Silver nanowires as surface plasmon resonators

Plasmon-enhanced two photon excited emission from edges of one-dimensional plasmonic hotspots with continuous-wave laser excitation

One-dimensional junctions between parallelly and closely arranged multiple silver nanowires (NWs) exhibit a large electromagnetic (EM) enhancement factor (FR) owing to both localized and surface plasmon resonances. Such junctions are referred to as one-dimensional (1D) hotspots (HSs). This study found that two-photon excited emissions, such as hyper-Rayleigh, hyper-Raman, and two-photon fluorescence of dye molecules, are generated at the edge of 1D HSs of NW dimers with continuous-wave near-infrared (NIR) laser excitation and propagated through 1D HSs; however, they were not generated from the centers of 1D HSs. Numerical EM calculations showed that FR of the NIR region for the edges of 1D HSs was larger than that for the centers by ∼102 times, resulting in the observation of two-photon excited emissions only from the edge of 1D HSs. The analysis of the NW dimer gap distance dependence of FR revealed that the lowest surface plasmon (SP) mode, compressed and localized at the edges of 1D HSs, was the origin of the large FR in the NIR region. The propagation of two-photon-excited emissions was supported by the higher-order coupled SP mode.

Growth of metal nanowire forests controlled through stress fields induced by grain gradients

Pure metal nanowires (NWs) are one-dimensional nanomaterials with distinctive properties for various applications. Nevertheless, mass-growth forests have not been developed because of vapor pressure limitations, chemical reduction problems, or both. We succeeded in the mass growth of aluminum (Al) NW forests at desired locations by controlling atomic diffusion within the solid film. Whereas prior attention has focused only on how to increase the driving force, we show that focused ion beam irradiation created localized regions of high stress, which provided pathways for atomic diffusion as well as nuclei and driving forces for vertical NW growth. The underlying growth process could in principle be extended to other metals.

Strong angular and spectral narrowing of electroluminescence in an integrated Tamm-plasmon-driven halide perovskite LED

Next-generation light-emitting applications such as displays and optical communications require judicious control over emitted light, including intensity and angular dispersion. To date, this remains a challenge as conventional methods require cumbersome optics. Here, we report highly directional and enhanced electroluminescence from a solution-processed quasi-2-dimensional halide perovskite light-emitting diode by building a device architecture to exploit hybrid plasmonic-photonic Tamm plasmon modes. By exploiting the processing and bandgap tunability of the halide perovskite device layers, we construct the device stack to optimise both optical and charge-injection properties, leading to narrow forward electroluminescence with an angular full-width half-maximum of 36.6° compared with the conventional isotropic control device of 143.9°, and narrow electroluminescence spectral full-width half-maximum of 12.1 nm. The device design is versatile and tunable to work with emission lines covering the visible spectrum with desired directionality, thus providing a promising route to modular, inexpensive, and directional operating light-emitting devices.

Ultraconfined Plasmons in Atomically Thin Crystalline Silver Nanostructures

The ability to confine light down to atomic scales is critical for the development of applications in optoelectronics and optical sensing as well as for the exploration of nanoscale quantum phenomena. Plasmons in metallic nanostructures with just a few atomic layers in thickness can achieve this type of confinement, although fabrication imperfections down to the subnanometer scale hinder actual developments. Here, narrow plasmons are demonstrated in atomically thin crystalline silver nanostructures fabricated by prepatterning silicon substrates and epitaxially depositing silver films of just a few atomic layers in thickness. Specifically, a silicon wafer is lithographically patterned to introduce on-demand lateral shapes, chemically process the sample to obtain an atomically flat silicon surface, and epitaxially deposit silver to obtain ultrathin crystalline metal films with the designated morphologies. Structures fabricated by following this procedure allow for an unprecedented control over optical field confinement in the near-infrared spectral region, which is here illustrated by the observation of fundamental and higher-order plasmons featuring extreme spatial confinement and high-quality factors that reflect the crystallinity of the metal. The present study constitutes a substantial improvement in the degree of spatial confinement and quality factor that should facilitate the design and exploitation of atomic-scale nanoplasmonic devices for optoelectronics, sensing, and quantum-physics applications.

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.

Superplastic Nanomolding of Aluminum Waveguides for Subwavelength Light Routing, Splitting, and Encryption

Plasmonic nanowires (NWs) due to their polarization-dependent optics and enhanced light-matter interactions have presented vibrant capabilities in functional nanophotonic devices. However, current demonstrations have largely been based on chemically synthesized Ag NWs, which are extremely unstable and poorly functional. Here we show single-crystalline Al NWs can be fabricated by a superplastic nanomolding (SPNM) technique on a centimeter scale, which are earth-abundant and highly stable. They present robust properties of multimode waveguiding with long-term stability, high efficiency of beam splitting in response to the polarization, and durable thermal optical modulation, which can be readily applied as nanophotonic routers, splitters, and information encryptors. Moreover, this SNPM technique is extendable to other metals, which are highly exploitable for functional nanophotonic devices and integrated optical chips.

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.

Plasmonic Cavity-Catalysis by Standing Hot Carrier Waves

Manipulating active sites of catalysts is crucial but challenging in catalysis science and engineering. Beyond the design of the composition and structure of catalysts, the confined electromagnetic field in optical cavities has recently become a promising method for catalyzing chemical reactions via strong light-matter interactions. Another form of confined electromagnetic field, the charge density wave in plasmonic cavities, however, still needs to be explored for catalysis. Here, we present an unprecedented catalytic mode based on plasmonic cavities, called plasmonic cavity-catalysis. We achieve direct control of catalytic sites in plasmonic cavities through standing hot carrier waves. Periodic catalytic hotspots are formed because of localized energy and carrier distribution and can be well tuned by cavity geometry, charge density, and excitation angle. We also found that the catalytic activity of the cavity mode increases several orders of magnitude compared with conventional plasmonic catalysis. We ultimately demonstrate that the locally concentrated long-lived hot carriers in the standing wave mode underlie the formation of the catalytic hotspots. Plasmonic cavity-catalysis provides a new approach to manipulate the catalytic sites and rates and may expand the frontier of heterogeneous catalysis.

