Atomically flat single-crystalline gold nanostructures for plasmonic nanocircuitry

Controlling wave-vector of propagating surface plasmon polaritons on single-crystalline gold nanoplates

Surface plasmon polaritons (SPPs) propagating at metal nanostructures play an important role in breaking the diffraction limit. Chemically synthesized single-crystalline metal nanoplates with atomically flat surfaces provide favorable features compared with traditional polycrystalline metal films. The excitation and propagation of leaky SPPs on micrometer sized (10-20 μm) and thin (30 nm) gold nanoplates are investigated utilizing leakage radiation microscopy. By varying polarization and excitation positions of incident light on apexes of nanoplates, wave-vector (including propagation constant and propagation direction) distributions of leaky SPPs in Fourier planes can be controlled, indicating tunable SPP propagation. These results hold promise for potential development of chemically synthesized single-crystalline metal nanoplates as plasmonic platforms in future applications.

Giant colloidal silver crystals for low-loss linear and nonlinear plasmonics

The development of ultrasmooth, macroscopic-sized silver (Ag) crystals exhibiting reduced losses is critical to fully characterize the ultimate performance of Ag as a plasmonic material, and to enable cascaded and integrated plasmonic devices. Here we demonstrate the growth of single-crystal Ag plates with millimetre lateral sizes for linear and nonlinear plasmonic applications. Using these Ag crystals, surface plasmon polariton propagation lengths beyond 100 μm in the red wavelength region are measured. These lengths exceed the predicted values using the widely cited Johnson and Christy data. Furthermore, they allow the fabrication of highly reproducible plasmonic nanostructures by focused ion beam milling. We have designed and fabricated double-resonant nanogroove arrays using these crystals for spatially uniform and spectrally tunable second-harmonic generation. In conventional ‘hot-spot'-based nonlinear processes such as surface-enhanced Raman scattering and second-harmonic generation, strong enhancement can only occur in random, localized regions. In contrast, our approach enables uniform nonlinear signal generation over a large area.

The typical small sizes of silver microcrystals prevent the full characterization of fundamental plasmonic properties and limits applications. Here, Wang et al. report the synthesis of large colloidal silver crystals with superior nonlinear properties and surface plasmon polariton propagation lengths beyond 100 μm.

Origin and Future of Plasmonic Optical Tweezers

Plasmonic optical tweezers can overcome the diffraction limits of conventional optical tweezers and enable the trapping of nanoscale objects. Extension of the trapping and manipulation of nanoscale objects with nanometer position precision opens up unprecedented opportunities for applications in the fields of biology, chemistry and statistical and atomic physics. Potential applications include direct molecular manipulation, lab-on-a-chip applications for viruses and vesicles and the study of nanoscale transport. This paper reviews the recent research progress and development bottlenecks and provides an overview of possible future directions in this field.

A gold nanorod-based localized surface plasmon resonance platform for the detection of environmentally toxic metal ions

Gold nanorods (Au NRs) are elongated nanoparticles with unique optical properties which depend on their shape anisometry. The Au NR-based longitudinal localized surface plasmon resonance (longitudinal LSPR) band is very sensitive to the surrounding local environment and upon the addition of target analytes, the interaction between the analytes and the surface of the Au NRs leads to a change in the longitudinal LSPR band. This makes it possible to devise Au NR probes with application potential to the detection of toxic metal ions with an improved limit of detection, response time, and selectivity for the fabrication of sensing devices. The effective surface modification of Au NRs helps in improving their selectivity and sensitivity toward the detection of toxic metal ions. In this review, we discuss different methods for the preparation of surface modified Au NRs for the detection of toxic metal ions based on the LSPR band of the Au NRs and the types of interactions between the surface of Au NRs and metal ions. We summarize the work that has been done on Au NR-based longitudinal LSPR detection of environmentally toxic metal ions, sensing mechanisms, and the current progress in various modified Au NR-based longitudinal LSPR sensors for toxic metal ions. Finally, we discuss the applications of Au NR-based longitudinal LSPR sensors to real sample analysis and some of the future challenges facing longitudinal LSPR-based sensors for the detection of toxic metal ions toward commercial devices.

