Showing papers by "Franz R. Aussenegg published in 2002"

Two-dimensional optics with surface plasmon polaritons

Abstract: We report the experimental realization of highly efficient optical elements built up from metal nanostructures to manipulate surface plasmon polaritons propagating along a silver/polymer interface. Mirrors, beamsplitters, and interferometers produced by electron-beam lithography are investigated. The plasmon fields are imaged by detecting the fluorescence of molecules dispersed in the polymer.

Controlling the optical response of regular arrays of gold particles for surface-enhanced Raman scattering

Abstract: This paper is intended to show how the control of the optical response of regular arrays of gold nanoparticles allows to improve our understanding of surface-enhanced Raman scattering (SERS). Regular particle arrays, designed by electron-beam lithography, exhibit remarkable optical properties and appear to be suitable substrates for the deepened study of the mechanisms at the origin of the SERS effect. Indeed, the resonance of the surface plasmons localized on the particles, which are at the origin of visible to near-infrared extinction spectra and the SERS effect, can be tuned to almost any desirable wavelength by varying the particle shape, size, and spacing, thus optimizing the Raman amplification. The optical extinction spectrum of various gratings was calculated in order to gain insight into their physical sense. The SERS study of trans-1,2-bis (4-pyridyl) ethylene (BPE) adsorbed on these arrays enabled us to determine the enhancement factors G of four bands in the BPE Raman spectrum. The G values thus deduced were found to be of the same order of magnitude as those calculated using a phenomenological relation derived from the electromagnetic theory. Furthermore, a photon scanning tunneling microscope enabled us to acquire near-field optical images of the arrays and to estimate the electric near-field enhancement resulting from plasmon excitation; the Raman enhancement factor thus obtained is of the same order of magnitude as that found from Raman experiments.

Non?diffraction-limited light transport by gold nanowires

Abstract: We show that metal nanowires sustaining collective electron oscillations (surface plasmon polaritons) can be used as optical waveguides. Thereby, the use of a metal allows to overcome the limitations of miniaturization imposed on conventional dielectric waveguides due to diffraction. To demonstrate this effect we investigate a 200 nm wide and 50 nm high gold nanowire locally excited at a light wavelength of 800 nm. By direct imaging the optical near-field with subwavelength-resolution photon scanning tunneling microscopy we observe light transport along the nanowire over a distance of a few μm. Besides the realization of unprecedented integration densities of photonic devices, metal nanowires could be effectively used to optically address individual nanostructures or molecules.

Fluorescence imaging of surface plasmon fields

Abstract: We demonstrate that surface plasmon fields can be imaged in real time by detecting the fluorescence of a molecular film close to the plasmon carrying metal surface. We use this method to image the field profile of surface plasmons launched at lithographically designed nanoscopic defects.

Enhanced substrate-induced coupling in two-dimensional gold nanoparticle arrays

Abstract: The influence of substrate induced coupling on plasmon excitations is probed by means of visible and near-infrared extinction microspectroscopy on various arrays of gold oblate spheroidal particles deposited onto a 20-nm-thick gold film. At zero incidence angle and for an interparticle spacing smaller than 250 nm, the spectra exhibit two bands instead of the single one observed for similar particles but deposited on indium tin oxide coated glass. We suggest that the short-wavelength band proceeds from two simultaneous mechanisms: (i) Excitation of a surface-plasmon resonance localized on the particles. (ii) Generation at the gold-glass interface of a propagating surface-plasmon wave due to the fact that the gold particles can act as a grating coupler. Surface-enhanced Raman-scattering experiments performed on oblate spheroid arrays give arguments in favor of the attribution of the long-wavelength band to surface-plasmon resonance of an ensemble of strongly coupled particles. When increasing the spacing between particles of the array beyond 250 nm, extinction spectra display the emergence of a new band. Calculations suggest that this new band proceeds from the excitation of a surface-plasmon standing wave on the film due to Bragg scattering. This assignment to Bragg scattering is supported by the investigation of the effect of varying the incidence angle under both transverse magnetic and transverse electric polarizations.

Patient Safety Technology for Microadsorbent Systems in Extracorporeal Blood Purification

Abstract: Alternative technologies for extracorporeal blood purification systems based on microadsorbents in suspension are discussed. Principally, microadsorbents offer higher efficiency and flexibility when compared to conventional column-based adsorption systems. Systems already clinically employed (e.g., BioLogic DT) or close to clinical application (e.g., the microspheres-based detoxification system, MDS) are described. The MDS technology, in particular, is characterized by efficiency and a high degree of flexibility with respect to both the use of different adsorbents as well as the combination with hemodialysis/hemofiltration therapy. It was designed for continuous use in intensive-care units, but enables also the removal of low-density lipoprotein, fibrinogen, autoimmune antibodies, immune complexes, and other pathophysiologically relevant substances. Alternative anticoagulation regimes and safety systems on fluorescence sensor technology have recently been developed for the MDS and are presented in this paper.