Nikolai E Kask

Fractal nanostructures arising under the ablation of graphite and silicon by millisecond-laser radiation

A mechanism for the formation of fractal nanostructures with micro- and macroscopic sizes under 10-ms laser-pulse irradiation of carbon, silicon, and their mixtures is investigated. The appearance of linear fragments containing nanoclusters is considered based on the diffusion-limited aggregation model with the dipole-dipole interaction. It is shown that the competition between the thermophoretic and gradient forces acting upon particles and their aggregates in a plume determines the area of congestion and the formation of the macroscopical fractal nanostructures.

Formation of silicon nanostructures when a target is ablated by a quasi-continuous laser pulse

When a target is ablated in an atmosphere of an inert buffer gas by low-intensity laser radiation (no greater than 105W/cm2), nanoparticles self-assemble into low-dimensional fractal structures. The dependences of the efficiency of the self-assembly process on the composition of the silicon-silica target and the pressure of the buffer gas manifest a distinct correlation with percolation in the plasma of the laser flare. A structure with a fractal dimension of df>2 is formed most efficiently close to the three-dimensional percolation threshold pc≈0.3, where pc is the ratio of the number density of silicon atoms to the number of all the atoms in the laser flare. Close to the two-dimensional percolation threshold (pc≈0.5), fractal structures are observed with df<2, assembled on a plane. The main structural elements of the observed objects are one-dimensional structures--chains of tens of nanoparticles, with a characteristic size of about 80nm. Results of a study of fractal nanostructures by scanning electron microscope are presented, an x-ray structural analysis is carried out, and the photoluminescence is studied.

Magnetic and electric dipole absorption in the dispersed plasma of a laser jet

Experimental results are presented of a study of the magnetic and electric dipole absorption of microwave radiation by a laser plasma which accompanying the evaporation of various metals. An analysis is made of the influence of the finely dispersed fraction on the absorption and it is shown that the model of a percolation cluster provides a good explanation for the entire set of experimental results.