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.

Periodic Distributions and Ultrafast Dynamics of Hot Electrons in Plasmonic Resonators

Understanding and managing hot electrons in metals are of fundamental and practical interest in plasmonic studies and applications. A major challenge for the development of hot electron devices requires the efficient and controllable generation of long-lived hot electrons so that they can be harnessed effectively before relaxation. Here, we report the ultrafast spatiotemporal evolution of hot electrons in plasmonic resonators. Using femtosecond-resolution interferometric imaging, we show the unique periodic distributions of hot electrons due to standing plasmonic waves. In particular, this distribution can be flexibly tuned by the size, shape, and dimension of the resonator. We also demonstrate that the hot electron lifetimes are substantially prolonged at hot spots. This appealing effect is interpreted as a result of the locally concentrated energy density at the antinodes in standing hot electron waves. These results could be useful to control the distributions and lifetimes of hot electrons in plasmonic devices for targeted optoelectronic applications.

Broadband plasmonic indium arsenide photonic antennas

An on-chip integrated mid-infrared Fabry-Perot (F-P) polariton resonator exhibits excellent biosensing, thermal emission, and quantum laser utility potential. However, the narrow optical response range and absence of optoelectronic tunability have hindered the development of a F-P phonon polariton resonator. The discovery of surface plasmons in semiconductor nanowires provides a novel route to F-P polariton resonator devices with a broadband optical response and multi-field tunability. Due to their high electron mobility and crystalline quality, InAs twinning superlattice (TSL) nanowires have become a promising candidate in plasmonic electronics. We systemically studied the F-P plasmonic resonance of individual InAs TSL nanowires with a scattering-type near-field optical microscope. Using a metallic AFM tip to excite surface plasmons, we can observe odd-order and even-order modes of F-P polariton resonance, breaking the symmetric selection rules. Through nano Fourier transform infrared spectroscopy, we found that InAs nanowires' F-P polariton resonances appear in a broadband frequency range (650-1100 cm^-1) and calculated that the corresponding Q factor is 5-10. This semiconductor F-P polariton resonator with inherent electrical tunability will be essential in integrated nanophotonic circuits.

Silver Nanoparticle Chains for Ultra-Long-Range Plasmonic Waveguides for Nd3+ Fluorescence

Plasmonic waveguides have been shown to be a promising approach to confine and transport electromagnetic energy beyond the diffraction limit. However, ohmic losses generally prevent their integration at micrometric or millimetric scales. Here, we present a gain-compensated plasmonic waveguide based on the integration of linear chains of Ag nanoparticles on an optically active Nd³⁺-doped solid-state gain medium. By means of dual confocal fluorescence microscopy, we demonstrate long-range optical energy propagation due to the near-field coupling between the plasmonic nanostructures and the Nd³⁺ ions. The subwavelength fluorescence guiding is monitored at distances of around 100 µm from the excitation source for two different emission ranges centered at around 900 nm and 1080 nm. In both cases, the guided fluorescence exhibits a strong polarization dependence, consistent with the polarization behavior of the plasmon resonance supported by the chain. The experimental results are interpreted through numerical simulations in quasi-infinite long chains, which corroborate the propagation features of the Ag nanoparticle chains at both excitation (λ_exc = 590 nm) and emission wavelengths. The obtained results exceed by an order of magnitude that of previous reports on electromagnetic energy transport using linear plasmonic chains. The work points out the potential of combining Ag nanoparticle chains with a small interparticle distance (~2 nm) with rare-earth-based optical gain media as ultra-long-range waveguides with extreme light confinement. The results offer new perspectives for the design of integrated hybrid plasmonic-photonic circuits based on rare-earth-activated solid-state platforms.

Subwavelength dichroic demultiplexer based on double Fabry-Perot cavities

Plasmonic demultiplexers hold promise for the realization of the subwavelength and high-splitting ratio dichroic splitter and have a wide range of applications from optical communication, and manipulation to ultrafast data treatment. However, this vision has not been realized for a long time due to lacking the suitable splitting structure design, which limits its further development of integrated photonic circuits. Here, we demonstrate a plasmonic demultiplexer with subwavelength feature size (0.54 µm) and broadband spectral (620-870 nm) range, and high-splitting ratio (17 dB in experiments and 20 dB in calculations). It consists of two adjacent Fabry-Perot cavities (covered by PMMA polymer) and coupling gratings, which are integrated with the Au waveguide. The relatively simple double cavities design of our device has a simple theoretical analysis and fabrication process. Our work has relevance for various optical applications, such as multiple wavelength photodetectors and optical multichannel interconnects.

