Optimizing substrate-mediated plasmon coupling toward high-performance plasmonic nanowire waveguides

Design of an ultrafast plasmonic nanolaser for high-intensity broadband emission operating at room temperature

We propose a plasmonic nanolaser based on a metal-insulator-semiconductor-insulator-metal (MISIM) structure, which effectively confines light on a subwavelength scale (∼λ/14). As the pump power increases, the proposed plasmonic nanolaser exhibits broadband output characteristics of 20 nm, and the maximum output power can reach 20 µW. Furthermore, the carrier lifetime at the upper energy level in our proposed structure is measured to be about 400 fs using a double pump-probe excitation. The ultrafast characteristic is attributed to the inherent Purcell effect of plasmonic systems. Our work paves the way toward deep-subwavelength mode confinement and ultrafast femtosecond plasmonic lasers in spaser-based interconnected, eigenmode engineering of plasmonic nanolasers, nano-LEDs, and spontaneous emission control.

Integrated Manipulation and Addressing of Spin Defect in Diamond

Scalable and addressable integrated manipulation of qubits is crucial for practical quantum information applications. Different waveguides have been used to transport the optical and electrical driving pulses, which are usually required for qubit manipulation. However, the separated multifields may limit the compactness and efficiency of manipulation and introduce unwanted perturbation. Here, we develop a tapered fiber-nanowire-electrode hybrid structure to realize integrated optical and microwave manipulation of solid-state spins at nanoscale. Visible light and microwave driving pulses are simultaneously transported and concentrated along an Ag nanowire. Studied with spin defects in diamond, the results show that the different driving fields are aligned with high accuracy. The spatially selective spin manipulation is realized. And the frequency-scanning optically detected magnetic resonance (ODMR) of spin qubits is measured, illustrating the potential for portable quantum sensing. Our work provides a new scheme for developing compact, miniaturized quantum sensors and quantum information processing devices.

Nanoscale Manipulation of Exciton-Trion Interconversion in a MoSe2 Monolayer via Tip-Enhanced Cavity-Spectroscopy

Emerging light-matter interactions in metal-semiconductor hybrid platforms have attracted considerable attention due to their potential applications in optoelectronic devices. Here, we demonstrate plasmon-induced near-field manipulation of trionic responses in a MoSe2 monolayer using tip-enhanced cavity-spectroscopy (TECS). The surface plasmon-polariton mode on the Au nanowire can locally manipulate the exciton (X0) and trion (X-) populations of MoSe2. Furthermore, we reveal that surface charges significantly influence the emission and interconversion processes of X0 and X-. In the TECS configuration, the localized plasmon significantly affects the distributions of X0 and X- due to the modified radiative decay rate. Additionally, within the TECS cavity, the electric doping effect and hot electron generation enable dynamic interconversion between X0 and X- at the nanoscale. This work advances our understanding of plasmon-exciton-hot electron interactions in metal-semiconductor-metal hybrid structures, providing a foundation for an optimal trion-based nano-optoelectronic platform.

Controlling Hot Charge Carrier Transfer in Monolithic AlSiAl Heterostructures for Plasmonic On-Chip Energy Harvesting

The generation of hot carriers by Landau damping or chemical interface damping of plasmons is of particular interest to the fundamental aspects of extreme light-matter interactions. Hot charge carriers can be transferred to an attached acceptor for photochemical or photovoltaic energy conversion. However, these lose their excess energy and relax to thermal equilibrium within picoseconds and it is difficult to extract useful work thereof with thermodynamic efficiencies that are of interest for practical devices. Without a detailed understanding of the underlying plasmon decay processes and transfer mechanisms, proper material matching and design considerations for novel plasmonic devices are extremely challenging. Here, a multifunctional AlSiAl heterostructure device with tunable Schottky barriers is presented to control plasmon-induced hot carrier injection at an abrupt metal-semiconductor interface. Light absorption, surface plasmon generation, and separation of hot carriers arising from the non-radiative decay of surface plasmons are realized in a monolithic Schottky barrier field effect transistor. Aside from barrier modulation, a virtual p-n junction can be emulated in the semiconductor channel with the distinct merit that carrier concentration and polarity are tunable by electrostatic gating. The investigations are carried out with a view to possible use for CMOS-compatible plasmonic photovoltaics, with versatile implementations for autonomous nanosystems.

