Rate Pulsed Laser Research Articles

Enhanced room temperature magnetoresistance response in textured La0.7Sr0.3MnO3 strips made by pulsed laser deposition

Magnetoresistive La0.7Sr0.3MnO3 polycrystalline films have been deposited by high laser pulse energy pulsed laser deposition that exhibit an extreme variability in the temperature of the resistance maximum, Tp, as a function of the deposition conditions. Films have been deposited that exhibit the resistance maximum at temperatures ranging from 155 to 360 K as a function of laser pulse energy and shadowing conditions. High laser pulse energies, shadowing to eliminate film particulates, and a specific laser pulse rate range, were necessary to synthesize films with the temperature of the resistance maximum elevated to room temperature. The room temperature low field magnetoresistive response of samples with Tp approximately room temperature was highly anisotropic with respect to applied magnetic fields in plane and perpendicular to the plane. The (110) textured films exhibited an enhanced low field magnetoresistance compared to films with appreciable (111) texture.

Pyroelectric Coefficient Optimization Through Grain Size Control in (Ba, Sr)TiO3 Thin Films

AbstractThin Ba0.2Sr0.8TiO3 films were pulsed laser deposited at laser pulse repetition rates of 1, 5 and 20 Hz in order to assess the effect of grain size on pyroelectric properties. Scanning electron microscopy reveals an increase in columnar grain size with decreasing laser pulse rate, as expected. Substrate bending measurements show an increase in tensile stress with increasing grain size, suggesting some form of stress relaxation has occurred. X-ray diffraction studies reveal all films to be epitaxial and oriented, while Rutherford Backscattering Spectroscopy finds no significant differences in film composition. The films grown at 5 and 20 Hz display a suppressed dielectric permittivity as a function of temperature, while the film deposited at 1 Hz displays an unusually sharp and large dielectric permittivity peak. A variation on current theory concerning phase transitions of stressed ferroelectric thin films is proposed to explain these results.

An analysis of surface roughness, surface morphology and composite/dentin bond strength of human dentin following the application of the Nd:YAG laser

Objectives: This study was conducted to investigate the surface effects to dentin following laser “etching” with an Nd:YAG laser, and to determine the shear bond strengths of composite to untreated and laser-treated dentin. Methods: Dentin surfaces from freshly extracted permanent molar teeth were subjected to laser treatment with an Nd:YAG laser using different laser pulse rates and power settings. The effect of application of a laser enhancing gel was also investigated. Surface profile analysis of the dentin surfaces was undertaken with a perthometer. The results were analysed by SPSS/PC multiple range test. Specimens were examined using a scanning electron microscope (SEM). The shear bond strengths of laser-treated dentin/composite and untreated dentin/composite by conventional dentin bonding (Gold Standard, Chameleon Dental Products) were also determined. These results were also compared by SPSS/PC multiple range test. Results: The laser-treated dentin specimens exhibited a qualitatively different surface morphology when compared with untreated dentin. Laser treatment at higher exposures resulted in the formation of microcracks and fissures. Bonding of resin composite to the laser-treated dentin was not significantly different from untreated dentin despite the fact that the surface roughness of laser-treated dentin was significantly higher than untreated dentin ( p<0.05). Significance: It was concluded from this study that the etched dentin surfaces produced by the Nd:YAG laser operated under the conditions described were not superior to those obtained with conventional dentin bonding.

Fully in plane aligned SmCo based films prepared by pulsed laser deposition

SmCo based films, ≈1–3 μm thick, have been directly crystallized onto moderately heated polycrystalline susbstrates, 375 °C, by high pulse energy pulsed laser deposition. Films have been deposited to compare shadow and direct deposition. Only for shadow deposition conditions has it been possible to deposit CaCu5-type structure films that exhibit hysteresis loops characteristic of single phase materials with high coercivities. The shadow deposited films are mirror-like, very fined grained (less that 0.2 μm), and with the crystallite c axes completely aligned onto the substrate plane. For optimally deposited films, room temperature hysteresis loops measured perpendicular to the film plane were closed with approximately zero coercivity. For such films, hysteresis loops measured in plane with fields to ±90 kOe at room temperature exhibited intrinsic coercivities of at least 22.5 kOe. Measurement fields of ±19 kOe yielded only minor loops that did not enclose the origin in some cases. The fully textured SmCo based films in the present study were deposited using λ=248 nm and a pulse energy of 1200 mJ. The magnetic properties are a function of the laser pulse rate and this has been studied from 5 to 50 Hz repetition rates.

