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Raman distributed optical fiber temperature sensors are based on the intensity ratio of the anti-Stokes to the Stokes Raman band at 440 cm−1 of silica. In this paper we predict that the sensitivity of the Raman measurements for low temperatures can be improved by considering the Boson peak in the low frequency Raman scattering domain at 60 cm−1. In this way Raman temperature sensors can be performed down to cryogenic temperatures. It is further shown that the Boson peak is less dependent than the 440 cm−1 band to the polarization of light. For the usual excitation at 1550 nm the anti-Stokes Boson peak at 1536 nm is in the low loss transmission window of the silica fibers.

Amorphous nanoheterogeneities of the size less than 100 A have been formed in glasses of the Li2O–Nb2O5–SiO2 (LNS) and Li2O–ZnO–Nb2O5–SiO2 (LZNS) systems at the initial stage of phase separation and examined by transmission electron microscopy, small-angle X-ray and neutron scattering. Both LNS and LZNS nanoheterogeneous glasses exhibit second harmonic generation (SHG) even when they are characterized by fully amorphous X-ray diffraction (XRD) patterns. Chemical differentiation and ordering of glass structure during heat treatments at appropriate temperatures higher Tg lead to drastic increase of SHG efficiency of LNS glasses contrary to LZNS ones in the frame of amorphous state of samples. Following heat treatments of nanostructured glasses result in crystallization of ferroelectric LiNbO3 and non-polar LiZnNbO4 in the LNS and LZNS glasses, respectively. Taking into account similar polarizability of atoms in LNS and LZNS glasses, the origin of the principal difference in the second-order optical non-linearity of amorphous LNS and LZNS samples is proposed to connect predominantly with the internal structure of formed nanoheterogeneities and with their polarity. Most probably, amorphous nanoheterogeneities in glasses may be characterized with crystal-like structure of polar (LiNbO3) phase initiating remarkable SHG efficiency or non-polar (LiZnNbO4) phase, which do not initiate SHG activity. It gives an opportunity to vary SHG efficiency of glasses in a wide rage without remarkable change of their transparency by chemical differentiation process at the initial stage of phase separation when growth of nanoheterogeneities is ‘frozen’. At higher temperatures, LiNbO3 crystals identified by XRD precipitate in LNS glasses initiating even more increase of SHG efficiency but visually observable transparency is impaired.

Glass isotropy could be broken by using the electric field of the 355 nm wavelength delivered by a nanosecond YAG: Nd 3+ laser. In this way, pulsed laser-induced birefringence was obtained in the La 2 O 3 –B 2 O 3 –GeO 2 (LBG) glassy system. Buried lines behaving like waveguides were written in glasses using this laser irradiation process. Micro Raman spectra performed in irradiated points of glasses revealed a rotation of the light polarization in agreement with previous results obtained on thermally-poled LBG glasses. However, no orientation effects of irradiated points related to the direction of the laser wave polarization (UV poling) could be clearly evidenced as suggested in our last paper. The interaction of the glass with the electromagnetic field of the UV laser beam can be more probably described by mechanisms based on a densification process.

The submicroscopic structure of lithium niobium silicate glasses of the compositions 2xLiNbO3 · (1 − x)(Li2O · 2SiO2) (x = 0.40, 0.45, 0.50) and 30Li2O · 25Nb2O5 · 45SiO2 in the initial state and after heat treatment for different times at temperatures in the vicinity of the glass transition point T g are investigated using X-ray powder diffraction, small-angle neutron scattering (SANS), synchrotron small-angle X-ray scattering (SAXS), and electron microscopy. A nanostructure with inhomogeneities ∼40 A in size is formed in glasses at the initial stages of phase separation at temperatures in the range 600–670°C. This structure is responsible for the appearance of the second-order optical nonlinearity. The SANS, SAXS, and electron microscopic data on the inhomogeneity size are in good agreement with each other. According to the X-ray diffraction, SANS, and SAXS data, the ordering of the glass structure and the difference between the density of inhomogeneities and the density of the matrix increase in the course of heat treatment. At the initial stage of amorphous phase separation, the glass decomposes into regions enriched in SiO2 and regions with an increased content of lithium and niobium. An increase in the temperature or time of heat treatment results in the precipitation of LiNbO3 ferroelectric crystals. The results obtained allow us, for the first time, to make the inference that nanoscale changes in the glass structure lead to considerable changes (by one order of magnitude and more) in the quadratic optical nonlinearity, which can be controlled by heat treatment. The origin of the second-order optical nonlinearity is associated with both the nanosized modulations of the polarizability due to the inhomogeneous glass structure and the polarity of structural nanoinhomogeneities from which the LiNbO3 phase precipitates at the later stages of phase separation.

