Spatial distribution of laser-ablated mateiral by probing a plasma plume in three dimensions

摘要

A Pb_0.2Bi_0.8Sr_2Ca_1Cu_2O_x target has been irradiated with 30 ns FWHM KrF excimer laser pulses having a wavelength of 248 nm and an energy density of approx 8 J/cm~2. An intensified charge coupled device camera was used to measure the overall luminescence of the resulting plume in three dimensions as it moved from the target to the substrate. For small enough laser spots the plumes were found to have an ellipsoidal shape with two distinct axes. On the other hand, for a large rectangular laser spot (3.0 X 1.2 mm~2 or 2.4 X 1.0 mm~2) the plumes were found to have an ellipsoidal shape with three distinct axes. For example, at a time delay of 1000 ns the extensions were z=34 mm (normal), y/2=12 mm, and x/2=8 mm, the lateral dimensions (x and y axes) being rotated at 90 deg with respect to the laser spot. The z extension is sufficiently greater than those for x/2 and y/2, and the z expansion-front velocity is sufficiently high, that both normal vaporization and normal boiling can be excluded as the mechanisms of the laser sputtering. Subsurface explosion can be eliminated on the grounds that the analysis demonstrating its existence was wrong. It follows that either an electronic process or else phase explosion (also termed explosive boiling) or else another still-to-be-defined but violent process may be involved. The results were finally compared with numerical solutions of the two-dimensional flow equations and a rotation effect was found essentially as observed. For example, it was largely absent at 200-700 ns, fully developed at 600-2000 ns, and, interestingly, starting to disappear at 3000-9000 ns.