Silver nanowires as surface plasmon resonators

Tailoring radiative and non-radiative losses of thin nanostructured plasmonic waveguides

Thin nanostructured metal films allow to control radiative and non-radiative losses of surface plasmon polariton modes without changing their group velocities. This effect is studied in plasmonic waveguides made of thin gold films drilled with very narrow slits and deposited on a GaAs substrate. The analysis is supported by high-resolution angle-resolved transmission measurements and rigorous electromagnetic calculations. We show that the excitation of air/gold and gold/GaAs surface waves leads to Fano-type resonances with specific light localization into the slits. As a result, gold/GaAs surface waves induce a modulation of radiative and non-radiative losses of air/gold surface waves. The minimum and maximum of the Fano-type resonance introduce two propagation regimes. In the radiative propagation regime, the losses due to the absorption are negligible, whereas an efficient inhibition of free-space coupling is demonstrated in low-loss propagation regime.

Subdiffraction limited, remote excitation of surface enhanced Raman scattering

We demonstrate that focused laser excitation at the end of silver nanowires of 50-150 nm diameter excites SERS hot-spots at points of nanoparticle adsorption many micrometers along the wire due to the plasmon waveguide effect. The total SERS intensity detected at the hot-spots following wire-end excitation correlates with the known wavelength, polarization, and distance dependences of surface plasmon polariton (SPP) propagation in nanowires. The SERS spectra obtained at the hot-spots following wire-end excitation show very little background compared to when excitation occurs directly at the hot-spot, suggesting that a much smaller SERS excitation volume is achieved by remote, waveguide excitation. The ability to transfer SERS excitation over several micrometers, through a structure with a subdiffraction limit diameter, is discussed with respect to potential high-resolution SERS imaging applications.

Integration of photonic and silver nanowire plasmonic waveguides

Future optical data transmission modules will require the integration of more than 10,000 x 10,000 input and output channels to increase data transmission rates and capacity. This level of integration, which greatly exceeds that of a conventional diffraction-limited photonic integrated circuit, will require the use of waveguides with a mode confinement below the diffraction limit, and also the integration of these waveguides with diffraction-limited components. We propose to integrate multiple silver nanowire plasmonic waveguides with polymer optical waveguides for the nanoscale confinement and guiding of light on a chip. In our device, the nanowires lay perpendicular to the polymer waveguide with one end inside the polymer. We theoretically predict and experimentally demonstrate coupling of light into multiple nanowires from the same waveguide, and also demonstrate control over the degree of coupling by changing the light polarization.

Sublinear and superlinear photoluminescence from Nd doped anodic aluminum oxide templates loaded with Ag nanowires

We first time prepared Nd(3+) ions doped anodic aluminum oxide (Nd:AAO) templates, reported linear, sublinear and superlinear photoluminescence (PL) from Nd:AAO templates loaded with Ag nanowires in different excitation power regions, in which, the excitation laser with wavelength 805 nm resonantly pumped the population to (4)F5/2 states of Nd(3+), and the radiative transitions (4)F(3/2) -->(4)I(9/2) of Nd(3+) centered at 880 nm. The excitation power dependences of emission polarization ratio and the spectral width were also investigated. The observed nonlinear amplifications of the PL intensity implied strong interaction between randomly-dispersed Nd(3+) ions and ordered-arrayed Ag nanowires in AAO templates.

Spectral properties of plasmonic resonator antennas

A theoretical study of the optical properties of metallic nano-strip antennas is presented. Such strips exhibit retardation-based resonances resulting from the constructive interference of counter propagating short-range surface plasmon-polaritons (SR-SPPs) that reflect from the antenna terminations. A Fabry-P erot model was formulated that successfully predicts both the peak position and spectral shape of their optical resonances. This model requires knowledge of the SR-SPP reflection amplitude and phase pickup upon reflection from the structure terminations. These quantities were first estimated using an intuitive Fresnel reflection model and then calculated exactly using full-field simulations based on the finite-difference frequency-domain (FDFD) method. With only three dimensionless scaling parameters, the Fabry-P erot model provides simple design rules for engineering resonant properties of such plasmonic resonator antennas.

Plasmon resonance in silver nanoparticles arrays grown by atomic terrace low-angle shadowing

The plasmonic properties of aligned silver nanoparticles grown by atomic terrace low-angle shadowing (ATLAS) were studied by optical transmission spectroscopy. ATLAS is a novel nanofabrication technique that combines the advantages of self-assembly with control of the nanostructure features. The silver nanoarrays were optically characterized and distinct optical responses with strong polarization dependence were obtained indicating the existence of a strong near-field coupling between the particles. We found that different morphologies gave rise to a similar optical response due to the local contributions to the overall response. Numerical model based on discrete dipole approximation was used to give a qualitative insight into the studied phenomena.

