PHYS 314 |
| We report the synergistic effect of a pair of ultrafast laser pulses on the ablation of materials and on the properties of the resulting plasma. Using Si<111> as an example, we compare the ratio of the fluorescence from discrete transitions of neutral and ionized Si produced by a pair of pulses to that produced by a single pulse of the same total energy. We find that the fluorescence enhancement ratio increases with the delay between the pulses and decreases with the total pulse energy. We explain these results by a mechanism in which the first pulse melts the surface to produce a nanoscale liquid film, and the second pulse is absorbed more strongly by the molten phase. This mechanism is supported by reflectivity and AFM measurements. Similar behavior is observed for Cu, whereas qualitatively different trends are observed for a variety of dielectrics. We further observe for Si that the fluorescence is strongly polarized. The polarization increases with delay and pulse energy and decreases with wavelength for discrete as well as continuum emission. Under appropriate conditions, greater than 90% polarization was observed, suggesting strong anisotropy of the plasma. |
|
Spectroscopy, Chemistry, and Imaging through Nanophotonics
1:20 PM-5:20 PM, Wednesday, April 9, 2008 Morial Convention Center -- Rm. 340/341, Oral
Division of Physical Chemistry |