Monolithically Programmed Stretchable Conductor by Laser-Induced Entanglement of Liquid Metal and Metallic Nanowire Backbone

Owing to its low mechanical compliance, liquid metal is intrinsically suitable for stretchable electronics and future wearable devices. However, its invariable strain-resistance behavior according to the strain-induced geometrical deformation and the difficulty of circuit patterning limit the extensive use of liquid metal, especially for strain-insensitive wiring purposes. To overcome these limitations, herein, novel liquid-metal-based electrodes of fragmented eutectic gallium-indium alloy (EGaIn) and Ag nanowire (NW) backbone of which their entanglement is controlled by the laser-induced photothermal reaction to enable immediate and direct patterning of the stretchable electrode with spatially programmed strain-resistance characteristics are developed. The coexistence of fragmented EGaIn and AgNW backbone, that is, a biphasic metallic composite (BMC), primarily supports the uniform and durable formation of target layers on stretchable substrates. The laser-induced photothermal reaction not only promotes the adhesion between the BMC layer and substrates but also alters the structure of laser-irradiated BMC. By controlling the degree of entanglement between fragmented EGaIn and AgNW, the initial conductivity and local gauge factor are regulated and the electrode becomes effectively insensitive to applied strain. As the configuration developed in this study is compatible with both regimes of electrodes, it can open new routes for the rapid creation of complex stretchable circuitry through a single process.

Correlations between incident and emission polarization in nanowire-particle coupled junctions

Plasmonic nanostructures with subwavelength confinement are of great importance for the development of integrated nanophotonic circuits and devices. Here, we experimentally investigate how the polarization of the emitted light from nanowire-particle junction relies on the incident polarization. We demonstrate that the correlations can be effectively modulated by the particle position relative to the wire. By varying the wire-particle gap with only several nanometers, the nanowire-particle junction can be changed from polarization maintainer to rotator. Then, by moving the particle along the wire within half of the surface plasmon polariton (SPP) beat, the polarization behaviors can be tuned from positive to negative correlation. The mechanism can be well understood by the hybridization of wire-particle coupled mode and propagating SPP modes, which is verified by finite-difference time-domain simulations. These findings would provide a new degree of freedom for manipulating light polarization at the nanometer scale and additional flexibility for constructing nanophotonic devices.

Decrypting Material Performance by Wide-field Femtosecond Interferometric Imaging of Energy Carrier Evolution

Energy carrier evolution is crucial for material performance. Ultrafast microscopy has been widely applied to visualize the spatiotemporal evolution of energy carriers. However, direct imaging of a small amount of energy carriers on the nanoscale remains difficult due to extremely weak transient signals. Here, we present a method for ultrasensitive and high-throughput imaging of energy carrier evolution in space and time. This method combines femtosecond pump-probe techniques with interferometric scattering microscopy (iSCAT), named Femto-iSCAT. The interferometric principle and unique spatially modulated contrast enhancement enable the exploration of new science. We address three important and challenging problems: transport of different energy carriers at various interfaces, heterogeneous hot-electron distribution and relaxation in single plasmonic resonators, and distinct structure-dependent edge-state dynamics of carriers and excitons in optoelectronic semiconductors. Femto-iSCAT holds great potential as a universal tool for ultrasensitive imaging of energy carrier evolution in space and time.

The advent of thermoplasmonic membrane distillation

Freshwater scarcity is a vital societal challenge related to climate change, population pressure, and agricultural and industrial demands. Therefore, sustainable desalination/purification of salty/contaminated water for human uses is particularly relevant. Membrane distillation is an emerging hybrid thermal-membrane technology with the potential to overcome the drawbacks of conventional desalination by a synergic exploitation of the water-energy nexus. Although membrane distillation is considered a green technology, efficient heat management remains a critical concern affecting the cost of the process and hindering its viability at large scale. A multidisciplinary approach that involves materials chemistry, physical chemistry, chemical engineering, and materials and polymer science is required to solve this problem. The combination of solar energy with membrane distillation is considered a potentially feasible low-cost approach for providing high-quality freshwater with a low carbon footprint. In particular, recent discoveries about efficient light-to-heat conversion in nanomaterials have opened unprecedented perspectives for the implementation of sunlight-based renewable energy in membrane distillation. The integration of nanofillers enabling photothermal effects into membranes has been demonstrated to be able to significantly enhance the energy efficiency without impacting on economic costs. Here, we provide a comprehensive overview on the state of the art, the opportunities, open challenges and pitfalls of the emerging field of solar-driven membrane distillation. We also assess the peculiar physicochemical properties and synthesis scalability of photothermal materials, as well as the strategies for their integration into polymeric nanocomposite membranes enabling efficient light-to-heat conversion and freshwater.

Elliptical gold nanowires: controlled fabrication and plasmonic Fabry-Pérot resonances

Nanowires (NWs) are essential building blocks of photonic devices for guiding light waves. However, the controlled synthesis of non-circular NWs remains challenging. Herein, we develop a bottom-up approach for the fabrication of high-quality elliptical gold NWs with finely tuned geometry engineering by using an advanced ion-track template technology. Compared to ordinary NWs, the rotational symmetry breaking leads to highly polarization-dependent plasmonic responses. Modal analysis shows that the lowest dipolar HE₁ mode splits into two branches where the attenuation of the long-range branch decreases by 40%, while the short-range branch has a stronger enhanced near-field. Novel, to the best of our knowledge, plasmonic Fabry-Pérot resonances on finite NWs are measured. Our method can be extended to fabricate non-circular NWs with other materials, holding potential for novel applications from quantum to collective scales.

Unified Scattering and Photoluminescence Spectra for Strong Plasmon-Exciton Coupling

The strong coupling between excitons and single plasmonic nanocavities enables plexcitonic states in nanoscale systems at room temperature. Here we demonstrate the strong coupling of surface plasmon modes of metal nanowires and excitons in monolayer semiconductors, with Rabi splitting manifested in both scattering and photoluminescence (PL) spectra. By utilizing the propagation properties of surface plasmons on the nanowires, the PL emitted through the scattering of plasmon-exciton hybrid modes is extracted. The analytically calculated scattering and PL spectra well reproduce the experimental results. These findings unify the scattering and PL spectra in the plexcitonic system and eliminate the ambiguities of PL emission, shedding new light on understanding the rich spectral phenomena in the plasmon-exciton strong coupling regime.

