Ablation Laser Fluence Research Articles

Defects and transport in complex oxide thin films

Epitaxial oxide thin films are at the heart of new “oxide electronic” applications, such as excitonic ultraviolet light-emitting diodes and resistive switching memories. Complex oxide films are often grown by pulsed laser deposition (PLD) because the technique is believed to be material agnostic. Here, we show that one of the fundamental premises used to justify the use of PLD, that material is transferred from an ablation target to the film without stoichiometry deviations, is incorrect even when no volatile elements are involved. Even more importantly, the commonly used solution of increasing the laser energy density above a material-specific threshold value to obtain stoichiometric films cannot be used in the case of low carrier density systems such as SrTiO3, where even minute 1018 cm−3 order cation nonstoichiometry can have a dramatic effect on transport. Lattice parameter deviations in oxide films, which are often incorrectly ascribed to oxygen loss, correlate very well with cation nonstoichiometry. We show that proper simultaneous choice of ablation laser fluence and ablation area is essential and often more important than the growth temperature and oxygen pressure for obtaining bulklike properties in oxide heterostructures.

Time-resolved dynamics analysis of nanoparticles applying dual femtosecond laser pulses

The evolution of a fs laser generated plume on copper was studied by dual laser pulses. The plume generated by the first pulse (620 nm) was excited by the second, delayed pulse (310 nm). The actual state of the plume was monitored by detecting the intensities of the emitted light and the reflected delayed pulse from the interaction range, i.e. 0–150 μm above the surface. Four peaks were observed in the 0–1400 ps time range and assigned to appearance of electrons, ions, atomic species and nanoparticles. Accordingly, the timing of the creation and ejection of these species was determined. By integrating the intensity of the nanoparticle peak, the production yield of the nanoparticles and its dependence on the ablating laser fluence was calculated. The ablation products were deposited on a Si substrate, too, and analyzed by atomic force microscopy and scanning electron microscopy. The results of the deposition and the dual-pulse experiments are in good agreement with the theory.

Emission and electron/ion analysis of the ablated plume from LiYF 4 crystals doped with Tm 3+ or Nd 3+ ions

We performed temporal as well as frequency-resolved emission spectroscopy of the plasma plume generated in vacuum by 355 nm pulsed laser ablation of a LiYF 4:Tm 3+ crystal (1% Tm 3+ concentration) and a LiYF 4:Nd 3+ crystal (1.5% Nd 3+ concentration). The plume emission spectrum is very rich in features and shows the elemental lines belonging to all the components of the host lattice, either neutral or ionized. The time-resolved analysis of some emission features is discussed and some stream velocities are derived. A possible model for the time evolution of the plume components is reported. The electronic detection technique was used to detect the onset of the plasma plume and to measure the ablation laser fluence threshold.

Laser ablation of optically thin absorbing liquid layer predeposited onto a transparent solid substrate

Ablation of optically thin liquid 2-propanol layers of variable thickness on IR-transparent solid Si substrate by a nanosecond CO2 laser has been experimentally studied using time-resolved optical interferometric and microscopy techniques. Basic ablation parameters—threshold fluences for surface vaporization and explosive homogeneous boiling of the superheated liquid, ablation depths, vaporization (ablation) rates, and characteristic ablation times versus laser fluence—were measured as a function of alcohol layer thickness. The underlying ablation mechanisms, their thermodynamics, and microscopic details are discussed.

Formation Process of Platelet Nanocomposites with Zinc Hydroxide and Sodium Dodecyl Sulfate Prepared by Laser Ablation in Solution

Abstract Layered zinc hydroxide nanocomposites (Zn-NC) were prepared by laser ablation of Zn in an aqueous solution of sodium dodecyl sulfate (SDS) with various ablation times and laser fluences. At the fluence of 6.7 J/cm2, the rectangular Zn-NC was obtained by 10 min ablation. Rectangular structures were found with different stages of Zn filling at the fluence of 2.0 J/cm2. These results suggest that an SDS monohydrate with rectangular shape acts as the template for Zn-NC formation.

Improved stoichiometry and misfit control in perovskite thin film formation at a critical fluence by pulsed laser deposition

Pulsed laser deposition (PLD) is a good method for growing high-quality functional oxide thin films because of the technical simplicity and the ease with which deposition can be switched from one material to another. However, the repeatability of film quality is often hard to achieve, especially when using several different PLD systems. Here we report the steps that we have taken to grow nearly bulk-equivalent defect-free thin films, with SrTiO3 as an example, by using PLD in a reproducible fashion. The ablation laser fluence was found to have a very strong effect on the lattice constant and defect structure of the films. Nonstoichiometric transfer of material from the ablation target was observed when either the laser fluence or the beam spot area was inadequate.

Onset of crater formation during short pulse laser ablation

We report morphologic changes of metallic surfaces at the onset of ablation, starting from gentle ablation to the emergence of ablation craters. The evolution of both observed melting zones and of ablation craters therein are investigated in dependence of the ablation laser fluence for nanosecond ultraviolet laser pulses. Further, consequences of crater formation for cluster synthesis within the released atomic vapor are pointed out.