The structure and crystallization behavior of glasses with 25La2O3 · 25B2O3 · 50GeO2 composition, melted in platinum (P glass) and corundum (A glass) crucibles, were studied by DTA, X-ray diffraction and FTIR spectroscopy. The Al2O3 dissolved from corundum crucible in the A glass was estimated to be in the range 5–7 wt%. This alumina content had almost no influence on glass transition temperature but strongly affected the structure and crystallization behavior of the A glass. In fact, the P glass showed good texture-forming ability: high quality textured glass-ceramic plates based on stillwellite-like LaBGeO5 crystals were easily obtained. On the contrary, the presence of alumina stabilized the A glass from which binary phases crystallize first, and only afterwards they are transformed in stillwellite by secondary crystallization: so in this glass texturing is hindered. Crystallization and texturing behavior of P and A glasses were well related to FTIR data. P glass contained both threefold and fourfold coordinated boron while in the A glass the presence of aluminum forced boron to assume almost exclusively threefold co-ordination. Hence the easier crystallization of stillwellite phase and the good textures obtained from the P glass contrary to the A glass, can be well understood since all boron atoms have tetrahedral co-ordination in stillwellite LaBGeO5 crystal.

UV assisted photoelectron emission measurements are carried out using a xenon lamp as the excitation source. Free-standing diamond surfaces treated at diverse experimental conditions give rise to varying photocurrent intensities. In order to gain a better understanding of these phenomena, different surfaces are prepared: as-grown, hydrogenated, oxidized and annealed. The hydrogenated film is obtained from optimised treatment in hydrogen plasma. The highest emission photocurrent is obtained with the hydrogenated diamond film in agreement with in situ surface conductivity values. Moreover, in situ photoelectric measurements performed in a vacuum chamber demonstrate that the emission photocurrent increases when the sample is exposed to air. Indeed, it is now agreed that the formation of an adsorbate layer on hydrogenated surfaces leads to a decrease of the work function. Two electrons emission thresholds are observed: the highest energy emission is close to the bandgap value (5.5 eV) and the lowest is located near 4.5 eV corresponding to defect levels in the bandgap. Both emissions are dependent on the surface coverage but the main striking result is the increase of the intensity of the 4.5 eV emission for hydrogenated diamonds in contact with air. Optical absorption experiments are compared with the photoconductivity measurements. Laser excited photoluminescence at 514 nm revealed defects corresponding to a 690 nm broad band emission sensitive to the surface coverage.

Photoelectric properties of carbon-based thin films are investigated by UV pulsed laser-induced photoelectric charge measurements. Photocathodes are thin films deposited on different substrates by vacuum deposition methods. Laser irradiation consists of fifth harmonic (λ=213 nm) picosecond pulses of a Nd-YAG laser. Samples are irradiated at low laser intensities (approx. a few MW/cm2) in order to avoid pronounced structural transformations at their surface. Quantum yield (QY) values are determined at 213 nm; UV laser-induced surface cleaning process and air contamination effects on electron emission are studied. The highest QY value is obtained for the hydrogenated diamond photocathode according to its low surface potential barrier (NEA emitter). This QY value is decreasing after air exposure of the post-irradiated sample; on the other hand, electron emission from nanostructured fullerene films can be enhanced by a UV surface cleaning process.

Films composed of Ni nanocrystals placed in an amorphous carbon matrix were investigated by photoelectric methods. Information about the structure of Ni nanocrystals and the amorphous carbon matrix were obtained from TEM and Raman spectroscopy, respectively. The lowering of the photoelectric threshold (from 2.61 to 2.55 eV) as well as an increase of the photoelectric charge sensitivity after film cleaning with UV laser light was found.