Optical near-field mapping of plasmonic nanoprisms

The optical local-field enhancement on nanometer length scales provides the basis for plasmonic metal nanostructures to serve as molecular sensors and as nanophotonic devices. However, particle morphology and the associated surface plasmon resonance alone do not uniquely reflect the important details of the local field distribution. Here, we use interferometric homodyne tip-scattering near-field microscopy for plasmonic near-field imaging of crystalline triangular silver nanoprisms. Strong spatial field variation on lengths scales as short as 20 nm are observed sensitively depending on structural details and environment. The poles of the dipole and quadrupole plasmon modes, as identified by phase-sensitive probing and calculations performed in the discrete dipole approximation (DDA), reflect the particle symmetry. Together with the observation that the largest enhancement is not necessarily found to be associated with the tips of the nanoprisms, our results provide critical information for the selection of particle geometries as building blocks for plasmonic device applications.

Spectroscopic mode mapping of resonant plasmon nanoantennas

We present spatially resolved spectral mode mapping of resonant plasmon gap antennas using two-photon luminescence microspectroscopy. The obtained maps are in good agreement with 3D calculations of the antenna modes. The evolution of the modal field with wavelength, both in the gap and along the two coupled gold nanowires forming the antenna, is directly visualized. At resonance, the luminescence for the gap area is enhanced at least 80 times and a comparison with the antenna extremities shows a dynamical charge redistribution due to the near-field coupling between the two arms.

Fabrication of surface plasmon resonators by nanoskiving single-crystalline gold microplates

This paper demonstrates the sectioning of chemically synthesized, single-crystalline microplates of gold with an ultramicrotome (nanoskiving) to produce single-crystalline nanowires; these nanowires act as low-loss surface plasmon resonators. This method produces collinearly aligned nanostructures with small, regular changes in dimension with each consecutive cross-section: a single microplate thus can produce a number of "quasi-copies" (delicately modulated variations) of a nanowire. The diamond knife cuts cleanly through microplates 35 microm in diameter and 100 nm thick without bending the resulting nanowire and cuts through the sharp edges of a crystal without deformation to generate nanoscale tips. This paper compares the influence of sharp tips and blunt tips on the resonator modes in these nanowires.

A hybrid plasmonic-photonic nanodevice for label-free detection of a few molecules

Noble metal nanowaveguides supporting plasmon polariton modes are able to localize the optical fields at nanometer level for high sensitivity biochemical sensing devices. Here we report on the design and fabrication of a novel photonic-plasmonic device which demonstrates label-free detection capabilities on single inorganic nanoparticles and on monolayers of organic compounds. In any case, we determine the Raman scattering signal enhancement and the device detection limits that reach a number of molecules between 10 and 250. The device can be straightforwardly integrated in a scanning probe apparatus with the possibility to match topographic and label-free spectroscopic information in a wide range of geometries.

Optoelectronic property modeling of carbon nanotubes grafted with gold nanoparticles

A three-dimensional (3D) electrodynamic model is built using the finite-difference time-domain (FDTD) method to investigate the optical response of carbon nanotubes grafted with gold nanoparticles. Theoretical characterizations suggest an anisotropic response, in line with previously observed absorption peaks of such systems in the optical range. An investigation of geometric and wavelength dependences is conducted, predicting the ability to tune the sub-wavelength intensity enhancement for efficient localization and propagation. The support of electric field enhancement along the nanotube walls raises the possibility of utilizing such systems as plasmon generators and waveguides for optical signal propagation.

Intraparticle surface plasmon coupling in quasi-one-dimensional nanostructures

The optical properties of two-component quasi-one-dimensional nanostructures consisting of Au and Ni blocks have been investigated. The optically inactive component Ni plays a relaying role in the surface plasmon coupling both for the dipole mode and for the higher-order modes of gold blocks. The experimental results exhibit that the free electrons in Ni participate in the optical coupling phenomenon and that plasmon excitations in the Au blocks induce the free electrons in Ni to oscillate.

Short range plasmon resonators probed by photoemission electron microscopy

Short range surface plasmon resonators are investigated at the nanometer scale. Gold nanorods (30 nm in diameter) were microfabricated and probed by photoemission electron microscopy under direct laser light excitation. Resonances presenting various numbers of lobes occur for specific rod lengths. A simple analytical model shows that the successive resonant lengths differ by a multiple of one-half of the wavelength of the supported short-range surface plasmon polariton.