Mode matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation

Boosting nonlinear frequency conversion in extremely confined volumes remains a challenge in nano-optics research, but can enable applications in nanomedicine, photocatalysis and background-free biosensing. To obtain brighter nonlinear nanoscale sources, approaches that enhance the electromagnetic field intensity and counter the lack of phase matching in nanoplasmonic systems are often employed. However, the high degree of symmetry in the crystalline structure of plasmonic materials (metals in particular) and in nanoantenna designs strongly quenches second harmonic generation. Here, we describe doubly-resonant single-crystalline gold nanostructures with no axial symmetry displaying spatial mode overlap at both the excitation and second harmonic wavelengths. The combination of these features allows the attainment of a nonlinear coefficient for second harmonic generation of ∼5 × 10(-10) W(-1), enabling a second harmonic photon yield higher than 3 × 10(6) photons per second. Theoretical estimations point toward the use of our nonlinear plasmonic nanoantennas as efficient platforms for label-free molecular sensing.

Single-Crystalline Aluminum Nanostructures on a Semiconducting GaAs Substrate for Ultraviolet to Near-Infrared Plasmonics

Aluminum, as a metallic material for plasmonics, is of great interest because it extends the applications of surface plasmon resonance into the ultraviolet (UV) region and is superior to noble metals in natural abundance, cost, and compatibility with modern semiconductor fabrication processes. Ultrasmooth single-crystalline metallic films are beneficial for the fabrication of high-definition plasmonic nanostructures, especially complex integrated nanocircuits. The absence of surface corrugation and crystal boundaries also guarantees superior optical properties and applications in nanolasers. Here, we present UV to near-infrared plasmonic resonance of single-crystalline aluminum nanoslits and nanoholes. The high-definition nanostructures are fabricated with focused ion-beam milling into an ultrasmooth single-crystalline aluminum film grown on a semiconducting GaAs substrate with a molecular beam epitaxy method. The single-crystalline aluminum film shows improved reflectivity and reduced two-photon photoluminescence (TPPL) due to the ultrasmooth surface. Both linear scattering and nonlinear TPPL are studied in detail. The nanoslit arrays show clear Fano-like resonance, and the nanoholes are found to support both photonic modes and localized surface plasmon resonance. We also found that TPPL generation is more efficient when the excitation polarization is parallel rather than perpendicular to the edge of the aluminum film. Such a counterintuitive phenomenon is attributed to the high refractive index of the GaAs substrate. We show that the polarization of TPPL from aluminum preserves the excitation polarization and is independent of the crystal orientation of the film or substrate. Our study gains insight into the optical property of aluminum nanostructures on a high-index semiconducting GaAs substrate and illustrates a practical route to implement plasmonic devices onto semiconductors for future hybrid nanodevices.

Transformation of truncated gold octahedrons into triangular nanoprisms through the heterogeneous nucleation of silver

Described is a straightforward procedure for forming organized substrate-immobilized nanoprisms which are single crystalline, surfactant-free and which form a heteroepitaxial relationship with the underlying substrate. The devised route utilizes truncated Au octahedrons formed through solid state dewetting techniques as high temperature heterogeneous nucleation sites for Ag adatoms which are arriving to the substrate surface in the vapour phase. Observed is a morphological and compositional transformation of the Au structures to triangular nanoprisms comprised of a homogeneous AuAg alloy. During this transformation, the localized surface plasmon resonance red-shifts, broadens and increases in strength. The shape transformation, which cannot be rationalized using thermodynamic arguments dependent on the surface energy minimization, is described in terms of a kinetically driven growth mode, previously predicted by molecular dynamic simulations. The so-formed structures, when coated with a thin layer of Pd, are demonstrated as plasmonic sensing elements for hydrogen detection.

Plasmonic Films Can Easily Be Better: Rules and Recipes

High-quality materials are critical for advances in plasmonics, especially as researchers now investigate quantum effects at the limit of single surface plasmons or exploit ultraviolet- or CMOS-compatible metals such as aluminum or copper. Unfortunately, due to inexperience with deposition methods, many plasmonics researchers deposit metals under the wrong conditions, severely limiting performance unnecessarily. This is then compounded as others follow their published procedures. In this perspective, we describe simple rules collected from the surface-science literature that allow high-quality plasmonic films of aluminum, copper, gold, and silver to be easily deposited with commonly available equipment (a thermal evaporator). Recipes are also provided so that films with optimal optical properties can be routinely obtained.

Nonlinear photoluminescence spectrum of single gold nanostructures

We investigate the multiphoton photoluminescence characteristics of gold nanoantennas fabricated from single crystals and polycrystalline films. By exciting these nanostructures with ultrashort pulses tunable in the near-infrared range, we observe distinct features in the broadband photoluminescence spectrum. By comparing antennas of different crystallinity and shape, we demonstrate that the nanoscopic geometry of plasmonic devices determines the shape of the emission spectra. Our findings rule out the contribution of the gold band structure in shaping the photoluminescence.