In this paper, we investigate UV picosecond laser-induced effects on undoped chemical vapor deposited (CVD) diamond surfaces as a function of irradiation time. Diamond films are negative electron affinity (NEA) emitters and contain defects (non-diamond phases). Surface analysis is performed before and after laser irradiation, in order to clearly demonstrate surface structural transformations. A diamond peak shift after pulsed laser irradiation is determined by micro-Raman analysis and correlated to stress variations in diamond. The post-irradiated diamond surface reactivity with ambient air is studied by X-ray photoelectron spectroscopy (XPS). XPS results show surface bonding modifications due to carbon–oxygen interaction enhanced by UV pulsed laser irradiation.

Hydrogenated amorphous carbon films (a-C:H) were deposited by plasma enhanced chemical vapour deposition using a planar RF discharge and pure methane gas. The influence of the negative self-bias (varying from 0 to 400 V) on physical properties is investigated. The density of the materials, characterized by X-ray reflectivity, varies from 1.3 to 1.8 g/cm 3 . The lowest density obtained for the grounded (0 V) film is related to the porosity. Curvature measurements reveal high compressive stress (2.6 GPa) for the film deposited with a self-bias close to 100 V. These different behaviours are related with the different structures (polymerlike, diamondlike, graphitelike) depending on the self-bias used during the deposition process. It is also shown that in spite of these different structures, the surface energies calculated from contact angle measurements using different liquids are similar whatever the used self-bias.

Dimethacrylate based networks usually have a poor impact resistance. In this work, we have tried to understand the origin of this brittleness. Two systems based on a same dimethacrylate monomer polymerised with styrene and divinylbenzene comonomers, respectively, have been chosen to correlate structural parameters with mechanical properties. The increase of structural heterogeneity, characterized by the width of relaxation time distribution, was measured as a function of double bonds conversion using dynamic mechanical tests. Atomic force microscopy observations of network structure after laser ablation show that the heterogeneity of networks is spacially organised due to the formation of microgels and their agglomeration. The presence of microgels strongly affects the polymerisation kinetics and controls the mechanical behaviour. Results show that the more densely crosslinked the network, the more heterogeneous is its structure. The impact resistance is shown to be related to the level of networks heterogeneities.

In situ reflectivity measurements were carried out at room temperature and atmospheric pressure to follow dynamic structural variations occurring during pulsed laser irradiation of diamond-like carbon (DLC) thin films. Laser irradiation consisted of second harmonic (λ = 532 nm) picosecond pulses of a Nd-YAG laser, leading to a modification of the carbon layer structure. Irradiation at this wavelength usually leads to graphitization, i.e. an increase of the number and/or size of sp 2 clusters dispersed in the amorphous carbon structure of DLC films. One of the consequences of such a transformation is a pronounced change in the optical properties. Upon graphitization, the optical absorption strongly increases in the near-infrared region. In order to monitor such changes we used a probe beam tunable in this wavelength region. An optical parametric generator (OPG) and amplifier (OPG/OPA) with low energy pulses tunable in the near-infrared range between 700 nm and 2 μm was used to record reflectivity changes at several wavelengths. Such an in situ characterization reveals how the structural transformations proceed in the early phase of graphitization.

In this paper the results of an investigation of two multi-phase film systems composed of Pd nanocrystals placed in carbonaceous or fullerenes matrices are presented. The films were obtained by thermal deposition (from two separated sources at the same time) of an Pd acetate and C60/C70 fullerenes mixture from two separate sources. Structural differences of the films were found by electron diffraction, transmission electron microscopy and Raman spectroscopy. The Raman D- and G-bands belonging to amorphous carbon vibrations and bands that could be attributed to polymerised fullerenes were observed for one kind of film. Raman bands that could be connected to a new fullerene crystalline structure were found for the second kind of film. The Raman spectra also suggest the presence of N2 molecules superficially bonded to the films. The photoelectric work function of the films was measured under ultra high vacuum conditions with Xe-lamp excitation. The measured value of the work function for both kinds of films was 2.75 eV. The I–U characteristics were also measured for these films. Fowler–Nordheim analysis results were compared with results of photoelectric measurements.

