Effect of varying electric potential on surface-plasmon resonance sensing

The high sensitivity of surface-plasmon resonance (SPR) sensors allows measurements of small variations in surface potentials to be made. We studied the changes of the SPR angle when an oscillating electric potential was applied to a gold film on which surface plasmons were excited. The shifts of the SPR resonance angle were observed for various aqueous solutions as an adjacent medium. A model that takes into account the redistribution of charges at the double layer near the metal-liquid interface as well as the oxidation of the gold film was developed. It was found that a change in the electronic density at voltages below the oxidation potential and, in addition, the oxidation of the gold surface above this potential are the main mechanisms that account for the observed dependences. It was shown that relatively slow oxidation-reduction processes can explain the observed hysteresis effect. Application of these techniques to studies of dielectric properties and conformational changes of polar biomolecules, such as tubulin, are discussed.     

Properties and sensing characteristics of surface-plasmon resonance in infrared light

Conditions of surface-plasmon resonance (SPR) production with use of IR pumping light (800-2300 nm) in the Kretschmann-Raether prism arrangement were investigated. Both calculations and experimental data showed that SPR characteristics in the IR are strongly influenced by the properties of the coupling prism material. Indeed, quite different regularities of plasmon excitation, polarity of sensing response, and sensitivity are observed for two different glasses and silicon. The observed differences in SPR properties are related to essentially different behavior of dispersion characteristics of materials near the SPR coupling point. Methods for improving sensor performance and miniaturizing the SPR technique using novel coupling materials (silicon) are discussed.     

Environment effects on surface-plasmon spectra in gold-island films potential for sensing applications

The effects of the local dielectric environment on the surface-plasmon resonances of annealed gold-island films as a potential for sensing applications are studied experimentally and modeled theoretically. Gold-island films were annealed at 600 degrees C to produce spheroidal shape particles that exhibit well-resolved resonances in polarized, angle-resolved, absorption spectra. These resonances are shifted in different amounts by the depolarization effect of the surrounding medium (liquids with various refraction indices). Cross-section calculations based on nonretarded, single-particle, dielectric interaction for these various configurations are presented and are found to be in good agreement with the experimental observations. The results show an interesting potential for biosensing or environmental monitoring applications.     

High-resolution scanning surface-plasmon microscopy

Surface plasmons (SP's) are electromagnetic surface waves that propagate along the interface between conductors and dielectrics. The k vector of these waves is larger than the free-space wave vector. The importance of SP's lies in the fact that they are extremely sensitive to small changes in the dielectric properties of substances that are in contact with the conductors. This property means that SP's have many sensor applications; however, when they are used in microscopic applications the lateral resolution is limited to several micrometers. We discuss how this limit can be overcome by use of defocused high-numerical-aperture liquid-immersion objectives. We also present SP images that demonstrate a resolution comparable with that expected from high-numerical-aperture optical microscopes. Finally, we discuss how ultrahigh-numerical-aperture objectives with numerical apertures greater than 1.5 can be expected to have considerable influence on biological imaging.     

Novel system for coupling to surface-plasmon polaritons

A novel system for coupling to surface-plasmon polaritons has been designed, fabricated, and assembled. Microcomputer controlled electronics provide for accurate and repeatable angular positioning of the waveguide coupler with respect to the incident beam and for precise angular registration of detector output signals. The optical components, combined with a retroreflecting spherical-surface coupling prism, give apertured control of the beam profile at the coupling interface.     

Evanescent-wave holography by use of surface-plasmon resonance

We report a method for evanescent-wave holography using surface-plasmon resonance from the illumination light. The device we have made consists of three layers: a prism of high refractive index, a thin metallic film, and a grating. Evanescent waves generated by the surface plasmons are diffracted with a prerecorded grating to reconstruct a three-dimensional image. The possibility of white-light illumination and the application to a flat display system with waveguides in the proposed method are discussed.     

Dual-mode surface-plasmon sensor based on bimetallic film

We propose a plasmonic structure, based on a silver-gold two-layered metallic film, where two surface plasmons (SPs) with equal propagation constants are excited simultaneously at different wavelengths. We show theoretically that the bimetallic film provides unique opportunities for manipulation of plasmons and optimization of the accuracy and cross-sensitivity. The structure can be used as an effective self-reference SP sensor in wavelength-interrogated design.     

