Femtosecond microscopy of surface plasmon polariton wave packet evolution at the silver/vacuum interface

A movie of the dispersive and dissipative propagation of surface plasmon polariton (SPP) wave packets at a silver/vacuum interface is recorded by the interferometric time-resolved photoemission electron microscopy with 60 nm spatial resolution and 330 as frame interval. The evolution of SPP wave packets is imaged through a two-path interference created by a pair of 10 fs phase correlated pump-probe light pulses at 400 nm. The wave packet evolution is simulated using the complex dielectric function of silver.     

Size-dependent surface plasmon resonance in silver silica nanocomposites

Silver silica nanocomposites were obtained by the sol-gel technique using tetraethyl orthosilicate (TEOS) and silver nitrate (AgNO(3)) as precursors. The silver nitrate concentration was varied for obtaining composites with different nanoparticle sizes. The structural and microstructural properties were determined by x-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopic (XPS) studies were done for determining the chemical states of silver in the silica matrix. For the lowest AgNO(3) concentration, monodispersed and spherical Ag crystallites, with an average diameter of 5?nm, were obtained. Grain growth and an increase in size distribution was observed for higher concentrations. The occurrence of surface plasmon resonance (SPR) bands and their evolution in the size range 5-10?nm is studied. For decreasing nanoparticle size, a redshift and broadening of the plasmon-related absorption peak was observed. The observed redshift and broadening of the SPR band was explained using modified Mie scattering theory.     

Investigation on an application of silver substrates for sensitive surface plasmon resonance imaging detection

A surface plasmon resonance (SPR) imaging biosensor based on silver substrates was investigated to demonstrate that silver could be used as a substrate material for sensitive detection of biomolecular interactions, despite its poor chemical stability. The calculation results showed that oxidation of silver film may lead to a decrease in the sensitivity due to a variation in SPR characteristics such as a broader curve width and shallower minimum reflectance at resonance. The effect of a change in the refractive index of target analytes on the sensitivity was also explored. In particular, it is noteworthy that Ag/Au bimetallic substrates with a thin gold protection layer to prevent oxidation of a silver film can provide a significant amplification of SPR imaging signals in comparison with conventional gold substrates.     

Wavelength-scanning surface plasmon resonance imaging

A new surface plasmon resonance (SPR) imaging technique was proposed. After measurements were conducted at varying wavelengths, the wavelength affording the minimum brightness (SPR wavelength) was determined at each pixel of the image. A two-dimensional map of the SPR wavelength could be converted to a thickness profile by use of a nonlinear calibration curve, which was obtained by Fresnel calculation. An array of protein thin layers on a gold film was evaluated in air to present the layers' surface structure in nanometer scale.     

Sequential monitoring of film thickness variations with surface plasmon resonance imaging and imaging ellipsometry constructed with a single optical system

Sequential surface plasmon resonance imaging (SPRI) and imaging ellipsometry (IE) measurements are realized with a single set of optical components mounted on a goniometer. The sample situated on top of a triangular prism is positioned in the center of the goniometer. The resultant setup (SPRI/IE) can be used to examine the same surface region above (IE) and below (SPRI) the sample. Thickness values of silver stripes sputtered onto a gold substrate were determined by SPRI and IE, and the results were compared to establish the validity of the method. The SPRI/IE setup was also used to monitor the thickness of phospholipid films of different layer numbers. SPRI measurements were found to be more accurate for ultrathin films, whereas the IE results are more reliable for films whose thicknesses approach or exceed the distance encompassed by the evanescent wave of the surface plasmon. Thus, utilizing these two techniques sequentially facilitates the continuous monitoring of film thickness variation over a wide thickness range with high fidelity and provides a viable approach to image the surface regions with large topographic fluctuations.     

Ultrafast dynamics of the real and imaginary permittivity parts of a photoexcited silver layer revealed by surface plasmon resonance

Dynamics of the real and imaginary parts of the dielectric permittivity of a photoexcited silver layer has been investigated by means of femtosecond pump-probe spectroscopy in a surface plasmon resonance Kretschmann configuration. Both real and imaginary permittivity parts experience changes in the visible-near-IR regions under silver excitation at 400 nm. The changes are stronger, particularly those of the imaginary part, at longer wavelengths. Three excited states are formed during the relaxation process, which are attributed to the nonequilibrium and equilibrium heated electron distributions and lattice heating. Different time evolutions of the real and imaginary dielectric permittivity parts are explained by different contributions from interband and intraband transitions caused by plasma frequency and electron scattering frequency variations.     

