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We employed the same measurement techniques that have proven successful for bulk
damage thresholds measurements to measure damage thresholds of bare silica surfaces polished
using various methods and to measure damage thresholds for antireflection coated silica, again for
various surface polishes. Light in a single transverse and longitudinal mode, from a Q-switched
Nd:YAG laser is focused to an 8 µm spot on the front and rear surfaces of silica windows polished
using ceria, alumina, or alumina/silica to find the damage threshold. We repeated the exercise for the
same surfaces anti reflection coated with silica/hafnia film stacks. We used surface third harmonic
generation to precisely place the focus on the surfaces. Key findings include:
1. The surface damage threshold can be made equal to the bulk damage threshold. There is a
large difference in single-pulse damage thresholds of bare silica surfaces polished using
ceria, alumina, and alumina followed by silica. The ceria polished samples have a statistical
damage threshold ranging from 50 to 450 GW/cm2. The alumina polished surfaces damage at
200-500 GW/cm2, with half the spots damaging at the bulk threshold of 500 GW/cm2. The
windows polished by alumina followed by silica damage almost universally at the bulk
damage threshold of 500 GW/cm2.
2. There are strong conditioning effects for these surfaces. The ceria polished surfaces have
reduced thresholds for multiple pulses. The alumina polished surfaces attain the bulk damage
threshold at most locations using multiple pulse annealing.
3. The underlying polishes strongly affect the damage thresholds for the AR coatings. The
alumina plus silica polished samples have the highest thresholds, with statistical variations
from 150-380 GW/cm2. The alumina polished samples damage at only 50 GW/cm2, but with
annealing the threshold rises to 200 GW/cm2, while the ceria polished samples damage at 50-200 GW/cm2 with no strong multiple shot effect.
4. We found there was no beam size variation of the damage threshold irradiance for the bare
alumina/silica polished samples.
5. We showed that air breakdown does not limit the surface irradiance, silica breakdown does.
6. We recorded damage morphologies for the different surfaces.
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