Nano-engineered optimal templates for surface-plasmon enhanced Raman scattering

We investigated the critical conditions to realize reliable and nano-engineered templates for surface-plasmon enhanced Raman scattering (SERS). Ultra-sensitive SERSs of thymine oligonucleotides were successfully realized on the template of Au nanoparticle arrays which were prepared by the combination of electron-beam lithography and post-chemical modification techniques. Drastic enhancement of Raman signal from the thymine oligonucleotides was only observed on the optimized templates, where the tuning of the plasmon resonance condition and the formation of the hot spots were both critical. Our results suggest that the artificial generation of reproducible and controlled hot spots can be achieved by our approach.     

Optical-fiber surface-plasmon resonance sensor with multiple resonance peaks

We report on an optical fiber surface plasmon resonance sensor that exhibits multiple resonance peaks. The sensor is based on a uniform-waist single-mode tapered fiber coated on one side with a thin metal layer. Owing to the asymmetry of the sensor structure, the different hybrid surface plasmon modes supported by the semicircular layer can be excited by the fundamental fiber mode. As a result, the sensor transmission spectrum exhibits several dips that depend on the taper waist diameter. The advantages of a plasmon resonance sensor with multiple dips are discussed.     

Characterization of surface-plasmon modes in metal-clad optical waveguides

Finite-element analysis, based on the vector H-field formulation and incorporating the perturbation technique, is used to calculate the complex propagation characteristics of metal-coated dielectric waveguides. The propagation and attenuation characteristics of the surface-plasmon modes at the metal/dielectric interfaces are presented. The effects on the optical properties of metal-clad optical fibers with infinite and finite cladding thickness and the formation of the supermodes due to the coupling between the surface-plasmon modes in the presence of different surrounding materials are also investigated.     

Hybridized surface-plasmon resonances of platinum colloid-adsorbed gold nanospheres

The surface-plasmon resonances of gold nanospheres dispersed in water split into two bands and shift to the red with the adsorption of colloidal platinum. These spectral changes are quite different from both the calculated and the experimental spectral variations of gold nanospheres with the thickness of coating platinum. Thus, these spectral changes have been attributed to the elementary plasmon interactions of the core gold and the adsorbed colloidal platinum as well as to the modification of the medium refraction index of the gold nanospheres. A simple and intuitive picture has been drawn to describe the hybridization plasmon interactions of a platinum colloid-adsorbed gold nanosphere.     

Approximate model for surface-plasmon generation at slit apertures

We present a semianalytical model that quantitatively predicts the scattering of light by a single subwavelength slit in a thick metal screen. In contrast to previous theoretical works related to the transmission properties of the slit, the analysis emphasizes the generation of surface plasmons at the slit apertures. The model relies on a two-stage scattering mechanism, a purely geometric diffraction problem in the immediate vicinity of the slit aperture followed by the launching of a bounded surface-plasmon wave on the flat interfaces surrounding the aperture. By comparison with a full electromagnetic treatment, the model is shown to provide accurate formulas for the plasmonic generation strength coefficients, even for metals with a low conductivity. Limitations are outlined for large slit widths (>lambda) or oblique incidence (>30 degrees ) when the slit is illuminated by a plane wave.     

Locally oxidized silicon surface-plasmon Schottky detector for telecom regime

We experimentally demonstrate an on-chip nanoscale silicon surface-plasmon Schottky photodetector based on internal photoemission process and operating at telecom wavelengths. The device is fabricated using a self-aligned approach of local-oxidation of silicon (LOCOS) on silicon on insulator substrate, which provides compatibility with standard complementary metal-oxide semiconductor technology and enables the realization of the photodetector and low-loss bus photonic waveguide at the same fabrication step. Additionally, LOCOS technique allows avoiding lateral misalignment between the silicon surface and the metal layer to form a nanoscale Schottky contact. The fabricated devices showed enhanced detection capability for shorter wavelengths that is attributed to increased probability of the internal photoemission process. We found the responsivity of the nanodetector to be 0.25 and 13.3 mA/W for incident optical wavelengths of 1.55 and 1.31 μm, respectively. The presented device can be integrated with other nanophotonic and nanoplasmonic structures for the realization of monolithic opto-electronic circuitry on-chip.     

Sensitivity of optical sensors based on laser-excited surface-plasmon waves.

