Enhancing the Sensitivity of Single-Particle Photothermal Imaging with Thermotropic Liquid Crystals

Individual molecules and nanoparticles can be imaged based on their absorption using photothermal microscopy. This technique relies on the heating-induced changes in the refractive index of the surrounding medium. Here, we demonstrate an order of magnitude larger enhancement of the signal-to-noise ratio in photothermal imaging of 20 nm gold nanoparticles when using a thermotropic liquid crystal (5CB). We show quantitatively that this increase is due to the large change in the thermo-optical properties of 5CB mainly along the nematic director. Enhancing the sensitivity is important for the further development of absorption-based single-molecule spectroscopy techniques

Mechanistic Study of Bleach-Imaged Plasmon Propagation (BlIPP)

Bleach-imaged plasmon propagation, BlIPP, is a far-field microscopy technique developed to characterize the propagation length of surface plasmon polaritons in metallic waveguides. To correctly extract the propagation length from the measured photobleach intensity, it is necessary to understand the mechanism by which dye photobleaching occurs. In particular, 1- vs 2-photon bleaching reactions yield different propagation lengths based on a kinetic model for BlIPP. Because a number of studies have reported on the importance of 2-photon processes for dye photobleaching, we investigate here the nature of the photobleaching step in BlIPP. We are able to demonstrate that only 1-photon absorption is relevant for typical BlIPP conditions as tested here for a thin film of indocyanine green fluorescent dye molecules coated over gold nanowires and excited at a wavelength of 785 nm. These results are obtained by directly measuring the excitation intensity dependence of the photobleaching rate constant of the dye in the presence of the metallic waveguide

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

Dye-Assisted Gain of Strongly Confined Surface Plasmon Polaritons in Silver Nanowires

Noble metal nanowires are excellent candidates as subwavelength optical components in miniaturized devices due to their ability to support the propagation of surface plasmon polaritons (SPPs). Nanoscale data transfer based on SPP propagation at optical frequencies has the advantage of larger bandwidths but also suffers from larger losses due to strong mode confinement. To overcome losses, SPP gain has been realized, but so far only for weakly confined SPPs in metal films and stripes. Here we report the demonstration of gain for subwavelength SPPs that were strongly confined in chemically prepared silver nanowires (mode area = λ²/40) using a dye-doped polymer film as the optical gain medium. Under continuous wave excitation at 514 nm, we measured a gain coefficient of 270 cm^–1 for SPPs at 633 nm, resulting in partial SPP loss compensation of 14%. This achievement for strongly confined SPPs represents a major step forward toward the realization of nanoscale plasmonic amplifiers and lasers

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

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

Using Particle Lithography to Tailor the Architecture of Au Nanoparticle Plasmonic Nanoring Arrays

The facile assembly of metal nanostructured arrays is a fundamental step in the design of plasmon enhanced chemical sensing and solar cell architectures. Here we have investigated methods of creating controlled formations of two-dimensional periodic arrays comprised of 20 nm Au nanoparticles (NPs) on a hydrophilic polymer surface using particle lithography. To direct the assembly process, capillary force and NP concentration both play critical roles on the resulting nanostructured arrays. As such, tuning these experimental parameters can directly be used to modify the nature of the nanostructures formed. To explore this, two different concentrations of Au NP solutions (∼7 × 10¹¹ or 4 × 10¹² NPs/mL) were used in conjunction with a fixed concentration of polystyrene microspheres (PS MS, ∼6 × 10⁹ PS MS/mL). Assembly at a relative humidity (RH) of 45% with the higher concentration resulted in the formation of well-defined Au nanorings of ca. 23 nm in height and 881 nm in diameter with a pitch of 2.5 μm. Assembly at 65% RH with the lower concentration of NPs resulted in Au nanodonut arrays comprised of isolated single Au NPs. To explore the extent of coupling in the well-defined structures, dark field scattering spectra were collected and showed a broad localized surface plasmon resonance (LSPR) peak with a shoulder, which full-wave electrodynamics modeling (finite-difference time domain (FDTD) method) attributed to be a result of pronounced particle–particle coupling along the circumference of the nanoring array

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

Scattering Properties of Individual Hedgehog Particles

“Hedgehog” particles (HPs) possess a micrometer-sized dielectric spherical core which is densely coated with nanoscale metal oxide spikes. This unique surface topography, resembling the appearance of a hedgehog, provides the particles with the exclusive physiochemical property to stably disperse in both polar and nonpolar solvents without the necessity of changing the surface chemistry. Optical extinction measurements of HP ensembles in aqueous solution indicate a broad spectral response in the visible range. However, there remains a dearth of fundamental knowledge about the cause of the broad optical resonance, as it can be a consequence of shape polydispersity in the many-particle system or intrinsic to each individual HP. In this paper, we present the first experimental study of the dark-field scattering of individual hydrophilic and hydrophobic HPs. Our measurements disclose that the expansive optical response in the visible spectral range is truly characteristic for the far-field scattering of a single HP. Our results also uncover how intrinsic particle features, such as spike length, as well as environmental changes affect the scattering of individual HPs. In particular, by changing the atmosphere around a hydrophilic HP from air to nitrogen and by completely immersing in water by employing a 3D optical trap, we discovered that the scattering from a hydrophilic HP is strongly modulated by excess water in its interstitial shell

Influence of Cross Sectional Geometry on Surface Plasmon Polariton Propagation in Gold Nanowires

We investigated the effects of cross sectional geometry on surface plasmon polariton propagation in gold nanowires (NWs) using bleach-imaged plasmon propagation and electromagnetic simulations. Chemically synthesized NWs have pentagonally twinned crystal structures, but recent advances in synthesis have made it possible to amplify this pentagonal shape to yield NWs with a five-pointed-star cross section and sharp end tips. We found experimentally that NWs with a five-pointed-star cross section, referred to as SNWs, had a shorter propagation length for surface plasmon polaritons at 785 nm, but a higher effective incoupling efficiency compared to smooth NWs with a pentagonal cross section, labeled as PNWs. Electromagnetic simulations revealed that the electric fields were localized at the sharp ridges of the SNWs, leading to higher absorptive losses and hence shorter propagation lengths compared to PNWs. On the other hand, scattering losses were found to be relatively uncorrelated with cross sectional geometry, but were strongly dependent on the plasmon mode excited. Our results provide insight into the shape-dependent waveguiding properties of chemically synthesized metal NWs and the mode-dependent loss mechanisms that govern surface plasmon polariton propagation