In-flight imaging of transverse gas jets injected into compressible crossflows

The development of and results from an airborne flight-test experiment to study nonreacting gas jets injected transversely into subsonic and slightly supersonic crossflows is presented. Freestream/crossflow Mach numbers in the study ranged from 0.6 to 1.2, and planar laser-induced fluorescence imaging of an iodine-seeded nitrogen jet was used to visualize the jet flow. Time-dependent images were obtained with a high-speed intensified video camera synchronized to the laser pulse rate. The entire experimental assembly was configured compactly inside a unique flight-test fixture mounted under the fuselage of NASA Dryden's F-104G research aircraft, which served as a flying wind tunnel. Recorded transverse gas jet images were then digitized to allow analysis of jet trajectory, spreading, and mixing characteristics. Comparisons of experimental results with model predictions in this transonic crossflow regime show reasonable agreement. Nomenclature AJ = cross-sectional area of jet nozzle at exit A6 = cross-sectional area of jet nozzle at transducer location; see Fig. 4 Mj = jet exit Mach number

Instantaneous temperature measurement in a rapid-compression machine using laser Rayleigh scattering

The instantaneous local temperature is measured in a Rapid-Compression Machine (RCM) after the compression. The technique that we have used is the laser Rayleigh scattering at 532 nm. Despite the important background noise due to the very confined RCM chamber, optimum optical conditions lead to a single-shot temperature accuracy of 30 K at the end of compression. The temperature history is sampled at the laser pulse rate, and it exhibits large temperature fluctuations just after the end of compression. Comparison with the extensively used calculated adiabatic core gas temperature shows excellent agreement, at least in the time interval corresponding to ignition delays ( < 100 ms). This first experimental assessment of core-gas assumption is important for chemical-kinetics numerical predictions in RCMs.

Basic elements for photodeposited high Tc thin film devices

Flat films, high quality insulating layers and adequately superconducting via contacts are basic elements for high T c device fabrication. We studied the influence of the process parameters of laser deposition on the occurrence of droplets and outgrowths in YBaCuO films. The droplet density is minimal when a laser fluence below about 1.0 J cm −2 is used. The outgrowth density decreases with increasing laser pulse rate or decreasing deposition temperature. High quality flat films were obtained with a rate of 10 Hz and at a temperature of 720 °C. Wet chemical etching and etching with an Argon ion source were used for structuring multilayers with SrTiO 3 as an insulating layer. Smooth edges were obtained with an argon gun. Bromine and EDTA etching are not adequate techniques for fabricating controllable well-defined edges. Cross-overs, via contacts and coils were prepared.

Microstructural evolution of laser deposited superconducting Y-Ba-Cu-O films: Effect of deposition rate

The microstructure of laser deposited, superconducting Y-Ba-Cu-O thin films grown on [001] SrTiO 3 substrates has been characterized as a function of laser pulse rate. Over a range of deposition rates from 1–50 Å/s, the films consist mainly of the c-axis oriented epitaxial, YBa 2Cu 3O 7− x phase with a high density of polytypoidic stacking defects. High transport critical current densities, J c, are attributed to the retention of the primarily c-axis oriented microstructure along with the distribution of stacking defects that provide good flux pinning. At higher deposition rates we observe new growth directions, specifically the a,b-axis and the [103] growth directions are favored. We conclude that within the deposition rate range of 1–150 Å/s, there is no significant fundamental change in microstructure except for outgrowth formation. Therefore, we are optimistic that much higher rates can be attained in the near future, although controlling the outgrowth formation may be difficult.

High power high repetition rate pulsed collisional laser using a He + Eu+ mixture

Studies have been made of the properties of an ionized europium collision laser withλ=1002 nm in the 1.5 to 6 kW range of pumping powers. For this purpose, a gas-discharge tube made of BeO ceramic, 50 cm long and 2.7 cm diameter was used. The pulse repetition rate ranged from 2 to 10 kHz at a pumping duration of 400 ns.