We investigate the effect of the divergence of a Gaussian laser beam on the resonance curve and the sensitivity of optical sensors based on surface-plasmon resonance (SPR). For He-Ne laser beams it is found that, for beams with a waist radius of less than 300 mum, the SPR-curve characteristics differ appreciably from the case in which a plane wave is considered. Simple expressions for the sensitivity of (bio)chemical sensors are given. A simple Lorentzian model is used to estimate the maximum possible sensitivity when a multilayer system is used to enhance the resonance peak. It was found that the sensitivity can reach its highest value when the width of the SPR curve is equal to the laser-beam divergence. The results could be particularly important when a SPR curve is used to measure the absolute value of the refractive index of a sample or the dielectric constant and the thickness of a metal layer.     

Excitation of surface-plasmon polaritons by use of a zeroth-order Bessel beam

We report an experimental result that shows the excitation of surface plasmon polaritons by use of a zeroth-order Bessel beam. From the viewpoint of energy efficiency, the zeroth order Bessel beam is ideal for the local excitation of surface plasmon polaritons on a metal film. We introduce an optical setup using an axicon element in order to carry out the excitation.     

Simulation of an absorption-based surface-plasmon resonance sensor by means of ellipsometry

Through numerical simulations, we point out that introduction of an ellipsometric measurement technique to an absorption-based surface-plasmon resonance (SPR) sensor enhances precision and sensitivity in measuring the imaginary part k of the complex refractive index of the sample. By measuring a pair of ellipsometric Delta-Psi parameters, instead of the conventional energy reflectance R(p) of p-polarized light in the Kretschmann optical arrangement, we can detect a small change of k that is proportional to that of the concentration of the sample, especially when k < 1. While one has difficulty in determining the value of k uniquely by the standard technique, when the thickness of Au under the prism is thin (20-30 nm), the ellipsometric technique (ET) overcomes the problem. Furthermore, the value of k and the thickness d(s) of the absorptive sample that is adsorbed on Au can be determined precisely. The ET based on the common-path polarization interferometer is robust against external disturbance such as mechanical vibration and intensity fluctuation of a light source. Although only the p-polarized light is responsible for the SPR phenomenon, we show that the introduction of the ET is significant for quantitative analysis.     

Sensitive detection of biological species through localized surface-plasmon resonance on gold nanodisks

In this paper, we are interested in the detection of biological molecules. This detection relies on the use of nanosensors based on localized surface-plasmon resonance (LSPR). We discuss the sensitivity of these nanosensors by studying the influence of the concentration of Biotin on the shift of the LSPR wavelength. Moreover, to study the selectivity of these sensors, the systems Biotin/Anti-Biotin (AB) and Biotin/Streptavidin (SA) are used to detect very weak concentration of AB or SA. We found respectively 7 pM and 3 nM for the limits of detection of SA and AB.     

Small-angle measurement based on surface-plasmon resonance and the use of magneto-optical modulation

A new, to our knowledge, optical method for small-angle measurement based on surface-plasmon resonance (SPR) is presented. In this method the high sensitivity of the phase of SPR to the angle of incidence is employed to improve the resolution of the measurement of the angle. Small-angle measurement is performed by the monitoring of the phase shift resulting from the minute change of the angle of incidence with the use of magneto-optical modulation. The validity of this method is demonstrated, and a measurement resolution of 0.2 arc sec is achieved experimentally.     

Sensitivity and detection limit of concentration and adsorption measurements by laser-induced surface-plasmon resonance

The shot noise limitation as well as other factors that influence the sensitivity of measurements with a surface plasmon resonance (SPR) sensor are considered. It is demonstrated that minute changes in the refractive index of a medium close to the surface of a metal film can be detected owing to a shift in the resonance angle. In particular, changes in the adsorption layer of only a fraction of a biomolecular monolayer could be measured. Data for SPR are presented with adjacent media of air, water, as well as aqueous solutions of ethanol and sodium chloride at different concentrations. The immobilization of the protein bovine serum albumin to a specially prepared surface was monitored with the SPR technique. Specific responses to changes in the concentration and thickness of the adsorption layer were determined. The angular resolution of the present apparatus is approximately 1 millidegree, corresponding to a detection limit for an adsorbed protein layer of 15 pg/mm(2), which is still 2 to 3 orders of magnitude larger than the shot-noise limit, and therefore a further improvement in sensitivity is possible.