Local large temperature difference and ultra-wideband photothermoelectric response of the silver nanostructure film/carbon nanotube film heterostructure

Photothermoelectric materials have important applications in many fields. Here, we joined a silver nanostructure film and a carbon nanotube film by van der Waals force to form a heterojunction, which shows excellent photothermal and photoelectric conversion properties. The local temperature difference and the output photovoltage increase rapidly when the heterojunction is irradiated by lasers with wavelengths ranging from ultraviolet to terahertz. The maximum temperature difference reaches 215.9 K, which is significantly higher than that of other photothermoelectric materials reported in the literature. The photothermal and photoelectric responsivity depend on the wavelength of lasers, which are 175~601 K W^-1 and 9.35~40.4 mV W^-1, respectively. We demonstrate that light absorption of the carbon nanotube is enhanced by local surface plasmons, and the output photovoltage is dominated by Seebeck effect. The proposed heterostructure can be used as high-efficiency sensitive photothermal materials or as ultra-wideband fast-response photoelectric materials.

Finding efficient photothermoelectric materials remains critical to the development of clean and renewable energy conversion technologies. Here, authors prepare a silver nanostructure film/carbon nanotube film heterojunction with excellent photothermal and photoelectric conversion performance.

Plasmon-enhanced photoresponse of single silver nanowires and their network devices

The photo-bolometric effect is critically important in optoelectronic structures and devices employing metallic electrodes with nanoscale features due to heating caused by the plasmonic field enhancement. One peculiar case is individual silver nanowires (Ag NWs) and their networks. Ag NW-networks exhibit excellent thermal, electrical, and mechanical properties, providing a simple yet reliable alternative to common flexible transparent electrode materials used in optoelectronic devices. To date, there have been no reports on the photoresponse of Ag NWs. In this study, we show that single Ag NWs and networks of such Ag NWs possess a significant, intrinsic photoresponse, thanks to the photo-bolometric effect, as directly observed and measured using scanning photocurrent microscopy. Surface plasmon polaritons (SPPs) created at the contact metals or plasmons created at the nanowire-metal structures cause heating at the junctions where a plasmonic field enhancement is possible. The local heating of the Ag NWs results in negative photoconductance due to the bolometric effect. Here an open-circuit response due to the plasmon-enhanced Seebeck effect was recorded at the NW-metal contact junctions. The SPP-assisted bolometric effect is found to be further enhanced by decorating the Ag NWs with Ag nanoparticles. These observations are relevant to the use of metallic nanowires in plasmonic applications in particular and in optoelectronics in general. Our findings may pave the path for plasmonics-enabled sensing without spectroscopic detection.

Atomically Smooth Single-Crystalline Platform for Low-Loss Plasmonic Nanocavities

Nanoparticle-on-mirror plasmonic nanocavities, capable of extreme optical confinement and enhancement, have triggered state-of-the-art progress in nanophotonics and development of applications in enhanced spectroscopies. However, the optical quality factor and thus performance of these nanoconstructs are undermined by the granular polycrystalline metal films (especially when they are optically thin) used as a mirror. Here, we report an atomically smooth single-crystalline platform for low-loss nanocavities using chemically synthesized gold microflakes as a mirror. Nanocavities constructed using gold nanorods on such microflakes exhibit a rich structure of plasmonic modes, which are highly sensitive to the thickness of optically thin (down to ∼15 nm) microflakes. The microflakes endow nanocavities with significantly improved quality factor (∼2 times) and scattering intensity (∼3 times) compared with their counterparts based on deposited films. The developed low-loss nanocavities further allow for the integration with a mature platform of fiber optics, opening opportunities for realizing nanocavity-based miniaturized photonic devices for practical applications.

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.

Remote Dual-Cavity Enhanced Second Harmonic Generation in a Hybrid Plasmonic Waveguide

On-chip nanoscale optical platforms capable of efficient second harmonic generation (SHG) are highly desired for optical sensing, subwavelength coherent sources, and quantum photonic devices. Here, we develop a remotely excited dual cavity resonance scheme to achieve significantly enhanced SHG in a CdSe nanobelt on Au film hybrid waveguide system. The SHG emission with superior efficiency originates from counter-propagating plasmonic modes interference in a horizontal Fabry-Pérot (FP) cavity enabled by remote excitation of propagating surface plasmons, which is further enhanced through a vertical FP cavity. With this effective cooperation of hybrid plasmon modes and FP cavity modes, 2 orders of magnitude enhancement of the conversion efficiency (3.5 × 10^-4 W^-1) is achieved compared to the off-resonance case. Our design provides new insight into the development of a multifunctional hybrid plasmonic device toward on-chip nonlinear nanophotonic applications.

CMOS-Compatible Electronic-Plasmonic Transducers Based on Plasmonic Tunnel Junctions and Schottky Diodes

To develop methods to generate, manipulate, and detect plasmonic signals by electrical means with complementary metal-oxide-semiconductor (CMOS)-compatible materials is essential to realize on-chip electronic-plasmonic transduction. Here, electrically driven, CMOS-compatible electronic-plasmonic transducers with Al-AlO_X -Cu tunnel junctions as the excitation source of surface plasmon polaritons (SPPs) and Si-Cu Schottky diodes as the detector of SPPs, connected via plasmonic strip waveguides of Cu, are demonstrated. Remarkably, the electronic-plasmonic transducers exhibit overall transduction efficiency of 1.85 ± 0.03%, five times higher than previously reported transducers with two tunnel junctions (metal-insulator-metal (MIM)-MIM transducers) where SPPs are detected based on optical rectification. The result establishes a new platform to convert electronic signals to plasmonic signals via electrical means, paving the way toward CMOS-compatible plasmonic components.

