The influence of substrate material and annealing procedure on the properties of superconducting thin films

Thin layers YBa₂Cu₃O_7–x. are deposited by a laser ablation technique using a pulsed excimer laser operating at 308 nm. The influence of the substrate material and the annealing procedure on the superconducting behaviour of the 123 film and the reactions between the film and the substrate are studied by resistance, X-ray patterns and TEM measurements. The best results are obtained for deposition on (100) SrTiO₃ substrates. The resistance of the 1 m thick film shows a metallic behaviour, an onset in superconductivity at a temperature of 90 K, and has zero resistance at 86 K. The 123 material has a preferential oriented c-axis perpendicular to the surface plane.     

CXMS study of mild steel laser alloyed with CrB2

Laser alloying of surfaces has attracted a great deal of attention for technical applications. By laser alloying of materials it is possible to improve hardness as well as wear and corrosion resistance of the surface without affecting the bulk material. The surface of a mild steel (C45) substrate was laser-alloyed with chromium-boride CrB₂. The chromium-boride was added to the substrate surface by powder injection during laser surface melting with a high power continuous-wave CO₂-laser. The resulting surface layers were studied by surface Mössbauer measurements. The backscattering geometry of Conversion X-ray Mössbauer Spectroscopy (CXMS) was used to study the phase formation in the laser alloyed surface. The results for the treated surfaces are discussed for different samples.     

Simulation of KrF laser ablation of Al2O3-TiC

Previous work by the authors on micromachining of Al₂O₃-TiC ceramics using excimer laser radiation revealed that a columnar surface topography forms under certain experimental conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations show that the columns develop from small globules of TiC, which appear at the surface of the material during the first laser pulses. To understand the mechanism of formation of these globules, a 2D finite element ablation model was developed and used to simulate the time evolution of the temperature field and of the surface topography when a sample of Al₂O₃-TiC composite is treated with KrF laser radiation. Application of the model showed that the surface temperature of TiC rises much faster than that of Al₂O₃, but since TiC has a very high boiling temperature, its vaporization is significant only for a short time. By contrast, the surface temperature of Al₂O₃ rises above its boiling temperature for a much longer period, leading to a greater ablation depth than TiC. As a result, a small TiC globule stands above the Al₂O₃ surface. The results of the model are compared with experimental measurements performed by AFM. After three pulses, the height of the globules predicted by the model is about 340 nm, in good agreement with the height measured experimentally, about 400 nm.     

Plasmonic enhancement of the vanadium dioxide phase transition induced by low-power laser irradiation

Nanocomposites consisting of gold nanoparticle (NP) arrays and vanadium dioxide (VO₂) thin films are noteworthy for the tunability of both their thermal and optical properties. The localized surface plasmon resonance (LSPR) of the Au can be tuned when its dielectric environment is modulated by the semiconducting-to-metal phase transition (SMT) of the VO₂; the LSPR itself can be altered by changing the shape of the NPs and the pitch of the NP array. In principle, then it should be possible to choose a combination of VO₂ film and Au LSPR properties that maximizes the overall optical response of the nanocomposite. To demonstrate this effect, transient transmission measurements were conducted on lithographically fabricated arrays of Au NPs of diameter 140?nm, array spacing 350 nm, and covered with a 60?nm thick films of VO₂ via pulsed laser deposition. Both Au::VO₂ nanocomposites and bare VO₂ film were irradiated with a shuttered 785?nm pump laser, and their optical response was probed at 1550?nm by a fixed-frequency diode laser. The Au::VO₂ nanocomposite exhibited an increased effective absorption coefficient 1.5 times that of the plain film and required 37?% less laser power to induce the SMT. The time-dependent temperature rise in the film as a function of laser intensity was calculated from these measurements and compared with both analytic and finite-element models. Our results suggest that Au::VO₂ nanocomposites may be useful in applications such as thermal-management coatings for energy efficient ??smart?? windows.     