Charge gradient-induced on-surface growth of ultralarge single-crystalline Ag nanomembranes for long surface plasmon propagation

The Ag nanomembrane with an atomically smooth surface and a large area has been prepared and demonstrated superior plasmonic properties.

A classical treatment of optical tunneling in plasmonic gaps: extending the quantum corrected model to practical situations

The optical response of plasmonic nanogaps is challenging to address when the separation between the two nanoparticles forming the gap is reduced to a few nanometers or even subnanometer distances. We have compared results of the plasmon response within different levels of approximation, and identified a classical local regime, a nonlocal regime and a quantum regime of interaction. For separations of a few Ångstroms, in the quantum regime, optical tunneling can occur, strongly modifying the optics of the nanogap. We have considered a classical effective model, so called Quantum Corrected Model (QCM), that has been introduced to correctly describe the main features of optical transport in plasmonic nanogaps. The basics of this model are explained in detail, and its implementation is extended to include nonlocal effects and address practical situations involving different materials and temperatures of operation.

Comparison of the plasmonic performances between lithographically fabricated and chemically grown gold nanorods

The plasmonic performances of lithographic and chemical gold nanorods are quantitatively examined and compared through both experiments and electrodynamic simulations.

Strongly coupled plasmonic modes on macroscopic areas via template-assisted colloidal self-assembly

We present ensembles of surface-ordered nanoparticle arrangements, which are formed by template-assisted self-assembly of monodisperse, protein-coated gold nanoparticles in wrinkle templates. Centimeter-squared areas of highly regular, linear assemblies with tunable line width are fabricated and their extinction cross sections can be characterized by conventional UV/vis/NIR spectroscopy. Modeling based on electrodynamic simulations shows a clear signature of strong plasmonic coupling with an interparticle spacing of 1-2 nm. We find evidence for well-defined plasmonic modes of quasi-infinite chains, such as resonance splitting and multiple radiant modes. Beyond elementary simulations on the individual chain level, we introduce an advanced model, which considers the chain length distribution as well as disorder. The step toward macroscopic sample areas not only opens perspectives for a range of applications in sensing, plasmonic light harvesting, surface enhanced spectroscopy, and information technology but also eases the investigation of hybridization and metamaterial effects fundamentally.

High speed e-beam lithography for gold nanoarray fabrication and use in nanotechnology

E-beam lithography has been used for reliable and versatile fabrication of sub-15 nm single-crystal gold nanoarrays and led to convincing applications in nanotechnology. However, so far this technique was either too slow for centimeter to wafer-scale writing or fast enough with the so-called dot on the fly (DOTF) technique but not optimized for sub-15 nm dots dimension. This prevents use of this technology for some applications and characterization techniques. Here, we show that the DOTF technique can be used without degradation in dots dimension. In addition, we propose two other techniques. The first one is an advanced conventional technique that goes five times faster than the conventional one. The second one relies on sequences defined before writing which enable versatility in e-beam patterns compared to the DOTF technique with same writing speed. By comparing the four different techniques, we evidence the limiting parameters for the writing speed. Wafer-scale fabrication of such arrays with 50 nm pitch allowed XPS analysis of a ferrocenylalkyl thiol self-assembled monolayer coated gold nanoarray.

The modulation effect of transverse, antibonding, and higher-order longitudinal modes on the two-photon photoluminescence of gold plasmonic nanoantennas

Plasmonic nanoantennas exhibit various resonant modes with distinct properties. Upon resonant excitation, plasmonic gold nanoantennas can generate strong two-photon photoluminescence (TPPL). The TPPL from gold is broadband and depolarized, and may serve as an ideal local source for the investigation of antenna eigenmodes. In this work, TPPL spectra of three arrays of single-crystalline gold nanoantennas are comprehensively investigated. We carefully compare the TPPL spectra with dark-field scattering spectra and numerically simulated spectra. We show the modulation effect of the transverse resonant mode and the nonfundamental longitudinal mode on the TPPL spectrum. We also demonstrate suppression of TPPL due to the subradiant antibonding modes and study the influence of antenna resonant modes on the overall TPPL yield. Our work provides direct experimental evidence on nanoantenna-mediated near-to-far-field energy coupling and gains insight into the emission spectrum of the TPPL from gold nanoantennas.

Hybridization effects on the out-of-plane electron tunneling properties of monolayers: is h-BN more conductive than graphene?