Picosecond pulsed laser irradiation of several metallic photocathodes (W, Ta, Al and Au) was carried out along with work function measurements during the irradiation process, work function measurements were performed before irradiation and after different irradiation periods. Metals used in this study behaved differently according to their affinity towards oxygen. Most of these metal surfaces were covered with a native oxide layer, except gold. Photolythic or thermal processes could occur depending on the laser wavelength, in the former case, the native oxide layer was removed pulse by pulse while in the latter, surface oxidation was enhanced. For the tungsten and tantalum samples, the kinetics of oxide removal occurred in distinct stages relative to the native oxide multilayer structure. In the case of the Au sample, no native oxide layer formed on its surface (noble metal) and the laser irradiation induced a surface smoothing only. A linear increase of the photoelectric signal was observed during laser irradiation of the aluminium sample probably due to the partial surface oxide removal and the sub-surface oxide incorporation. The presence of various solid phases of the oxide layer could be determined from pulsed laser irradiation measurements.

International audience; In this paper, amorphous titanium dioxide (TiO2) thin films have been deposited on a commercially available Klarite substrate using the sol-gel process to produce surface-enhanced Raman scattering (SERS). The substrate consists of square arrays of micrometer-sized pyramidal pits in silicon with a gold coating. Several thin TiO2 layers have been deposited on the surface to study the influence of film thickness. Ultimately, we obtained information on SERS of an amorphous TiO2 layer by gold nanostructures, whose range is less than a few nanometers. Mechanisms responsible for the enhancement are the product of concomitant chemical and electromagnetic effects with an important contribution from plasmon-induced charge transfer

In this study, we investigate the photoelectric emission from CVD diamond films. These diamond samples present NEA properties due to their as-grown surfaces terminated with hydrogen atoms. Photocathodes are characterised by UV pulsed laser-induced photoelectric measurements and photoelectric threshold measurements because the photoelectric emission is strongly dependant on the electron affinity of the diamond surface. Photoelectric threshold measurements show the existence of a sub-bandgap signal associated to a defect-band level for both samples, with the lowest value obtained for the highest defect-density diamond film. Moreover, the quantum efficiency of undoped diamond is measured at 213 nm as a function of the CH4 concentration. The highest quantum efficiency value is measured for the highest defect-density diamond film. Surface bonds modifications occurring during a prolonged laser irradiation are responsible for the decrease in the photoemissive performances of diamond films. # 2000 Elsevier Science B.V. All rights reserved.

By using laser-induced photoemission and complementary in situ monitoring of photoelectric response spectra, damage threshold values have been determined for three wavelengths (213, 266 and 532 nm) of a pulsed picosecond Nd-YAG laser. The photoelectric signal is extremely sensitive on the surface state of the diamond-like carbon (DLC) film, therefore laser-induced surface transformations could be observed. Measuring the photoelectric signal and the charge of ionized species stemming from ablation processes, laser damage threshold values could be determined in situ with a very high sensitivity surpassing usual optical methods. Results were analyzed in terms of applied power density and number of shots, and structural changes were characterized by ex situ Raman measurements before and after illumination.

Laser-induced surface damage introduces a dramatic change in the photoelectric properties of metallic and semiconductor substrates. Hence, by varying the applied laser intensity, ultra-short- pulsed laser-induced photoemission (in the mono- or multiphotonic regime according to the applied wavelength) can be used to monitor in situ surface structural changes. Results for a model system (thin diamond-like carbon (DLC) layer on an Si carrier) are presented, where, after ablation of the DLC layer, we observed a rapid increase in the photoelectric contribution from the underlying substrate. High-sensitivity measurements are presented for Au and W substrates where due to laser- induced damage within the escape depth of electrons and because of the apparition of ionized species upon ablation, the measured charge can indicate the onset of surface damage. Results are presented for various wavelengths (213 nm, 266 nm and 532 nm) and discussed in the light of previous results of threshold measurements obtained by optical methods.

K+-implanted W samples with various implantation depths were investigated. Generally, implantation of alkali ions gives rise to two competitive effects: it lowers the surface work function, however it enhances surface oxidation too which in turn leads to a slight work function increase. In opposite to alkali overlayers, implanted species confined within a `metallic cage' resist the applied laser irradiation and alkali removal occurs only to a small extent. Measurements of laser-induced photoelectric charge pulses indicate a variation of the photoemission yield as a function of surface oxide thickness. Moreover, following the charge pulse evolution over a longer period (2–3 h), one finds that the laser-induced oxide removal characteristics depends on the implantation parameters as well. Results are compared to those obtained for pure, non-implanted W and the mechanisms responsible for the work function lowering are discussed.

