Active Modulation of Nanorod Plasmons

Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogues of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100% using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogeneous to a twisted nematic phase of the liquid crystal covering the nanorods. With our method it will be possible to actively control optical antennas as well as other plasmonic elements

In Situ Measurement of Bovine Serum Albumin Interaction with Gold Nanospheres

We present in situ observations of adsorption of bovine serum albumin (BSA) on citrate-stabilized gold nanospheres. We implemented scattering correlation spectroscopy as a tool to quantify changes in the nanoparticle Brownian motion resulting from BSA adsorption onto the nanoparticle surface. Protein binding was observed as an increase in the nanoparticle hydrodynamic radius. Our results indicate the formation of a protein monolayer at similar albumin concentrations as those found in human blood. Additionally, by monitoring the frequency and intensity of individual scattering events caused by single gold nanoparticles passing the observation volume, we found that BSA did not induce colloidal aggregation, a relevant result from the toxicological viewpoint. Moreover, to elucidate the thermodynamics of the gold nanoparticle–BSA association, we measured an adsorption isotherm which was best described by an anticooperative binding model. The number of binding sites based on this model was consistent with a BSA monolayer in its native state. In contrast, experiments using poly­(ethylene glycol)-capped gold nanoparticles revealed no evidence for adsorption of BSA

Active Modulation of Nanorod Plasmons

Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogues of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100% using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogeneous to a twisted nematic phase of the liquid crystal covering the nanorods. With our method it will be possible to actively control optical antennas as well as other plasmonic elements

Plasmon Emission Quantum Yield of Single Gold Nanorods as a Function of Aspect Ratio

We report on the one-photon photoluminescence of gold nanorods with different aspect ratios. We measured photoluminescence and scattering spectra from 82 gold nanorods using single-particle spectroscopy. We found that the emission and scattering spectra closely resemble each other independent of the nanorod aspect ratio. We assign the photoluminescence to the radiative decay of the longitudinal surface plasmon generated after fast interconversion from excited electron–hole pairs that were initially created by 532 nm excitation. The emission intensity was converted to the quantum yield and was found to approximately exponentially decrease as the energy difference between the excitation and emission wavelength increased for gold nanorods with plasmon resonances between 600 and 800 nm. We compare this plasmon emission to its molecular analogue, fluorescence

Selective Entrapment of Pb2+ from Fresh Thunbergia laurifolia Leaves Extract and Thunbergia laurifolia Tea Extract

The leaves of Thunbergia laurifolia and its tea were extracted by water. The abilities to chelate heavy metal ions of the extracts were studied by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and atomic absorption spectroscopy (AAS). The results showed that the extracts exhibited high selectivity for Pb2+ chelation via a favourable-selective precipitation to Pb2+ in aqueous solutions compared to other metal ions, such as Zn2+, Cu2+ and Fe3+. The Pb2+ removal capability of the extracts were 51-52%. The selective Pb2+-trapping process of the extracts of Thunbergia laurifolia could be attributed to predominant presence of the phytochemical compounds, tannin and saponin, in the Thunbergia laurifolia leave and tea. Moreover, Thunbergia laurifolia tea also exhibited antioxidant property as demonstrating by the 2,2-diphenyl-1-picryl-hydrazyl (DPPH) free radical method.</div

Seeing Double: Coupling between Substrate Image Charges and Collective Plasmon Modes in Self-Assembled Nanoparticle Superstructures

The interaction between adjacent metal nanoparticles within an assembly induces interesting collective plasmonic properties. Using dark-field imaging of plasmon scattering, we investigated rings of gold nanoparticles and observed that the images were dependent on the substrate. In particular, for nanoparticles assembled on carbon and gold substrates, intensity line sections perpendicular to the ring revealed a significant broadening beyond the optical resolution accompanied by an intensity dip in the middle of the line profile. Overall, this appeared in the image as a “splitting” into two offset circles along the direction of the scattered light polarization. This effect was not observed for a substrate with a low permittivity, such as glass. By varying the substrate as well as selecting different detected wavelengths and polarization components of the excitation light, we were able to confirm that the observed effect was due to coupling of collective plasmon modes with their induced image charges in the supporting substrates. These results suggest that plasmon scattering in extended nanostructures can be spatially modulated by tuning the permittivity of the substrate

