Fast imaging of the plume expansion produced by laser ablation of LiNbO3

Fast photography of the plume produced by laser ablation of LiNbO₃ in vacuum has been performed using an image intensified CCD (ICCD) camera in a time interval up to 2 7s after the laser pulse. Two differently oriented single crystalline LiNbO₃ targets were used. The results show that although the emission intensity of the laser-generated plume initially depends on the crystalline orientation of the target, it reaches a stationary state after several minutes which is the same for both targets orientations. Under these stationary conditions, the angular distribution of the Li atoms is found to be broader than that of Nb atoms. The observed less forward directed expansion of the Li species may explain the poor Li content normally observed in films grown by laser ablation of LiNbO₃ in vacuum.     

Influence of surface effects on the performance of lead-niobium-germanate optical waveguides

Lead-niobium-germanate planar waveguides have been produced by pulsed laser deposition. The composition of the waveguides is found to be relatively weakly dependent on the laser fluence, while their surface morphology is affected dramatically. Smooth surfaces are obtained for a narrow fluence range centered at 2.0 J/cm², while particulates having typical diameters of <0.5 μm or droplets with typical diameters of <10 μm are observed at lower and higher fluences, respectively. The refractive index of the waveguides increases with fluence up to 2.1 at 2.0 J/cm², which is close to the value of the bulk glass, and remains constant at higher fluences. Propagation losses show instead a minimum (≈6.5 dB/cm) at 2.0 J/cm². The characteristics of the ablation process that leads to the ejection of solid particulates or molten droplets as well as the increase of the waveguides density on increasing the fluence are discussed to be responsible for the observed optical behavior.     

Ultrashort-laser-pulse-driven rewritable phase-change optical recording in Sb-based films

This paper reviews the work we have carried out over the last years on the development of ultrashort-laser-pulse-driven, rewritable, phase-change optical memories. The materials we have tailored for this application are non-stoichiometric, Sb-rich amorphous thin films, which can be crystallized upon irradiation with ultrashort laser pulses, showing a large optical contrast upon transformation. This result makes them very promising for the development of rewritable phase-change optical memories under ultrashort pulses, since the reversibility of the process has also been demonstrated. Adequate control of the heat-flow conditions has allowed us to achieve a full transformation time faster than 400 ps for crystallization/amorphization using 30-ps pulses. The crystallization threshold fluence has been found to decrease upon irradiation with pulses shorter than 800 fs, thus suggesting the relevance of high-electronic-excitation-induced processes in the amorphous-to-crystalline phase transition. This has been confirmed by the observation of an ultrafast, non-thermal phase transition occurring 200–300 fs after the arrival of the laser pulse at the surface, for fluences above the crystallization threshold. The presence of this transient phase conditions the final state induced therefore enabling the applicability of this material as a rewritable recording medium using femtosecond pulses. Received: 11 October 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/564-5557, E-mail: J.Solis@io.cfmac.csic.es     

Interdiffusion studies in Bi-based layered systems with nanosecond laser pulses

Interdiffusion processes are induced by nanosecond laser pulses from an excimer laser. The Bi-based systems studied are formed by a Bi layer and a Sb or Ge layer. Configurations with Bi at the surface layer or at the innermost layer are both studied. Real-time reflectivity measurements are performed during the irradiation to determine the process kinetics and times and Rutherford backscattering spectrometry is used to obtain the concentration depth profiles. It will be shown that there is an interfacially initiated diffusion process in the Bi-Sb system and that the diffusion coefficients of this system within the liquid phase are in the 10^–5–10^–6 cm²/s range. The Bi-Ge system shows instead little mixing, the diffusion coefficients of the system within the liquid phase being at least two orders of magnitude lower. The differences observed when Bi is the surface layer or the innermost one are related to the different thermal responses of the system.     