Photocurrent measurements yielded new data, which were used to determine the surface work function of a native oxide-covered tungsten photocathode. The photoemission was generated by a continuous UV source. The surface work function has been measured at various stages of a pulsed UV laser-induced oxide removal in order to characterize the process. Since the measured surface work function can be correlated with the surface coverage, along with the measurements of charge of laser-induced photoelectrons a description of the laser–oxide interaction is presented. A fully computer-controlled experimental setup is demonstrated, achieving photoelectric measurements at the spot of laser illumination. This apparatus enables us to follow the work function temporal dependence during laser illumination, thus giving a detailed description for the surface coverage dynamics.

A native oxide film on a metallic surface corresponds generally to a multilayer. We describe a method using a number of picosecond laser pulses at 213 nm to achieve partial removal of the oxide overlayer on a polycrystalline W substrate under UHV conditions. The process of oxide removal was monitored by measuring the charge of emitted photoelectrons by using an appropriate anode (collector)–cathode (target) setup, and simultaneously, the surface reflectivity for each pulse. The temporal evolution of the photoelectric charge shows – as expected – an increasing tendency due to the lowering of the surface work function, but moreover – under certain experimental conditions – it shows the presence of several kinks, which are interpreted as a sign of changes in the surface morphology. The results are discussed within the frame of a model describing mono/multilayer desorption phenomena.

The fifth harmonic (213 nm) of a picosecond Nd 3+ :YAG laser was used to remove small fractions of the covering oxide overlayer from polycrystalline metallic photocathodes. The evolution of the photoemitted charge (high-density electron pulses) was followed along with the surface reflectivity change, and they were interpreted in terms of the work function change of the surface. The careful choice of the laser fluence enables us to investigate the cleaning kinetics within a relatively narrow coverage range. Moreover, after recontaminating the surface at different pressures, we observe distinct kinetics as well. This latter effect is explained by the various rates of oxygen incorporation into the oxide lattice, resulting in a looser, rougher surface for high pressures and a more compact oxide for low pressures.

The nanostructure and nonlinear optical properties of high-niobium glasses in the (1 - x)KNbO3-xSiO2 system with an SiO2 content x = 0.05-0.35 have been studied by small-angle neutron scattering (SANS), electron microscopy (EM), and second-optical-harmonic generation (SHG). Vitreous samples are manufactured by the methods of fast melt cooling (pressing with metal plates and quenching between rotating rolls). Glasses with x < 0.12 have been established to form a micro-inhomogeneous structure in the form of silica-enriched regions at the cooling rates used. According to SANS data, quenched glasses with x > 0.2 are homogeneous, but form a silica-enriched nanostructure after thermal treatments. At temperatures below ∼T g + 50 C, silica-enriched regions manifest a very slight tendency to grow, whereas, according to SANS and X-ray diffraction data, their chemical composition is observed to shift appreciably towards SiO2 with thermal treatment. The obtained data on an inhomogeneous structure allows us to clarify the complicated character of the previously revealed dependence T g (x). Nano-inhomogeneous transparent samples produce a weak SHG signal, which indicates the quasi-periodic modulation of the chemical composition and, correspondingly, polarizability, in the volume of glass. The nonlinear optical phase KNbO3 precipitates at later stages of crystallization, when a glass loses its transparency. In this case, the SHG signal is amplified by several orders of magnitude. © 2013 Pleiades Publishing, Ltd.

The influence of N, W, Mo and Cu addition, alone or combined, on the active and passive corrosion behaviour of type 17Cr-13Ni stainless steels was investigated in de-aerated 2 N H 2 SO 4 solutions. Potentiodynamic polarization curves and current decay curves were determined in this medium. Surface characterization was examined by X-ray photo-electron spectroscopy (XPS).

The pitting corrosion of unimplanted and Ne, N and Mo-implanted 304 stainless steel specimens was studied in a neutral chloride 0.02 M environment. The effect of ageing of the oxide layers during immersion in chloride solution was investigated with respect to pitting corrosion resistance. Pitting potential measurements were performed and showed large differences on both the nature of implanted ion and the ageing treatment: implanted samples exhibited lower pitting potentials than the bare alloy but an improved behaviour was observed after ageing. Surface analyses (XPS and SIMS) were used to detect compositional changes induced by ageing and to explain the experimental results.