Maskless Laser Etching of Alumina and Ceramic Substrates

AbstractMaskless patterning of alumina (A12O3) films of various thicknesses and ceramic (TiC-Al2O3) substrate material was accomplished by using pulsed excimer and Nd:YAG laser systems. Etched structure size was defined through the computer software which also controls the workpiece position and laser operating parameters. Etch profile, materials removal rate and surface roughness was found to be dependent on laser emission wavelength, fluence, pulse rate and relative scan speed. Smoother etching was obtained with the excimer lasers. The etch characteristics are compared with those of the polymeric and metallic materials. Various potential applications of this laser based etching process in microelectronic and micromagnetic device fabrication are identified.

Laser-assisted chemical etching of copper

Copper films several microns thick can be rapidly etched (0.1–0.5 μm/s) in a chlorine atmosphere using 308 nm pulsed excimer laser radiation. Etch rates exhibit a practical threshold of ∼0.1 J/cm2 and saturate above 0.3 J/cm2. The etching mechanism involves reaction of copper and chlorine in the time between the laser pulses to form a nonstoichiometric chloride which is thermally ablated by the laser. Photochemical effects appear to be unimportant. The etch rate increases roughly as the square root of the laser pulse rate, suggesting that reactive diffusion is a limiting factor. As the substrate temperature increases, the etch rate rises, then falls, indicating the competition of processes such as adsorption, desorption, and reaction of chlorine molecules on copper chloride surfaces. A simple model based on chemical kinetics suggests that the copper-chlorine reaction at pressures above 0.1 Torr is controlled by a physisorbed intermediate state.

Direct Determination of Copper in Solids by Direct Current Argon Plasma Emission Spectrometry with Sample Introduction Using Laser Ablation

A direct-current argon plasma emission spectrometric system with sample introduction using laser ablation has been developed and evaluated for the determination of copper in pelletized powder and solid samples. The use of a low-energy and high-repetition-rate Nd: YAG laser gave a nearly steady-state signal that could be integrated over a relatively long time period of 30 s. This allowed a larger portion of the sample to be probed, and high % RSD—often obtained with single shot laser ablation because of sample inhomogeneities—reduced to 2–6%. The emission intensity was dependent on sample preparation technique, particle size, laser pulse rate, and copper anion. The system was used to directly determine copper in pelletized ore samples with 2–6% RSD, a detection limit of 20 μg g−1, and acceptable accuracy, once the factors described above were controlled.

Evaporation and condensation rates of liquid droplets deduced from structure resonances in the fluorescence spectra

A new optical technique, based on morphology-dependent peaks in the fluorescence spectra, is-used to determine the evaporation and condensation rates of a linear stream of ethanol droplets. The droplets are monodispersed, in close proximity to one another, and impregnated with fluorescent dye molecules. On irradiation of the droplets with a single N(2) laser pulse, the evaporation or condensation rates can be deduced from the wavelength shift (to the blue or to the red, respectively) of the spectrally narrow (<0.1-nm) structure-resonance peaks in the fluorescence spectra.

High repetition rate pulsed gas lasers and their applications in chemistry and isotope separation

Presented in this paper are the results of experimental studies of pulsed high repetition rate XeCl, CO 2 , NH 3 , and CF 4 lasers with a closed gas cycle. Some applications of these lasers in chemistry and isotope separation are discussed.

Field evaporation of silicon and field desorption of hydrogen from silicon surfaces