Single-Crystal Nanostructure Arrays Forming Epitaxially through Thermomechanical Nanomolding

For nanostructures in advanced electronic and plasmonic systems, a single-crystal structure with controlled orientation is essential. However, the fabrication of such devices has remained challenging, as current nanofabrication methods often suffer from either polycrystalline growth or the difficulty of integrating single crystals with substrates in desired orientations and locations to create functional devices. Here we report a thermomechanical method for the controlled growth of single-crystal nanowire arrays, which enables the simultaneous synthesis, alignment, and patterning of nanowires. Within such diffusion-based thermomechanical nanomolding (TMNM), the substrate material diffuses into nanosized cavities under an applied pressure gradient at a molding temperature of ∼0.4 times the material's melting temperature. Vertically grown face-centered cubic (fcc) nanowires with the [110] direction in an epitaxial relationship with the (110) substrate are demonstrated. The ability to control the crystal structure through the substrate takes TMNM a major step further, potentially allowing all fcc and body-centered cubic (bcc) materials to be integrated as single crystals into devices.

Plasmon-Directed On-Wire Growth of Branched Silver Nanowires with Chiroptic Activity

Silver nanowires (Ag NWs) present prominent waveguiding properties of subwavelength light due to their nanoconfinement with propagating surface plasmons, which is of great importance for on-chip integration of nanophotonic devices and optical computation. Such propagating plasmons also exert plasmonic forces, which can be utilized to manipulate nanoparticles (NPs) beyond the diffraction limit. However, such controllability is spatially limited to the near fields, whereas a large portion of uncontrolled particles are randomly deposited on the chips, which could be detrimental to the integrated optical devices. Herein we shine continuous wave laser at one end of the Ag NW immersed in AgNO₃ solution to launch the propagating surface plasmons. The laser irradiation also induces the photoreduction of Ag⁺ ions to locally generate tiny Ag NPs, which evolve into large Ag flake branches closer to the other end of the Ag NW. Such a peculiar growth is due to the synergistic effect of plasmonic forces and the thermophoretic/thermo-osmosis forces induced by temperature gradient. These branched Ag NWs with sharp angles are intrinsically chiral, which can be partially controlled by changing the irradiation location, forming plasmonic chiral enantiomers. The circular differential scattering (CDS) response of these branched Ag NWs can be as large as 40%, which can be used for chiral enantiomer sensing with spectral dissymmetric factor up to 4 nm induced by phenylalanine. This plasmon-directed on-wire growth not only offers a facile approach for generating plasmonic chiral nanostructures with remote controllability, but also provides significant insights on the synergistic effect of plasmonic forces and thermal-induced forces, which has great implications for self-assembly and integration of on-chip optics.

In situ TEM observations of void movement in Ag nanowires affecting the electrical properties under biasing

In this study we investigated the electromigration (EM) of metal electrodes and the effect of stacking faults on the EM in Ag nanowires (NWs). We used the galvanic replacement method to synthesize these NWs by controlling the concentration of silver nitrate. In situ transmission electron microscopy (TEM) revealed the presence of both intrinsic and extrinsic stacking faults in the Ag NWs. We found that planar defects increased the lifetime of the devices with an intrinsic change in the material properties. Our EM measurements involved examinations of the change in electrical resistance (arising from void formation in the NW as a result of electromigration) as well as direct visual observation of the shape (using in situ TEM).

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.

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.

Controlling plasmon propagation and enhancement via reducing agent in wet chemistry synthesized silver nanowires

Silver nanowires with varying diameters and submillimeter lengths were obtained by changing a reducing agent used during hydrothermal synthesis. The control over the nanowire diameter turns out to play a critical role in determining their plasmonic properties, including fluorescence enhancement and surface plasmon polariton propagation. Advanced fluorescence imaging of hybrid nanostructures assembled of silver nanowires and photoactive proteins indicates longer propagation lengths for nanowires featuring larger diameters. At the same time, with increasing diameter of the nanowires, we measure a substantial reduction of fluorescence enhancement. The results point at possible ways to control the influence of plasmon excitations in silver nanowires by tuning their morphology.

Self-Assembly of Plasmonic Nanoantenna-Waveguide Structures for Subdiffractional Chiral Sensing

Spin-momentum locking is a peculiar effect in the near-field of guided optical or plasmonic modes. It can be utilized to map the spinning or handedness of electromagnetic fields onto the propagation direction. This motivates a method to probe the circular dichroism of an illuminated chiral object. In this work, we demonstrate local, subdiffraction limited chiral coupling of light and propagating surface plasmon polaritons in a self-assembled system of a gold nanoantenna and a silver nanowire. A thin silica shell around the nanowire provides precise distance control and also serves as a host for fluorescent molecules, which indicate the direction of plasmon propagation. We characterize our nanoantenna-nanowire systems comprehensively through correlated electron microscopy, energy-dispersive X-ray spectroscopy, dark-field, and fluorescence imaging. Three-dimensional numerical simulations support the experimental findings. Besides our measurement of far-field polarization, we estimate sensing capabilities and derive not only a sensitivity of 1 mdeg for the ellipticity of the light field, but also find 10³ deg cm²/dmol for the circular dichroism of an analyte locally introduced in the hot spot of the antenna-wire system. Thorough modeling of a prototypical design predicts on-chip sensing of chiral analytes. This introduces our system as an ultracompact sensor for chiral response far below the diffraction limit.

