driven modeling. The results may provide valuable reference for building comprehensive datasets of plasmonic architectures, facilitating theoretical investigations and large-scale designs of nanophotonic components and devices.

Subwavelength-resolution imaging of surface plasmon polaritons with up-conversion fluorescence microscopy

Imaging and characterization of surface plasmon polaritons (SPPs) are crucial for the research and development of the plasmonic devices and circuits. Here, we report on direct imaging of SPPs propagation on SiO₂/metal interface with subwavelength spatial resolution using up-conversion fluorescence microscopy, that exploits rare-earth ions, such as Er³⁺, Yb³⁺, and Nd³⁺, doped nanoparticles as the fluorophores. We demonstrated that by further taking the intensity ratio of the image obtained with fluorescent emission at different wavelengths, we are able to substantially enhance the features associated to the SPP wavefronts in the image for quantitative analysis, such as the wavevector and propagation direction of the SPPs. Our results agree with the theoretic prediction of the SPP wavelengths quantitatively. We further demonstrate the evolution of the SPP wavefronts due to refraction SPPs, and reproduced the experiment with finite difference time domain (FDTD) method simulations. The relative refractive index of SPP estimated from the experiment also agrees quantitatively with those extracted from the theory and the simulation.

Surface plasmon polaritons of higher-order mode and standing waves in metallic nanowires

The surface plasmon polaritons (SPPs) of higher-order mode propagating along a plasmonic nanowire (NW) or an elongated nanorod (NR) are studied theoretically. The dispersion relations of SPPs in NWs of different radii, obtained from a transcendental equation, show that the propagation lengths of SPPs of mode 1 and 2 at a specific frequency are longer than that of mode 0. For the higher-order mode, the spatial phase of the longitudinal component of electric field at a cross section of a NW exhibits the topological singularity, which indicates the optical vortex. Of importance, the streamlines of Poynting vector of these SPPs exhibit a helical winding along NW, and the azimuthal component of orbital momentum density exists in the nearfield of NW to produce a longitudinal orbital angular momentum (OAM). Two types of standing wave of counter-propagating SPPs of mode 1 and 2 are also studied; they perform as a string of beads or twisted donut depending on whether the handedness of two opposite-direction propagating SPPs is same or opposite. In addition, a SPP of mode 1 propagating along an elongated NR can be generated by means of an end-fire excitation of crossed electric bi-dipole with 90° phase difference. If the criterion of a resonator for a mode-1 standing wave (string of beads) is met, the configuration of a plasmonic NR associated with a pair of bi-dipoles with a phase delay (0° or 180°) at the two ends can be applied as a high-efficiency nanoantenna of transmission. Our results may pave a way to the further study of SPPs of higher-order mode carrying OAM along plasmonic waveguides.

Gold Nanowires from Nanorods

Gold nanowires (AuNWs) possess strong potential application in micro- and nanoelectronics as well as in plasmonic waveguides because of their low electrical resistance. However, the synthesis of pure solvent-dispersible AuNWs with full control over their length still remains a challenge. All the previously reported methods produce AuNWs with other impurities such as smaller nanorods, platelets, and spherical particles and are limited to a certain length (typically below 10 μm). This article describes a one-step synthesis of extremely long AuNWs (up to 25 μm) with great control over their dimensions by using pentahedrally twinned gold nanorods (AuNRs) as seed particles. To induce the AuNW growth, the reduction of Au(I) to Au(0) was carried out on the surface of AuNRs at a very low pH by introducing HCl into the growth solution. The slow conversion of Au(I) to Au(0) due to the increase in reduction potential at lower pH promoted the preferential deposition of metallic gold on the more reactive tips of AuNRs compared to their sides, resulting in the formation of AuNWs. In analogy to the "living" polymerization reaction, the length of the AuNWs was proportional to the amount of Au(I) added to the growth solution; thus, the desired length of AuNWs was achieved by controlling the supply of Au(I) ions in the reaction mixture. The AuNWs longer than 6 μm were found to be responsive to microwave radiation. When an aqueous solution of AuNWs was exposed to microwaves, the formation of sharp kinks was observed in several locations of AuNWs without their disintegration into smaller pieces.

Direct Visualization of Gap-Plasmon Propagation on a Near-Touching Nanowire Dimer

Dimers of metallic nanowires (NWs) with nanometric gaps could be an alternative to overcome the limitations of existing plasmonic waveguides. The gap-surface plasmon polaritons (gap-SPPs) of the dimers may propagate along the NW without crosstalk and greatly enhance the coupling efficiency with an emitter, enabling ultracompact optical circuits. Such a possibility has not been realized, and we experimentally show its possibility. The gap-SPPs of the AgNW-molecule-AgNW structure, with a gap of 3-5 nm defined by the molecules, are visualized using the surface-enhanced Raman scattering (SERS) of the molecules. The SERS images, representing the gap-field intensity distribution, reveal the decay and beating of the monopole-monopole and dipole-dipole gap modes. The propagation lengths of the two (l₁ = 0.5-2 μm and l₂ = 5-8 μm) closely follow the model prediction with a uniform gap, confirming that the scattering loss induced by the gap irregularities is surprisingly low.