QCD phase transitions from relativistic hadron models

The models of translationally invariant infinite nuclear matter in the relativistic mean field models are very interesting and simple, since the nucleon can connect only to a constant vector and scalar meson field. Can one connect these to the complicated phase transitions of QCD? For an affirmative answer to this question, one must consider models where the coupling contstants to the scalar and vector fields depend on density in a nonlinear way, since as such the models are not explicitly chirally invariant. Once this is ensured, indeed one can derive a quark condensate indirectly from the energy density of nuclear matter which goes to zero at large density and temperature. The change to zero condensate indicates a smooth phase transition.     

Critical Evaluation of Kinetic Method Measurements: Possible Origins of Nonlinear Effects

The kinetic method is a widely used approach for the determination of thermochemical data such as proton affinities (PA) and gas-phase acidities (ΔH° _acid ). These data are easily obtained from decompositions of noncovalent heterodimers if care is taken in the choice of the method, references used, and experimental conditions. Previously, several papers have focused on theoretical considerations concerning the nature of the references. Few investigations have been devoted to conditions required to validate the quality of the experimental results. In the present work, we are interested in rationalizing the origin of nonlinear effects that can be obtained with the kinetic method. It is shown that such deviations result from intrinsic properties of the systems investigated but can also be enhanced by artifacts resulting from experimental issues. Overall, it is shown that orthogonal distance regression (ODR) analysis of kinetic method data provides the optimum way of acquiring accurate thermodynamic information.      

Early stages of laser mixing process in Sb/Ge multilayer thin films

The onset of mixing at the interfaces between Sb and Ge in thin multilayered films containing two or four layers has been studied. The films were irradiated with nanosecond laser pulses in order to trigger mixing, and in situ reflectivity measurements were used to follow the transformation in real-time. Cross sectional transmission electron microscopy analysis was used to study both the structure and the composition profile before and after irradiation.A threshold irradiation energy exists for the onset of mixing, below which roughening of the interface between the layers is observed, together with recrystallization of the surface Sb layer following melting. The results are consistent with a melting/diffusion process which is inhomogeneously nucleated at the interface between the top Sb and Ge layers. Once mixing is initiated an amorphous Sb-Ge layer of constant thickness is formed, corresponding to mixing along a well defined planar melt front. Voids are observed at the former Sb/Ge interface, which may be related to interfacial stress in the as-grown configuration.     

Interdiffusion at Sb/Ge interfaces induced in thin multilayer films by nanosecond laser irradiation

Thin films consisting of 3 or 4 Sb and Ge alternating layers are irradiated with single nanosecond laser pulses (12 ns, 193 nm). Real time reflectivity (RTR) measurements are performed during irradiation, and Rutherford backscattering spectrometry (RBS) is used to obtain the concentration depth profiles before and after irradiation. Interdiffusion of the elements takes place at the layer interfaces within the liquid phase. The reflectivity transients allow to determine the laser energy thresholds both to induce and to saturate the process being both thresholds dependent on the multilayer configuration. It is found that the energy threshold to initiate the process is lower when Sb is at the surface while the saturation is reached at lower energy densities in those configurations with thinner layers.     

Thermal characterization of the solid state and raw material fluconazole by thermal analysis and pyrolysis coupled to GC/MS

This article had studied the thermal characterization of the raw material and different fluconazole crystals, obtained through recrystallization with different solvents using thermoanalytical techniques (TG, DTA, DSC-50, DSC Photovisual, DSC-60) and Pyr-GC/MS. The results confirmed that the fluconazole volatilizes without decomposition until 250 °C. Pyr-GC/MS showed hexachlorobenzene like impurities in fluconazole raw material.     

Pyridines and pyridine derivatives from vinyl allenes and imines

The Lewis acid mediated reaction of acyclic vinyl allenes and imines, yielding tetrahydropyridines has been carried out. Only one cycloadduct was observed in each case, and thus, the reaction progressed in all cases with total regio-, face- and endo/exo selectivities. The cycloadducts were transformed into polysubstituted pyridines, including bipyridines, by catalytic transfer hydrogenation using cyclohexene as hydrogen donor in good yield. The reaction was extended to the aromatization of the bicyclic cycloadducts prepared in previous works from the reaction of semicyclic vinyl allenes and imines, yielding tetrahydropyridines.