Heat generation in plasmonic nanostructures has attracted enormous attention due to the ability of these nanostructures
to generate high temperatures in nanoscale volumes using far-field irradiation, enabling applications ranging from
photothermal therapy to fast (sub-nanosecond) thermal optical switching. Here we investigate the optical and thermal
response of a heterogeneous trimer structure composed of a gold nanoparticle surrounded by two larger silver
nanoparticles analytically and numerically. We observe that this type of multi-scale multi-material plasmonic oligomer
can produce temperature changes over two orders of magnitude higher than possible with isolated gold nanoparticles.
Plasmon resonant metal nanoparticles on substrates have been considered for use in several nanophotonic applications
due to the combination of large field enhancement factors, broadband frequency control, ease of fabrication, and
structural robustness that they provide. Despite the existence of a large body of work on the dependence of the
nanoparticle plasmon resonance on composition and particle-substrate separation, little is known about the role of
substrate roughness in these systems. This is in fact an important aspect, since particle-substrate gap sizes for which large
resonance shifts are observed are of the same order of typical surface roughness of deposited films. In the present study,
the plasmon resonance response of 80 nm diameter gold nanoparticles on a thermally evaporated gold film are
numerically calculated based on the measured surface morphology of the gold film. By combining the measured surface
data with electromagnetic simulations, it is demonstrated that the plasmon resonance wavelength of single gold
nanoparticles is blueshifted on a rough gold surface compared the response on a flat gold film. The anticipated degree of
spectral variation of gold nanoparticles on the rough surface is also presented. This study demonstrates that nanoscale
surface roughness can become an important source of spectral variation for substrate tuned resonances that use small gap
sizes.
Optical field enhancement in coupled plasmonic nanostructures has attracted significant attention because of field
enhancement factors that significantly exceed those observed in isolated nanostructures. While previous studies
demonstrated the existence of such cascaded field enhancement in coupled nanospheres with identical composition, this
effect has not yet been studied in systems containing multiple materials. Here, we investigate the polarization-dependent
optical response of multi-material trimer nanostructures composed of Au nanoparticles surrounded by two Ag
nanoparticles as a function of nanoparticle size and inter-particle spacing. We observe field enhancement factors that are
ten times larger than observed in isolated Au nanoparticles.
Substrate-based tuning of plasmon resonances on gold nanoparticles (NP) is a versatile method of achieving plasmon
resonances at a desired wavelength, and offers reliable nanogap sizes and large field enhancement factors. The
reproducibility and relative simplicity of these structures makes them promising candidates for frequency-optimized
sensing substrates. The underlying principle in resonance tuning of such a structure is the coupling between a metal
nanoparticle and the substrate, which leads to a resonance shift and a polarization dependent scattering response. In this
work, we experimentally investigate the optical scattering spectra of isolated 60 nm diameter gold nanoparticles on
aluminum oxide (Al2O3) coated gold films with various oxide thicknesses. Dark-field scattering images and scattering
spectra of gold particles reveal two distinct resonance modes. The experimental results are compared with numerical
simulations, revealing the magnitude and phase relationships between the effective dipoles of the gold particle and the
gold substrate. The numerical approach is described in detail, and enables the prediction of the resonance responses of a
particle-on-film structure using methods that are available in many available electromagnetics simulation packages. The
simulated scattering spectra match the experimentally observed data remarkably well, demonstrating the usefulness of
the presented approach to researchers in the field.
Frequency control of plasmon resonances is important for optical sensing applications such as Surface Enhanced Raman
Spectroscopy. Prior studies that investigated substrate-based control of noble metal nanoparticle plasmon resonances
mostly relied on metal substrates with organic or oxide spacer layers that provided a fixed resonance frequency after
particle deposition. Here we present a new approach enabling continuous resonance tuning through controlled substrate
anodization. Localized Surface Plasmon tuning of single gold nanoparticles on an Al film is observed in single-particle
microscopy and spectroscopy experiments. Au nanoparticles (diameter 60 nm) are deposited on 100 nm thick Al films
on silicon. Dark field microscopy reveals Au nanoparticles with a dipole moment perpendicular to the aluminum surface.
Subsequently an Al2O3 film is formed with voltage controlled thickness through anodization of the particle coated
sample. Spectroscopy on the same particles before and after various anodization steps reveal a consistent blue shift as the
oxide thickness is increased. The observed trends in the scattering peak position are explained as a voltage controlled
interaction between the nanoparticles and the substrate. The experimental findings are found to closely match numerical
simulations. The effects of particle size variation and spacer layer dielectric functions are investigated numerically. The
presented approach could provide a post-fabrication frequency tuning step in a wide range of plasmonic devices, could
enable the investigation of the optical response of metal nanostructures in a precisely controlled local environment, and
could form the basis of chemically stable frequency optimized sensors.
