The laser beam was focused onto the sample with a fused silica
plano-convex lens. The focal length of the lens was 200 mm at
589 nm; at 1064 nm, the effective focal length is 203.9 mm. The
lens was mounted on a pair of translation stages for precise and
reproducible positioning. In this study, the laser pulse energy
was kept constant and the lens-to-target distance was varied to
obtain a range of laser power densities at the sample surface. The
sample was always placed before the focal point of the lens to
avoid gas breakdown by the focused laser beam. Laser power
density can be calculated from the laser beam spot area, pulse
energy, and pulse duration. The laser beam spot area was
calculated using simple geometric optical principles based on the
lens-to-target distance, lens focal length, and laser beam size. The
laser beam spot size was also determined by measuring the crater
diameter using a microscope and profilometer (for craters generated
at lower power densities when their diameter is approximately
equal to the laser beam diameter). The calculated and measured
spot sizes were found to agree within 10%. The laser power
density at the target surface was varied from 7 to 70 GW/cm2.
The ICP is a Plasma Therm 20P operated with a forward power
of 1250 W (reflected power 1-2 W). All gas flow rates were
regulated using Matheson mass flow controllers (model 8274).
The argon outer and intermediate gas flow rates were 14 and 1
L/min, respectively.