Preparation and properties of sputtered Ho x Mo6S8−y thin films

Films of Ho _x Mo₆S_8–y , in which ferromagnetism and superconductivity can coexist, have been made by a d.c. getter sputtering method. These films have been characterized by x-rays, ion beams, measurements of resistance versus external magnetic fields and temperature, critical temperature and a.c. susceptibility. Resistance measurements in zero magnetic field did not show reentrance to the normal state in contrast to the a.c. susceptibility measurements which did show reentrant behaviour at a temperatureT _{c 2}=0.6 K.Resistance measurements in external magnetic fields parallel to the film surface show that this field induces reentrance to the normal state atT _{c 2}=0.6 K whereas in perpendicular magnetic field up to 2 kOe the films were still superconducting down to 70 mK. Such anomalous behaviour is explained in terms of a model by Tachiki et al.     

Dynamics of oxide phases on the surface of single- and polycrystalline Pb1 − x Sn x Te films upon their investigation by the raman light scattering method

We have studied Raman spectra of single- and polycrystalline Pb_{1 ? x} Sn _x Te (0 ≤ x ≤ 1) films on different substrates in relation to the intensity of the laser action. The composition of oxide phases on the surface of lead-tin telluride films has been described, and their modification as a result of photostimulated oxidation of the surface during measurements of spectra has been analyzed. We have shown that, for films with a small mole fraction of tin telluride (x ≤ 0.26), irrespective of the crystalline state, predominant oxidation of tellurium with the formation of the compound TeO₂ takes place during the laser action. In films with a high content of tin, at a laser-action intensity higher than 1000 μW, tellurium dioxide TeO₂ on the surface is replaced with tin dioxide SnO₂.     

Plasma dynamics from laser ablated solid lithium

Emission plasma plume generated by pulsed laser ablation of a lithium solid target by a ruby laser (694 nm, 20 ns, 3 J) was subjected to optical emission spectroscopy: time and space resolved optical emission was characterised as a function of distance from the target surface. Propagation of the plume was studied through ambient background of argon gas. Spectroscopic observations can, in general, be used to analyse plume structure with respect to an appropriate theoretical plasma model. The plume expansion dynamics in this case could be explained through a shock wave propagation model wherein, the experimental observations made were seen to fit well with the theoretical predictions. Spectral information derived from measurement of peak intensity and line width determined the parameters, electron temperature (T _e) and electron number density (n _e), typically used to characterise laser produced plasma plume emission. These measurements were also used to validate the assumptions underlying the local thermodynamic equilibrium (LTE) model, invoked for the high density laser plasma under study. Some interesting results pertaining to the analysis of plume structure and spatio-temporal behaviour of T _e and n _e along the plume length will be presented and discussed.     

Heterodyne frequency measurements of carbonyl sulfide transitions at 26 and 51 THz. Improved OCS,O13CS,and OC34S molecular constants

Heterodyne frequency measurements were made on selected absorption features of carbonyl sulfide (OCS) near 26 THz (860 cm^?1) and 51 THz (1700 cm^?1). Frequency differences were measured between a tunable diode laser (TDL) locked to carbonyl sulfide absorption lines and either a stabilized ¹³CO₂ laser or a CO laser which was referred to stabilized CO₂ lasers. These measurements are combined with conventional TDL measurements and published microwave measurements to obtain new, more reliable molecular constants for OCS, O¹³CS, and OC³⁴S. New frequency measurements are given for nine CO laser transitions between 1686 and 1726 cm^?1.     

Behavioral studies of gain and saturation energy density in a N2 laser with corona preionization

By the use of an oscillator-amplifier (OSC-AMP) TE N₂ laser system, both operating with corona preionizers, the laser parameters (small-signal gain g ₀ and saturation energy density E _s) have been measured at different N₂ gas pressure and for different states of the AMP preionizer. The details of our measurements are presented. In addition, the effect of He buffer gas on the laser parameters has also been investigated; it was found that both laser parameters remain almost constant up to 150 Torr of He gas pressure, indicating that He mainly affects the discharge uniformity. Finally, with the variation of the N₂ laser gain values in the literature, we found that the g ₀-N₂-laser parameter depends strongly on the length of the laser channel. Based on the most recent measurements, a graph showing this dependency is introduced.     

