Raman scattering in sillenite-type crystals

The Raman spectra of Bi₂₄AlPO₄₀ and Bi₃₈ZnO₅₈ crystals have been studied for the first time. The polarization Raman spectra of Bi₁₂GeO₂₀, Bi₁₂TiO₂₀, Bi₂₄AlPO₄₀, and Bi₃₈ZnO₅₈ crystals are also studied. The lines in the Raman spectra of Bi₁₂TiO₂₀, Bi₂₄AlPO₄₀, and Bi₃₈ZnO₅₈ are identified and compared with the lines of the Bi₁₂GeO₂₀ spectra studied earlier. It is shown that the differences in the Raman spectra of the crystals studied are associated with the specific features of their atomic structures.     

Infrared lattice vibrations in Cu2GeSe3

Infrared reflectivity spectra of Cu₂GeSe₃ are measured at room temperature in the wavenumber range from 180 to 4000 cm⁻¹. From an analysis of the spectra the parameters of four vibrational modes are determined. The experimental results are compared with predictions from group theory. From a comparison of the results for Cu₂GeSe₃ with the vibrational characteristics of other chalcogenides it follows that the force constants of cation-chalcogen bonds increase with increasing valence of the cation.     

Raman scattering in low wavenumber region as a new probe to structural properties of microcrystalline silicon

The structural properties of microcrystalline silicon (μc-Si) are studied by Raman scattering. It is found that the intensity of each Raman band closely correlates with the absorption coefficient in the interband region and that the Raman band at ca. 150 cm^?1 is a sensitive probe to randomness of Si-Si bonding structure in μc-Si.     

Surface enhanced Raman scattering phenomenon

Surface enhanced Raman scattering and surface enhanced fluorescence phenomena are the result of resonance excitation of localized surface plasmons in nanoparticles of some metals. By appropriate choice of the shape and size of metallic nanoparticles, the kind of adsorbed molecules and the wavelength of the exciting light, it is possible to achieve enhancement of the intensity of Raman scattering equal to 10¹²‐10¹⁴. This means that only some molecules inside the exciting light beam are sufficient to record the Raman spectrum. Electromagnetic and chemical mechanisms of enhancement of Raman scattered light and methods of roughness investigation of surfaces exhibiting this promising phenomenon are described in this paper. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface enhanced Raman scattering in an amorphous matrix for Raman amplification

In order to improve the efficiency of Raman Amplifiers, the Surface Enhanced Raman Scattering (SERS) effect of an amorphous matrix of TiO₂ was studied. First, optimisation of the amorphous layer quality was performed by depositing thin films on glass substrates at different temperatures. Then, thin films of amorphous TiO₂ were deposited on silicon commercial gold SERS substrates (Klarite®) by a dip-coating process. The SERS effect was demonstrated by the great difference of Raman intensities of the amorphous TiO₂ matrix dip-coated on active and inactive parts of Klarite® substrate under 633 nm and 780 nm laser excitations in the tail of the Surface Plasmon Resonance band of gold nanoparticles.     

Low frequency Raman scattering in chalcogenide glasses

Low frequency Raman spectra of chalcogenide glasses are analyzed in terms of matrix element effects and modes of a layered structure. The spectra of GeSe₂ at low temperature shows no peaks which can be assigned to layer modes. The reduced spectra indicates that the density of states exhibits nearly ω² dependence for ω < 60 cm^?1, and the coupling constant approaches ω² dependence at frequencies less than 20 cm^?1.     

The Raman coupling function in v-GeO2 and v-SiO2: A new light and neutron scattering study

We report inelastic Raman and neutron scattering spectra for the network glass formers vitreous silica (v-SiO₂) and vitreous germania (v-GeO₂) measured at temperature from 10 to 300 K. The ability to determine the temperature dependence of the luminescence background in Raman scattering has allowed to obtain the Raman coupling function C(ω) and in particular, its low-frequency limit. This study indicates that C(ω) has a linear behavior near the Boson peak maximum and below.     