Arrays of 3D micro-columns generated by laser ablation of Ta and steel: modelling of a black body emitter

Three-dimensional extended arrays of micro-columns are generated on the surface of Ta and several stainless steels by their ablation by radiation of a Cu vapor laser either in vacuum or in air. The reflectivity of the arrays is tested in both visible and near-IR regions using the facilities at NASA Johnson Space Center. The reflectivity of the laser-treated areas was found to be very low (0.03–0.08) in the range 250–2800 nm. The emissivity of 3D arrays measured at elevated temperatures is close to the emissivity of a calibrated black body emitter. The effects of the experimental conditions of ablation (laser fluence, environment, etc.) on the integral optical characteristics of the generated arrays are discussed.

Fabrication of LiNbO3 thin films by pulsed laser deposition and investigation of nonlinear properties

Thin films of LiNbO3 were deposited by pulsed-laser deposition (PLD). Crystalline and transparent films were deposited on a sapphire substrate at 400 °C, in 100 mTorr of oxygen, with a fluence lower than 1.2 J/cm2. Droplet free films were deposited with low ablation laser fluence by the eclipse method, and waveguide losses were 15.9 dB/cm and 3.1 dB/cm. Subsequently, second-harmonic generation (SHG) was achieved by waveguide mode phase matching. The center wavelength of the matching spectrum was 853 nm, and the full width at half maximum (FWHM) was 19 nm.

Biocompatible nanocrystalline octacalcium phosphate thin films obtained by pulsed laser deposition

We extended for the first time pulsed laser ablation to the deposition of octacalcium phosphate Ca 8H 2(PO 4) 6·5H 2O (OCP) thin films. The depositions were performed with a pulsed UV laser source ( λ=248 nm, τ⩾20 ns) in a flux of hot water vapors. The targets were sintered from crystalline OCP powder and the laser ablation fluence was set at values of 1.5–2 J/cm 2. During depositions the collectors, Si or Ti substrates, were maintained at a constant temperature within the range 20–200°C. The resulting structures were submitted to heat treatment in hot water vapors for up to 6 h. The best results were obtained at a substrate temperature of 150°C during both deposition and post-deposition treatment. High-resolution electron microscopy and XRD at grazing incidence indicated that the coatings obtained were made of nanocrystalline OCP. Cross-section TEM investigations showed that the coatings contained droplets stacked on Ti substrates as well as distributed across the entire thickness of the arborescence-like structure layers. The results of WST-1 assay, cell adherence, DNA replication, and caspase-1 activity confirmed the good biocompatibility of the coatings.

Femtosecond laser pulse ablation of GaAs and InP: studies utilizing scanning and transmission electron microscopy

Single pulse laser ablation of GaAs and InP using 130 fs light pulses at ≈800 nm was studied with various techniques, in particular, scanning and transmission electron microscopies. The final state of the material near the laser-ablated region following femtosecond ablation was characterized in detail for selected laser fluences. Threshold ablation laser fluences were also obtained for both compounds.

Single- and multi-pulse femtosecond laser ablation of optical filter materials

Ablation experiments employing Ti:sapphire laser pulses with durations from 30 to 340 fs (centre wavelength 800 nm, repetition rate 1 kHz) were performed in air. Absorbing filters (Schott BG18 and BG36) served as targets. The direct focusing technique was used under single- and multi-pulse irradiation conditions. Ablation threshold fluences were determined from a semi-logarithmic plot of the ablation crater diameter versus laser fluence. The threshold fluence decreases for a shorter pulse duration and an increasing number of pulses. The multi-pulse ablation threshold fluences are similar to those of undoped glass material (∼1 J cm −2). That means that the multi-pulse ablation threshold is independent on the doping level of the filters. For more than 100 pulses per spot and all pulse durations applied, the threshold fluence is practically constant. This leads to technically relevant ablation threshold values.

Laser irradiation effects on the resistance of SmS films

Samarium monosulfide (SmS) films deposited by flash evaporation to realise pressure sensors have been processed by ultraviolet laser radiation at 308 nm to modulate their electrical resistance. The laser irradiation effects on the resistance of SmS films have been investigated at different laser fluences. It is shown that the SmS film resistance can be significantly modulated, even at fluences lower than the laser ablation fluence. The effects induced by laser processing on the film surface structure have also been investigated.