Surface plasmon resonance imaging detection of silver nanoparticle-tagged immunoglobulin

The detection sensitivity of silver nanoparticle (AgNP)-tagged goat immunoglobulin G (gIgG) microarrays was investigated by studying surface plasmon resonance (SPR) images captured in the visible wavelength range with the help of a Kretchmann-configured optical coupling set-up. The functionalization of anti-gIgG molecules on the AgNP surface was studied using transmission electron microscopy, photon correlation measurements and UV–visible absorption spectroscopy. A value of 1.3 × 10⁷ M⁻¹ was obtained for the antibody–antigen binding constant by monitoring the binding events at a particular resonance wavelength. The detection limit of this SPR imaging instrument is 6.66 nM of gIgG achieved through signal enhancement by a factor of larger than 4 owing to nanoparticle tagging with the antibody.     

Localized surface plasmon resonance spectroscopy of single silver nanocubes

In this work, we use dark-field microscopy to observe a new plasmon resonance effect for a single silver nanocube in which the plasmon line shape has two distinct peaks when the particles are located on a glass substrate. The dependence of the resonance on nanocube size and shape is characterized, and it is found that the bluer peak has a higher figure of merit for chemical sensing applications than that for other particle shapes that have been studied previously. Comparison of the measured results with finite difference time domain (FDTD) electrodynamics calculations enables us to confirm the accuracy of our spectral assignments.     

Doubly localized surface plasmon resonance in bimodally distributed silver nanoparticles

Growth of bimodally distributed silver nanoparticles using sequential physical vapour deposition (PVD) is reported. Growth conditions of nanoparticles are defined in the following three steps: In the first step, nanoparticles are grown at a heated substrate and then exposed to atmosphere, in the second step, nanoparticles are vacuum annealed and finally re-deposition of silver is performed in the third step. This special way of deposition leads to the formation of bimodally distributed nanoparticles. It has been investigated that by changing the deposition time, different sets of bimodally distributed nanoparticles can be grown. Localized surface plasmon resonance (LSPR) of such bimodally distributed nanoparticles generates double plasmon resonance peaks with overlapped absorption spectra. Double plasmon resonance peaks provide a quick indication of the existence of two sets of nanoparticles. LSPR spectra of such bimodally distributed nanoparticles could be modeled with double Lorentz oscillator model. Inclusion of double Lorentz oscillator model indicates that there exist two sets of non-interacting nanoparticles resonating at different plasma frequencies. It is also reported that silver nanoparticles grown at a heated substrate, again attain the new shape while being exposed to atmosphere, followed by vacuum annealing at the same temperature. This is because of physisorption of oxygen at the silver surface and change in surface free energy. The re-shaping due to the adsorbed oxygen on the surface is responsible for bimodal size distribution of nanoparticles.     

Development of UV-visible multiple-angle incidence resolution spectrometry and application study of anisotropic surface plasmon excitation in a silver thin film on a glass substrate

Multiple-angle incidence resolution spectrometry (MAIRS) that was originally developed in an infrared region for analysis of molecular orientation in a thin film on a substrate has been extended to develop a novel UV-visible spectrometry. The new technique, named Vis-MAIRS, has been employed for analysis of anisotropic surface plasmon excitation in a silver thin film with a thickness of 5 nm deposited on a glass slide. The Vis-MAIRS spectra yield two spectra at a time, which correspond to absorption spectra whose transition moments are parallel and perpendicular to the film surface. The two spectra of the silver thin film were largely different from each other in shape, which strongly suggested that the silver nanoparticles in the thin film were in an ellipsoidal shape. In addition, absorption due to long-range surface plasmon propagation across the nanoparticles aligned parallel to the film surface, which is a result of the dipole or quadruple couplings of plasmon in each particle, was clearly monitored for the first time in the Vis-MAIRS spectra. In this manner, Vis-MAIRS is expected to be a useful tool to study aggregates of metal nanoparticles in a film.     