Realization of a near-infrared active Fano-resonant asymmetric metasurface by precisely controlling the phase transition of Ge2Sb2Te5

A metasurface is one of the most effectual platforms for the manipulation of complex optical fields. One of the current challenges in the field is to develop active or reconfigurable functionalities to extend its operation band which is limited by its intrinsic resonant nature. Here we demonstrate a kind of active Fano-resonant asymmetric metasurface in the near-infrared (NIR) region with heterostructures made of a layer of asymmetric split-ring resonators and a thin layer of phase-change material (PCM). In the asymmetric metasurface, significant tunability in the frequency, Q-factor and strength of the Fano resonance are all achieved by precisely controlling the phase transition of the contained PCM Ge₂Sb₂Te₅ (GST), together with changing the geometric asymmetry of the split-ring resonators. Moreover, we provide a complete transition process of the optical properties for GST and an optimized modulation on the active Fano-resonant metasurface. Our approach to dynamically control a Fano-resonant metasurface paves the way to realizing various active photonic meta-devices involving PCM.

Visualization of Surface Plasmons Propagating at the Buried Organic/Metal Interface with Silver Nanocluster Sensitizers

Visualization of surface plasmon polariton (SPP) propagation at dielectric/metal interfaces is indispensable in providing opportunities for the precise designing and controlling of the functionalities of future plasmonic nanodevices. Here, we report the visualization of SPPs propagating along the buried organic/metal interface of fullerene (C₆₀)/Au(111), through dual-colored two-photon photoemission electron microscopy (2P-PEEM) which precisely visualizes the SPP propagation of plasmonic metal nanostructures. Although SPPs excited by near-infrared photons at the few monolayer C₆₀/Au(111) interface are clearly visualized as interference beat patterns between the SPPs and incident light, faithfully reflecting SPP properties modulated by the overlayer, photoemission signals are suppressed for thicker C₆₀ films, due to less valence electrons participating in 2P-photoemission processes. With the use of silver (Ag_n (n = 21 and 55)) nanoclusters, which exhibit enhancement of overall photoemission intensities due to localized surface plasmons functioning as SPP sensitizers, it is revealed that the 2P-PEEM is applicable to the imaging of SPPs for thick C₆₀/Au(111) interfaces, where SPP properties are hardly modulated by the added small amount (∼0.1 monolayer) of Ag_n sensitizers.

A method to simulate the optical image from far-field scattering numerical data and its application to the total internal reflection microscopy of metallic nanowires

Computational electrodynamics modelling plays an important role in understanding and designing new photonic devices. The results offered by these simulations are usually close-range field distributions or angular power emission plots. We describe a procedure to compute the optical microscopy image from simulated far-field scattering data using three-dimensional discrete Fourier transforms that can be used when the simulation software package do not include proper far-field to optical imaging projection routines. The method is demonstrated comparing simulated images with real images of nanowires obtained with a total internal reflection microscope.

A Single-Crystalline Silver Plasmonic Circuit for Visible Quantum Emitters

Plasmonic waveguides are key elements in nanophotonic devices, serving as optical interconnects between nanoscale light sources and detectors. Multimode operation in plasmonic two-wire transmission lines promises important degrees of freedom for near-field manipulation and information encoding. However, highly confined plasmon propagation along gold nanostructures is typically limited to the near-infrared region due to ohmic losses, excluding all visible quantum emitters from plasmonic circuitry. We report on the top-down fabrication of complex plasmonic nanostructures in single-crystalline silver plates. We demonstrate the controlled remote excitation of a small ensemble of fluorophores by a set of waveguide modes and the emission of the visible luminescence into the waveguide with high efficiency. This approach opens up the study of a nanoscale light-matter interaction between complex plasmonic waveguides and a large variety of quantum emitters available in the visible spectral range.

Optical Properties of Submillimeter Silver Nanowires Synthesized Using the Hydrothermal Method

We report on the synthesis of long silver nanowires using the hydrothermal method, with H₂O₂ as the reducing agent. Our approach yields nanowires with an average diameter and length of about 100 nm and 160 µm, respectively, reaching the maximum length of 800 µm. Scanning electron microscopy (SEM) measurements revealed the presence of a thick, inhomogeneous poly(vinylpyrrolidone) (PVP) layer covering the nanowires, which with time becomes much more uniform, leading to well-defined extinction peaks in the ultraviolet-visible (UV-Vis) spectra. This change in morphology is evidenced also by the fluorescence enhancement behavior probed using protein complexes. Wide-field and confocal fluorescence microscopy measurements demonstrate strong, 10-fold enhancement of the protein emission intensity, accompanied by a reduction of the fluorescence decay time. In addition, for the aged, one-month-old nanowires, the uniformity of the intensity profile along them was substantially improved as compared with the as-synthesized ones. The results point towards the importance of the morphology of plasmonically active silver nanowires when considering their application in enhancing optical properties or achieving energy propagation over submillimeter distances.

