Kalman Pelhos | PubMed Health

PubMed2006

Focusing characteristics of a planar solid-immersion mirror.

The focusing characteristics of a planar waveguide solid-immersion mirror with parabolic design have been investigated. The solid-immersion mirror is integrated into an optical waveguide, and light focusing is achieved with a parabolic mirror parallel to the waveguide plane and waveguide mode confinement normal to the waveguide plane. Optical-quality tantala silica planar waveguides can be obtained by evaporation. The parabolic sidewall reflects over 50% of the incident waveguide mode and generates a diffraction-limited focus. The measured spot size for the solid-immersion mirror described here is less than one third of the wavelength. Polarization analysis shows that the electric field near the focal region has components parallel and normal to the polarization state of the incident beam. The planar solid-immersion mirror is essentially free of chromatic aberration, and the alignment of the illumination beam is within a fraction of degrees.

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PubMed2002

Nanoscale surface chemistry.

We report evidence in several experiments for nanometer-size effects in surface chemistry. The evidence concerns bimetallic systems, monolayer films of Pt or Pd on W(111) surfaces. Pyramidal facets with [211] faces are formed on annealing on physical monolayer of Pt, Pd on a W(111) substrate, and facet sizes increase with annealing temperature. We used synchrotron radiation-based soft x-ray photoemission to show that monolayer films of Pt, Pd, on W "float" on the outer surface, whereas multilayer films form alloys on annealing. Acetylene reactions over bimetallic planar and faceted Pd/W surfaces exhibit size effects on the nanometer scale, that is, thermal desorption spectra of reactively formed benzene and ethylene (after acetylene adsorption) change systematically with facet size. In the second case, the decomposition of C(2)H(2) over planar and faceted Ir(210) surfaces also exhibits structure sensitivity; temperature programmed desorption of H(2) from C(2)H(2) dissociation depends on the nanoscale surface structure. Finally, we have characterized interactions of Cu with the highly ordered S(4 x 4)/W(111) surface. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4 x 4) periodicity, having broad planar terraces (approximately 30 nm in width). Fractional monolayers of vapor-deposited Cu grow as three dimensional clusters on the S(4 x 4) surface over a wide coverage range. At low Cu coverage (< or = 0.1 ML), Cu nanoclusters nucleate preferentially at characteristic 3-fold hollow sites; we find a clear energetic preference for one type of site over others, and evidence for self-limiting growth of nanoclusters.

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