Raman detection of nitrogen gas is very difficult without a multi-pass arrangement and high laser power. Hollow-core photonic bandgap fibers (HC-PBF) provide an excellent means of concentrating light energy in a very small volume and long interaction path between gas and laser. One particular commercial fiber with a core diameter of 4.9 microns offers losses of about 1dB/m for wavelengths between 510 and 610 nm. If 514nm laser is used for excitation, the entire Raman spectrum up to above 3000 cm-1 will be contained within the transmission band of the fiber. A standard Raman microscope launches mW level 514nm laser light into the PBF and collects backscattered Raman signal exiting the fiber. The resulting spectra of nitrogen gas in air at ambient temperature and pressure exhibit a signal enhancement of about several thousand over what is attainable with the objective in air and no fiber. The design and fabrication of a flow-through cell to hold and align the fiber end allowed the instrument calibration for varying concentrations of nitrogen. The enhancement was also found to be a function of fiber length. Due to the high achieved Raman signal, rotational spectral of nitrogen and oxygen were observed in the PBF for the first time to the best of our knowledge.

The authors show that a carbon nanotube p-n diode is a very sensitive probe of optical transitions in individual single-walled carbon nanotubes. In the photocurrent spectra, an alternating sequence of resonant peaks from dissociation of excitons and exciton-phonon bound states, for the lowest and higher electronic subbands, is observed. At an intermediate energy, the onset of continuum is observed that allows measurement of exciton binding energies. Both the binding energy and the onset of continuum follow the inverse diameter relation as expected from general theory of optical transitions in nanotubes.

The broadband optical absorption properties of silicon nanowire (SiNW) films fabricated on glass substrates by wet etching and chemical vapor deposition (CVD) have been measured and found tobe higher than solid thin films of equivalent thickness. The observed behavior is adequately explained by light scattering and light trapping though some of the observed absorption is due to a high density of surface states in the nanowires films, as evidenced by the partial reduction in high residual sub-bandgap absorption after hydrogen passivation. Finite difference time domain simulations show strong resonance within and between the nanowires in a vertically oriented array and describe the experimental absorption data well. These structures may be of interest in optical films and optoelectronic device applications.

The optical properties of silicon nanowire films fabricated on bulk Si and glass substrates are reported. The total reflectance of aligned nanowire arrays formed by a wet etch process on bulk Si is lower than the control over all wavelengths below the bandgap, varying from ~1% at 300 nm to les 10% at 1,000 nm. Similar results are observed for nanowire thin films. The observed reflectance is related to the so-called "moth-eye" effect. Modeling work shows strong resonance of light within and between nanowires

We report the growth and characterization of bulk GaN single crystals by temperature-gradient recrystallization at high pressure and high temperature (HPHT), using apparatus adapted from that used to synthesize gem-grade diamond crystals. The bulk crystals are grown on seeds that were synthesized by hydride vapor phase epitaxy (HVPE) and subsequently removed from their sapphire substrate. Our largest crystals to date are 15×18 mm in diameter; however, the process is scalable to 50 mm and above. The crystals are transparent and well faceted, and dislocation densities below 100 cm −2 have been achieved. Additional characterization of the GaN crystals is also presented.

The Raman spectrum of tungsten silicide has been observed and is reported for the first time. It was obtained on the MOLE■ Raman microprobe during the examination of an annealed sample of tungsten deposited over crystalline silicon. A similar examination of fine tungsten lines, 8 μm wide by 20 nm thick, selectively deposited on a crystalline silicon surface using laser-induced chemical vapor deposition techniques, produced an identical spectrum superimposed with that of the silicon substrate. This observation demonstrates the capability of the Raman microprobe to analyze the formation of solid silicide phases on a microscopic scale. The technique offers a rapid and nondestructive method for the identification of tungsten silicide either in the bulk or as a component in an integrated circuit.