Low-loss aluminum epitaxial film for scalable and sustainable plasmonics: direct comparison with silver epitaxial film

Aluminum is a plasmonic material well known for its excellent stability, complementary metal-oxide-semiconductor compatibility and wide availability as compared to popular plasmonic materials such as gold and silver. Aluminum can support surface plasmon resonances in a broad spectral range, including the deep ultra-violet, a regime where no other plasmonic materials can work. However, conventional aluminum films suffer from high losses in the visible region and low fidelity and reproducibility in nanofabrication, making aluminum plasmonics non-ideal for applications. Herein, we report the experimental results of consistent surface plasmon propagation length measurements for epitaxially grown aluminum and silver films (epifilms), using three different methods (white light interferometry, laser scattering and spectroscopic ellipsometry) in the full visible spectrum. In order to avoid losses caused by inferior material quality, we used single-crystalline aluminum and silver films for direct comparison. We found that, on directly comparing with the silver epifilm, the aluminum epifilm possesses reasonably long plasmon propagation lengths in the full visible range and outperforms silver in the deep blue region. These results illustrate the great potential of epitaxial aluminum films for visible-spectrum plasmonic applications, resulting from their superior crystallinity and excellent surface and interface properties.

Comparative electron and photon excitation of localized surface plasmon resonance in lithographic gold arrays for enhanced Raman scattering

The ability to tune the localized surface plasmon resonance (LSPR) of nanostructures is desirable for surface enhanced Raman spectroscopy (SERS), plasmon-assisted chemistry and other nanophotonic applications. Although historically the LSPR is mainly studied by optical techniques, with the recent advancement in electron monochromators and correctors, it has attracted considerable attention in transmission electron microscopy (TEM). Here, we use electron energy loss spectroscopy (EELS) in a scanning TEM to study individual gold nanodiscs and bowties in lithographic arrays with variable LSPRs by adjusting the size, interspacing, shape and dielectric environment during the nanofabrication process. We observe the strongest Raman signal enhancement when the LSPR frequency is close to the incident laser frequency in Raman spectroscopy. A simplified harmonic oscillator model is used to estimate SERS enhancement factor (EF) from EELS, bridging the connection between electron and photon excitation of plasmonic arrays. This work demonstrates that STEM-EELS shows promise for revealing the contributions of specific LSPR modes to SERS EF. Our results provide guidelines to fine-tune nanoparticle parameters to deliver the maximum signal enhancement in biosensing applications, such as early cancer detection.

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.

Integration of Single-Photon Emitters in 2D Materials with Plasmonic Waveguides at Room Temperature

Efficient integration of a single-photon emitter with an optical waveguide is essential for quantum integrated circuits. In this study, we integrated a single-photon emitter in a hexagonal boron nitride (h-BN) flake with a Ag plasmonic waveguide and measured its optical properties at room temperature. First, we performed numerical simulations to calculate the efficiency of light coupling from the emitter to the Ag plasmonic waveguide, depending on the position and polarization of the emitter. In the experiment, we placed a Ag nanowire, which acted as the plasmonic waveguide, near the defect of the h-BN, which acted as the single-photon emitter. The position and direction of the nanowire were precisely controlled using a stamping method. Our time-resolved photoluminescence measurement showed that the single-photon emission from the h-BN flake was enhanced to almost twice the intensity as a result of the coupling with the Ag nanowire. We expect these results to pave the way for the practical implementation of on-chip nanoscale quantum plasmonic integrated circuits.

Characterisation and Manipulation of Polarisation Response in Plasmonic and Magneto-Plasmonic Nanostructures and Metamaterials

Optical properties of metal nanostructures, governed by the so-called localised surface plasmon resonance (LSPR) effects, have invoked intensive investigations in recent times owing to their fundamental nature and potential applications. LSPR scattering from metal nanostructures is expected to show the symmetry of the oscillation mode and the particle shape. Therefore, information on the polarisation properties of the LSPR scattering is crucial for identifying different oscillation modes within one particle and to distinguish differently shaped particles within one sample. On the contrary, the polarisation state of light itself can be arbitrarily manipulated by the inverse designed sample, known as metamaterials. Apart from polarisation state, external stimulus, e.g., magnetic field also controls the LSPR scattering from plasmonic nanostructures, giving rise to a new field of magneto-plasmonics. In this review, we pay special attention to polarisation and its effect in three contrasting aspects. First, tailoring between LSPR scattering and symmetry of plasmonic nanostructures, secondly, manipulating polarisation state through metamaterials and lastly, polarisation modulation in magneto-plasmonics. Finally, we will review recent progress in applications of plasmonic and magneto-plasmonic nanostructures and metamaterials in various fields.

Nanoplasmonic Nanorods/Nanowires from Single to Assembly: Syntheses, Physical Mechanisms and Applications

Gold nanorods are anisotropic and exhibit different optical characteristics in both transverse and longitudinal directions, so the plasmon resonance in the near-infrared region will reflect two absorption peaks. Because of strong enhancements of electromagnetic fields of gold nanorods, gold nanorods are widely used in medical treatment, biological detection, sensors, solar cells and other fields. Since rapid developments of gold nanorods, it is necessary to sort out the recent achievements. In this review, we select three classifications of single nanorods/nanowires, dimers and assembled nanorods to introduce their syntheses methods, optical properties and applications respectively. We firstly overview the history of nanorods/nanowires syntheses and summarize the improvement of the commonly utilized seed-mediated growth synthesis method; and then, physically, nano-plasmonic and optical properties of single and assembled nanorod/nanowires are concluded in detail. Lastly, we mainly summarize the recent advances in applications and provide perspective in different fields.