Electron transport properties through multilayers of hexagonal boron nitride (h-BN) sandwiched between gold electrodes is investigated by density functional theory together with the non-equilibrium Green's function method. The calculated results find that despite graphene being a gapless semimetal and h-BN two-dimensional layer being an insulator, the transmission function perpendicular to the atomic layer plane in both systems is nearly identical. The out-of-plane tunnel current is found to be strongly dependent on the interaction at the interface of the device. As a consequence, single layer h-BN coupled with atomically flat weakly interacting metals such as gold may not work as a good dielectric material, but the absence of sharp resonances would probably lead to more stable out-of-plane electronic transport properties compared to graphene.

Toward plasmonics with nanometer precision: nonlinear optics of helium-ion milled gold nanoantennas

Plasmonic nanoantennas are versatile tools for coherently controlling and directing light on the nanoscale. For these antennas, current fabrication techniques such as electron beam lithography (EBL) or focused ion beam (FIB) milling with Ga(+)-ions routinely achieve feature sizes in the 10 nm range. However, they suffer increasingly from inherent limitations when a precision of single nanometers down to atomic length scales is required, where exciting quantum mechanical effects are expected to affect the nanoantenna optics. Here, we demonstrate that a combined approach of Ga(+)-FIB and milling-based He(+)-ion lithography (HIL) for the fabrication of nanoantennas offers to readily overcome some of these limitations. Gold bowtie antennas with 6 nm gap size were fabricated with single-nanometer accuracy and high reproducibility. Using third harmonic (TH) spectroscopy, we find a substantial enhancement of the nonlinear emission intensity of single HIL-antennas compared to those produced by state-of-the-art gallium-based milling. Moreover, HIL-antennas show a vastly improved polarization contrast. This superior nonlinear performance of HIL-derived plasmonic structures is an excellent testimonial to the application of He(+)-ion beam milling for ultrahigh precision nanofabrication, which in turn can be viewed as a stepping stone to mastering quantum optical investigations in the near-field.

HNO₃-assisted polyol synthesis of ultralarge single-crystalline Ag microplates and their far propagation length of surface plasmon polariton

We developed a HNO3-assisted polyol reduction method to synthesize ultralarge single-crystalline Ag microplates routinely. The edge length of the synthesized Ag microplates reaches 50 μm, and their top facets are (111). The mechanism for dramatically enlarging single-crystalline Ag structure stems from a series of competitive anisotropic growths, primarily governed by carefully tuning the adsorption of Ag(0) by ethylene glycol and the desorption of Ag(0) by a cyanide ion on Ag(100). Finally, we measured the propagation length of surface plasmon polaritons along the air/Ag interface under 534 nm laser excitation. Our single-crystalline Ag microplate exhibited a propagation length (11.22 μm) considerably greater than that of the conventional E-gun deposited Ag thin film (5.27 μm).

Mode conversion in high-definition plasmonic optical nanocircuits

Symmetric and antisymmetric guided modes on a plasmonic two-wire transmission line have distinct properties and are suitable for different circuit functions. Being able to locally convert the guided modes is important for realizing multifunctional optical nanocircuits. Here, we experimentally demonstrate successful local conversion between the symmetric and the antisymmetric modes in a single-crystalline gold plasmonic nanocircuit with an optimally designed mode converter for optical signals at 194.2 THz. Mode conversion may find applications in controlling nanoscale light-matter interaction.

Encapsulated annealing: enhancing the plasmon quality factor in lithographically-defined nanostructures

Lithography provides the precision to pattern large arrays of metallic nanostructures with varying geometries, enabling systematic studies and discoveries of new phenomena in plasmonics. However, surface plasmon resonances experience more damping in lithographically–defined structures than in chemically–synthesized nanoparticles of comparable geometries. Grain boundaries, surface roughness, substrate effects, and adhesion layers have been reported as causes of plasmon damping, but it is difficult to isolate these effects. Using monochromated electron energy–loss spectroscopy (EELS) and numerical analysis, we demonstrate an experimental technique that allows the study of these effects individually, to significantly reduce the plasmon damping in lithographically–defined structures. We introduce a method of encapsulated annealing that preserves the shape of polycrystalline gold nanostructures, while their grain-boundary density is reduced. We demonstrate enhanced Q–factors in lithographically–defined nanostructures, with intrinsic damping that matches the theoretical Drude damping limit.

(Gold nanorod core)/(polyaniline shell) plasmonic switches with large plasmon shifts and modulation depths

(Gold nanorod core)/(polyaniline shell) nanostructures are prepared for functioning as active plasmonic switches. The single core/shell nanostructures exhibit a remarkable switching performance, with the modulation depth and scattering peak shift reaching 10 dB and 100 nm, respectively. The nanostructures are also deposited on substrates to form macroscale monolayers with remarkable ensemble plasmonic switching performances.