Excitation of local field enhancement on silicon nanowires

The interaction between light and reduced-dimensionality silicon attracts significant interest due to the possibilities of designing nanoscaled optical devices, highly cost-efficient solar cells, and ultracompact optoelectronic systems that are integrated with standard microelectronic technology. We demonstrate that Si nanowires (SiNWs) possessing metal-nanocluster coatings support a multiplicatively enhanced near-field light-matter interaction. Raman scattering from chemisorbed probing molecules provides a quantitative measure of the strength of this enhanced coupling. An enhancement factor of 2 orders of magnitude larger than that for the surface plasmon resonance alone (without the SiNWs) along with the attractive properties of SiNWs, including synthetic controllability of shape, indicates that these nanostructures may be an attractive and versatile material platform for the design of nanoscaled optical and optoelectronic circuits.

Surface plasmon mediated interference phenomena in low-q silver nanowire cavities

Optical excitation of surface plasmons in wet-chemically grown monocrystalline silver nanowires ( approximately 100 nm diameter and up to a few tens of micrometers length) is studied by broadband imaging spectroscopy. Surface plasmons excited by an incident light beam in the so-called Kretschmann-Raether configuration give optical interference phenomena in the spectral domain. These spectral oscillations are interpreted in terms of Fabry-Perot cavity modes for surface plasmons in silver nanowires and allow for a direct experimental determination of the surface plasmon group velocity and cavity losses.

Surfactantless synthesis of silver nanoplates and their application in SERS

Silver nanoplates with thicknesses of 50-70 nm and edge lengths ranging from 200 nm to 1 mum have been grown on semiconductor wafers at room temperature through a simple galvanic reaction between an aqueous solution of AgNO(3) and n-type GaAs. The as-grown Ag structures have chemically clean surfaces due to no surfactant or coordinating molecules being involved in the synthesis. Electron microscopy characterizations indicate that each Ag plate has rough surfaces and a half-moon morphology with one straight edge and one arclike edge. Systematic studies on varying reaction conditions reveal that the oxide (i.e., Ga(2)O(3) and As(2)O(3)) layers of GaAs, generated in situ in the reactions, play an important role in assisting the growth of anisotropic nanoplates. The cleanliness of the surfaces of the Ag nanoplates is beneficial to attachment of interesting molecules on their surfaces for various applications, such as plasmonic-enhanced photophysical and photochemical processes and surface-enhanced spectroscopies.

Efficient and spurious-free integral-equation-based optical waveguide mode solver

Modal analysis of waveguides and resonators by integra-lequation formulations can be hindered by the existence of spurious solutions. In this paper, spurious solutions are shown to be eliminated by introduction of a Rayleigh-quotient based matrix singularity measure. Once the spurious solutions are eliminated, the true modes may be determined efficiently and reliably, even in the presence of degeneracy, by an adaptive search algorithm. Analysis examples that demonstrate the efficacy of the method include an elliptical dielectric waveguide, two unequal touching dielectric cylinders, a plasmonic waveguide, and a realistic micro-structured optical fiber. A freely downloadable version of an optical waveguide mode solver based on this article is available.

Exciton-plasmon-photon conversion in plasmonic nanostructures

A silver-nanowire cavity is functionalized with CdSe nanocrystals and optimized towards cavity quantum electrodynamics by varying the nanocrystal-nanowire distance d and cavity length L. From the modulation of the nanocrystal emission by the cavity modes a plasmon group velocity of v (gr) approximately 0.5c is derived. Efficient exciton-plasmon-photon conversion and guiding is demonstrated along with a modification in the spontaneous emission rate of the coupled exciton-plasmon system.

Direct observation of plasmonic modes in au nanowires using high-resolution cathodoluminescence spectroscopy

We use cathodoluminescence imaging spectroscopy to excite and investigate plasmonic eigenmodes of Au nanowires with lengths of 500-1200 nm and approximately 100 nm width. We observe emission patterns along the Au nanowire axis that are symmetric and strongly wavelength dependent. Different patterns correspond to different resonant modes of the nanowire. From the observed patterns, we derive the spatial and spectral properties of the wire eigenmodes and determine the dispersion relation for plasmonic Au nanowire modes.

Distributed feedback gratings for surface-plasmon polaritons based on metal nano-groove/ridge arrays

The existence of gap states within the first stop band is theoretically investigated for surface-plasmon polaritons propagating along metal surfaces structured with finite arrays of nano-grooves/ridges with imperfections. By means of rigorous calculations of all scattering channels on the basis of the reduced Rayleigh equation, it is shown that a mismatch of the grating period, rather than simple vacant/defect sites, favors the opening of gap states (located at midgap for half-period shifts) as a phase step does in distributed feedback gratings. Quality factors of the resulting surface-plasmon polaritons' resonances are limited by radiative leakage and ohmic losses.