Dynamics of surface-plasmon lasing in planar metal gratings on semiconductor

We investigate the dynamics of surface plasmon (SP) lasing in Au gratings fabricated on InGaAs with a period of around 400 nm, which locates the SP resonance near the semiconductor energy gap and facilitates efficient energy transfer. By optically pumping the InGaAs to reach the population inversion required for the amplification and the lasing, we observe SP lasing at specific wavelengths that satisfy the SPR condition depending on the grating period. The carrier dynamics in semiconductor and the photon density in the SP cavity was investigated from the time-resolved pump-probe measurement and the time resolved photoluminescence spectroscopy, respectively. Our results reveal that the photon dynamics is strongly correlated with the carrier dynamics and the lasing build-up is accelerated as the initial gain proportional to the pumping power increases, and this trend is satisfactorily explained using the rate equation model.

Substrate-mediated plasmon hybridization toward high-performance light trapping

High-performance light trapping in metamaterials and metasurfaces offers prospects for the integration of multifunctional photonic components at subwavelength scales. However, constructing these nanodevices with reduced optical losses remains an open challenge in nanophotonics. Herein, we design and fabricate aluminum-shell-dielectric gratings by integrating low-loss aluminum materials with metal-dielectric-metal designs for high-performance light trapping featuring nearly perfect light absorption with broadband and large angular tuning ranges. The mechanism governing these phenomena is identified as the occurrence of substrate-mediated plasmon hybridization that allows energy trapping and redistribution in engineered substrates. Furthermore, we strive to develop an ultrasensitive nonlinear optical method, namely, plasmon-enhanced second-harmonic generation (PESHG), to quantify the energy transfer from metal to dielectric components. Our studies may provide a mechanism for expanding the potential of aluminum-based systems in practical applications.

Ultra-confined Propagating Exciton-Plasmon Polaritons Enabled by Cavity-Free Strong Coupling: Beating Plasmonic Trade-Offs

Hybrid coupling systems consisting of transition metal dichalcogenides (TMD) and plasmonic nanostructures have emerged as a promising platform to explore exciton-plasmon polaritons. However, the requisite cavity/resonator for strong coupling introduces extra complexities and challenges for waveguiding applications. Alternatively, plasmonic nano-waveguides can also be utilized to provide a non-resonant approach for strong coupling, while their utility is limited by the plasmonic confinement-loss and confinement-momentum trade-offs. Here, based on a cavity-free approach, we overcome these constraints by theoretically strong coupling of a monolayer TMD to a single metal nanowire, generating ultra-confined propagating exciton-plasmon polaritons (PEPPs) that beat the plasmonic trade-offs. By leveraging strong-coupling-induced reformations in energy distribution and combining favorable properties of surface plasmon polaritons (SPPs) and excitons, the generated PEPPs feature ultra-deep subwavelength confinement (down to 1-nm level with mode areas ~ 10^-4 of λ²), long propagation length (up to ~ 60 µm), tunable dispersion with versatile mode characters (SPP- and exciton-like mode characters), and small momentum mismatch to free-space photons. With the capability to overcome the trade-offs of SPPs and the compatibility for waveguiding applications, our theoretical results suggest an attractive guided-wave platform to manipulate exciton-plasmon interactions at the ultra-deep subwavelength scale, opening new horizons for waveguiding nano-polaritonic components and devices.

Transfer Learning for Modeling Plasmonic Nanowire Waveguides

Retrieving waveguiding properties of plasmonic metal nanowires (MNWs) through numerical simulations is time- and computational-resource-consuming, especially for those with abrupt geometric features and broken symmetries. Deep learning provides an alternative approach but is challenging to use due to inadequate generalization performance and the requirement of large sets of training data. Here, we overcome these constraints by proposing a transfer learning approach for modeling MNWs under the guidance of physics. We show that the basic knowledge of plasmon modes can first be learned from free-standing circular MNWs with computationally inexpensive data, and then reused to significantly improve performance in predicting waveguiding properties of MNWs with various complex configurations, enabling much smaller errors (~23-61% reduction), less trainable parameters (~42% reduction), and smaller sets of training data (~50-80% reduction) than direct learning. Compared to numerical simulations, our model reduces the computational time by five orders of magnitude. Compared to other non-deep learning methods, such as the circular-area-equivalence approach and the diagonal-circle approximation, our approach enables not only much higher accuracies, but also more comprehensive characterizations, offering an effective and efficient framework to investigate MNWs that may greatly facilitate the design of polaritonic components and 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.

