471 research outputs found
Coarsening Dynamics of Crystalline Thin Films
The formation of pyramid-like structures in thin-film growth on substrates
with a quadratic symmetry, e.g., {001} surfaces, is shown to exhibit
anisotropic scaling as there exist two length scales with different time
dependences. Analytical and numerical results indicate that for most
realizations coarsening of mounds is described by an exponent n=0.2357.
However, depending on material parameters, n may lie between 0 (logarithmic
coarsening) and 1/3. In contrast, growth on substrates with triangular
symmetries ({111} surfaces) is dominated by a single length scale and an
exponent n=1/3.Comment: RevTeX, 4 pages, 3 figure
Coarsening of Surface Structures in Unstable Epitaxial Growth
We study unstable epitaxy on singular surfaces using continuum equations with
a prescribed slope-dependent surface current. We derive scaling relations for
the late stage of growth, where power law coarsening of the mound morphology is
observed. For the lateral size of mounds we obtain with . An analytic treatment within a self-consistent mean-field
approximation predicts multiscaling of the height-height correlation function,
while the direct numerical solution of the continuum equation shows
conventional scaling with z=4, independent of the shape of the surface current.Comment: 15 pages, Latex. Submitted to PR
Asymptotic step profiles from a nonlinear growth equation for vicinal surfaces
We study a recently proposed nonlinear evolution equation describing the
collective step meander on a vicinal surface subject to the Bales-Zangwill
growth instability [O. Pierre-Louis et al., Phys. Rev. Lett. (80), 4221
(1998)]. A careful numerical analysis shows that the dynamically selected step
profile consists of sloped segments, given by an inverse error function and
steepening as sqrt(t), which are matched to pieces of a stationary
(time-independent) solution describing the maxima and minima. The effect of
smoothening by step edge diffusion is included heuristically, and a
one-parameter family of evolution equations is introduced which contains
relaxation by step edge diffusion and by attachment-detachment as special
cases. The question of the persistence of an initially imposed meander
wavelength is investigated in relation to recent experiments.Comment: 4 pages, 5 included figures. Typo in Eq.(5) corrected, section
headlines added and Ref.[12] update
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Identification and Elimination of Fluorescent Surface-Damage Precursors on DKDP Optics
Fluorescing surface defects that led to damage upon 351-nm laser exposure below 7 J/cm{sup 2} (3-11s) in DKDP optics were reported in these proceedings by this group a year ago. Subsequent laser damage experiments have correlated the density of these damage precursors to single-point diamond finishing conditions. Every diamond-finishing schedule contains brittle-mode cutting and ductile-mode cutting in a taper-down sequence. Finishing experiments have traced the occurrence of these defects to insufficient ductile-mode removal of subsurface damage incurred during prior brittle-mode cutting. Additionally, a correlation between defect fluorescence, laser-induced damage, and defect morphology has been established. Laser-induced damage tests also suggest a correlation between growth method and damage probability. Current experiments indicate that damage-prone defects can be minimized with the proper choice of diamond finishing conditions
Results of applying a non-evaporative mitigation technique to laser-initiated surface damage on fused-silica
We present results from a study to determine an acceptable CO{sub 2} laser-based non-evaporative mitigation protocol for use on surface damage sites in fused-silica optics. A promising protocol is identified and evaluated on a set of surface damage sites created under ICF-type laser conditions. Mitigation protocol acceptability criteria for damage re-initiation and growth, downstream intensification, and residual stress are discussed. In previous work, we found that a power ramp at the end of the protocol effectively minimizes the residual stress (<25 MPa) left in the substrate. However, the biggest difficulty in determining an acceptable protocol was balancing between low re-initiation and problematic downstream intensification. Typical growing surface damage sites mitigated with a candidate CO{sub 2} laser-based mitigation protocol all survived 351 nm, 5 ns damage testing to fluences >12.5 J/cm{sup 2}. The downstream intensification arising from the mitigated sites is evaluated, and all but one of the sites has 100% passing downstream damage expectation values. We demonstrate, for the first time, a successful non-evaporative 10.6 {micro}m CO{sub 2} laser mitigation protocol applicable to fused-silica optics used on fusion-class lasers like the National Ignition Facility (NIF)
Competing mechanisms for step meandering in unstable growth
The meander instability of a vicinal surface growing under step flow
conditions is studied within a solid-on-solid model. In the absence of edge
diffusion the selected meander wavelength agrees quantitatively with the
continuum linear stability analysis of Bales and Zangwill [Phys. Rev. B {\bf
41}, 4400 (1990)]. In the presence of edge diffusion a local instability
mechanism related to kink rounding barriers dominates, and the meander
wavelength is set by one-dimensional nucleation. The long-time behavior of the
meander amplitude differs in the two cases, and disagrees with the predictions
of a nonlinear step evolution equation [O. Pierre-Louis et al., Phys. Rev.
Lett. {\bf 80}, 4221 (1998)]. The variation of the meander wavelength with the
deposition flux and with the activation barriers for step adatom detachment and
step crossing (the Ehrlich-Schwoebel barrier) is studied in detail. The
interpretation of recent experiments on surfaces vicinal to Cu(100) [T.
Maroutian et al., Phys. Rev. B {\bf 64}, 165401 (2001)] in the light of our
results yields an estimate for the kink barrier at the close packed steps.Comment: 8 pages, 7 .eps figures. Final version. Some errors in chapter V
correcte
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Optimizing Blocker Usage On NIF Using Image Analysis And Machine Learning*
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Polarization Smoothing on the National Ignition Facility
We have recently implemented polarization smoothing (PS) on one quad of the NIF laser. Specially cut KDP and DKDP crystals at 420 x 420 mm sizes were used to scramble the incident 351 nm laser polarization over the beam aperture. The intensity contrast deduced from the measured focal spot images for one of the NIF beams is in very good agreement with the expected contrast. KDP and DKDP crystals are known to produce considerable amount of transverse stimulated Raman scattering (SRS) when irradiated with large beams at {approx}1-2 GW/cm2. In order to measure the transverse SRS, we attached optical fibers on the side of one of the PS crystals. The KDP PS crystal showed > 1 J/cm2 side scattered SRS at irradiances of 1.2 GW/cm2. The DKDP (70% deuteration level) PS crystal showed significantly less SRS. Detailed analysis of the SRS scattering in the PS crystal is in progress
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