Basic Study on Laser Ablation Surface Cleaning of Nuclear Facility

An ultra short pulsed Nd:YAG laser was employed to study the ablation effects and laser beam transport in optical fibres. Using 13 µm Zn coated carbon steel as a target, the relationship between ablation rate and laser fluence was investigated, and optimal processing conditions were determined. The laser induced products consisted of vapour and liquid particles. It was found that a smaller incident laser angle was more suitable for protecting the focusing lens from the pollution of the ablated products. The collection rate of the ablated products almost reached 100%. Additionally, a laser beam of over 5·6 W in power with a transportation rate of nearly 90% was output at the end of a 20 m long optical fibre. By this output power, a Zn coating of the same thickness could be completely ablated. A decontamination factor was introduced to estimate the cleaning effects of laser ablation, and satisfactory cleaning was obtained

Spatial distribution of carbon species in laser ablation of graphite target

We report on the temporal evolution and spatial distribution of C2 and C3 molecules produced by KrF laser ablation of a graphite target using laser induced fluorescence imaging and optical emission spectroscopy. Spatial density profiles of C2 were measured using two-dimensional fluorescence in various pressures of different ambient (vacuum, nitrogen, oxygen, hydrogen, helium, and argon) gases at various ablation laser fluences and ablation area. A large yield of C2 is observed in the central part of the plume and near the target surface and its density and distribution was affected by the laser fluence and ambient gas. Fluorescent C3 was studied in Ar gas and the yield of C3 is enhanced at higher gas pressure and longer delay times after ablation.

Fabrication of Nd 3+, Cr 4+ co-doped Gd 3Ga 5O 12 thin film waveguide by two-target pulsed laser deposition

The growth of Nd 3+,Cr 4+ co-doped gadolinium gallium garnet (Gd 3Ga 5O 12:GGG) (1 1 1) thin films on YAG (1 1 1) substrate has been demonstrated with two-target pulsed laser deposition and post-annealing. The concentrations of Nd and Cr in the co-doped thin film are well controlled by changing respective KrF laser ablation fluence for Nd:GGG and Cr,Ca:GGG sintered targets. The structure of Nd,Cr:GGG thin films on the YAG substrate shows a waveguide structure with high numerical aperture. It is confirmed that the Nd 3+ and Cr 4+ ions in co-doped films act as the laser active ion and saturable absorber, respectively, at 1.06 μm to be able to be used for monolithic self Q-switched waveguide laser to generate high peak power and short pulse output.

Simulation study and experiment on laser-ablation surface cleaning

A study of the ablation effects of an ultra-short pulsed Nd : YAG laser was carried out. Using a 21 μm Zn-coated carbon steel plate as the target, the relationship between ablation rate and laser fluence was investigated through computer simulation and experiment, and the optimal processing conditions were determined. The tendency of the scanning operation curve was confirmed and the data obtained were taken as a guide for the practical utilization of this technique. Finally, a decontamination factor was introduced and satisfactory cleaning was obtained.

LiNbO 3 optical waveguides deposited on sapphire by electric-field-assisted pulsed laser deposition

Oriented LiNbO 3 films have been grown on sapphire (1 1 0 2) substrates by low electric-field-assisted pulsed laser deposition. KrF excimer laser (248 nm) has been utilized for the ablation of monocrystalline stoichiometric target. The deposition was performed at all substrate temperatures in the range of 550–800°C in an oxygen atmosphere. The ablation threshold of 2.5 J/cm 2 for the monocrystalline target was determined from the experimental results presenting ablation rate vs. laser fluence dependence. Electro-optical properties of these films appeared to be strongly dependent on the process parameters (substrate temperature, oxygen partial pressure, electric field), demonstrating the importance of the epitaxial growth control during the deposition. The films structure and morphology were characterized by XRD and AFM, respectively. The waveguide properties of the films were studied experimentally.

Short pulse UV laser ablation of solid and liquid gallium

UV laser ablation of gallium from the solid (300 K) and liquid (330K) phase is reported. The ablation is performed by using a 248nm excimer laser with pulse durations of 15 ns and 0.5 ps. The ablated neutrals are ionised by resonant multiphoton ionization with a time-delayed laser (605 nm, 15 ns) and are detected by time-of-flight mass spectroscopy. The ablation threshold fluences, the ablation rates, and the velocity distributions of Ga atoms from the solid and liquid phase for irradiation with 0.5 ps and 15 ns pulses are determined. The threshold fluence for 0.5 ps laser ablation is found to be 7 times lower than that for 15 ns laser ablation and is identical for the solid and liquid samples of gallium. For ns pulses, the threshold fluence is 1.5 times higher for the solid compared to the liquid phase. This difference between the sub-picosecond and nanosecond laser ablation behaviour can be explained by the longer and deeper heat diffusion during the longer laser pulses. The time-of-flight measurements show the thermal behaviour of the removed Ga atoms in both 0.5 ps and 15 ns laser ablation and are fitted to Maxwell-Boltzmann distributions giving translational temperatures of several thousand Kelvin. An increase in the temperature as a function of the ablation laser fluence is observed.

Electrical and optical characteristics of organic thin films fabricated by laser ablation

Non-polymer organic thin films, copper-phthalocyanine (CuPc) and 4-dialkylamino-4′-nitrostilbene (DANS), were fabricated by KrF laser ablation. Crystalline thin films with almost the same composition as the target material were deposited when reducing the laser ablation fluence close to the ablation threshold of ∼ 20 mJ/cm2. Higher laser fluences deplete some weakly bound atoms in the thin film, such as nitrogen in CuPc. Semiconductor characteristics of CuPc thin film were examined in a Schottky diode. DANS-doped amorphous fluoropolymer was also fabricated by combining two ablation deposition processes. A crystalline feature of DANS disappears at low DANS concentrations in a fluoropolymer matrix.