The influence of single layer MoS2flake on the propagated surface plasmons of silver nanowire

We demonstrate enhancing the excitation and transmission efficiency of the propagated surface plasmon (SP) of an Ag nanowire (Ag NW) in hybrid Ag-MoS₂structures by contrasting the SP propagation of the Ag NW on different substrates, including SiO₂and monolayer MoS₂, or partially overlapping the Ag NW on MoS₂flakes. The simulation results show that the leaky radiation of the hybrid plasmonic modes H₁and H₂can be prominently suppressed due to the high refractive index dielectric layer of MoS₂, which provides an optical barrier for blocking the leaky radiation, resulting in reduced propagation loss. This paper provides a feasible and effective method to improve the SP propagation length.

Graphene Nanoribbon Gap Waveguides for Dispersionless and Low-Loss Propagation with Deep-Subwavelength Confinement

Surface plasmon polaritons (SPPs) have been attracting considerable attention owing to their unique capabilities of manipulating light. However, the intractable dispersion and high loss are two major obstacles for attaining high-performance plasmonic devices. Here, a graphene nanoribbon gap waveguide (GNRGW) is proposed for guiding dispersionless gap SPPs (GSPPs) with deep-subwavelength confinement and low loss. An analytical model is developed to analyze the GSPPs, in which a reflection phase shift is employed to successfully deal with the influence caused by the boundaries of the graphene nanoribbon (GNR). It is demonstrated that a pulse with a 4 μm bandwidth and a 10 nm mode width can propagate in the linear passive system without waveform distortion, which is very robust against the shape change of the GNR. The decrease in the pulse amplitude is only 10% for a propagation distance of 1 μm. Furthermore, an array consisting of several GNRGWs is employed as a multichannel optical switch. When the separation is larger than 40 nm, each channel can be controlled independently by tuning the chemical potential of the corresponding GNR. The proposed GNRGW may raise great interest in studying dispersionless and low-loss nanophotonic devices, with potential applications in the distortionless transmission of nanoscale signals, electro-optic nanocircuits, and high-density on-chip communications.

Modal and wavelength conversions in plasmonic nanowires

We show that plasmonic nanowire-nanoparticle systems can perform nonlinear wavelength and modal conversions and potentially serve as building blocks for signal multiplexing and novel trafficking modalities. When a surface plasmon excited by a pulsed laser beam propagates in a nanowire, it generates a localized broadband nonlinear continuum at the nanowire surface as well as at active locations defined by sites where nanoparticles are absorbed (enhancement sites). The local response may couple to new sets of propagating modes enabling a complex routing of optical signals through modal and spectral conversions. Different aspects influencing the optical signal conversions are presented, including the parameters defining the local formation of the continuum and the subsequent modal routing in the nanowire.

A top-down fabricated gold nanostrip on a silicon-on-insulator wafer: a promising building block towards ultra-compact optical devices

A plasmonic waveguide is a fundamental building block for high speed, large data transmission capacity, low energy consumption optical communication and sensing. Controllable fabrication and simultaneous optimization of propagation loss and coupling efficiency with free space light are essential for the realization of ultra-compact passive and active plasmonic components. Here, we proposed gold nanostrips on a silicon-on-insulator wafer as plasmonic waveguides and first demonstrated the direct free-space light coupling and end-scattering detection of the top-down fabricated plasmonic waveguide. The scattering intensity from the terminal of a 6 μm long nanostrip can be improved experimentally over 34 times larger than that on the silica substrate. The controllable fabrication process renders the gold nanostrip on a silicon-on-insulator substrate a promising building block for ultracompact, monolithic integration and CMOS-compatible plasmonic devices in optical communication and sensing.

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.

The Basic Properties of Gold Nanoparticles and their Applications in Tumor Diagnosis and Treatment

Gold nanoparticles (AuNPs) have been widely studied and applied in the field of tumor diagnosis and treatment because of their special fundamental properties. In order to make AuNPs more suitable for tumor diagnosis and treatment, their natural properties and the interrelationships between these properties should be systematically and profoundly understood. The natural properties of AuNPs were discussed from two aspects: physical and chemical. Among the physical properties of AuNPs, localized surface plasmon resonance (LSPR), radioactivity and high X-ray absorption coefficient are widely used in the diagnosis and treatment of tumors. As an advantage over many other nanoparticles in chemicals, AuNPs can form stable chemical bonds with S-and N-containing groups. This allows AuNPs to attach to a wide variety of organic ligands or polymers with a specific function. These surface modifications endow AuNPs with outstanding biocompatibility, targeting and drug delivery capabilities. In this review, we systematically summarized the physicochemical properties of AuNPs and their intrinsic relationships. Then the latest research advancements and the developments of basic research and clinical trials using these properties are summarized. Further, the difficulties to be overcome and possible solutions in the process from basic laboratory research to clinical application are discussed. Finally, the possibility of applying the results to clinical trials was estimated. We hope to provide a reference for peer researchers to better utilize the excellent physicochemical properties of gold nanoparticles in oncotherapy.