Inelastic neutron and low-frequency Raman scattering in a niobium-phosphate glass for Raman gain applications

We present measurements of the vibrational spectrum of a binary niobium-phosphate glass in the THz frequency range using inelastic neutron and Raman scattering. The spectra of these glasses show a low-frequency enhancement of the vibrational density of states (“boson peak”). Using a recently developed theory of vibrational excitations in disordered solids we are able to reconcile the measured neutron and Raman spectra using fluctuating elastic and Pockels constants as a model concept. As the spontaneous Raman susceptibility is a key parameter for Raman amplification our results suggest a significant gain profile for application of niobium-phosphate glasses in Raman amplifiers.     

Low-frequency Raman scattering and small-angle X-ray scattering of glasses inclined to phase decomposition

Low-frequency (<1000 cm⁻¹) Raman scattering of lithium aluminosilicate (12Li₂O : 15Al₂O₃ : 73SiO₂ with 4 mol% TiO2) glasses with addition of titanium dioxide has been studied. With a heat treatment at temperatures 660°C, 700°C, 720°C and 820°C and for various times and sequences of temperature, our samples decompose into nanometer sized dispersed aluminotitanate particles. In Raman spectra of these glasses an evolution of a boson peak was observed. The width of the relatively broad boson band decreases as does the frequency of the band. From small-angle X-ray scattering data we conclude that the boson peak is connected with elastic vibrations of amorphous or crystalline regions of inhomogeneity with a dimension of ∼1.7 nm in initial glasses or larger depending on the heat treatment sequences.     

Resonant scattering from liquid crystal devices: an in-situ structural probe for the smectic phases

Resonant X-ray scattering is a relatively new technique in the field of liquid crystals and in recent years has been used to investigate the structures of the SmC* sub-phases. This work has been carried out on several materials which include either a sulfur or selenium atom and has revealed the detailed biaxial structure of the four layer intermediate (ferrielectric) phase. Resonant X-ray scattering has also been used to produce the first in-situ measurements of the chiral smectic sub-phases in a liquid crystal device. Both the antiferroelectric and the four-layer intermediate phases have been observed via this technique in a device geometry. Electric field studies have also been carried out in these phases whilst monitoring phase structure simultaneously. In this paper I will summarize the most up-to-date results from resonant scattering in free-standing films and discuss resonant scattering experiments on liquid crystal devices.     

Raman study of liquid sulfur

The Raman spectra of liquid sulfur up to 300°C were measured and discussed. The spectral features were apparently changed over 160°C and these were explained in terms of the ratio between S₈ rings and sulfur chains. The spectral feature of sulfur chains were discussed.     

Observation of Raman scattering from N2 in a silica optical fiber

We have observed Raman scattering from N₂ in a silica optical fiber which had been pressurized in an N₂ atmosphere. The Raman shift is 2190 cm^?1 and the half-width of the line is 90 cm^?1 (fwhm).     

Low-energy excitations in polyvinyl chloride: Raman scattering and thermal properties

The low-energy excitations of a very fragile glass-former (in the sense of Angell), poly(vinyl chloride), have been studied by low-frequency Raman scattering and by measurements of the low-temperature heat capacity and thermal conductivity. Two different samples were investigated : one with a crystallinity of about 15%, the other quenched and amorphous, as measured by small-angle neutron scattering. The boson peak in Raman scattering was observed in both samples even at room temperature. A clear correspondance between the Raman boson peak, the excess of heat capacity and the plateau of thermal conductivity was shown. It is confirmed that the boson peak or the heat capacity excess are relatively small in this very fragile glass-former. However it is deduced that the high concentration of tunnelling systems in the amorphous sample is the result of a rapid quenching rather than an intrinsic property of fragile glass-formers.     