Squeezed mode conversion in hybrid plasmon polariton waveguide using spin-coated silver film

We designed, fabricated, and characterized a hybrid surface plasmon polariton waveguide (SPP_wg) for mode conversion. The 20-nm-thick silver SPP_wg was fabricated via spin-coating with an aqueous silver ionic complex solution. The structure of the SPP_wg consists of a straight Insulator-Metal-Insulator waveguide (IMl_wg), a lateral tapered Insulator-Metal-Insulator-Metal-Insulator waveguide (tapered_IMIMI_wg), and a straight IMIMI waveguide (IMIMI_wg). An s0 mode size of 12.90 microm x 8.08 microm at a 6-microm-wide IMI_wg was excited by a butt-coupling method at a wavelength of 1550 nm. The s0 mode was converted into an Ss0 mode size of 8.08 microm x 5.65 microm at a 3-microm-wide IMIMI_wg. The mode size was squeezed by approximately 2/3 via a 15-microm-long lateral tapered_IMIMI_wg with a 500-nm-thick central insulator. The coupling loss for mode conversion between the straight IMI_wg and the straight IMIMI_wg was 5.49 dB. The hybrid SPP_wg for mode conversion has the potential to bridge the gap between micron and sub-micron scales in nano plasmonic integrated circuits. In addition, the use of the spin coating method is very cost-effective because films are formed at a low temperature in a short period of time without the need for a vacuum system.     

Determination of ribonuclease H surface enzyme kinetics by surface plasmon resonance imaging and surface plasmon fluorescence spectroscopy

The kinetics of the ribonuclease H (RNase H) surface hydrolysis of RNA-DNA heteroduplexes formed on DNA microarrays was studied using a combination of real-time surface plasmon resonance imaging (SPRI) and surface plasmon fluorescence spectroscopy (SPFS). Time-dependent SPRI and SPFS data at various enzyme concentrations were quantitatively analyzed using a simple model that couples diffusion, enzyme adsorption, and surface enzyme kinetics. This model is characterized by a set of three rate constants, enzyme adsorption (k(a)), enzyme desorption (k(d)), enzyme catalysis (k(cat)), and one dimensionless diffusion parameter (beta). Values of k(a) = 3.15 (+/-0.20) x 10(6) M(-1).s(-1), k(d) = 0.10 (+/-0.05) s(-1), and k(cat) = 0.95 (+/-0.10) s(-1) were determined from fitting all of the SPRI and SPFS data sets. One of the most interesting kinetic parameters is the surface RNase H hydrolysis reaction rate constant (k(cat)), which was found to be approximately 10 times slower than that observed in solution, but approximately 100 times faster than that recently observed for the exonuclease III surface hydrolysis of double-stranded DNA microarrays (k(cat) = 0.009 s(-1)). Moreover, the surface coverage of the intermediate enzyme-substrate complex (ES) was found to be extremely small during the course of the reaction because k(cat) is much larger than the product of k(a) and the bulk enzyme concentration.     

Parallel scan spectral surface plasmon resonance imaging

We describe a parallel scan spectral surface plasmon resonance (SPR) imaging technique. We demonstrate experimentally, with a line-shaped light illumination, that an image acquired with an area CCD detector provides both SPR wavelength information and one-dimensional spatial distribution. Thus two-dimensional distribution of the refractive index of the entire sensing plane can be obtained with a one-dimensional optical line parallel scan. The technique offers advantages of both high sensitivity and high throughput, and could have potential applications in biochips analysis.     

High-throughput immunophenotyping by surface plasmon resonance imaging

Cell binding assays on antibody arrays permit the rapid immunophenotyping of living cells. The throughput of the analysis, however, is still limited due to our inability to perform parallel and quantitative detection of cells captured on the array. To address this limitation, we employed here an imaging technique based on surface plasmon resonance (SPR). SPR has been frequently used to monitor capture of proteins on antibody microarrays, while few cases were reported for capture of cells. Antibody arrays were prepared through the photopatterning of an alkanethiol monolayer on a gold-evaporated glass plate and the subsequent immobilization of various antibodies onto 4-9 separate spots created by photopatterning. A glass slip was mounted onto the array with a thin spacer to construct a parallel-plate chamber. Leukemia cells were injected into the chamber to conduct a binding assay, while refractive index changes at the vicinity of the array surface were monitored by SPR imaging. We observed that SPR signals were intensified on specific antibody spots but not on nonspecific spots. Confocal laser scanning microscopy revealed that the observed SPR signals were attributed to cell deformations caused by multivalent interactions with immobilized antibody, which effectively elevated the refractive index of a medium phase within an evanescent field. This effect could be suitably utilized to monitor quantitatively cell binding to multiple spots from a heterogeneous cell population.     