Chemical interface damping for propagating surface plasmon polaritons in gold nanostripes

Leakage radiation microscopy has been used to examine chemical interface damping (CID) for the propagating surface plasmon polariton (PSPP) modes of Au nanostripes-nanofabricated structures with heights of 40 or 50 nm, widths between 2 and 4 µm, and 100 µm lengths. Real space imaging was used to determine the propagation lengths L_SPP of the leaky PSPP modes, and back focal plane measurements generated ω vs k dispersion curves, which yield the PSPP group velocities v_g. The combination of these two experiments was used to calculate the PSPP lifetime via T₁ = L_SPP/v_g. The difference in T₁ times between bare and thiol coated nanostripes was used to determine the dephasing rate due to CID Γ_CID for the adsorbed thiol molecules. A variety of different thiol molecules were examined, as well as nanostripes with different dimensions. The values of Γ_CID are similar for the different systems and are an order-of-magnitude smaller than the typical values observed for the localized surface plasmon resonances (LSPRs) of Au nanoparticles. Scaling the measured Γ_CID values by the effective path length for electron-surface scattering shows that the CID effect for the PSPP modes of the nanostripes is similar to that for the LSPR modes of nanoparticles. This is somewhat surprising given that PSPPs and LSPRs have different properties: PSPPs have a well-defined momentum, whereas LSPRs do not. The magnitude of Γ_CID for the nanostripes could be increased by reducing their dimensions, principally the height of the nanostructures. However, decreasing dimensions for the leaky PSPP mode increases radiation damping, which would make it challenging to accurately measure Γ_CID.

Direct Near-Field Observation of Surface Plasmon Polaritons on Silver Nanowires

Surface plasmon polaritons on (silver) nanowires are promising components for future photonic technologies. Here, we study near-field patterns on silver nanowires with a scattering-type scanning near-field optical microscope that enables the direct mapping of surface waves. We analyze the spatial pattern of the plasmon signatures for different excitation geometries and polarization and observe a plasmon wave pattern that is canted relative to the nanowire axis, which we show is due to a superposition of two different plasmon modes, as supported by electromagnetic simulations including the influence of the substrate. These findings yield new insights into the excitation and propagation of plasmon polaritons for applications in nanoplasmonic devices.

Routing a Chiral Raman Signal Based on Spin-Orbit Interaction of Light

Spontaneous Raman scattering is a second-order perturbation process with two photons linking the internal structures of the matter. The frequency-shifted Raman peaks are sharp and carry rich information about the internal structures. However, encoding and manipulating this information have been barely explored up to now. Here, we report the high-fidelity routing of a chiral Raman signal into propagating surface plasmon polaritons along a silver nanowire based on spin-orbit interaction of light. A directionality up to 91.5±0.5% is achieved and can be quantitatively controlled by tuning the polarization of the incident laser and the position of excitation. The deterministic routing of the Raman signal is sensitively dependent on the local spin density of the plasmon field and the polarization of the Raman modes. This study extends the spin-orbit interaction of light to the Raman scattering regime and proposes a new perspective for the remote readout of local optical chirality, helicity-related directional sorting, and quantum information processing.

Making waves: Radiation damping in metallic nanostructures

Metal nanostructures display several types of resonances. In the visible and near-IR spectral regions, there are localized surface plasmon resonances (LSPRs) that involve the coherent oscillation of the conduction electrons. Extended metal nanostructures, such as nanowires or nanoplates, also exhibit propagating surface plasmon polaritons (PSPPs), which are motions of the electrons at the surface of the structure that have a well-defined momentum. In addition, the vibrational normal modes of metal nanostructures give rise to low frequency resonances in the gigahertz to terahertz range. These different types of motions/resonances suffer energy losses from internal effects and from interactions with the environment. The goal of this perspective is to describe the part of the energy relaxation process due to the environment. Even though the plasmon resonances and acoustic vibrational modes arise from very different physics, it turns out that environmental damping is dominated by radiation of waves. The way the rates for radiation damping depend on the size of the nanostructure and the properties of the environment will be discussed for the different processes. For example, it is well known that for LSPRs, the rate of radiation damping increases with particle size. However, the radiation damping rate decreases with increasing dimensions for PSPPs and for the acoustic vibrational modes.