Single-Particle Spectroscopy of Gold Nanorods beyond the Quasi-Static Limit: Varying the Width at Constant Aspect Ratio

We have examined how the surface plasmon resonances (SPRs) of chemically grown gold nanorods with tunable widths and lengths evolve due to phase retardation. For nanorods with diameters d > 30 nm, the aspect ratio is not a sufficient parameter for determining the energy of the longitudinal SPR. To rigorously study the effects of the size, we performed correlated scanning electron microscopy and single-particle spectroscopy on broad gold nanorods that were chemically grown wider (d > 100 nm) and longer while maintaining the surface chemistry and hemispherical end-cap geometry as the slim rods we compared them to (d d >100 nm displayed higher order plasmon modes that were not observed for slim nanorods of similar aspect ratio. To measure the full spectrum of the largest nanorods, we implemented a new strategy for acquiring single-particle extinction spectra with an extended window of 500−1700 nm by combining a Si CCD camera and an InGaAs array detector. This experiment revealed that changing the width from 25 to 120 nm while maintaining an aspect ratio of only 3.1 caused the longitudinal dipole SPR to red shift 560 nm to 1300 nm. The spectroscopic studies were complemented by theoretical modeling using the discrete dipole approximation. While we found excellent agreement between the measured and predicted maxima of the longitudinal dipole SPR, the intensities of the multipolar plasmon modes were significantly enhanced in the single-particle spectra compared to calculations

Quadrupole-Enhanced Raman Scattering

Dark, nonradiating plasmonic modes are important in the Raman enhancement efficiency of nanostructures. However, it is challenging to engineer such hotspots with predictable enhancement efficiency through synthesis routes. Here, we demonstrate that spiky nanoshells have designable quadrupole resonances that efficiently enhance Raman scattering with unprecedented reproducibility on the single particle level. The efficiency and reproducibility of Quadrupole Enhanced Raman Scattering (QERS) is due to their heterogeneous structure, which broadens the quadrupole resonance both spatially and spectrally. This spectral breadth allows for simultaneous enhancement of both the excitation and Stokes frequencies. The quadrupole resonance can be tuned by simple modifications of the nanoshell geometry. The combination of tunability, high efficiency, and reproducibility makes these nanoshells an excellent candidate for applications such as biosensing, nanoantennaes, and photovoltaics

A Plasmonic Fano Switch

Plasmonic clusters can support Fano resonances, where the line shape characteristics are controlled by cluster geometry. Here we show that clusters with a hemicircular central disk surrounded by a circular ring of closely spaced, coupled nanodisks yield Fano-like and non-Fano-like spectra for orthogonal incident polarization orientations. When this structure is incorporated into an uniquely broadband, liquid crystal device geometry, the entire Fano resonance spectrum can be switched on and off in a voltage-dependent manner. A reversible transition between the Fano-like and non-Fano-like spectra is induced by relatively low (∼6 V) applied voltages, resulting in a complete on/off switching of the transparency window

Turning the Corner: Efficient Energy Transfer in Bent Plasmonic Nanoparticle Chain Waveguides

For integrating and multiplexing of subwavelength plasmonic waveguides with other optical and electric components, complex architectures such as junctions with sharp turns are necessary. However, in addition to intrinsic losses, bending losses severely limit plasmon propagation. In the current work, we demonstrate that propagation of surface plasmon polaritons around 90° turns in silver nanoparticle chains occurs without bending losses. Using a far-field fluorescence method, bleach-imaged plasmon propagation (BlIPP), which creates a permanent map of the plasmonic near-field through bleaching of a fluorophore coated on top of a plasmonic waveguide, we measured propagation lengths at 633 nm for straight and bent silver nanoparticle chains of 8.0 ± 0.5 and 7.8 ± 0.4 μm, respectively. These propagation lengths were independent of the input polarization. We furthermore show that subradiant plasmon modes yield a longer propagation length compared to energy transport via excitation of super-radiant modes