Enhancing nanoparticle electrodynamics with gold nanoplate mirrors

Mirrors and optical cavities can modify and enhance matter-radiation interactions. Here we report that chemically synthesized Au nanoplates can serve as micrometer-size mirrors that enhance electrodynamic interactions. Because of their plasmonic properties, the Au nanoplates enhance the brightness of scattered light from Ag nanoparticles near the nanoplate surface in dark-field microscopy. More importantly, enhanced optical trapping and optical binding of Ag nanoparticles are demonstrated in interferometric optical traps created from a single laser beam and its reflection from individual Au nanoplates. The enhancement of the interparticle force constant is ≈20-fold more than expected from the increased intensity due to standing wave interference. We show that the additional stability for optical binding arises from the restricted axial thermal motion of the nanoparticles that couples to and reduces the fluctuations in the lateral plane. This new mechanism greatly advances the photonic synthesis of ultrastable nanoparticle arrays and investigation of their properties.

Shape-selective deposition and assembly of anisotropic nanoparticles

We report the large-area assembly of anisotropic gold nanoparticles into lithographically defined templates with control over their angular position using a capillary force-based approach. We elucidate the role of the geometry of the templates in the assembly of anisotropic nanoparticles consisting of different shapes and sizes. These insights allow us to design templates that immobilize individual triangular nanoprisms and concave nanocubes in a shape-selective manner and filter undesired impurity particles from a mixture of triangular prisms and other polyhedra. Furthermore, by studying the assembly of two particles in the same template, we elucidate the importance of interparticle forces in this method. These advances allow for the construction of face-to-face and edge-to-edge nanocube dimers as well as triangular nanoprism bowtie antennas. As an example of the fundamental studies enabled by this assembly method, we investigate the surface-enhanced Raman scattering (SERS) of face-to-face concave cube dimers both experimentally and computationally and reveal a strong polarization dependence of the local field enhancement.

Tip-enhanced near-field optical microscopy

Tip-enhanced near-field optical microscopy (TENOM) is a scanning probe technique capable of providing a broad range of spectroscopic information on single objects and structured surfaces at nanometer spatial resolution and with highest detection sensitivity. In this review, we first illustrate the physical principle of TENOM that utilizes the antenna function of a sharp probe to efficiently couple light to excitations on nanometer length scales. We then discuss the antenna-induced enhancement of different optical sample responses including Raman scattering, fluorescence, generation of photocurrent and electroluminescence. Different experimental realizations are presented and several recent examples that demonstrate the capabilities of the technique are reviewed.

A twin-free single-crystal Ag nanoplate plasmonic platform: hybridization of the optical nano-antenna and surface plasmon active surface

Surface plasmons based on metallic nanostructures enable light manipulation beyond the optical diffraction limit. We have epitaxially synthesized twin-free single-crystal Ag nanoplates on SrTiO3 substrates. Unlike the nanoplates synthesized in a solution phase, these nanoplates have perfectly clean surfaces as well as a quite large size of tens of micrometers. As-synthesized defect-free single-crystal Ag nanoplates have an atomically flat surface and sides with well-defined angles, allowing long distance propagation of surface plasmons and highly reliable plasmonic integration. By spatially separating receiving and transmitting antennas and plasmonically interfacing them, the signal quality of transmission/reception can be largely improved. Furthermore, by combining sub-dimensional nanostructures onto the two-dimensional space effective hierarchical plasmonic nano-complexes can be built up. Theoretical simulations well reproduced unique experimental results of coupling between SPPs and free-space radiation by the nanoplate antenna sides, low-loss long-range SPP propagation, and tunneling or scattering of SPPs at a nano-gap as well as a nano-structure introduced on the nanoplate. The single-crystal Ag nanoplate will find superb applications in plasmonic nano-circuitry and lab-on-a-chip for biochemical sensing.

Direct coupling of photonic modes and surface plasmon polaritons observed in 2-photon PEEM

We report the direct microscopic observation of optical energy transfer from guided photonic modes in an indium tin oxide (ITO) thin film to surface plasmon polaritons (SPP) at the surfaces of a single crystalline gold platelet. The photonic and SPP modes appear as an interference pattern in the photoelectron emission yield across the surface of the specimen. We explore the momentum match between the photonic and SPP modes in terms of simple waveguide theory and the three-layer slab model for bound SPP modes of thin metal films. We show that because the gold is thin (30-40 nm), two SPP modes exist and that momentum of the spatially confined asymmetric field mode coincides with the dominant mode of the ITO waveguide. The results demonstrate that photoemission electron microscopy (PEEM) can be an important tool for the observation of photonic to SPP interactions in the study of integrated photonic circuits.