Cascaded optical field enhancement in coupled plasmonic nanostructures has attracted significant attention because of
field enhancement factors that dramatically exceed those observed in isolated nanostructures. While previous studies
demonstrated the existence of cascaded enhancement, little work has been done to identify the requirements for
achieving maximum field enhancement. Here, we investigate cascaded field enhancement in silver nanosphere dimers as
a function of volume ratio and center-to-center separation, and show the requirements for achieving the ultimate
cascading limit in nanoparticle dimers. We observe field enhancements that are a factor 75 larger than observed in
isolated silver nanoparticles.
The optical properties of cascaded plasmon resonant metallic nanocomposites are investigated. Plasmon resonances and
their related field distributions are numerically evaluated in two-dimensional arrays of spherical silver nanoparticles
embedded in a dielectric host. The field distributions in structures with identical particle sizes indicate the presence of a
largely dipolar particle response, with a small multipole resonance contribution at high frequency. However, in arrays
consisting of particles with dissimilar sizes, an additional coupled mode appears in which the dipole moment in adjacent
particles is found to be anti-parallel. For increasing size-dissimilarity a higher electric field enhancement is observed
inside the small metal nanospheres, indicative of a cascaded field enhancement effect. This effect may be used to
enhance the nonlinear optical response of an effective medium composed of particles with engineered size dispersion
and particle placement.
Three-dimensional (3-D) imaging is demonstrated using an electronically controlled liquid crystal (LC) optical lens to accomplish a no-moving-parts depth-section scanning in a modified commercial 3-D confocal microscope. Specifically, 3-D views of a standard CDC blood vessel (enclosed in a glass slide) have been obtained using the modified confocal microscope operating at the red 633-nm laser wavelength. The image sizes over a 25-µm axial scan depth were 50×50 µm and 80×80 µm, using 60× and 20× micro-objectives, respectively. The transverse motion step was 0.1 µm for the 60× data and 0.2 µm for the 20× data. As a first-step comparison, image processing of the standard and LC electronic-lens microscope images indicates correlation values between 0.81 and 0.91. The proposed microscopy system within aberration limits has the potential to eliminate the mechanical forces due to sample or objective motion that can distort the original sample structure and lead to imaging errors.
A plasmonic coupling device consisting of an array of ellipsoidal silver nanoparticles embedded in silica in close
proximity to a silver surface is studied. By tuning the inter-particle spacing, the shape of the particles in the array,
and the height of the array above the silver film, the array-mediated surface plasmon excitation is studied. Finite
Integration Technique simulations of such a plasmon coupler optimized for operation at a free space wavelength of
676 nm are presented. Plane wave normal incidence excitation of the system results is seen to result in resonantly
enhanced fields near the nanoparticles, which in turn excite surface plasmons on the metal film. The existence of an
optimum particle-surface separation for maximum surface plasmon excitation efficiency is demonstrated. Analysis
of the frequency dependent electric field in the simulation volume as a function of particle aspect ratio reveals the
influence of the particle resonance and the surface plasmon resonance on the excitation efficiency.
To the best of our knowledge, for the first time, biological Three Dimensional (3-D) imaging has been achieved using an electronically controlled optical lens to accomplish no-moving parts depth section scanning in a modified commercial 3-D confocal microscope. Specifically, full 3-D views of a standard CDC blood vessel (enclosed in a glass slide) have been obtained using the modified confocal microscope operating at the red 633 nm laser wavelength.
Recent work in plasmon nanophotonics has shown the successful fabrication of surface plasmon (SP) based optical elements such as waveguides, splitters, and multimode interference devices. These elements enable the development of plasmonic integrated circuits. An important challenge lies in the coupling of conventional far-field optics to such nanoscale optical circuits. To address this coupling issue, we have designed structures that employ local resonances for far-field excitation of SPs. The proposed coupler structure consists of an array of ellipsoidal silver nanoparticles embedded in SiO2 and placed close to a silver surface. To study the performance of the coupler we have performed simulations using the Finite Integration Technique. Our simulations show that normal incidence illumination at a free-space wavelength of 676 nm leads to the resonant excitation of SP oscillations in the Ag nanoparticles, accompanied by coherent near-field excitation of propagating SPs on the Ag film. The excitation efficiency can by maximized by tuning the aspect ratio of the nanoparticles, showing optimum coupling at an aspect ratio of 3.0 with the long axis (75 nm) along the polarization of the excitation signal. We discuss the origin of these observations.