Near field excited state imaging via stimulated electron energy gain spectroscopy of localized surface plasmon resonances in plasmonic nanorod antennas

Continuous wave (cw) photon stimulated electron energy loss and gain spectroscopy (sEELS and sEEGS) is used to image the near field of optically stimulated localized surface plasmon resonance (LSPR) modes in nanorod antennas. An optical delivery system equipped with a nanomanipulator and a fiber-coupled laser diode is used to simultaneously irradiate plasmonic nanostructures in a (scanning) transmission electron microscope. The nanorod length is varied such that the m = 1, 2, and 3 LSPR modes are resonant with the laser energy and the optically stimulated near field spectra and images of these modes are measured. Various nanorod orientations are also investigated to explore retardation effects. Optical and electron beam simulations are used to rationalize the observed patterns. As expected, the odd modes are optically bright and result in observed sEEG responses. The m = 2 dark mode does not produce a sEEG response, however, when tilted such that retardation effects are operative, the sEEG signal emerges. Thus, we demonstrate that cw sEEGS is an effective tool in imaging the near field of the full set of nanorod plasmon modes of either parity.

Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

Plasmonics is a rapidly growing field spanning research and applications across chemistry, physics, optics, energy harvesting, and medicine. Ultrafast photoemission electron microscopy (PEEM) has demonstrated unprecedented power in the characterization of surface plasmons and other electronic excitations, as it uniquely combines the requisite spatial and temporal resolution, making it ideally suited for 3D space and time coherent imaging of the dynamical plasmonic phenomena on the nanofemto scale. The ability to visualize plasmonic fields evolving at the local speed of light on subwavelength scale with optical phase resolution illuminates old phenomena and opens new directions for growth of plasmonics research. In this review, we guide the reader thorough experimental description of PEEM as a characterization tool for both surface plasmon polaritons and localized plasmons and summarize the exciting progress it has opened by the ultrafast imaging of plasmonic phenomena on the nanofemto scale.

Visualizable detection of nanoscale objects using anti-symmetric excitation and non-resonance amplification

Why can we not see nanoscale objects under a light microscope? The textbook answers are that their relative signals are weak and their separation is smaller than Abbe’s resolution limit. Thus, significant effort has gone into developing ultraviolet imaging, oil and solid immersion objectives, nonlinear methods, fluorescence dyes, evanescent wave tailoring, and point-spread function engineering. In this work, we introduce a new optical sensing framework based on the concepts of electromagnetic canyons and non-resonance amplification, to directly view on a widefield microscope λ/31-scale (25-nm radius) objects in the near-field region of nanowire-based sensors across a 726-μm × 582-μm field of view. Our work provides a simple but highly efficient framework that can transform conventional diffraction-limited optical microscopes for nanoscale visualization. Given the ubiquity of microscopy and importance of visualizing viruses, molecules, nanoparticles, semiconductor defects, and other nanoscale objects, we believe our proposed framework will impact many science and engineering fields.

The authors introduce the concept of electromagnetic canyons and non-resonance amplification for optical detection of nanoscale objects. They demonstrate that a pair of nanowire sensors enable detection of 25-nm radii objects with a standard widefield microscope.

Plasmon-Driven Modulation of Reaction Pathways of Individual Pt-Modified Au Nanorods

Understanding the underlying kinetic mechanism of plasmon-enhanced catalysis is important for designing optimized bimetal nanostructures. Here, we characterize product formation rate at both the single-particle and ensemble level. The single-particle measurement allows us to reveal the underlying catalytic kinetic mechanisms of a bimetal nanostructure. Combining this with ensemble observations of two different catalytic behaviors of this catalyst with and without illumination shows that energetic charge carriers induce a transition from a competitive reactant adsorption type to a noncompetitive adsorption type, which leads to the suppression of catalytic rate decay at high reactant concentration. Theoretical modeling as well as analysis of hole acceptability of scavengers on Pt and Au surfaces indicates that the Pt light absorptivity is enhanced near Au and the energetic charges may form directly from the Pt part of the Au-Pt nanostructure. The presented study deepens our understanding of plasmon-enhanced catalysis by bimetal nanostructures.

Interferometric near-field characterization of plasmonic slot waveguides in single- and poly-crystalline gold films

Single-crystalline gold films show superior plasmonic properties compared to their poly-crystalline counterparts. However, this advantage comes at the cost of a more complex preparation process. It is thus crucial to validate whether the impact of the material quality on the performance of the respective plasmonic device justifies this additional effort. In order to address this question for the case of plasmonic slot waveguides, we present interferometric near-field measurements at telecommunication wavelengths on slot waveguides in single- and poly-crystalline gold films. We observe significantly larger propagation lengths in the case of single-crystalline gold films for slot widths below 100 nm. In contrast for larger widths, both gold films give rise to comparable propagation lengths.