Evaluating plasmonic transport in current-carrying silver nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support. In this context, metal nanowires are especially desirable for realizing dense routing networks. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires randomly distributed on a glass substrate. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.

Multimode plasmon excitation and in situ analysis in top-down fabricated nanocircuits

We experimentally demonstrate synthesis and in situ analysis of multimode plasmonic excitations in two-wire transmission lines supporting a symmetric and an antisymmetric eigenmode. To this end we irradiate an incoupling antenna with a diffraction-limited excitation spot exploiting a polarization- and position-dependent excitation efficiency. Modal analysis is performed by recording the far-field emission of two mode-specific spatially separated emission spots at the far end of the transmission line. To illustrate the power of the approach we selectively determine the group velocities of symmetric and antisymmetric contributions of a multimode ultrafast plasmon pulse.

Helium focused ion beam fabricated plasmonic antennas with sub-5 nm gaps

We demonstrate a reliable fabrication method to produce plasmonic dipole nanoantennas with gap values in the range of 3.5-20 nm. The method combines electron beam lithography to create gold nanorods and helium focused ion beam milling to cut the gaps. Results show a reproducibility within 1 nm. Scattering spectra of antennas show a red shift of resonance wavelengths and an increase of the intensity of resonance peaks with a decrease of the gap size, which is in agreement with finite element simulations. The measured refractive index sensitivity was about 250 nm per refractive index unit for antennas with gap values below 5 nm.

Gold nanorods and their plasmonic properties

Gold nanorods have been receiving extensive attention owing to their extremely attractive applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices. The growth methods and plasmonic properties of Au nanorods have therefore been intensively studied. In this review, we present a comprehensive overview of the flourishing field of Au nanorods in the past five years. We will focus mainly on the approaches for the growth, shape and size tuning, functionalization, and assembly of Au nanorods, as well as the methods for the preparation of their hybrid structures. The plasmonic properties and the associated applications of Au nanorods will also be discussed in detail.

Seed-mediated growth of ultralong gold nanorods and nanowires with a wide range of length tunability

This study reports a systematic approach to synthesize ultralong gold nanorods and nanowires using a seed-mediated growth approach. In the first series, the effect of growth solution pH on the lengths of nanorods prepared was investigated. Interestingly, although shorter rods (230-310 nm) were produced in a basic solution environment than in an acidic condition (330-410 nm), the nanorod yield is greatly improved with relatively few nanoplate byproducts formed. Nanorod growth proceeds quickly in a basic solution as evidenced by the fast solution color changes. By adjusting several experimental parameters with the aim to elongate the nanorod length in a tunable fashion, gold nanorods and nanowires with average lengths from 580 to 2850 nm can be synthesized by progressively increasing the HNO3 concentration in the final growth solution. Nanowire growth in a highly acidic solution is slower, and a substantially longer time is needed to reach long lengths. Further extension of the nanowire length can be achieved simply by reducing the volume of second growth solution transferred to the final growth solution. Nanorods and nanowires with lengths spanning from 700 nm to 4.5 μm were prepared in this series of experimental conditions. The longest nanowires can reach a length of up to 6 μm. The nanowires still maintain thin average diameters of 33-53 nm. The ability to make gold nanorods and nanowires over this exceptionally wide and useful length range is exciting because applications and demonstrations using ultralong gold nanorods and nanowires of most suitable lengths are now possible.

Detecting 20 nm wide defects in large area nanopatterns using optical interferometric microscopy

Due to the diffraction limited resolution and the presence of speckle noise, visible laser light is generally thought to be impractical for finding deep subwavelength defects in patterned semiconductor wafers. Here, we report on a nondestructive low-noise interferometric imaging method capable of detecting nanoscale defects within a wide field of view using visible light. The method uses a common-path laser interferometer and a combination of digital image processing techniques to produce 70 μm by 27 μm panoramic phase and amplitude images of the test nanopattern. Significant noise reduction and high sensitivity are achieved, which enables successful detection of several different types of sparse defects with sizes on the order of 20 nm wide by 100 nm long by 110 nm tall.