The diffraction limit is the major stumbling block in pushing optical lithography to feature sizes smaller than ~50 nm. One approach to circumvent the diffraction limit in optical lithography has been to use optical near-field probes to perform local writing of resist layers. This approach suffers from low writing speeds due to the sequential nature of the process. We discuss two near-field optical illumination schemes that are compatible with broad-beam exposure and high throughput nanofabrication. The first approach concerns a method that can be used to print patterns with feature sizes below 50 nm using standard photoresist. The method relies on the plasmon resonance occurring in nanoscale metallic particles. Nanoparticle surface plasmons can be excited resonantly, producing a strongly enhanced dipole field around the particle. This enhanced near field can be used to locally expose a thin resist layer. Experiments and simulations show that feature sizes < 50 nm can be produced using an exposure wavelength of 400 nm. The second approach involves projecting near-field patterns using planar metal films. It has been predicted that thin metal films may be used to generate images with a spatial resolution better than the diffraction limit. We present simulations that reveal the role of surface plasmons in such near-field imaging with planar metal films.
We investigate the possibility of using arrays of closely spaced metal nanoparticles as plasmon waveguides for electromagnetic energy below the diffraction limit of light. Far-field spectroscopy on arrays of closely spaced 50 nm Au particles fabricated using electron beam lithography reveals the presence of near-field optical particle interactions that lead to shifts in the plasmon resonance frequencies for longitudinal and transverse excitations. We link this observation to a point-dipole model for energy transfer in plasmon waveguides and give an estimate of the expected group velocities and energy decay lengths for the fabricated structures. A near-field optical excitation and detection scheme for energy transport is proposed and demonstrated. The fabricated structures show a high propagation loss of about 3 dB / 15 nm which renders a direct experimental observation of energy transfer impossible. The nature of the loss and ways to decrease it by an order of magnitude are discussed. We also present finite-difference time-domain simulations on the energy transfer properties of plasmon waveguides.
We have recently proposed a new approach to optical lithography that could be used to replicate arrays of metal nanoparticles using broad beam illumination with visible light and standard photoresist. The method relies on resonant excitation of the surface plasmon oscillation in the nanoparticles. When excited at the surface plasmon frequency, a resonantly enhanced dipole field builds up around the nanoparticles. This dipole field is used to locally expose a thin layer of photoresist, generating a replica of the original pattern in the resist. Silver nanoparticles on photoresist can be resonantly excited at wavelengths ranging from 410 nm to 460 nm, allowing for resonantly enhanced exposure of standard g-line photoresist. Finite Difference Time Domain (FDTD) simulations of isolated silver particles on a thin resist layer show that broad beam illumination with p-polarized light at a wavelength of 439 nm can produce features as small as 30 nm, or λ/14. Depending on exposure time lateral spot sizes ranging from 30 to 80 nm with exposure depths ranging from 12 to 45 nm can be achieved. We discuss the effect of particle-particle interactions in the replica formation process. Experiments on low areal density Ag nanoparticle arrays are discussed. Resist layers (thickness 75 nm) in contact with 40 nm Ag nanoparticles were exposed using 410 nm light and were subsequently developed. Atomic Force Microscopy on these samples reveals nanoscale depressions in the resist, providing evidence for plasmon-enhanced resist exposure.
We investigate the possibility of using arrays of closely spaced metal nanoparticles as waveguides for electromagnetic energy below the diffraction limit of visible light. Coupling between adjacent particles sets up coupled plasmon modes that give rise to coherent propagation of energy along the array. A point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission through chain networks such as corners and tee structures is possible at high efficiencies. Although radiation losses into the far field are negligible due to the near-field nature of the coupling, resistive heating leads to transmission losses of about 3 dB/500 nm for gold and silver particles. We confirmed the predictions of this analytical model using numeric finite difference time domain (FDTD) simulations. Also, we have fabricated gold nanoparticle arrays using electron beam lithography to study this type of electromagnetic energy transport. A modified illumination near field scanning optical microscope (NSOM) was used as a local excitation source of a nanoparticle in these arrays. Transport is studied by imaging the fluorescence of dye-filled latex beads positioned next to the nanoparticle arrays. We report on initial experiments of this kind.
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