Surface Plasmon Nanolaser: Principle, Structure, Characteristics and Applications

Photonic devices are becoming more and more miniaturized and highly integrated with the advancement of micro-nano technology and the rapid development of integrated optics. Traditional semiconductor lasers have diffraction limit due to the feedback from the optical system, and their cavity length is more than half of the emission wavelength, so it is difficult to achieve miniaturization. Nanolasers based on surface plasmons can break through the diffraction limit and achieve deep sub-wavelength or even nano-scale laser emission. The improvement of modern nanomaterial preparation processes and the gradual maturity of micro-nano machining technology have also provided technical conditions for the development of sub-wavelength and nano-scale lasers. This paper describes the basic principles of surface plasmons and nano-resonators. The structure and characteristics of several kinds of plasmonic nanolasers are discussed. Finally, the paper looks forward to the application and development trend of nanolasers.

Low Threshold Plasmonic Nanolaser Based on Graphene

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

Design of Surface Plasmon Nanolaser Based on MoS2

The paper has proposed a new structure based on MoS2. The electric field distribution, the locality and the loss of the mode, and the threshold under different geometric shapes and parameters are investigated using COMSOL Multiphysics software, based on the finite element method. The different influenced degree of each component is also analyzed. Simulation results reveal that this kind of nanolaser has a low loss and high field confinement ability, the radius of CdS and Ag make a major contribution to the low loss and low threshold, and field confinement ability is mainly affected by the height of air gap. Under optimal parameters, effective propagation loss is only 0.00013, and the lasing threshold can be as low as 0.11 μm−1. The results provide theory and technique support to the field of new nanolaser design.

Tunable Low Loss 1D Surface Plasmons in InAs Nanowires

Due to the ability to manipulate photons at nanoscale, plasmonics has become one of the most important branches in nanophotonics. The prerequisites for the technological application of plasmons include high confining ability (λ₀ /λ_p ), low damping, and easy tunability. However, plasmons in typical plasmonic materials, i.e., noble metals, cannot satisfy these three requirements simultaneously and cause a disconnection to modern electronics. Here, the indium arsenide (InAs) nanowire is identified as a material that satisfies all the three prerequisites, providing a natural analogy with modern electronics. The dispersion relation of InAs plasmons is determined using the nanoinfrared imaging technique, and show that their associated wavelengths and damping ratio can be tuned by altering the nanowire diameter and dielectric environment. The InAs plasmons possess advantages such as high confining ability, low loss, and ease of fabrication. The observation of InAs plasmons could enable novel plasmonic circuits for future subwavelength applications.

An on-chip hybrid plasmonic light steering concentrator with ∼96% coupling efficiency

We, for the first time, propose and theoretically study a plasmonic light steering concentrator (PLSC) that is based on a hybrid photonic-plasmonic sandwich structure. In this device, a transverse electric (TE) polarization guided mode supported by a silicon-on-insulator (SOI) waveguide is vertically coupled to a metal-dielectric-metal sandwich structure, while the structure steers the light to a perpendicular metal taper and focuses the light on the apex of the taper with a small radius of 15 nm. Based on the coupled-mode theory, the two supermodes (quasi-TM modes) are clarified to illustrate the coupling mechanism of the device. We numerically obtain over 96% coupling efficiency at the 1500 nm telecommunication wavelength, and the mode width supported by the apex is limited laterally within the range of ∼110 nm, where the field enhancement calculated is found to be more than 10⁷ compared to that of light in the silicon waveguide.

