Through computer simulations and surface plasmon resonance (SPR) measurements, we establish optimum parameters for the design and fabrication of SPR sensors of high sensitivity, resolution, stability, and long decay-length evanescent fields. We present simulations and experimental SPR data for variety of sensors fabricated by using bimetal (Ag/Au) and multilayer waveguide-coupled Ag/Si3N4/Au structures. The simulations were carried out by using the transfer matrix method in MATLAB environment. Results are presented as functions of the thickness of the metal (Ag or Au) and the waveguide dielectric used in Ag/Si3N4/Au structures. Excellent agreement is observed between the simulations and experiments. For optimized thickness of the Si3N4 waveguide (150 nm), the sensor exhibits very high sensitivity to changes in the refractive index of analytes, Sn≈52°/RIU, extremely high resolution (FWHM≤0.28°), and long penetration depth of evanescent fields (δ≥305nm).
Direct measurements of doubly differential (angle and energy) distributions of backscattered positrons are reported using an electrostatically guided slow-positron beam. Backscattering yields for 35-keV positrons incident normally on the target were measured as a function of both scattering angle 8 and target atomic number Z (4~ZS 82). Absolute backscattering coefficients for positrons are com-
Positron beam lifetime spectroscopy has been utilized to study the depth distribution of vacancy-type defects in molecular beam epitaxy GaAs grown at low temperature. Lifetime spectra were measured as a function of positron energy. From the analysis of the positron lifetime in as-grown and annealed low temperature grown GaAs, the concentrations of Ga monovacancies and voids are estimated. Our results show that in an as-grown sample the Ga monovacancy concentration is >3×1018 cm−3. It is also known that vacancy-cluster concentration in an annealed sample exceeds 1018 cm−3 with a nonuniform spatial distribution.
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