Nanoparticle Mediates Improvement in Si Electroluminescence Efficiency
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(FWHM), at a position 93 cm downstream from the CRL using 6.5-keV photons. The spot diameter was larger than expected, which was attributed to surface irregularities or the uncertainty of the source. The Gaussian transmission profile FWHM of the Be CRL was also measured by scanning a 25 µm × 25 µm beam across the CRL and was determined to be 321 µm with on-axis transmission of 9% at 6.5 keV photon energy. The attenuation aperture was thus calculated to be 607 µm. Consequently, using the experimental data, a gain of 1.5 was calculated for this lens, compared with a theoretical value of 6. The obtainable gain is dependent on the source size used for the experiments, and much higher gains would be expected from thirdgeneration and beyond x-ray facilities. According to the researchers, the use of CRLs made of beryllium “can achieve submeter focal lengths at lower x-ray energies (~6.5 keV) than for previously reported CRLs and still have what are believed to be the largest reported apertures (~600 µm). Thus, it is expected that Be CRLs can outperform lenses constructed of higher-atomic-number materials at energies below 30 keV.” CALIN MICLAUS
Heating Allows Birefringence Tuning in Microstructured Optical Fibers Partially Filled with a Polymer Recently, researchers have shown that microstructured photonic-crystal optical fibers (MOFs) formed by incorporating air holes running along the length of the fiber can enable additional control of birefringence, dispersion, and nonlinearity of the fibers. Among other novel techniques, introducing active materials into the air holes of microstructured optical fibers (MOFs) improves the capabilities of the devices fabricated. As they demonstrate in their article in the May 15 issue of Optics Letters, C. Kerbage, B.J. Eggleton, and coworkers from OFS Fitel Laboratories (now OFS Laboratories) in New Jersey, were able to induce and tune birefringence in MOFs. The researchers chose MOFs comprised of a central germanium-doped core, 8 µm in diameter, encircled symmetrically by six air holes. They fabricated an all-fiber polarization controller, 125 µm in diameter but tapered along 1 cm of the length down to ~30 µm in diameter. This was to ensure that the mode to guide light was by total internal reflection at the silica-airhole interface. The fiber was tapered adiabatically to achieve low intrinsic loss. Since MOFs with a rotational symmetry larger than twofold are not birefringent, the scientists had to break the sixfold symmetry 432
by filling two opposite air holes with an acrylate-based polymer. The monomer filling, limited to the tapered area, was fed at 0.01 cm/s and cured using UV rays for 15 min. The resulting polymer had a refractive index of 1.434 at ambient temperature and at a wavelength of 1550 nm, although it is temperature-dependent (dn/dT ~ -4 × 10-4 °C-1), and hence is controlled by heating. After placing the device in a capillary heater, the research team applied a 1550-nm polarized laser beam to the fiber to yield an output light beam, which was analyzed for its polar
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