I. Weaver

Polar distribution of ablated atomic material during the pulsed laser deposition of Cu in vacuum: Dependence on focused laser spot size and power density

Experiments have been carried out to investigate the polar distribution of atomic material ablated during the pulsed laser deposition of Cu in vacuum. Data were obtained as functions of focused laser spot size and power density. Thin films were deposited onto flat glass substrates and thickness profiles were transformed into polar atomic flux distributions of the form f(θ)=cosn θ. At constant focused laser power density on target, I=4.7±0.3×108 W/cm2, polar distributions were found to broaden with a reduction in the focused laser spot size. The polar distribution exponent n varied from 15±2 to 7±1 for focused laser spot diameter variation from 2.5 to 1.4 mm, respectively, with the laser beam exhibiting a circular aspect on target. With the focused laser spot size held constant at φ=1.8 mm, polar distributions were observed to broaden with a reduction in the focused laser power density on target, with the associated polar distribution exponent n varying from 13±1.5 to 8±1 for focused laser power density va...

Three-dimensional number density mapping in the plume of a low-temperature laser-ablated magnesium plasma

Simultaneous optical absorption and laser-induced fluorescence measurements have been used to map the three-dimensional number densities of ground-state ions and neutrals within a low-temperature KrF laser-produced magnesium plasma expanding into vacuum. Data is reported for the symmetry plane of the plasma, which includes the laser interaction point at a delay of I μs after the 30 ns KrF laser ablation pulse and for a laser fluence of 2 J cm -2 on target. The number density distributions of ion and neutral species within this plane indicate that two distinct regions exist within the plume; one is a fast component containing ions and neutrals at maximum densities of ∼ 3 x 10 13 cm -3 and ∼ 4 X 10 12 cm -3 . respectively and the second is a high-density region containing slow neutral species, at densities up to ∼ 1 x 10 15 cm -3 .

Electron number density measurements in magnesium laser produced plumes

Abstract Interferometry has been used to investigate the spatio-temporal evolution of electron number density following 248 nm laser ablation of a magnesium target. Fringe shifts were measured as a function of laser power density for a circular spot obtained using a random phase plate. Line averaged electron number densities were obtained at delay times up to ∼100 ns after the laser pulse. Density profiles normal to the target surface were recorded for power densities on target in the range 125–300 MW cm −2 .

He-like x-ray line emission from laser irradiated sources

Photon yields for the 1s2–1s2p (He-α) transition of He-like ions have been measured for laser irradiated, thin foils of Ti, V and Fe. The laser pulses were of 0.527 μm wavelength and of either 80 or 300 ps duration. The data shows significant shot-to-shot variation but the Ti data is broadly consistent with previous results. In this work, we extend the previous results to include, new elements, longer pulse lengths and yields measured for emission from both surfaces of the foils. We compare our data to simulations using a hydrodynamic code and a collisional radiative model.

Spectral simulation of laser ablated magnesium plasmas

We describe a collisional-radiative equilibrium model for predicting the optical emission spectrum of low-temperature magnesium plasmas, specifically those created by laser ablation. In the model, levels are populated by a balance of collisional and radiative rates. We include Stark widths of lines and trapping of radiation in the calculations. By use of this model we discuss various issues of importance in spectral analysis of laser ablated plasma plumes, such as the partial local thermodynamic equilibrium approximation, line trapping and time dependence.

Aspects of particulate production from metals exposed to pulsed laser radiation

Abstract Experiments have been carried out to investigate particulate production from Al and Cu targets during pulsed laser ablation in vacuum. Ablated material, comprising atomic and particulate fluxes, was deposited onto flat glass and concave mylar substrates. Optical and atomic force microscopy was used to obtain particulate number densities, size and polar distributions as a function of laser focused spot size and power density. Particulate size distributions were fitted to Boltzmann probability distributions, with the increase in energy of the system related to the increase in surface energy during the formation and emission of a spherical particulate from the target surface, qualitatively explaining the observed difference in particulate emission from Al and Cu targets. The laser power density threshold for particulate emission occurred coincident with that calculated for atomic material removal, with a further increase in emission occurring at power densities above 109 W/cm2. Polar particulate emission was fitted to cosθ distributions, indicating that particulates were not emitted in any preferred direction. Particulate production was consistent with a subsurface boiling/cavitation emission process.

PLD fabrication of a soft X-ray multilayer mirror and LPP reflectance test

Abstract A soft X-ray mirror based on a molybdenum–silicon (Mo/Si) multilayer structure has been fabricated by the pulsed laser deposition (PLD) technique. The multilayer was designed to reflect at 196 A for normal incidence operation. An iterative graphical procedure was used to calculate the optimum periodic multilayer structure. The normal incidence reflectance of the multilayer was determined using a continuum source of soft X-rays from a laser-produced plasma (LPP). The multilayer peak reflectance was 7.3% at 190 A, with the magnitude of reflectance consistent with an effective interfacial roughness within the structure equal to ∼15 A.

A case study with a novel geometry for pulsed laser deposition

Aluminium thin films were deposited by a laser ablation technique from solid cylindrical targets rotated on a lead screw. Both smooth-surfaced targets and targets with screw threads cut into the area to be irradiated were used. The targets were irradiated with a focused circular spot size of 2.5 mm in diameter, at a power density equal to 5.4?0.2 ? 108 W cm-2 . The polar distribution of the ablated atomic material was found to vary as a function of the target screw thread pitch, with the exponent n , in the polar distribution f ( ) = cosn , varying from 13.5?1.3 for no pitch to a minimum of 5?0.7 for a screw thread pitch of 0.2 mm. The use of such novel target geometries forms a possible basis for increasing thickness uniformity during the pulsed laser deposition of thin films.