Extending the Propagation Distance of a Silver Nanowire Plasmonic Waveguide with a Dielectric Multilayer Substrate

Chemical-synthesized silver nanowires have been proven as an efficient architecture for plasmonic waveguides, but the high propagation loss prevents their widely applications. Here, we demonstrate that the propagation distance of the plasmons along a silver nanowire can be extended if this nanowire was placed on a dielectric multilayer substrate containing a photonic band gap but not placed on a commonly used glass substrate. The propagation distance at 630 nm wavelength can reach 16 μm, even when the silver nanowire is as thin as 90 nm in diameter. Experimental and simulation results further show that the polarization of this propagating plasmon mode was nearly parallel to the surface of the dielectric multilayer, so it can be excited by a transverse-electric polarized Bloch surface wave propagating along a polymer nanowire with diameter at only about 170 nm on the same dielectric multilayer. Numerical simulations were also carried out and are consistent with the experiment results. Our work provides a platform with which to extend the propagation distance of the plasmonic waveguide and also for the integration between photonic and plasmonic waveguides on the nanometer scale.

Manipulating Propagation Constants of Silver Nanowire Plasmonic Waveguide Modes Using a Dielectric Multilayer Substrate

Experiments and numerical simulations demonstrate that when a silver nanowire is placed on a dielectric multilayer, but not the commonly used bare glass slide, the effective refractive index of the propagating surface plasmons along the silver nanowire can be controlled. Furthermore, by increasing the thickness of the top dielectric layer, longer wavelength light can also propagate along a very thin silver nanowire. In the experiment, the diameter of the silver nanowire can be as thin as 70 nm, with the incident wavelength as long as 640 nm. The principle of this control is analysed from the existence of a photonic band gap and the Bloch surface wave with this dielectric multilayer substrate.

Transversely Divergent Second Harmonic Generation by Surface Plasmon Polaritons on Single Metallic Nanowires

Coherently adding up signal wave from different locations are a prerequisite for realizing efficient nonlinear optical processes in traditional optical configurations. While nonlinear optical processes in plasmonic waveguides with subwavelength light confinement are in principle desirable for enhancing nonlinear effects, so far it has been difficult to improve the efficiency due to the large momentum mismatch. Here we demonstrate, using remotely excited surface plasmon polaritons (SPPs), axial collimated but transversely divergent second harmonic (SH) generation in a single silver nanowire-monolayer molybdenum disulfide hybrid system. Fourier imaging of the generated SH signal confirms the momentum conservation conditions between the incident and reflected SPPs and reveals distinct features inherent to the 1D plasmonic waveguides: (i) the SH photons are collimated perpendicular to the nanowire axis but are divergent within the perpendicular plane; (ii) the collimation (divergence) is inversely proportional to the length of the active region (lateral confinement of the SPPs); and (iii) the SH emission pattern resembles that of an aligned dipole chain on top of the substrate with an emission peak at the critical angle. Our results pave the way to generate and manipulate SH emission around subwavelength waveguides and open up new possibilities for realizing high efficiency on-chip nonlinear optics.

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.

Selective excitation of surface plasmon modes propagating in Ag nanowires

Surface plasmon modes propagating in metal nanowires are conveniently excited by focusing a laser beam on one extremity of the nanowire. We find that the precise positioning of the nanowire inside the focal region drastically influences the excitation efficiency of the different SPP modes sustained by the plasmonic waveguide. We demonstrate a spatially selective excitation of bound and leaky surface plasmon modes with excitation maps that strongly depend on the orientation of the incident linear polarization. We discuss this modal selection by considering the inhomogeneous distribution of the field components inside the focus. Our finding provides a way to discriminate the effective indices of the modes offering thus an increased coupling agility for future nanowire-based plasmonic architectures.

Super-Resolution Imaging and Plasmonics

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

Modeling of diameter-dependent Fe and Co ultrathin nanowires from first-principles calculations

We present the electronic, magnetic, thermoelectric and optical properties of ferromagnetic metal nanowires (NWs) made of iron (Fe) and cobalt (Co) atoms with different diameter using a first principles approach.

Direction-resolved radiation from polarization-controlled surface plasmon modes on silver nanowire antennas

Metallic nanowires (NWs) support multiple surface plasmon (SP) modes, which lead to extraordinary SP propagation behaviors. The leaky SP modes in metallic NWs connect the guiding and radiation of light at the nanometer scale. Understanding and controlling these modes are of vital importance for various nanophotonic applications. Here, we investigate the radiation from two polarization-controlled SP modes on supported silver NWs by using leakage radiation imaging and Fourier imaging techniques. The radiation directions from these modes can be clearly resolved from the Fourier images. The radiation polarization of the SP modes is related to the polarization of the excitation light. By depositing thin Al₂O₃ films onto silver NWs or decreasing the excitation wavelength, the radiation angles and wave vectors of the two modes are increased, and the longitudinal mode is more sensitive to Al₂O₃ thickness. Moreover, the propagation length of the longitudinal mode is obtained by analyzing the leakage radiation images, which is decreased with the decrease of the excitation wavelength and the increase of the Al₂O₃ layer thickness. These results show that leakage radiation from different SP modes on silver NWs can be resolved directly and controlled effectively. The supported silver NWs can thus be applied to designing plasmonic circuits, nanoantennas and nanosensors.