Short range order in anomalous liquid metals

Liquid metals with anomalous physical properties such as increasing sound velocity with temperature or density anomalies, exhibit a complex structure in their one dimensional experimental diffraction patterns. Typically, their radial distribution functions are characterized by an asymmetric first peak and a subsidiary peak or shoulder on the right hand side of the first main peak. It has been hypothesized that the complex structure is associated with short range ordering the liquid. Specifically in the liquid pnictides, it has been proposed that such order may be associated with the underlying solid A7 structure. We present an analysis of the short range order in liquids using a modified quasi-crystalline model of liquid structure. This model is shown to fit the experimental radial distribution function very well and to reproduce the experimentally observed structure factor. Using this model we find that the short range order in the liquid pnictides is dominated by an A7-like structure with two types of bonds, in close agreement with the underlying solid phase. The existence of two bond lengths is necessary within this model to explain the asymmetry in the first peak as well as the change in coordination number along the pnictide series. The quasi-crystalline model is discussed and shown to correlate with the Lindemann melting criterion.     

Diffuse scattering and chemical short range order in binary metallic glasses

X-ray and neutron scattering from binary metallic glasses is discussed in the context of the Bhatia-Thornton equations for the structure factor. For neutrons a generalised form of the Bhatia-Thornton equation must be used when there are different isotopes present. By making a distinction between correlated and uncorrelated scattering centres an expression can be derived which shows clearly the different contributions to the total scattered intensity. Information about the relative visibility of the alloy components can be obtained from this expression and a figure of merit or degree of contrast between the species defined, which is helpful in understanding results of experiments performed to date.     

Pressure dependence of the Boson peak in glassy As2S3 studied by Raman scattering

A detailed pressure dependence study of the low energy excitations of glassy As₂S₃ is reported over a wide pressure range, up to 10 GPa. The spectral features of Boson peak are analyzed as a function of pressure. Pressure effects on the Boson peak are manifested as an appreciable shift of its frequency to higher values, a suppression of its intensity, as well as a noticeable change of its asymmetry leading to a more symmetric shape at high pressures. The pressure-induced Boson peak frequency shift agrees very well with the predictions of the soft potential model over the whole pressure range studied. As regards the pressure dependence of the Boson peak intensity, the situation is more complicated. It is proposed that in order to reach proper conclusions the corresponding dependence of the Debye density of states must also be considered. Comparing the low energy modes of the crystalline counterpart of As₂S₃, as well as the experimental data concerning the pressure dependencies of the Boson peak frequency and intensity, a structural or glass-to-glass transition seems to occur at the pressure ∼4 GPa related to a change of local dimensionality of the glass structure. Finally, the pressure-induced shape changes of the Boson peak can be traced back to the very details of the excess (over the Debye contribution) vibrational density of states.     

Raman scattering and nonlinear refractive index measurements of optical glasses

The absolute Stokes-Raman scattering cross section is measured for optical glasses of varying composition, from which the nuclear contribution to the nonlinear polarizability is determined. The nonlinearities increase progressively from fluoroberyllate compositions to fluorophosphate to phosphate to silicate and finally to heavy-metal oxide glasses. Although values of the total nonlinear polarizability vary by nearly two orders-of-magnitude it is determined that the ratio of the nuclear to the electronic contribution is nearly constant and is independent of the glass composition. The electrostrictive contribution to the nonlinear polarizability is also calculated using previously measured elasto-optic constants. Correlations in the atomic structure were determined by measuring the position of the lowest-frequency Raman scattering peak and it is found that the correlation length for pure one-component glasses is larger than multi-component glasses. Examination of the low-frequency scattering region shows that there is an 0xcess light-scattering below 30 cm^?1 in all glasses tested. This excess scattering in LaSF-7 was fitted between 2 to 20 cm^?1 to a Lorentzian centered at zero frequency with a halfwidth of 2.5 ± 1 cm^?1, while the fit in pure silica between 5 and 30 cm^?1 yields a halfwidth of 8 ± 3 cm^?1.