Plasmonic polarization beam splitting based on single silver nanowire

Plasmonic waveguides have been indispensable "building-blocks" to construct functional elements for future integrated nano-photonic devices and circuits. In this paper, we demonstrate that a thick silver nanowire with well-defined end facets can provide multiple outcoupling channels, and the controllable beam splitting is realized. The propagating surface plasmons emission at nanowire end are split into two parts: I₁ and I₂, with the polarizations nearly perpendicular to the respective emitting facets. By changing incident polarization, the splitting ratio (I₁/I₂) can be tuned in the range of 1.52~0.36. Electromagnetic simulations indicate that polarization beam splitting mechanisms in this single thick nanowire are the interference of propagating surface plasmon modes and the superposition of excited dipoles at the nanowire end. These findings would deepen the understanding of manipulation of surface plasmons propagation/emission, and advance the development of plasmonic waveguide-based nano-photonic devices.

Macro-scale transport of the excitation energy along a metal nanotrack: exciton-plasmon energy transfer mechanism

Presently we report (i) excited state (exciton) propagation in a metal nanotrack over macroscopic distances, along with (ii) energy transfer from the nanotrack to adsorbed dye molecules. We measured the rates of both of these processes. We concluded that the effective speed of exciton propagation along the nanotrack is about 8 × 107 cm/s, much lower than the surface plasmon propagation speed of 1.4 × 1010 cm/s. We report that the transmitted energy yield depends on the nanotrack length, with the energy emitted from the surface much lower than the transmitted energy, i.e. the excited nanotrack mainly emits in its end zone. Our model thus assumes that the limiting step in the exciton propagation is the energy transfer between the originally prepared excitons and surface plasmons, with the rate constant of about 5.7 × 107 s-1. We also conclude that the energy transfer between the nanotrack and the adsorbed dye is limited by the excited-state lifetime in the nanotrack. Indeed, the measured characteristic buildup time of the dye emission is much longer than the characteristic energy transfer time to the dye of 81 ns, and thus must be determined by the excited state lifetime in the nanotrack. Indeed, the latter is very close to the characteristic buildup time of the dye emission. The data obtained are novel and very promising for a broad range of future applications.

Ultrafast measurements of the dynamics of single nanostructures: a review

The ability to study single particles has revolutionized nanoscience. The advantage of single particle spectroscopy measurements compared to conventional ensemble studies is that they remove averaging effects from the different sizes and shapes that are present in the samples. In time-resolved experiments this is important for unraveling homogeneous and inhomogeneous broadening effects in lifetime measurements. In this report, recent progress in the development of ultrafast time-resolved spectroscopic techniques for interrogating single nanostructures will be discussed. The techniques include far-field experiments that utilize high numerical aperture (NA) microscope objectives, near-field scanning optical microscopy (NSOM) measurements, ultrafast electron microscopy (UEM), and time-resolved x-ray diffraction experiments. Examples will be given of the application of these techniques to studying energy relaxation processes in nanoparticles, and the motion of plasmons, excitons and/or charge carriers in different types of nanostructures.

Mesoscale surface plasmons: modelling and imaging using near-field scanning optical microscopy

Meso-scale plasmons are supported by structures with dimensions on the order of tens of plasmon wavelengths. Metal structures at this length-scale are promising for the design and engineering of structures to direct the flow of optical energy and generate high intensity, localized electric fields. The near-field optical properties of mesoscale crystalline gold plates were examined using near-field scanning optical microscopy with a focus on the effects of modifying morphology and excitation conditions. Excitation of surface plasmon polaritons (SPPs) at plate edges and their subsequent propagation and interference as radial waves across the surface results in nodes of enhancement of the near-field on the plate surface at specific positions within the plate. The spatial position of the near-field enhancement may be directed by controlling either, or both, the boundary conditions (plate shape) and polarization of the excitation light.

Remote activation and detection of up-converted luminescence via surface plasmon polaritons propagating in a silver nanowire

In this paper, we demonstrate remote activation and detection of the 2-photon up-conversion luminescence via surface plasmon polaritons propagating in a long silver nanowire. The hybrid nanostructure was assembled by locally depositing a submicron droplet of nanocrystal-containing colloidal solution on one of the ends of the metallic nanowire. When - using a classic confocal microscope - the second end of the nanowire, without the nanocrystals, is illuminated with infrared laser light, we observe strong emission from the same end. Therefore, it indicates that surface plasmon polaritons activated with infrared light at the second end of the nanowire propagate along it and can excite nanocrystals in the droplet at the opposite end. Subsequently, the excited nanocrystals up-convert the energy and by launching surface plasmon polaritons can guide the up-converted luminescence back to the starting point. The emergence of this effect is much more pronounced for a laser polarized along the nanowire. The spectral and temporal character of this emission reveals strong interactions between surface plasmon polaritons and electronic states of the nanocrystals. The details of local and non-local aspects of the effects of remote excitation and guiding of energy in a silver nanowire are elucidated using a unique experimental setup, based on two microscope objectives for spatial separation and control of both excitation and emission beams.

