Višnja Henč-Bartolić

Droplet formation during laser sputtering of silicon

Abstract High purity single-crystal silicon was ablated with nitrogen laser radiation wavelength 337 nm, pulse length 6 ns, maximal energy density 1.1 J/cm 2 , nonuniform target energy distribution. Many droplets were observed around the damaged target area, which seem to be ejected out of it and splashed vigorously onto the surrounding target surface. Their diameters are found to be in the range of a micrometer. The droplets were most probably produced within a single laser pulse as a result of hydrodynamical instability of the molten surface layer. Intense splashing occurs as a consequence of the large plume pressure generated by the most intense parts of the laser beam. The irregular power distribution on the target seems to enhance droplet formation significantly, since their abundance is drastically lower or even missing in similar experimental conditions but with uniform power distribution.

Dynamics of aluminum plasma produced by a nitrogen laser

13 mJ laser pulses from a nitrogen laser were focused onto an aluminum target in air. The target surface was perpendicular to the axis of the laser beam. A peak energy density of 1.3 J/cm2 and a power density of 80 MW/cm2 were achieved with a laser pulse duration of 16 ns. The high power density produced a transient plasma cloud that expanded explosively into the surrounding atmosphere. An initial electron density of about 1 × 1019 cm3 and an electron temperature of about 2eV were determined by optical spectroscopy. The line of sight was parallel to the surface and perpendicular to the laser beam axis. The height of the line of sight above the target surface was varied in order to gather data about the whole plasma cloud. In about 500 ns the plasma cloud expands to about 0.5 mm above the target surface, cools down to about 1.2eV and is tenfold reduced in electron density. The initial expansion velocity was determined to be about 2km/s. The experimentally determined plasma parameters were input into numerical models of target heating and plasma expansion. The numerical results outrule the so called outflow model of plasma expansion and show reasonable agreement with an effusion model. The observed discrepancies in observed and calculated plasma parameters are attributed to the fact that the theoretical models describe the plasma expansion in vacuum only.

DYNAMICS OF LASER-PRODUCED CARBON PLASMA

Carbon plasmas produced by radiation from a ruby laser (wavelength 694.3 nm) focussed onto a carbon target in vacuum are studied spectroscopically with a time resolution of 40 ns. Measured line profiles of several ionic species (CI-CIV) were used to infer electron density and temperature at several positions above the target surface as function of time elapsed after the beginning of the laser pulse. The particle density at several positions above the target surface as function of time was judged from corrected line intensities. Experimental data are compared with theoretical predictions made with the effusion model of plasma expansion (Kelly R and Braren B 1991 Appl. Phys. B 53 160). The effusion model provided the relative particle density in the expanding plasma cloud as a function of initial target temperature. By comparing predicted and measured time evolution of particle density, an initial target temperature of about 125eV was inferred. The coupling of the laser beam energy to the plasma itself was inferred from the failure of the model of the direct target surface heating (Andreic Z, Henc-Bartolic V and Kunze H-J 1993 Physica Scripta 48 331) to produce the required target temperature.

Aluminum plasma produced by a nitrogen laser

13 mJ laser pulses from a nitrogen laser were focused onto an aluminum target in air. The target surface was perpendicular to the axis of the laser beam. A peak energy density of 1.3 J/cm2 and a power density of 80 MW/cm2 were achieved with a laser pulse duration of 16ns. This high power density produced a transient plasma cloud that expanded explosively into the surrounding atmosphere. An initial electron density of about 1 × 1019 cm−3 and an electron temperature of about 2 eV were determined by optical spectroscopy. The line of sight was parallel to the surface and perpendicular to the laser beam axis. The height of the line of sight above the target surface was varied in order to gather data about the whole plasma cloud. In about 500 ns the plasma cloud expands to about 0.5 mm above the target surface, cools down to about 1.2 eV and is tenfold reduced in electron density. The initial expansion velocity was determined to be about 2 km/s.

Titanium plasma produced by a nitrogen laser

Titanium plasmas produced in vacuum and in air by radiation from a nitrogen laser focused onto a solid titanium target are studied spectroscopically. The energy deposition is more effective than in other cases since the wavelength of the laser is in the vicinity of Ti resonance lines.

Some properties of atomic beams from laser irradiated zirconium

Pulses from a CO2- and a N2-laser are focused onto a zirconium target, and the released atomic beams are analysed by the laser-induced fluorescence technique at various distances from the target surface. The average beam velocity exceeds quite drastically the velocity expected from any thermal model; the populations of the fine structure levels of the ground-state, on the other hand, correspond to a temperature below the expected lattice temperature. The influence of surface oxygen is investigated.

Simple device for the measurement of light beam penetration through river water

A device for the depth penetration measurement based on the transmition of He-Ne laser light power through samples of river water was developed.