Electrochemical etching of gold within nanoshaved self-assembled monolayers

Wet etching of metal substrates with patterned self-assembled monolayers (SAMs) is an inexpensive and convenient method to produce metal nanostructures. For this method to be relevant to the fabrication of high precision plasmonic structures, the kinetics of nanoscale etching process, particularly in the lateral direction, must be elucidated and controlled. We herein describe an in situ atomic force microscopy (AFM) study to characterize the etching process within patterned SAMs with nanometer resolution and in real time. The in situ study was enabled by several unique elements, including single crystalline substrates to minimize the variability of facet-dependent etch rate, high-resolution nanoshaved SAM patterns, electrochemical-potential-controlled etching, and AFM kymographs to improve temporal resolution. Our approach has successfully quantified the extent of both lateral etching and vertical etching at different potentials. Our study reveals the presence of an induction period prior to the onset of significant lateral etching, which would be difficult to observe with the limited time resolution and sample-to-sample variation of ex situ studies. By increasing the vertical etch rate during this induction period with higher potentials, gold was etched up to 40 nm in the vertical direction with minimal lateral etching. High-resolution etching was also demonstrated on single crystal gold microplates, which are high quality gold thin films suitable for plasmonics studies.

Aligned Linear Arrays of Crystalline Nanoparticles

Fabrication of one-dimensional arrays of crystalline nanoparticles with tunable particle size and spacing (down to 20 nm) is demonstrated. The individual nanocrystals are pentagonal prisms, and the arrays are up to 11 μm in length, with some arrays containing >50 nanocrystals. Precise particle morphology and interparticle spacing can be maintained down the array. The far-field scattering spectra of the arrays show the near-fields of the nanocrystals are coupled. The method is fast and produces precise, well-defined, coupled plasmonic arrays with optical properties that match well to theory.

Reaching the theoretical resonance quality factor limit in coaxial plasmonic nanoresonators fabricated by helium ion lithography

Optical antenna structures have revolutionized the field of nano-optics by confining light to deep subwavelength dimensions for spectroscopy and sensing. In this work, we fabricated coaxial optical antennae with sub-10-nanometer critical dimensions using helium ion lithography (HIL). Wavelength dependent transmission measurements were used to determine the wavelength-dependent optical response. The quality factor of 11 achieved with our HIL fabricated structures matched the theoretically predicted quality factor for the idealized flawless gold resonators calculated by finite-difference time-domain (FDTD). For comparison, coaxial antennae with 30 nm critical dimensions were fabricated using both HIL and the more common Ga focus ion beam lithography (Ga-FIB). The quality factor of the Ga-FIB resonators was 60% of the ideal HIL results for the same design geometry due to limitations in the Ga-FIB fabrication process.

Size-dependent permittivity and intrinsic optical anisotropy of nanometric gold thin films: a density functional theory study

Physical properties of materials are known to be different from the bulk at the nanometer scale. In this context, the dependence of optical properties of nanometric gold thin films with respect to film thickness is studied using density functional theory (DFT). We find that the in-plane plasma frequency of the gold thin film decreases with decreasing thickness and that the optical permittivity tensor is highly anisotropic as well as thickness dependent. Quantitative knowledge of planar metal film permittivity's thickness dependence can improve the accuracy and reliability of the designs of plasmonic devices and electromagnetic metamaterials. The strong anisotropy observed may become an alternative method of realizing indefinite media.

Tailoring and imaging the plasmonic local density of states in crystalline nanoprisms

Surface plasmon (SP) technologies exploit the spectral and spatial properties of collective electronic oscillations in noble metals placed in an incident optical field. Yet the SP local density of states (LDOS), which rule the energy transducing phenomena between the SP and the electromagnetic field, is much less exploited. Here, we use two-photon luminescence (TPL) microscopy to reveal the SP-LDOS in thin single-crystalline triangular gold nanoprisms produced by a quantitative one-pot synthesis at room temperature. Variations of the polarization and the wavelength of the incident light redistribute the TPL intensity into two-dimensional plasmonic resonator patterns that are faithfully reproduced by theoretical simulations. We demonstrate that experimental TPL maps can be considered as the convolution of the SP-LDOS with the diffraction-limited Gaussian light beam. Finally, the SP modal distribution is tuned by the spatial coupling of nanoprisms, thus allowing a new modal design of plasmonic information processing devices.

Contrast between surface plasmon polariton-mediated extraordinary optical transmission behavior in epitaxial and polycrystalline Ag films in the mid- and far-infrared regimes

In this Letter we report a comparative study, in the infrared regime, of surface plasmon polariton (SPP) propagation in epitaxially grown Ag films and in polycrystalline Ag films, all grown on Si substrates. Plasmonic resonance features are analyzed using extraordinary optical transmission (EOT) measurements, and SPP band structures for the two dielectric/metal interfaces are investigated for both types of film. At the Si/Ag interface, EOT spectra show almost identical features for epitaxial and polycrystalline Ag films and are characterized by sharp Fano resonances. On the contrary, at the air/Ag interface, dramatic differences are observed: while the epitaxial film continues to exhibit sharp Fano resonances, the polycrystalline film shows only broad spectral features and much lower transmission intensities. In corroboration with theoretical simulations, we find that surface roughness plays a critical role in SPP propagation for this wavelength range.