The field evaporation of silicon in ultrahigh vacuum and in hydrogen has been studied with the use of the pulsed-laser atom probe. Measurements of the ion yields of various field-evaporated and field-desorbed species were made as a function of applied voltage, laser power, hydrogen background pressure, and laser pulse rate. The results indicate that, in ultrahigh vacuum and above cryogenic temperatures, field evaporation of silicon is qualitatively the same as for metals. In hydrogen, however, the field-evaporation process is quite different from that of metals, with the rate-limiting step being the field-enhanced formation of surface hydrides. Field-desorbed ${\mathrm{H}}^{+}$ and $\mathrm{H}_{2}^{}{}_{}{}^{+}$ ions are shown to arise from a field-adsorbed binding state, and the voltage range where $\mathrm{H}_{2}^{}{}_{}{}^{+}$ dissociates to ${\mathrm{H}}^{+}$ is used to calibrate the electric field strength. The low-temperature evaporation field of silicon is estimated from this calibration to be 3.3-3.6 V/\AA{}, which is considerably higher than the currently accepted value of 2.0 V/\AA{}. Field-desorbed $\mathrm{H}_{3}^{}{}_{}{}^{+}$ ions are detected only when oxide contamination is present on the silicon surface. If we assume that the mechanism of $\mathrm{H}_{3}^{}{}_{}{}^{+}$ formation is the same as that proposed for metals, this observation suggests that weakly bound, chemisorbed hydrogen atoms exist on silicon only in the presence of surface contamination.

Ultra‐fast polymerization of epoxy‐acrylate resins by pulsed laser irradiation

AbstractThe 337.1‐nm emission of a pulsed nitrogen laser was shown to initiate the crosslinking polymerization of epoxy‐acrylate photoresists effectively. We evaluated the extent of curing from the amount of insoluble polymer formed and by the decrease in infrared (IR) absorption of the reactive double bond at 810 cm−1. With the large power density available in the laser pulse (0.5 MW cm−2) rates of polymerization as high as 108 mol L−1 s−1 were observed in the presence of air. Quantum yield measurements indicated that each photon absorbed can create as many as 450 crosslinks; the kinetic chain length was calculated as ca. 4000 double bonds polymerized per initiating radical. During the induction period due to oxygen inhibition each photoinitiator radical consumed 1 O2 molecule. The influence of the monomer and photoinitiator used on the sensitivity of the resin was examined; the best performing formulation contained the epoxy‐acrylate oligomer, pentaerythritol triacrylate, as monomer and 2,2 dimethoxy‐2‐phenyl‐acetophenone as photoiniatior. All the formulations studied can be cured by a single 500‐kW laser pulse of 8 ns duration, provided that the irradiation is carried out in an inert atmosphere or with a focused laser beam.

Active attenuation of acoustic disturbances in high energy pulsed lasers

The potential advantages of the active attenuation of acoustic disturbances in high energy pulsed lasers have been analyzed, using the theory developed for the control of sound in long ducts [M. A. Swinbanks, J. Sound Vib. 27, 411–436 (1973)]. The results show that active attenuators, which are especially effective at the lower audio frequencies, could be used in conjunction with passive mufflers to significantly reduce the total muffler volume required to damp the acoustic disturbances to an acceptable level. Only modest acoustical power would be required because the active attenuation system would be used to damp just the fraction of the acoustical disturbance which is reflected back toward the optical cavity by the thermal interface created by the previous laser pulse. The additional attenuation provided by an active system could also result in higher laser pulse rates, and thus higher average power. The design requirements for an experimental demonstration of the potential benefits of active attenuation for pulsed laser operation will also be discussed.

Study of a 1-Watt Repetitive Dye Laser

A repetitive flashtube pumped dye laser (pulse rate up to 30 Hz) is described. The untuned laser output average power with rhodamine 6G is 1 W. When tuned by a Perot-Fabry etalon, 0.36 W is obtained in a bandwidth of 3 A. Output power is given for several dyes emitting between 440 nm and 680 nm. The behavior of flashtubes and dye solutions after long use is discussed.

FREQUENCY PULLING AND PULSE POSITION MODULATION OF PULSING cw GaAs INJECTION LASERS

The frequency pulling and locking of intensity pulsations from continuously operating GaAs injection lasers have been studied by varying the frequency of the externally applied locking signal in the vicinity of the self-induced pulse rate or one of its harmonics. The ability of the laser pulse rate to follow a rapidly varying locking signal has led to the first realization of optical pulse position modulation with microwave repetition rates. Modulation rates attainable with this effect are expected to be as high as one-half the self-induced pulse rate.

Five kilohertz repetition rate pulsed YAG Nd laser

Five kilohertz repetition rate pulsed YAG Nd laser