Target density and surface state effects on the compositional changes in iron oxide particle aggregated films prepared by laser ablation

Iron oxide nanoparticle aggregated films were prepared using the excimer laser ablation technique by adopting an off-axis configuration and the gas condensation process. Sintered iron oxide (-Fe₂O₃) targets were ablated in oxygen ambient by an ArF excimer laser. The product of ablation comprised Fe₂O₃ at lower pressure and a mixture of Fe₂O₃ and FeO at higher pressure by X-ray-diffraction measurements. The maximum ambient oxygen pressure, P_S(O₂), at which the product composition was still a single Fe₂O₃ phase was higher for the higher-density target than for its lower-density counterpart. The target surface state affected the product composition only if the pressure was set to the pressure P_S(O₂) of 40 Pa for a high-density target. When the fluence was high (200 mJ/pulse, 3.3-mm² spot size), the product composition varied at the initial stage of laser irradiation with the number of laser pulses from a mixture of Fe₂O₃ and FeO to only Fe₂O₃ along with the target surface morphological change from a grooved structure to a smooth surface. Product composition was practically independent of the number of pulses by low-fluence laser irradiation even at this particular pressure of 40 Pa. PACS 81.07.Bc; 81.15.Fg; 61.10.Nz     

Distorted surface oxygen structure on UO2(100)

Low-energy electron diffraction (LEED) has been combined with ion-scattering spectroscopy (ISS) measurements of He⁺ at 500 eV to characterize experimentally the surface structure formed by oxygen atoms on UO₂(100). Insight into the surface geometry required to generate the LEED features was gained via laser transform simulation and kinematical diffraction analysis of two-dimensional arrays. Integrating the above approaches leads to a UO₂(100) surface model consisting of a monolayer of oxygen atoms arranged in distorted bridge-bond, zig-zag chains along 〈100〉 directions. Configurational energies were calculated which support the distorted UO₂(100) zig-zag structure.     

Absolute frequency measurement of CO2 laser lines with a femtosecond laser comb and new determination of the CO2 molecular constants and frequency grid

Absolute frequency measurements of a CO₂ laser stabilized on saturated absorption resonances of CO₂ laser lines are reported. They were performed using a femtosecond-laser frequency comb generator and two laser diodes at 852 and 782 nm as intermediate oscillators, with their frequency difference phase-locked to the CO₂ laser. Twenty ¹²C¹⁶O₂ laser lines in the P and R bands at 9 μm were measured with a relative uncertainty of a few 10⁻¹² limited by the CO₂ frequency reproducibility. A new determination of the CO₂ molecular constants was obtained from these data and previous measurements in the 10 μm band. The CO₂ frequency grid was also calculated, with an improvement of two orders of magnitude compared to the previous grid of Maki et al. [J. Mol. Spectrosc. 167 (1994) 211].     

On-line monitoring of cyclotron target-gas output by means of tunable lead salt diode laser absorption spectroscopy

We demonstrate what is, to our knowledge, the first use of mid-infrared laser absorption spectroscopy for trace-gas measurements of cyclotron target outputs used for the generation of radioactive carbon-11 in positron emission tomography (PET). The spectrometer was based upon a liquid-nitrogen-cooled lead salt diode laser generating single-mode radiation in the wavenumber range of 2230–2240 cm^?1. The sample flowed to a multiple-pass optical cell with a total path length of 15.23 m and the laser radiation was detected by two liquid-nitrogen-cooled InSb photodetectors. We present the results of CO, N₂O and CO₂ measurements on PET trace cyclotron output and discuss future work on ¹¹CO and ¹¹CO₂ detection.     

New approaches for the fabrication of photonic structures of nonlinear optical materials

We revisited two different strategies to fabricate 1D photonic crystals of nonlinear optical dielectric materials based on ultrafast laser ablation of the surface of an RbTiOPO₄ crystal, and selective etching of ferroelectric domains of the surface of a periodically poled LiNbO₄ crystal. We evaluated their behaviour as Bragg diffraction gratings. We also presented the recent advances we developed in a new procedure of fabrication of 2D and 3D photonic crystals of KTiOPO₄ (KTP) grown on the surface of a KTP substrate by liquid phase epitaxial means within the pores of a silicon macroporous template. Optical, structural, morphological, and compositional characterization for the photonic crystals produced through this technique are presented.     