Surface plasmon polariton microscope with parabolic reflectors

We report the realization of a two-dimensional optical microscope for surface plasmons polaritons (SPPs) based on parabolic Bragg mirrors. These mirrors are built from lithographically fabricated gold nanostructures on gold thin films. We show by direct imaging by leakage radiation microscopy that the magnification power of the SPP microscope follows basic predictions of geometrical optics. Spatial resolution down to the value set by the diffraction limit is demonstrated.

Theory of molecular excitation and relaxation near a plasmonic device

The new optical concepts currently developed in the research field of plasmonics can have significant practical applications for integrated optical device miniaturization as well as for molecular sensing applications. Particularly, these new devices can offer interesting opportunities for optical addressing of quantum systems. In this article, we develop a realistic model able to explore the various functionalities of a plasmon device connected to a single fluorescing molecule. We show that this theoretical method provides a useful framework to understand how quantum and plasmonic entities interact in a small area. Thus, the fluorescence signal evolution from excitation control to relaxation control depending on the incident light power is clearly observed.

Directed electroless growth of metal nanostructures on patterned self-assembled monolayers

The directed placement of Cu nanostructures on surfaces has been studied using a combination of scanning probe lithography and electroless metal deposition onto nanopatterned SAMs of 16-mercaptohexadecanoic acid (16-MHA) on Au. In situ studies using nanoscale molecular gradients reveal how controlling the areal density of the 16-MHA molecules dictates the nucleation and growth of the metal nanostructures. The influence of controlling pattern line spacing and tip path on pattern feature fidelity is also discussed.

Surface plasmon mediated near-field imaging and optical addressing in nanoscience

We present an overview of recent progress in "plasmonics". We focus our study on the observation and excitation of surface plasmon polaritons (SPPs) with optical near-field microscopy. We discuss in particular recent applications of photon scanning tunnelling microscope (PSTM) for imaging of SPP propagating in metal and dielectric wave guides. We show how near-field scanning optical microscopy (NSOM) can be used to optically and actively address remote nano objects such as quantum dots. Additionally we compare results obtained with near-field microcopy to those obtained with other optical far-field methods of analysis such as leakage radiation microscopy (LRM).

Enhanced nonlinear optical effects with a tapered plasmonic waveguide

Infrared surface plasmon polaritons (SPPs) are concentrated in a laterally tapered planar Ag waveguide. The near field of SPPs excited with 1490 nm light at a Ag-sapphire interface is probed using the photoluminescence of upconverted Er ions at 550 and 660 nm. SPP interference patterns are observed that exhibit clear evidence of SPP concentration toward the taper end. The concentration leads to an enhancement of the upconversion luminescence intensity from Er energy levels that are populated by multiphoton processes.

Quantum optics with surface plasmons

We describe a technique that enables strong, coherent coupling between individual optical emitters and guided plasmon excitations in conducting nanostructures at optical frequencies. We show that under realistic conditions optical emission can be almost entirely directed into the plasmon modes. As an example, we describe an application of this technique involving efficient generation of single photons on demand, in which the plasmon is efficiently outcoupled to a dielectric waveguide.

Observation of plasmon propagation, redirection, and fan-out in silver nanowires

We report the coupling of free-space photons (vacuum wavelength of 830 nm) to surface plasmon modes of a silver nanowire. The launch of propagating plasmons, and the subsequent emission of photons, is selective and occurs only at ends and other discontinuities of the nanowire. In addition, we observe that the nanowires redirect the plasmons through turns of radii as small as 4 microm. We exploit the radiating nature of discontinuities to find a plasmon propagation length >3 +/- 1 microm. Finally, we observe that interwire plasmon coupling occurs for overlapping wires, demonstrating plasmon fan-out at subwavelength scales.

Local-field confinement in three-pair arrays of metallic nanocylinders

Confinement of light in nano-scale region of three silver nanocylinder pairs is studied by finite-difference time-domain simulations. Light is confined in gaps between nanocylinders due to localized plasmon excitation and the strongest local-field enhancement exhibits in the gap of the second pair. The surface plasmon resonance has red-shift for nanocylinders of larger radius. The resonance wavelength and local-field enhancement are nearly proportional to the radius of nanocylinders in visible light region, i.e., the plasmon resonance of nanocylinder pairs is predictable and controllable. An open cavity model is proposed to understand the linear relation between the resonant wavelength and the radius of nanocylinders.