Surface Plasmon Polariton Interference in Gold Nanoplates

Transient absorption microscopy (TAM) measurements have been used to study the optical properties of surface plasmon polariton (SPP) modes in gold nanoplates on a glass substrate. For thin gold nanoplates, the TAM images show an oscillation in the signal across the plate due to interference between the "bound" and "leaky" SPP modes. The wavelength of the interference pattern is given by λ = 2π/Δk, where Δk is the difference between the wavevectors for the bound and leaky modes and is sensitive to the dielectric constant of the material above the gold nanoplate. Back focal plane imaging was also used to measure the wavevector of the leaky mode, which, in combination with the Δk information from the TAM images, enabled the bound mode wavevector to be determined. These experiments represent the first far-field optical measurement of the wavevector for the bound mode in metal nanostructures.

Enhancement of Plasmonic Performance in Epitaxial Silver at Low Temperature

We report longer surface plasmon polariton propagation distance based on crystalline crystal silver at low temperature. Although enhanced plasmonic performance at low temperature has been predicted for a long time, it has not been directly observed on polycrystalline silver films which suffer from significant plasmonic losses due to grain boundaries and rough silver surface. Here we show that longer propagation distance can be achieved with epitaxial silver at low temperature. Importantly, the enhancement at low temperature are consistent across silver films grown with different methods.

Radiation losses and dark mode at light guiding by a linear chain of nanoparticles

A new general formula is presented for a collective extinction cross section of a dielectric or a metallic nanoparticle ensemble in terms of incident electric field work on currents excited inside particles. The formula is obtained by identical transformation of the well-known expression for the summing power of electromagnetic field energy losses caused by particle ensemble scattering and absorption. The derived formula is applied to the problem of radiation losses at electromagnetic excitation transfer along a straight chain of particles. Our general formula predicts a zero collective extinction cross section for an infinite straight chain of nonabsorbing dielectric particles providing that the projection of the wave vector of an incident electromagnetic wave on the chain axis does not coincide with its counterpart of the Bloch wave vector of propagating excitation. In another case of a finite chain of particles, with only the first particle of the chain irradiated by an incident narrow electromagnetic wave beam, the derived formula shows that only the irradiated particle directly contributes to the collective extinction cross section despite how large the total number of particles can be, which makes a direct summing contribution of all other particles to wave scattering as if they were unviewed (dark mode). Using a recently developed quasi-separable T-scattering operator approach that leads to the equation system for self-consistent currents excited inside particles by an incident electromagnetic wave field and restricting ourselves to the electric dipole single scattering and neighbor coupling approximation, we revealed a few gigahertz transparency band in the terahertz frequency range (orange color) in the spectra of a straight chain of closely spaced gold nanospheres of a certain radius and a length of a few millimeters. A resonant mechanism of filtering the dark mode from radiation losses established in this work allowed us to reveal a few-fold-more narrow passband in the spectra of a longer gold particle chain with the full length of a few centimeters.

Controlled electrochemical growth of ultra-long gold nanoribbons

This paper describes the electrochemical growth of branchless gold nanoribbons with ∼40 nm × ∼300 nm cross sections and >100 μm lengths (giving length-to-thickness aspect ratios of >10³). These structures are useful for opto-electronic studies and as nanoscale electrodes. The 0.75-1.0 V voltage amplitude range is optimal for branchless ribbon growth. Reduced amplitudes induce no growth, possibly due to reversible redox chemistry of gold at reduced amplitudes, whereas elevated amplitudes, or excess electrical noise, induce significant side-branching. The inter-relatedness of voltage-amplitude, noise, and side-branching in electrochemical nanoribbon growth is demonstrated.

Influence of symmetry breaking degrees on surface plasmon polaritons propagation in branched silver nanowire waveguides

Surface plasmon polaritons (SPPs)-based nanowire (NW) waveguides demonstrate promising potentials in the integrated nanophotonic circuits and devices. The realization of controlling SPPs propagation in NWs is significant for the performance of nanophotonic devices when employed for special function. In this work, we report the effect of symmetry breaking degrees on SPPs propagation behavior in manually fabricated branched silver NW structures. The symmetry breaking degree can be tuned by the angle between main NW and branch NW, which influences the emissions at the junction and the main NW terminal in a large extent. Our results illustrate the significance of symmetry breaking degree on SPPs propagation in NW-based waveguides which is crucial for designing the future nanophotonic circuits.

