Raman

To achieve high-spectral-resolution multiplex coherent anti-Stokes Raman scattering (CARS), one typically uses a narrowband pump pulse and a broadband Stokes pulse. This is to ensure a correspondence between anti-Stokes and vibrational frequencies. We obtain high-resolution CARS spectra of isopropanol, using a broadband chirped pump pulse and a broadband Stokes pulse, by detecting the anti-Stokes pulse with spectral interferometry. With the temporally resolved anti-Stokes signal, we can remove the chirp of the anti-Stokes pulse and restore high spectral resolution while also rejecting nonresonant scattering. (c) 2006 Optical Society of America.

A major impediment to the use of coherent anti-Stokes Raman scattering (CARS) to identify biological molecules is that the illumination levels required to produce a measurable signal often also produce significant nonresonant background from the medium, especially from water. We present a method of nonlinear interferometry to differentiate between which components of the anti-Stokes signal are resonant and nonresonant. The technique takes advantage of the persistence of intermediate states involved in the resonant process. This method is applicable to most existing pulsed CARS illumination methods and provides for identification of resonant CARS. We demonstrate the method by examining the signals produced by acetone, which exhibits resonance, and water, which does not. (C) 2004 American Institute of Physics.

The polymorphism of WO, powder samples, resulting from mild mechanical treatments and from temperature changes between 30 K and room temperature, has been investigated by using Raman spectroscopy and X-ray diffraction. A transition from the monoclinic (I) gamma-phase to the triclinic delta-phase after moderate mechanical treatments has been observed for untreated powder, just what happens when the same samples are rapidly cooled to low-temperature. Evidences of the low temperature monoclinic (II) polar epsilon-phase have been found at room temperature in samples after a stronger milling treatment. The sequence of the low-temperature phase transitions appears to be strongly dependent on the mechanical history of the powders. A new low-temperature N-phase has been observed below about 200 K in different samples: it is the main phase in commercial untreated powders, having the monoclinic (I) gamma-phase at room temperature, but constitutes only a small fraction in moderately treated powders, having the triclinic delta-phase at room temperature. (C) 1999 Academic Press.

The relationship between structural transformations and colour centres creation is discussed for deeply coloured hydrogen tungsten bronzes and for pure WO(3) powders, acquiring less intense colour after mechanical treatments of variable duration. A comparative study on coloration is made also for mixed compounds (1-x)WO(3-y)center dot xReO(2), where an evidence of a resonance effect for a particular Raman band at 970 cm(-1), attributed to the color centres, is observed. Besides, it is found that even moderate milling treatments result in a quite different structural evolution of tungsten trioxide upon cooling.

An extensive investigation of the temperature dependence of Raman spectra has been carried out on WO3 powders from room temperature to 800 degrees C. In particular the orthorhombic-to-tetragonal phase transition occurring at about 740 degrees C has been studied for the first time. The Raman active mode at 710 cm(-1) of the orthorhombic phase disappears from the spectrum at temperature below the phase transition point and the Raman activity in the tetragonal phase results very low. A comparative study of hydrogenated tungsten bronzes HxWO3 (x less than or equal to 0.23), where the same transition sequence is driven by an increase of the proton concentration from x = 0 to 0.23, reveals similar behaviour of the high frequency Raman active modes, which are better resolved due to the weaker anharmonic interactions. (C) 1999 Elsevier Science B.V. All rights reserved.

The precipitation of germanium nanocrystals in amorphous carbon-germanium films allows for the development of innovative devices, but the accurate control of both size and size distribution of Ge quantum dots in these matrices still constitutes a challenging step. In this paper, both the structure and morphology of amorphous carbon-germanium films (a-Ge1-xCx), deposited by d.c. magnetron sputtering onto silicon substrates and annealed in vacuum at temperatures up to 550 degrees C, are investigated by Raman spectroscopy and scanning electron microscopy. The main features of Raman spectra obtained from carbon-rich films (x > 0.43) are the D and G bands, characteristic of graphitic carbon films. The ratio between the intensities of the bands, I-D/I-G, increases with the annealing temperature, suggesting a progressive increase of the graphitic domains within the films. Raman spectra obtained in the low frequency region from both as-deposited and annealed germanium-rich films (x < 0.43) show broad bands associated with transverse acoustic and transverse optic Ge-Ge modes. Cre-Ge optic modes merge up into a well-shaped peak at 300 cm(-1) in the germanium-richest sample, and underwent annealing treatment at 550 degrees C, thus indicating the precipitation of crystalline Ge. Scanning electron microscopy analysis shows an apparently uniform nucleation of Ge crystallites at the sample surface. Microprobe Raman scattering results suggest the formation of a nearly homogeneous distribution of Ge nanocrystals in germanium-rich films annealed at 550 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.

Pure ground tungsten trioxide WO3 and (1-x) WO3-y. xReO(2) mixtures were studied by x-ray absorption spectroscopy, x-ray powder diffraction and Raman spectroscopy in comparison with hydrogen bronzes HxWO3 and hydrogenated calcium tungstate CaWO4:H. It was found that a grinding of pure WO3 leads to a decrease of the crystallites size and a development of the bluish coloration. The color change was found to be reversible under moderate heat treatment or after storage in oxidizing atmosphere and is attributed to the reduced W5+ ions, located at the surface of freshly ground powder. The (1-x) WO3-y. xReO(2) mixtures were found to be composed of monoclinic/orthorhombic WO3 and orthorhombic ReO2 phases with a grain boundary containing reduced W5.7+ ions which are mainly responsible for the compound color at low rhenium ion concentrations. In both cases, the W(6-z+) (0<z less than or equal to 1) color centers are responsible for strong optical absorption resulting in the dramatic decrease of the total Raman intensity. The structural models of free surface in pure ground WO3 and bulk WO3/ReO2 intragrain boundary in (1-x) WO3-y. xReO(2) mixtures are proposed and discussed. (C) 1998 American Institute of Physics. [S0021-8979(98)07322-8].