Localized surface plasmon resonance spectroscopy of single silver triangular nanoprisms

The plasmonic properties of single silver triangular nanoprisms are investigated using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) are observed. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approximation (DDA). The dipole resonance is found to be very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chemical sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is found to be as high as 205 nm RIU(-1) and the plasmon line width as narrow as approximately 0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 3.3. In addition, the LSPR shift response to alkanethiol chain length was found to be linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.     

Enhanced third-order optical nonlinearity of silver nanoparticles with a tunable surface plasmon resonance

Poly(vinylalcohol) (PVA)-stabilized silver nanoparticles were prepared by a seeding method. Nanoparticles of varying morphology were obtained by controlled variation of the reaction conditions, and this method allowed the tailoring of the position of the surface plasmon resonance. The samples show two bands in the visible absorption spectrum: one in the 410-440-nm region and a second peak between 500 and 600 nm. This tunable surface plasmon resonance serves to increase the third-order optical nonlinearity (chi 3) of the nanoparticles (measured at 532 nm) by a factor of 16.     

Transmission characteristics of surface plasmon polaritons through a metallic rectangle above a metallic film

The effects of a silver rectangle on the transmission characteristics of surface plasmon polaritons (SPPs) that propagate at the air–silver interface are investigated using the finite-element method. Results show that the structural parameters of the rectangle and distance between rectangle and film significantly influence SPP-transmission characteristics. These effects are due to the restriction of SPPs at the air–silver interface and resonance around the rectangle.     

Determination of surface selection rule of surface plasmon resonance near-infrared spectroscopy by using a Langmuir-Blodgett film

Near-infrared (NIR) absorption spectra of a cadmium arachidate Langmuir-Blodgett (LB) film were measured by surface plasmon resonance near-infrared spectroscopy (SPR-NIRS) based on the Kretschmann configuration with a 18.8-nm gold film. An NIR spectrum enhanced severalfold was obtained as a top ridge of the SPR-NIR spectra measured at different incident angles by using the principle of absorption-sensitive SPR. In order to determine the surface selection rule of SPR-NIRS, the enhanced NIR absorption spectrum of the LB film was compared to an unenhanced one without the gold film and to a normal incidence transmission spectrum. Moreover, a pair of out-of-plane (OP) and in-plane (IP) spectra were obtained by multiangle infrared spectroscopy analysis from a series of oblique incidence transmission measurements in the NIR region. It became obvious that the salient feature of the enhanced NIR absorption spectrum, i.e., the top ridge of the SPR-NIR spectra is almost equivalent to that of the OP spectrum. On the other hand, the unenhanced spectrum showed IP modes. These experimental results were well explained by calculation of the mean-square electric field based on the Fresnel formula.     

Long-range surface plasmon resonance imaging for bioaffinity sensors

A novel bioaffinity sensor based on surface plasmon resonance (SPR) imaging measurements of a multiple-layered structure that supports the generation of long-range surface plasmons (LRSPs) at the water-metal interface is reported. LRSPs possess longer surface propagation lengths, higher electric field strengths, and sharper angular resonance curves than conventional surface plasmons. LRSPR imaging is a version of SPR imaging that requires a symmetric dielectric arrangement around the gold thin film. This arrangement is created using an SF10 prism/Cytop/gold/water multilayer film structure where Cytop is an amorphous fluoropolymer with a refractive index very close to that of water. LRSPR imaging experiments are performed at a fixed incident angle and lead to an enhanced response for the detection of surface binding interactions. As an example, the hybridization adsorption of a 16-mer single-stranded DNA (ssDNA) onto a two-component ssDNA array was monitored with LRSPR imaging. The ssDNA array was created using a new fabrication technology appropriate for the LRSPR multilayers.