Orientation-and polarization-dependent optical properties of the single Ag nanowire/glass substrate system excited by the evanescent wave

As an important plasmon one-dimensional material, orientation- and polarization-dependent properties of single Ag nanowires/glass substrate system are investigated by a powerful platform consisting of evanescent wave excitation, near-/far-field detection and a micromanipulator. In the case of the nanowire perpendicular or parallel to the incident plane and p- ors-polarized evanescent excitation respectively, optical properties of the nanowire is measured both in far-field and near-field. For the perpendicular situation, scattering light from the nanowire shows strong dependence on the polarization of incident light, and period patterns along the nanowire are observed both in the near- and far-field. The chain of dipole model is used to explain the origin of this pattern. The discrepancy of the period patterns observed in the near- and far-field is due to the different resolution of the near- and far-field detection. For the parallel case, light intensity from the output end also depends on the incident polarization. Both experimental and calculation results show that the polarization dependence effect results from the surface plasmon excitation. These results on the orientation- and polarization-dependent properties of the Ag nanowires detected by the combination of near- and far-field methods would be helpful to understand interactions of one-dimensional plasmonic nanostructures with light.

Coupling Emitters and Silver Nanowires to Achieve Long-Range Plasmon-Mediated Fluorescence Energy Transfer

The development of quantum plasmonic circuitry requires efficient coupling between quantum emitters and plasmonic waveguides. A major experimental challenge is to simultaneously maximize the surface plasmon propagation length, the coupling efficiency into the plasmonic mode, and the Purcell factor. Addressing this challenge is also the key to enabling long-range energy transfer between quantum nanoemitters. Here, we use a dual-beam scanning confocal microscope to carefully investigate the interactions between fluorescent nanoparticles and surface plasmons on single-crystalline silver nanowires. By exciting the fluorescent nanoparticles via nanowire surface plasmons, we maximize the light-matter interactions and reach coupling efficiencies up to 44% together with 24× lifetime reduction and 4.1 μm propagation lengths. This improved optical performance enables the demonstration of long-range plasmon-mediated fluorescence energy transfer between two nanoparticles separated by micrometer distance. Our results provide guidelines toward practical realizations of efficient long-range fluorescence energy transfer for integrated plasmonics and quantum nano-optics.

Role of Resonances in the Transmission of Surface Plasmon Polaritons between Nanostructures

Understanding how surface plasmon polaritons (SPPs) propagate in metal nanostructures is important for the development of plasmonic devices. In this paper, we study the transmission of SPPs between single-crystal gold nanobars on a glass substrate using transient absorption microscopy. The coupled structures were produced by creating gaps in single nanobars by focused ion beam milling. SPPs were launched by focusing the pump laser at the end of the nanobar, and the transmission across the gaps was imaged by scanning the probe laser over the nanostructure. The results show larger losses at small gap sizes. Finite element method calculations were used to investigate this effect. The calculations show two main modes for nanobars on a glass surface: a leaky mode localized at the air-gold interface, and a bound mode localized at the glass-gold interface. At specific gap sizes (approximately 50 nm for our system), these SPP modes can excite localized surface plasmon modes associated with the gap, which dissipate energy. This increases the energy losses at small gap sizes. Experiments and simulations were also performed for the nanobars in microscope immersion oil, which creates a more homogeneous optical environment, and consistent results were observed.

Investigation of plasmonic properties of graphene multilayer nano-ribbon waveguides

In this paper, we investigate the plasmonic properties of a graphene-silica-silicon (G-SiO₂-Si) multilayer nano-ribbon waveguide in the mid-IR spectral range using the finite element method. Numerical results show that single-mode operation and modal cut-off properties of the G-SiO₂-Si are highly sensitive to the width and chemical potential. In particular, we demonstrate that by properly tuning the geometric and material parameters of the spacer layer or by decreasing the operation frequency, the graphene-based waveguide exhibits a propagation length higher than that of its metal-based counterpart. We believe that this study will provide a valuable reference for designing ultra-compact and low-loss graphene-based novel integrated plasmonic devices.

Low-Power Light Guiding and Localization in Optoplasmonic Chains Obtained by Directed Self-Assembly

Optoplasmonic structures contain plasmonic components embedded in a defined photonic environment to create synergistic interactions between photonic and plasmonic components. Here, we show that chains of optical microspheres containing gold nanoparticles in their evanescent field combine the light guiding properties of a microsphere chain with the light localizing properties of a plasmonic nanoantenna. We implement these materials through template guided self-assembly and investigate their fundamental electromagnetic working principles through combination of electromagnetic simulations and experimental characterization. We demonstrate that optoplasmonic chains implemented by directed self-assembly achieve a significant reduction in guiding losses when compared with conventional plasmonic waveguides and, at the same time, retain the light localizing properties of plasmonic antennas at pre-defined locations. The results reinforce the potential of optoplasmonic structures for realizing low-loss optical interconnects with high bandwidth.

