Experimental and simulation studies of femtosecond laser stimulated electrical discharges in small gaps and surface modifications

Femtosecond (FS) laser-stimulated discharges in nanoscale and microscale gaps between etched nanoprobe tip cathodes and metal anodes with applied DC potential were experimentally studied to define parameter ranges for their controlled formation and resulting surface modifications. For appropriate values of gap length, applied potential and laser irradiance, breakdown discharges could be reliably stimulated by FS laser pulses and the mean breakdown field was approximately an order of magnitude smaller than for breakdown without laser stimulation. For 500nm gaps, controllable gold film surface melting by laser-stimulated discharge was detected. Minor cathode tip ablation could be observed for FS laser pulses that reliably stimulated discharges, suggesting that cathode material played an important role in stimulation of breakdown discharges in nanoscale gaps. Surface melting produced features as small as 70nm on gold film anodes when discharge current was limited by 1 MO series resistor. Numerical simulations of FS laser-stimulated electrical discharges without current-limiting in submicron gaps were performed by using a particle-in-cell - Monte Carlo collision model and the results were compared to experiments. The effect of laser-ablated cathode materials (partially-ionized platinum and electrons) at various densities on gap breakdown was simulated. Breakdown discharges were predicted to occur at a much lower applied potential with initial laser-ablated material than for gaps without mainly due to enhanced local electric field at the cathode tip. Simulated discharge current pulses had nearly equal peak magnitudes and shorter durations than experimental discharges at the same applied potential. The heat flux distributions on the anode surface at various applied potentials were predicted from the energy deposited by incident particles. The heat flux distributions were approximated by Gaussian distributions and their time axes were scaled for anode surface temperature calculations using analytical solutions of thermal diffusion in a semi-infinite solid. Anode melting predictions were compared to experiments. The predicted discharge potential for onset of anode melting and the diameters of anode melting for various applied potentials were comparable to the experimental results.