Effect of Thermal Anneal on Growth Behavior of Laser-Induced Damage Sites on the Exit Surface of Fused Silica Page: 4 of 12
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driven by capillary flow [(Hrma, Han et al. 1988)]. Increasing the anneal temperature and time revealed a
treatment regime of strengthening followed by weakening.
Thermal anneal of laser-induced damage on SiO2 surface has also been achieved by exposure to 10.6 m
wavelength CO2 laser irradiation (SiO2 absorption coefficient -103 cm'), leading to significant surface
smoothing and an arrest of growth [(Temple, Lowdermilk et al. 1982), (Brusasco, Penetrante et al. 2002)].
While CO2 laser heating permits the observation of local, in situ healing of damage sites [(Yang, Matthews et
al. 2010)], in this study we employ an oven anneal for simultaneous treatment of a large number of sites at
identical conditions which is necessary for elucidating the statistical nature of damage growth. In addition to
treatment homogeneity, an oven treatment generates smaller spatial thermal gradients and subsequently less
residual stress than that compared to a spatially finite CO2 laser beam. The primary metric we employ to
characterize growth behavior is the rate at which a damage site grows laterally following a laser shot (growth
rate). In this work, we investigate the effect of isothermal anneal on growth rate of both as-initiated and
grown damage sites. Furthermore, we explore the relationship between growth rate and measurable anneal-
induced changes in surface and sub-surface damage structure, relative concentration of ultraviolet (UV) light
absorbers, and stress-induced birefringence, in reference to the known thermal properties of fused silica.
II. EXPERIMENTAL PROCEDURE
2.1 Sample preparation
Two UV-grade fused silica rounds (Corning 7980, [OH] -1000 ppm wt. %) were used in this study. Sample
A, intended for large aperture damage growth, was 1 cm in thickness and 2 inches in diameter. Sample B,
intended for side-viewing of damage site features extending from the surface into the bulk, began as an
identical cylinder and was then cut to produce a 1-cm thick square with rounded corners. By keeping the
corners rounded the sample could still be held in standard 2-inch optics mounts. All four edges were polished
to allow imaging though the sides of the sample.
Both samples were then exposed to a chemical etchant to render the surfaces with state-of-the-art
resistance to laser-induced damage [(Suratwala, Miller et al. 2011)]. Damage sites were initiated on the bare
surface using a 1-mJ, single laser pulse from a 355 nm, 3.5-ns Full Width at Half Maximum of intensity
(FWHM) Gaussian pulsed laser operating at 10 Hz (EKSPLA). The laser beam was focused down to a -50
m diameter spot at the exit surface of the flat using a 300 mm focal lens. The laser fluence at the sample's
surface exceeded the surface damage threshold (-60 J/cm2) and resulted in damage with -100% probability.
The damage manifested itself mostly as single pits with a diameter of 40 + 10 m (mean + standard
deviation). Each sample was translated to expose a pristine location to each subsequent pulse. For Sample A,
a grid of damage sites was generated across the surface within a 20 mm x 20 mm region of interest located
centered on the round [(Laurence, Bude et al. 2012)]. To establish an internal control, initially only one set (5
rows) of damage sites (n=50 with 2 mm nearest neighbor spacing) was created on the lower half of this region
of interest (designated as Set 1 or control). In this manner, site growth characteristics before heat-treatment
could be determined. For Sample B, one row of damage sites per edge was initiated 5 mm from the edge,
yielding a total of 40 sites with 2 mm nearest neighbor spacing.
Following initiation using the small laser beam, the damage sites on Sample A were simultaneously grown
using a large aperture (-30 mm) laser beam at the Optical Sciences Laboratory at LLNL. This laser, described
in detail elsewhere [(Nostrand, Weiland et al. 2003)] is a Nd:glass amplifier laser system outputting a 3rd
harmonic, 100 J pulse tunable in width and shape. Damage growth proceeded in near-vacuum (<10-6 Torr)
and at room temperature by exposing the sample to a series of five, nearly identical laser pulses at 351-nm,
10.0 2.0 J/cm2 (mean SD), 5-ns flat-in-time (FIT). For each growth shot, the spatial distribution of the
fluence within the large aperture beam was recorded and subsequently registered to the sample, providing the
local fluence at each site (standard error -1% of the mean value obtained by averaging over a 500 pm x 500
pm square patch). Between laser exposures, the sites were characterized with a number of methods described
in 2.2. Upon completion of the damage growth sequence and site characterization, additional sites (n=60)
were initiated as described above in the remaining half of the region of interest (designated as Set 2).2
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Raman, R N; Negres, R A; Matthews, M J & Carr, C W. Effect of Thermal Anneal on Growth Behavior of Laser-Induced Damage Sites on the Exit Surface of Fused Silica, article, December 14, 2012; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc870221/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.