Femtosecond laser-triggered electrical discharges at nanoprobe tips for nanoprocessing

Characteristics of and surface modifications by nano-scale laser-triggered electrical discharges are reported. The electrical discharges were stimulated by femtosecond laser pulses in nanoscale gaps between sharpened metal tips and gold film. Laser pulses with intensity 5.6×10¹¹ W/cm ² reliably triggered discharges in sub-micron and microscale gaps. For gaps shorter than a threshold value, the discharge stimulation probability was near unity, independent of applied potential. At longer gaps the discharge probability abruptly decreased and then more gradually approached zero probability at a gap length that depended on voltage. At a lower laser intensity of 3.5×10¹¹ W/cm², the discharge stimulation probability characteristics were similar to those at higher intensity, but only for the higher applied potentials (60V - 80V) used. At lower applied potential, the discharge probability was also lower and depended on potential. With current limiting diode, the discharge current reached a peak value in about 2ns, and extinguished after an additional 2-3ns, irrespective of whether or not the discharge was laser stimulated. Scanning Electron Microscopy studies show gold surface regions was melted and re-solidified after electrical discharge. The shapes of modifications were similar but the size increased from about 500nm to 1μm as voltage varied from 40V to 80V.