Theoretical and Experimental Investigation of Neon-Hydrogen Penning Recombination Laser

A theoretical and experimental study of Ne - H₂ Penning Recombination Laser operating on the Nel 585.3 nm line in the negative glow of a helical hollow cathode discharge is presented. A detailed kinetic model of the plasma-chemical reactions determining the inversion population on the Nel (2p₁-1s₂) transition is developed. The time dependent behaviour of the main plasma and laser parameters is numerically calculated. The behaviour of the plasma and laser parameters as a function of the discharge conditions is investigated and compared with the experimental results.     

Time-resolved nonlinear surface plasmon optics

A new surface-sensitive method of time-resolved optical studies is proposed. The method consists in the independent excitation of several surface electromagnetic waves (SEW) by two laser femtosecond pulse beams with varied time delay Δτ and distance Δr between corresponding excitation regions on the surface. To fulfill the phase-matching condition for plasmon-photon coupling, metal grating is used. Due to nonlinear plasmon interaction, the optical radiation with ω₁ + ω₂ and 2ω₁ ? ω₂ (where ω₁, ω₂ are corresponding laser beam frequencies) is generated. The intensity of this nonlinear response versus Δτ and Δr are studied. The direct measurements of the SEW temporal properties are presented. Experiments of this type are important for the development of femtosecond surface plasmon optics.     

Frequency domain measurements on Na2B4O7−V2O5 semiconducting glass

Summary Dielectric measurements on Na₂B₄O₇(99.5%)−V₂O₅(0.5%) glass system, in the frequency range 10⁻³ to 10⁴ Hz and temperature range 300 to 500 K, have been carried out. The normalized plots of complex capacitance have shown a single mechanism responsible for conduction for both volume and surface measurements with their close values of activation energies (0.67±0.03) eV and (0.64±0.03) eV, respectively. The low-frequency dispersion (LFD) behaviour has been observed to be perturbed by the presence of more than one competing process. The impedance plots have shown a parallel combination of a capacitor (C) and a resistor (R), with some contribution of a dispersive element due to charge accumulation in the vicinity of the electrodes. The values ofR andC were found to be of the same order of magnitude, for both surface and volume measurements. The observedR has shown a decrease with an increase in temperature due to an increase in mobility of Na⁺ ions, whereasC remains practicaly constant. The complex capacitance surface behaviour is dominated by volume, due to hygroscopy of this glass system.     

Optical characterization of thin thermal oxide films on copper by ellipsometry

A complete optical characterization in the visible region of thin copper oxide films has been performed by ellipsometry. Copper oxide films of various thicknesses were grown on thick copper films by low temperature thermal oxidation at 125 °C in air for different time intervals. The thickness and optical constants of the copper oxide films were determined in the visible region by ellipsometric measurements. It was found that a linear time law is valid for the oxide growth in air at 125 °C. The spectral behaviour of the optical constants and the value of the band gap in the oxide films determined by ellipsometry in this study are in agreement with the behaviour of those of Cu₂O, which have been obtained elsewhere through reflectance and transmittance methods. The band gap of copper oxide, determined from the spectral behaviour of the absorption coefficient was about 2 eV, which is the generally accepted value for Cu₂O. It was therefore concluded that the oxide composition of the surface film grown on copper is in the form of Cu₂O (cuprous oxide). It was also shown that the reflectance spectra of the copper oxide–copper structures exhibit behaviour expected from a single layer antireflection coating of Cu₂O on Cu. Received: 19 July 2001 / Accepted: 27 July 2001 / Published online: 17 October 2001     

Control of electrical resistance of TiO2 films by short-pulse laser irradiation

Titanium dioxide (TiO₂) films were irradiated with a femtosecond laser beam to alter their electrical resistances. The TiO₂ film was produced by aerosol beam deposition. The wavelength, pulse duration, and repetition rate of the femtosecond laser scanned across the sample surface were 800 nm, 100 fs, and 1 kHz, respectively. By attenuating the laser fluence on the TiO₂ film, a range was found in which the electrical resistance of the TiO₂ film was varied even though the morphology of the film surface was not changed.