Metallic Carbon Nanotube Nanocavities as Ultracompact and Low-loss Fabry-Perot Plasmonic Resonators

Plasmonic resonators enable deep subwavelength manipulation of light matter interactions and have been intensively studied both in fundamental physics as well as for potential technological applications. While various metallic nanostructures have been proposed as plasmonic resonators, their performances are rather limited at mid- and far-infrared wavelengths. Recently, highly confined and low-loss Luttinger liquid plasmons in metallic single-walled carbon nanotubes (SWNTs) have been observed at infrared wavelengths. Here, we tailor metallic SWNTs into ultraclean nanocavities by advanced scanning probe lithography and investigate plasmon modes in these individual nanocavities by infrared nanoimaging. The dependence of mode evolutions on cavity length and excitation wavelength can be captured by a Fabry-Perot resonator model of a plasmon nanowaveguide terminated by highly reflective ends. Plasmonic resonators based on SWNT nanocavities approach the ultimate plasmon confinement limit and open the door to the strong light-matter coupling regime, which may enable various nanophotonic applications.

Geometrical optimization for silver nanowire mesh as a flexible transparent conductive electrode

We report the effect of the geometric parameters on transparency and conductivity in a metallic nanowire mesh as a transparent electrode. Today, indium tin oxide and fluorine-doped tin oxide are used as the transparent electrode for displays and solar cells. Still, there is a definite need for their replacement due to drawbacks such as brittleness, scarcity, and adverse environmental effects. Metallic nanowire mesh is likely the best replacement option, but the main issue is how to find the optimal structure and how to get the best performance. Since the interaction of light with nanowire mesh is complicated, there is no straightforward rule with a simple analytical solution. We developed a kit based on wave optics for calculating the optical transmission of metallic nanowire mesh, which, unlike previous works, includes the interaction of light with the nanowire mesh, such as localized surface plasmon resonance (LSPR), surface plasmon polariton (SPP), and Rayleigh anomaly (RA). So, it is possible to accurately predict the effect of these phenomena and the transmission of mesh. Using the mentioned kit, we will be able to investigate the different geometrical structures of meshes to achieve optimal geometry. This kit is based on the classical Maxwell theory and empirical data and uses finite-difference time-domain for solving equations and experiential results for validation. Comparing the results by a redefined figure of merit shows that LSPR has the most significant reduction on transparency, whereas increasing the thickness (t) to width (w) ratio of the nanowire in the metallic mesh can reduce the LSPR effect and/or shifts it to the invisible region. The wire pitch (p) has no tangible impact on LSPR, but p can be chosen higher than 700 or lower than 350 nm to remove the extinction effects of the first-order RA. If p was larger than 150 nm, SPP could appear in the visible region of the spectrum. In small p, lower modes of SPP with higher intensities occur; therefore, there is an optimum value for p around 300 nm. The reduction of t and w reduces the intensity of SPP and causes it to red shift. By comparing the 900 different structures, the highest figure of merit is obtained in a p of 300 nm with a minimum w (10 nm) and maximum t (100 nm).

Principle and Applications of the Coupling of Surface Plasmons and Excitons

Surface plasmons have been attracting increasing attention and have been studied extensively in recent decades because of their half-light and half-material polarized properties. On the one hand, the tightly confined surface plasmonic mode may reduce the size of integrated optical devices beyond the diffraction limit; on the other hand, it provides an approach toward enhancement of the interactions between light and matter. In recent experiments, researchers have realized promising applications for surface plasmons in quantum information processing, ultra-low-power lasers, and micro-nano processing devices by using plasmonic structures, which have demonstrated their superiority over traditional optics structures. In this paper, we introduce the theoretical principle of surface plasmons and review the research work related to the interactions between plasmons and excitons. Some perspectives with regard to the future development of plasmonic coupling are also outlined.

Photothermal Imaging of Individual Nano-Objects with Large Scattering Cross Sections

Photothermal (PT) microscopy enables the efficient detection of absorbing nano-objects with high sensitivity and stability. The PT signal in the current PT microscopy usually comes from the interaction of the probe laser beam with the heating laser beam-induced thermal lens, and the contribution of the scattering field from the imaged nano-object is usually not taken into account. Here, in this paper, we systematically studied the influence of the scattering field from the imaged nanoparticles on the obtained PT signal by using Ag nanowires (NWs) on a glass substrate surrounded by glycerol as an example. Under the excitation of a heating laser beam at 532 nm wavelength, the rise of local temperature around the Ag NW results in the intensity variation of the interferometric scattering probe light at 730 nm wavelength which includes the scattering light from the Ag NW and the reflection light from the glass-glycerol interface. We found that the PT signal on the NW are positive and negative for the probe beam polarized parallel and perpendicular to the NW axis, respectively. Numerical simulations confirm that the heat-induced intensity variation of the pure scattering light from the NW and the thermal lens-induced intensity increase of the reflection light both contribute to the obtained PT signal. Our work provides the basic guidance for the analysis of PT signal from nano-objects with large scattering cross sections.

Facile fabrication and formation mechanism of aluminum nanowire arrays

Anodized alumina membranes (AAMs) have proven effective at making vertically-oriented and well-ordered metal nanowire arrays, which are useful in plasmonics and electrochemistry. Here, we produced Al nanowires via directed AAM pore nucleation: a patterned oxide mask on a flat Al surface directed where pores did and did not form, the pores acting to oxidize Al around the sites without pores. This left Al nanowires embedded in the AAM, and produced freestanding Al nanowires after etching the AAM. The nanowire tops had two distinct contours, smooth bowls and flat rough surfaces-suggesting that nanowires with bowl tops result from slow pore development relative to pattern-nucleated pores, not pore blockage as prior literature suggests. The observed low porosity of ∼2%, as opposed to the more typical 10%, suggests pore nucleation in the electrolyte employed may need greater local variations in electric field or pH, possibly explaining the electrolyte's peculiar ability to make Al nanowires. Finally, a soft nano-imprint lithography process was developed here to pattern the mask without damaging the stamp, avoiding a stamp degradation problem in previous work that utilized hard nano-imprint lithography.