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.

Long range hybrid tube-wedge plasmonic waveguide with extreme light confinement and good fabrication error tolerance

We studied a novel long range hybrid tube-wedge plasmonic (LRHTWP) waveguide consisting of a high index dielectric nanotube placed above a triangular metal wedge substrate. Using comprehensive numerical simulations on guiding properties of the designed waveguide, it is found that extreme light confinement and low propagation loss are obtained due to strong coupling between dielectric nanotube mode and wedge plasmon polariton. Comparing with previous studied hybrid plasmonic waveguides, the LRHTWP waveguide has longer propagation length and tighter mode confinement. In addition, the LRHTWP waveguide is quite tolerant to practical fabrication errors such as variation of the wedge tip angle and the horizontal misalignment between the nanotube and the metal wedge. The proposed LRHTWP waveguide could have many application potentials for various high performance nanophotonic components.

Routing of surface plasmons in silver nanowire networks controlled by polarization and coating

Controllable propagation of electromagnetic energy in plasmonic nanowaveguides is of great importance for building nanophotonic circuits. Here, we studied the routing of surface plasmons in silver nanowire structures by combining experiments and electromagnetic simulations. The superposition of different plasmon modes results in the tunable near field patterns of surface plasmons on the nanowire. Using the quantum dot fluorescence imaging technique, we experimentally demonstrate that the near field distribution on the nanowire controls the surface plasmon transmission in the nanowire networks. By controlling the polarization of the input light or by controlling the dielectric coating on the nanowire to modulate the plasmon field distribution and guarantee the strong local field intensity at the connecting junction, the surface plasmons can be efficiently routed to the connected nanowires. Depositing a thin layer of Al2O3 film onto the nanowires can reverse the polarization dependence of the output intensity at the nanowire terminals. These results are instructive for designing functional plasmonic nanowire networks and metal-nanowire-based nanophotonic devices.

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 and manipulation of ultrahigh order plasmon resonances in visible and near-infrared region

Optical properties of periodic nanorings with built-in V-shaped nanowedges (NRBV) are investigated theoretically. Tunable ultrahigh order Fano resonances are achieved and they are found to be sensitive to geometric parameters and surrounding dielectric environment of the planar nanostructure. High order Fano resonances can be suppressed or enhanced by adjusting the opening angle of the nanowedge, the size of the nanoring and the aspect ratio of the nanowedge. Moreover, manipulating the offset of the built-in nanowedge, or filling dielectrics asymmetrically can revive suppressed Fano resonances when the V-shaped nanowedge develops into a straight nanorod. Meanwhile, stronger plasmon resonances emerge alternately in the two parts of this planar nanostructure. This periodic plasmonic nanostructure produces ultrahigh order plasmon resonances and stronger electric field enhancement, which have great potential applications in multi-wavelength surface enhanced spectroscopy and biochemical sensing.

Metallic-nanowire-loaded silicon-on-insulator structures: a route to low-loss plasmon waveguiding on the nanoscale

The simultaneous realization of nanoscale field localization and low transmission loss remains one of the major challenges in nanophotonics. Metal nanowire waveguides can fulfill this goal to a certain extent by confining light within subwavelength space, yet their optical performances are still restricted by the tradeoff between confinement and loss, which results in quite limited propagation distances when their mode sizes are reduced down to the nanometer scale. Here we introduce a class of low-loss guiding schemes by integrating silicon-on-insulator (SOI) waveguides with plasmon nanowire structures. The closely spaced silicon and metal configurations allow efficient light squeezing within the nanometer, low-index silica gaps between them, enabling deep-subwavelength light transmission with low modal attenuation. Optimizations of key structural parameters unravel the wide-range existence of the high-performance hybrid nanowire plasmon mode, which demonstrates improved guiding properties compared to the conventional hybrid and nanowire plasmon polaritons. The excitation strategy of the guided mode and the feasibility of the waveguide for compact photonic integration as well as active components are also discussed to lay the foundation for its practical implementation. The remarkable properties of these metallic-nanowire-loaded SOI waveguides potentially lend themselves to the implementation of high performance nanophotonic components, and open up promising opportunities for a variety of intriguing applications on the nanoscale.