International audience; In this paper, we present results on UV pulsed laser-induced birefringence in La2O3- B2O3-GeO2 (LBG) glasses. Samples are irradiated at a wavelength of 355 nm delivered by an Nd-YAG laser operating in the nanosecond regime. After irradiation, glasses are analyzed by Micro Raman spectroscopy and Small Angle X-ray Scattering (SAXS). Raman spectra show figures characteristic of a light polarization rotation effect in agreement with an anisotropic distribution of the scattering intensity observed by SAXS measurements. These results are interpreted as the interaction of the glass with the electromagnetic field of the UV beam (UV poling).

The electrochemical behaviour of a-C:H thin films (1.25 μm) deposited on germanium substrates is studied in aerated solutions by means of rest potential, voltammetric and impedance measurements. All tests indicate that there is no contact between the germanium and the electrolyte after immersion of the electrodes and show the notable stability in aerated salt solutions of a-C:H layers deposited on germanium. According to these results a new equivalent circuit is proposed to simulate the electrochemical behaviour of the electrode/electrolyte interface.

We present results of photoelectric properties of carbonaceous films containing metallic nanocrystals, investigated by pulsed laser-induced photoelectric charge emission and in situ work function measurements. Samples are multiphase carbonaceous thin films composed of Ni nanocrystals embedded in a graphite–fullerene matrix. Photoelectric measurements are performed in a vacuum chamber. Picosecond laser-induced photoelectric charge measurements are performed at 213 nm at a low laser intensity (∼1 MW cm−2). At this intensity, surface cleaning effects are clearly demonstrated by a photoelectric charge increase during pulsed irradiation correlated to an in situ photoelectric threshold lowering. Surface contamination is also observed after the exposition of samples to the ambient air. MicroRaman measurements are carried out before and after laser irradiation in order to control that no structural changes have occurred under the pulsed laser beam.

Glasses at early stages of phase separation, while remaining amorphous, are shown to possess or not to possess second-order optical nonlinearity (SON) depending on specific features of their short-range/medium range order and nanoinhomogeneous structure characterized with IR and Raman spectroscopy, X-ray diffraction, small-angle X-ray and neutron scattering and second harmonic generation (SHG). In the present work it is demonstrated by the example of potassium titanium phosphate glasses of composition near KTiOPO4 stoichiometry. Formation of nanoinhomogeneities, structure of which is similar to the structural motif of some non-linear optical (NLO) crystal, favors SON. Heat treatment of such glasses at temperatures near glass-transition temperature Tg stimulates remarkable SON enhancement keeping transparency and amorphous state. While temperature is increasing SHG grows up to values, comparable with those of NLO crystals with rise of Bragg reflections in the X-ray diffraction pattern of the glass and decrease of its transparency. Revealing correlations between structural peculiarities of glasses and their SON argues preference of description of short- and medium-range order in glasses with composition close to boundaries of glass-forming regions from the position of quasi-crystallite models.

In spite of the interest in nonlinear optical (NLO) properties in glasses, the composite method has been successful only for the b-BaB2O4 polar crystal incorporated in the borosilicate glass matrix. In the present paper, transparent composites based on low melting borate and boron silicate glasses containing PbO or TeO2 and KNbSi2O7 ferroelectric crystal are obtained by a remelting technology. Depending on glass composition, NLO composites are subdivided in two types. The first are composites containing micron or submicron scaled crystals with a high second harmonic generation (SHG) activity, where refractive indexes of crystals and glass matrix are very similar. Such a type of transparent composites is expected to achieve SHG activity in the future comparable with that of single crystals. The second type are ‘nanocomposites’ where initial crystals are dissolved or transformed up to nanoscaled sizes. They are considered to be an analogue of nanostructured glasses exhibiting moderate second order optical nonlinearity.

The effect of dc poling on lanthanum boron germanate glasses has been investigated by means of Raman spectroscopy and birefringence experiments. The results show that the dc poling induces in the glass a breakage of its macroscopic centre of inversion and this effect extends to the whole bulk of the poled glasses.