Photonic Applications of Metal-Dielectric Heterostructured Nanomaterials

Metal materials, supporting plasmon modes on their surface, can confine the optical field at deep subwavelength scale, which is desired for photonic integration. However, their intrinsic high Ohmic losses make it impossible to construct the whole circuit solely with the metal materials. Integrating the plasmonic components with dielectric materials may offer a solution to this dilemma. With outstanding active optical performance, these dielectric components not only can greatly reduce the optical losses of the entire circuits but also offer an efficient way to launch the surface plasmon polaritons through the evanescent field coupling or the direct exciton-plasmon conversion. Furthermore, the cooperative interaction between metal and dielectric materials would bring vast novel optical phenomena and functional photonic devices. In this review, the synergistic effects among metal and dielectric materials in various heterostructures as well as their related applications are highlighted. Comprehensive understanding on their synergistic interactions would offer useful guidance for the design and fabrication of the ultracompact novel optical devices.

Propagation and localization of quantum dot emission along a gap-plasmonic transmission line

Plasmonic transmission lines have great potential to serve as direct interconnects between nanoscale light spots. The guiding of gap plasmons in the slot between adjacent nanowire pairs provides improved propagation of surface plasmon polaritons while keeping strong light confinement. Yet propagation is fundamentally limited by losses in the metal. Here we show a workaround operation of the gap-plasmon transmission line, exploiting both gap and external modes present in the structure. Interference between these modes allows us to take advantage of the larger propagation distance of the external mode while preserving the high confinement of the gap mode, resulting in nanoscale confinement of the optical field over a longer distance. The performance of the gap-plasmon transmission line is probed experimentally by recording the propagation of quantum dots luminescence over distances of more than 4 μm. We observe a 35% increase in the effective propagation length of this multimode system compared to the theoretical limit for a pure gap mode. The applicability of this simple method to nanofabricated structures is theoretically confirmed and offers a realistic way to combine longer propagation distances with lateral plasmon confinement for far field nanoscale interconnects.

Wedge Waveguides and Resonators for Quantum Plasmonics

Plasmonic structures can provide deep-subwavelength electromagnetic fields that are useful for enhancing light-matter interactions. However, because these localized modes are also dissipative, structures that offer the best compromise between field confinement and loss have been sought. Metallic wedge waveguides were initially identified as an ideal candidate but have been largely abandoned because to date their experimental performance has been limited. We combine state-of-the-art metallic wedges with integrated reflectors and precisely placed colloidal quantum dots (down to the single-emitter level) and demonstrate quantum-plasmonic waveguides and resonators with performance approaching theoretical limits. By exploiting a nearly 10-fold improvement in wedge-plasmon propagation (19 μm at a vacuum wavelength, λvac, of 630 nm), efficient reflectors (93%), and effective coupling (estimated to be >70%) to highly emissive (~90%) quantum dots, we obtain Ag plasmonic resonators at visible wavelengths with quality factors approaching 200 (3.3 nm line widths). As our structures offer modal volumes down to ~0.004λvac(3) in an exposed single-mode waveguide-resonator geometry, they provide advantages over both traditional photonic microcavities and localized-plasmonic resonators for enhancing light-matter interactions. Our results confirm the promise of wedges for creating plasmonic devices and for studying coherent quantum-plasmonic effects such as long-distance plasmon-mediated entanglement and strong plasmon-matter coupling.

Imaging nano-objects by linear and nonlinear optical absorption microscopies

Absorption based microscopy measurements are emerging as important tools for studying nanomaterials. This review discusses the three most common techniques for performing these experiments: transient absorption microscopy, photothermal heterodyne imaging, and spatial modulation spectroscopy. The focus is on the application of these techniques to imaging and detection, using examples taken from the authors' laboratory. The advantages and disadvantages of the three methods are discussed, with an emphasis on the unique information that can be obtained from these experiments, in comparison to conventional emission or scattering based microscopy experiments.

Generation of Subwavelength Plasmonic Nanovortices via Helically Corrugated Metallic Nanowires

We demonstrate that plasmonic helical gratings consisting of metallic nanowires imprinted with helical grooves or ridges can be used efficiently to generate plasmonic vortices with radius much smaller than the operating wavelength. In our proposed approach, these helical surface gratings are designed so that plasmon modes with different azimuthal quantum numbers (topological charge) are phase-matched, thus allowing one to generate optical plasmonic vortices with arbitrary topological charge. The general principles for designing plasmonic helical gratings that facilitate efficient generation of such plasmonic vortices are derived and their applicability to the conversion of plasmonic vortices with zero angular momentum into plasmonic vortices with arbitrary angular momentum is illustrated in several particular cases. Our analysis, based both on the exact solutions for the electromagnetic field propagating in the helical plasmonic grating and a coupled-mode theory, suggests that even in the presence of metal losses the fundamental mode with topological charge m = 0 can be converted to plasmon vortex modes with topological charge m = 1 and m = 2 with a conversion efficiency as large as 60%. The plasmonic nanovortices introduced in this study open new avenues for exciting applications of orbital angular momentum in the nanoworld.

