Surface Potential as a Laser Damage Diagnostic

Abstract

We investigated the relationship between surface potential changes and N-on-1 laser surface damage on a wide range of materials. The surface potential or work function difference was measured as a function of position by a small non-contacting Kelvin type probe. This design and operation of the probe is described. Using this probe, the change in surface potential due to laser irradiation was mapped with a ∼1 mm resolution. Although no consistent pre-damage changes in potential were observed, all larger damage features had surface potential changes associated with them. The insulating materials studied, fluoride and oxide thin films, bare fused silica and magnesium fluoride substrates, all showed the accumulation of negative charge in areas more than ten times larger in diameter than the laser beam spot or damage area. This initial charge was observed to decay on the time scale of hours to a lower fixed value of potential associated with permanent damage to the surface.