Prospects for plasmonic hot spots in single molecule SERS towards the chemical imaging of live cells

Single molecule surface enhanced Raman scattering (SM-SERS) is a highly local effect occurring at sharp edges, interparticle junctions and crevices or other geometries with a sharp nanoroughness of plasmonic nanostructures (“hot spots”) for an analyte detection.

Light transfer from quantum-dot-doped polymer nanowires to silver nanowires

The plasmons of two silver nanowires are simultaneously excited by photoluminescence of the quantum-dot-doped nanowire under 532 nm laser excitation.

Coupled nanowire-based hybrid plasmonic nanocavities on thin substrates

We theoretically analyze nanowire-based hybrid plasmonic nanocavities on thin substrates at visible wavelengths. In the presence of thin suspended substrates, the hybrid plasmonic modes, formed by the coupling between a metal nanowire and a dielectric nanowire with optical gain, exhibit negligible substrate-mediated characteristics and overlap better with the gain region. Consequently, the confinement factor of the guided hybrid modes is enhanced by more than 42%. However, the presence of significant mirror loss remains the main challenge to lasing. By adding silver coatings with a sufficient thickness range on the two end facets, we show that the reflectivity is substantially enhanced to above 50%. For a coating thickness of 50 nm and cavity length of about 4 μm, the quality factor is above 100.

Single nanoparticle couplers for plasmonic waveguides

A single nanoparticle antenna, can be used as an efficient coupler for plasmonic nanowire waveguides. The coup-ling of light into the surface plasmon polaritons on a nanowire can be suppressed or enhanced depending on the surface plasmon resonances of the nanoantenna. The coupler is compacted and can be simply controlled using focus ion beam.

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.

Tuning the hybridization of plasmonic and coupled dielectric nanowire modes for high-performance optical waveguiding at sub-diffraction-limited scale

We report the realization of low-loss optical waveguiding at telecommunication wavelength by exploiting the hybridization of photonic modes guided by coupled all-dielectric nanowires and plasmon waves at planar metal-dielectric interfaces. The characteristics of the hybrid plasmon polaritons, which are yielded by the coupling between two types of guided modes, can be readily tuned through engineering key structural parameters of the coupled nanowires and their distances to the metallic surfaces. In addition to exhibiting significantly lower attenuations for similar degrees of confinement as compared to the conventional hybrid waves in single-dielectric-nanowire-based waveguides, these hybridized plasmonic modes are also capable of enabling reduced waveguide crosstalk for comparable propagation distances. Being compatible with semiconductor fabrication techniques, the proposed guiding schemes could be promising candidates for various integrated photonic devices and may lead to potential applications in a wide variety of related areas.

Local spectroscopy of silver nanowire in different environments excited with a halogen lamp

We report a propagation spectrum detection system in which one end of a plasmonic silver nanowire is locally illuminated from a normal halogen lamp and the scattered light is recorded spectroscopically at the other end. The system is applied to investigate surface plasmon polariton-Fabry-Perot (SPP-FP) modes of silver nanowires with different lengths at air-glass and oil-glass interfaces. The generalized FP model is used to analyze the spectrum, which fits well with the experimental results. The influence of nanowire length and environment on the properties of the FP resonances is discussed. The propagation spectrum detection system will find applications for integrated optical circuits and plasmonic sensing.

Dye-assisted gain of strongly confined surface plasmon polaritons in silver nanowires

Noble metal nanowires are excellent candidates as subwavelength optical components in miniaturized devices due to their ability to support the propagation of surface plasmon polaritons (SPPs). Nanoscale data transfer based on SPP propagation at optical frequencies has the advantage of larger bandwidths but also suffers from larger losses due to strong mode confinement. To overcome losses, SPP gain has been realized, but so far only for weakly confined SPPs in metal films and stripes. Here we report the demonstration of gain for subwavelength SPPs that were strongly confined in chemically prepared silver nanowires (mode area = λ(2)/40) using a dye-doped polymer film as the optical gain medium. Under continuous wave excitation at 514 nm, we measured a gain coefficient of 270 cm(-1) for SPPs at 633 nm, resulting in partial SPP loss compensation of 14%. This achievement for strongly confined SPPs represents a major step forward toward the realization of nanoscale plasmonic amplifiers and lasers.