Coupling of individual quantum emitters to channel plasmons

Efficient light-matter interaction lies at the heart of many emerging technologies that seek on-chip integration of solid-state photonic systems. Plasmonic waveguides, which guide the radiation in the form of strongly confined surface plasmon-polariton modes, represent a promising solution to manipulate single photons in coplanar architectures with unprecedented small footprints. Here we demonstrate coupling of the emission from a single quantum emitter to the channel plasmon polaritons supported by a V-groove plasmonic waveguide. Extensive theoretical simulations enable us to determine the position and orientation of the quantum emitter for optimum coupling. Concomitantly with these predictions, we demonstrate experimentally that 42% of a single nitrogen-vacancy centre emission efficiently couples into the supported modes of the V-groove. This work paves the way towards practical realization of efficient and long distance transfer of energy for integrated solid-state quantum systems.

Rapid synthesis of ultra-long silver nanowires for tailor-made transparent conductive electrodes: proof of concept in organic solar cells

Rapid synthesis of ultralong silver nanowires (AgNWs) has been obtained using a one-pot polyol-mediated synthetic procedure. The AgNWs have been prepared from the base materials in less than one hour with nanowire lengths reaching 195 μm, which represents the quickest synthesis and one of the highest reported aspect ratios to date. These results have been achieved through a joint analysis of all reaction parameters, which represents a clear progress beyond the state of the art. Dispersions of the AgNWs have been used to prepare thin, flexible, transparent and conducting films using spray coating. Due to the higher aspect ratio, an improved electrical percolation network is observed. This allows a low sheet resistance (RS = 20.2 Ω/sq), whilst maintaining high optical film transparency (T = 94.7%), driving to the highest reported figure-of-merit (FoM = 338). Owing to the light-scattering influence of the AgNWs, the density of the AgNW network can also be varied to enable controllability of the optical haze through the sample. Based on the identification of the optimal haze value, organic photovoltaics (OPVs) have been fabricated using the AgNWs as the transparent electrode and have been benchmarked against indium tin oxide (ITO) electrodes. Overall, the performance of OPVs made using AgNWs sees a small decrease in power conversion efficiency (PCE), primarily due to a fall in open-circuit voltage (50 mV). This work indicates that AgNWs can provide a low cost, rapid and roll-to-roll compatible alternative to ITO in OPVs, with only a small compromise in PCE needed.

Ultrafast imaging of surface plasmons propagating on a gold surface

We record time-resolved nonlinear photoemission electron microscopy (tr-PEEM) images of propagating surface plasmons (PSPs) launched from a lithographically patterned rectangular trench on a flat gold surface. Our tr-PEEM scheme involves a pair of identical, spatially separated, and interferometrically locked femtosecond laser pulses. Power-dependent PEEM images provide experimental evidence for a sequential coherent nonlinear photoemission process, in which one laser source launches a PSP through a linear interaction, and the second subsequently probes the PSP via two-photon photoemission. The recorded time-resolved movies of a PSP allow us to directly measure various properties of the surface-bound wave packet, including its carrier wavelength (783 nm) and group velocity (0.95c). In addition, tr-PEEM images reveal that the launched PSP may be detected at least 250 μm away from the coupling trench structure.

Directional out-coupling of light from a plasmonic nanowire-nanoparticle junction

We experimentally show how a single Ag nanoparticle (NP) coupled to an Ag nanowire (NW) can convert propagating surface plasmon polaritons to directional photons. By employing dual-excitation Fourier microscopy with spatially filtered collection-optics, we show single- and dual-directional out-coupling of light from NW-NP junction for plasmons excited through glass-substrate and air-superstrate. Furthermore, we show NW-NP junction can influence the directionality of molecular-fluorescence emission, thus functioning as an optical antenna. The results discussed herein may have implications in realizing directional single-photon sources and quantum plasmon circuitry.

Excitation of surface plasmons from silver nanowires embedded in polymer nanofibers

We report an excitation of surface plasmons in silver nanowires (AgNWs) which were embedded in flexible polymer nanofibers. Using waveguiding excitation, surface plasmons in AgNWs were excited and propagated. By directing light of 650, 532, and 473 nm wavelengths into the nanofiber, surface plasmons in an embedded single AgNW (average diameter 400 nm, length 4.3 μm) were excited and the corresponding propagation lengths for the three wavelengths are 10.6, 7.7, and 5.1 μm. It was also found that, when a spatially incoherent white light of a halogen lamp with an excitation optical power of 80 μW was coupled into the polymer nanofiber, a surface plasmon mediated interference fringe was observed. In addition, on the basis of surface plasmon excitation, two adjacent AgNWs embedded in the polymer nanofibers were demonstrated to serve as coupled plasmonic waveguides.

Silver nanowires as receiving-radiating nanoantennas in plasmon-enhanced up-conversion processes

We demonstrate efficient coupling between plasmons in a single silver nanowire and nanocrystals doped with rare earth ions, α-NaYF4:Er(3+)/Yb(3+). Plasmonic interaction results in a sevenfold increase of the up-converted emission of nanocrystals located in the vicinity of the nanowires as well as much faster luminescence decays. The enhancement of the emission can be precisely controlled by the polarization of the excitation laser and is significantly stronger for polarization parallel to the nanowire antennas. Imaging of angular-resolved emission patterns in the Fourier plane reveals plasmon-mediated luminescence, where the up-converted radiation is emitted via the nanowire antennas as leakage radiation.