Reflective interferometry for optical metamaterial phase measurements

The unambiguous determination of optical refractive indices of metamaterials is a challenging task for device applications and the study of new optical phenomena. We demonstrate here simple broadband phase measurements of metamaterials using spectrally and spatially resolved interferometry. We study the phase response of a π-shaped metamaterial known to be an analog to electromagnetically induced transparency. The measured broadband interferograms give the phase delay or advance produced by the metamaterial in a single measurement. The presented technique offers an effective way of characterizing optical metamaterials including nonlinear and gain-metamaterial systems.

Plasmonic mode converter for controlling optical impedance and nanoscale light-matter interaction

To enable multiple functions of plasmonic nanocircuits, it is of key importance to control the propagation properties and the modal distribution of the guided optical modes such that their impedance matches to that of nearby quantum systems and desired light-matter interaction can be achieved. Here, we present efficient mode converters for manipulating guided modes on a plasmonic two-wire transmission line. The mode conversion is achieved through varying the path length, wire cross section and the surrounding index of refraction. Instead of pure optical interference, strong near-field coupling of surface plasmons results in great momentum splitting and modal profile variation. We theoretically demonstrate control over nanoantenna radiation and discuss the possibility to enhance nanoscale light-matter interaction. The proposed converter may find applications in surface plasmon amplification, index sensing and enhanced nanoscale spectroscopy.

Toward plasmonic polymers

We establish the concept of a plasmonic polymer, whose collective optical properties depend on the repeat unit. Experimental and theoretical analyses of the super- and sub- radiant plasmon response of plasmonic polymers comprising repeat units of single nanoparticles or dimers of gold nanoparticles show that (1) the redshift of the lowest energy coupled mode becomes minimal as the chain approaches the infinite chain limit at a length of ∼10 particles, (2) the presence and energy of the modes are sensitive to the geometries of the constituents, that is, repeat unit, but (3) spatial disorder and nanoparticle heterogeneity have only small effects on the super-radiant mode.

Electrically connected resonant optical antennas

Electrically connected resonant optical antennas hold promise for the realization of highly efficient nanoscale electro-plasmonic devices that rely on a combination of electric fields and local near-field intensity enhancement. Here we demonstrate the feasibility of such a concept by attaching leads to the arms of a two-wire antenna at positions of minimal near-field intensity with negligible influence on the antenna resonance. White-light scattering experiments in accordance with simulations show that the optical tunability of connected antennas is fully retained. Analysis of the electric properties demonstrates that in the antenna gaps direct current (DC) electric fields of 10(8) V/m can consistently be achieved and maintained over extended periods of time without noticeable damage.

Plasmonic nanolaser using epitaxially grown silver film

A nanolaser is a key component for on-chip optical communications and computing systems. Here, we report on the low-threshold, continuous-wave operation of a subdiffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium. The atomic smoothness of the metallic film is crucial for reducing the modal volume and plasmonic losses. Bimodal lasing with similar pumping thresholds was experimentally observed, and polarization properties of the two modes were used to unambiguously identify them with theoretically predicted modes. The all-epitaxial approach opens a scalable platform for low-loss, active nanoplasmonics.

Single-crystalline silver films for plasmonics

A simple route to flat, large-area, single-crystalline films for plasmonics is demonstrated by sputter deposition of silver onto mica substrates at elevated temperatures. The films exhibit improved dielectric properties and allow more precise patterning of high-quality nanostructures for plasmonic applications.

Adiabatic nanofocusing on ultrasmooth single-crystalline gold tapers creates a 10-nm-sized light source with few-cycle time resolution

We demonstrate adiabatic nanofocusing of few-cycle light pulses using ultrasharp and ultrasmooth single-crystalline gold tapers. We show that the grating-induced launching of spectrally broad-band surface plasmon polariton wavepackets onto the shaft of such a taper generates isolated, point-like light spots with 10 fs duration and 10 nm diameter spatial extent at its very apex. This nanofocusing is so efficient that nanolocalized electric fields inducing strong optical nonlinearities at the tip end are reached with conventional high repetition rate laser oscillators. We use here the resulting second harmonic to fully characterize the time structure of the localized electric field in frequency-resolved interferometric autocorrelation measurements. Our results strongly suggest that these nanometer-sized ultrafast light spots will enable new experiments probing the dynamics of optical excitations of individual metallic, semiconducting, and magnetic nanostructures.