Interferometric Plasmonic Lensing with Nanohole Arrays

Nonlinear photoemission electron microscopy (PEEM) of nanohole arrays in gold films is used to map propagating surface plasmons (PSPs) launched from lithographically patterned structures. Strong near-field photoemission patterns are observed in the PEEM images, recorded following low angle of incidence irradiation of nanohole arrays with sub-15 fs laser pulses centered at 780 nm. The recorded photoemission patterns are attributed to constructive and destructive interference between PSPs launched from the individual nanoholes which comprise the array. By exploiting the wave nature of PSPs, we demonstrate how varying the array geometry (hole diameter, pitch, and number of rows/columns) ultimately yields intense localized photoemission. Through a combination of PEEM experiments and finite-difference time-domain simulations, we identify the optimal array geometry for efficient light coupling and interferometric plasmonic lensing. We show a preliminary application of inteferometric plasmonic lensing by enhancing the photoemission from the vertex of a gold triangle using a nanohole array.

Effect of substrate discontinuities on the propagating surface plasmon polariton modes in gold nanobars

The surface plasmon polariton (SPP) modes of gold nanobars (nanowires with rectangular dimensions) have been investigated by scanning pump-probe microscopy. In these experiments the nanobars were suspended over trenches cut in glass coverslips, and propagating SPP modes were launched in the supported portion of the nanobar by focusing a near-IR pump laser beam at the end of the nanobar. Transient absorption images were then collected by scanning the probe laser over the nanobar using a galvo-mirror system. The images show that the trench has a large effect on the SPP modes, specifically, for approximately half the nanowires the propagation length is significantly reduced after the trench. Finite element calculations were performed to understand this effect. The calculations show that the pump laser excites bound and leaky modes (modes that have their fields localized at the nanobar/glass or nanobar/air interfaces, respectively) in the supported portions of the nanobars. These modes propagate along the nanobar. When they meet the trench their field distributions are altered. The modes that derive from the bound mode are strongly damped over the trench. Thus, the bound mode is not reconstituted on the opposite side of the trench, and only the leaky mode contributes to the signal. Because the bound and leaky modes can have different propagation lengths, the propagation lengths measured in our experiments can change from one side of the trench to the other. The results show how the substrate can be engineered to control the SPP modes in metal nanostructures.

Assembling single gold nanorods into large-scale highly aligned nanoarrays via vacuum-enhanced capillarity

We report a simple, straightforward, and efficient approach to assemble single gold nanorods (AuNRs) into highly aligned arrays, via a unique vacuum-enhanced capillarity. The assembled AuNR arrays demonstrate both an excellently unidirectional ordering and a wonderful single-rod resolution. The key role of vacuum in this approach enables high-aspect-ratio (10 to 22) AuNR alignment and efficiently facilitates large-area alignment. Further investigation of one- and two-dimensional AuNR arrays would undoubtedly be beneficial to their potential applications.

Self-organization of mesoscopic silver wires by electrochemical deposition

Long, straight mesoscale silver wires have been fabricated from AgNO3 electrolyte via electrodeposition without the help of templates, additives, and surfactants. Although the wire growth speed is very fast due to growth under non-equilibrium conditions, the wire morphology is regular and uniform in diameter. Structural studies reveal that the wires are single-crystalline, with the [112] direction as the growth direction. A possible growth mechanism is suggested. Auger depth profile measurements show that the wires are stable against oxidation under ambient conditions. This unique system provides a convenient way for the study of self-organization in electrochemical environments as well as for the fabrication of highly-ordered, single-crystalline metal nanowires.

Tunable plasmon modes in single silver nanowire optical antennas characterized by far-field microscope polarization spectroscopy

Performing far-field microscope polarization spectroscopy and finite element method simulations, we investigated experimentally and theoretically the surface plasmon modes in single Ag nanowire antennas. Our results show that the surface plasmon resonances in the single Ag nanowire antenna can be tuned from the dipole plasmon mode to a higher order plasmon mode, which would result in the emission with different intensities and polarization states, for the semiconductor quantum dots coupled to the nanowire antenna. The fluorescence polarization is changed with different polarized excitation of the 800 nm light beam, while it remains parallel to the Ag nanowire axis at the 400 nm excitation. The 800 nm incident light interacts nonresonantly with the dipole plasmon mode with the polarized excitation parallel to the Ag nanowire axis, while it excites a higher order plasmon mode with the perpendicular excitation. Under excitation of 400 nm, either the parallel or perpendicular excitation can only result in a dipole plasmon mode. In addition, we demonstrate that the single Ag nanowire antenna can work as an energy concentrator for enhancing the two-photon excited fluorescence of semiconductor quantum dots.