Subwavelength, low-haze, anti-reflective (AR) nano-textured surfaces are an effective replacement for thin-film AR coatings (TFARCs) with the potential to increase reliability and minimize thermo-optic effects in kW-class diamond-based laser systems. Etched directly into optical surfaces, AR nano-textured surfaces can yield high optical damage resistance combined with high transmission, low back reflection, and low absorption values equivalent to the bulk substrate material. In this initial study, Random AR (RAR) nano-structures were etched into monocrystalline chemical vapor deposited (CVD) diamond windows. Photothermal common-path interferometry (PCI) measurements at 1064nm were conducted in order to characterize the level of absorption at the surfaces and through the bulk of diamond substrates. Nano-second pulsed laser induced damage threshold (LiDT) measurements at 1064nm were conducted, and damage sites were analyzed via scanning electron microscopy (SEM) to understand damage mechanisms in both as-polished and RAR nano-textured diamond samples.
The pulsed laser induced damage threshold (LiDT) of Random Anti-Reflective (RAR) nano-textured fused silica optics has been shown to be many times higher than thin-film AR coated optics at wavelengths ranging from the near UV through the NIR. Because an RAR nano-texture is formed by a plasma etch process that removes part of the optic surface, the observed increase in damage resistance has kept track with the LiDT advances attained by low roughness super-polishing and damage pre-cursor mitigation techniques. In this work, nano-second pulse LiDT testing of RAR nano-textured optics was conducted at the deep UV wavelength of 266nm. The effect on 266nm LiDT of the uniform removal of additional surface material from fused silica optics using a dry plasma etch process was investigated. This plasma-polishing (PP), pre-RAR process was varied using fluorine-based chemistries that removed 100-300nm of material from each test surface, with surface roughness then characterized using white-light interferometry. Photothermal interferometry confirmed that no surface absorption was added by the PP, RAR, and PP-RAR plasma etching. Both standard grade, and ultra-low bulk absorption (low-OH) fused silica were included in the tests. RAR nanotextured surfaces showed an average damage threshold of 8.4 J/cm2, a level 3 times higher than a commercially available thin-film AR coated surface. Unexpected from pulsed LiDT testing at many longer wavelengths, all plasma etched surfaces exhibited less than half the damage threshold of the untreated, as-polished fused silica surfaces, and there was no observed correlation with surface roughness or plasma etch depth. From work by others it was theorized that exposure to the deep UV photons generated by the plasma might induce absorptive electronic defects in the fused silica material that could explain the reduced damage resistance relative to non-exposed surfaces. As an initial test of this concept an RAR nano-textured sample was baked at 400C to remove the suspected electronic defect. The subsequent pulsed LiDT of this one annealed sample was found to be 15.5 J/cm2, nearly double that of all other plasma etched samples. Further work to confirm this result is on-going.
Non-diffracting surface relief grating structures combined with high refractive index films were designed as high efficiency, narrow-band, polarization selective high reflectors for the near infrared wavelength region. Such nanostructure polarizers (NSP) have the potential for increased laser damage resistance due to reduced absorption and the ability to create arbitrary refractive index layers with fewer defects and reduced electric field enhancement. Three NSP designs based on gratings in fused silica combined with tantala and magnesium fluoride films, were prototyped and characterized for efficiency, surface absorption and pulsed laser damage resistance at a wavelength of 1064nm. Most NSP prototypes exhibited <99.7% reflectivity for linearly polarized illumination over a several nm bandwidth with high transmission of the orthogonal polarization leading to extinction ratios greater than 300:1 for the best performers. NSP prototype performance was worse than predicted by the design models due to the imprecise replication of the fused silica grating surface in the film layers resulting from the deposition system configuration. Surface absorption measurements showed the expected low absorption in the 4 ppm range for film layers deposited on non-structured control substrates, but voids and growth defects revealed through scanning electron microscopy in the same films deposited over gratings, likely caused an observed 5 fold increase in NSP prototype surface absorption. Initial 1064nm wavelength, 6.2ns pulsed laser damage testing also showed a reduced damage resistance for NSP prototypes compared to the films deposited on non-structured control substrates. Follow-on work to eliminate the film defects for NSP designs is underway.
A study of the continuous wave (CW) laser induced damage threshold (LiDT) of fused silica and yttrium aluminum garnet (YAG) optics was conducted to further illustrate the enhanced survivability within high power laser systems of an anti-reflection (AR) treatment consisting of randomly distributed surface relief nanostructures (RAR). A series of three CW LiDT tests using the 1070nm wavelength, 16 KW fiber laser test bed at Penn State Electro-Optic Center (PSEOC) were designed and completed, with improvements in the testing protocol, areal coverage, and maximum exposure intensities implemented between test cycles. Initial results for accumulated power, stationary site exposures of RAR nano-textured optics showed no damage and low surface temperatures similar to the control optics with no AR treatment. In contrast, optics with thin-film AR coatings showed high surface temperatures consistent with absorption by the film layers. Surface discriminating absorption measurements made using the Photothermal Common-path Interferometry (PCI) method, showed zero added surface absorption for the RAR nanotextured optics, and absorption levels in the 2-5 part per million range for thin-film AR coated optics. In addition, the surface absorption of thin-film AR coatings was also found to have localized absorption spikes that are likely pre-cursors for damage. Subsequent CW LiDT testing protocol included raster scanning an increased intensity focused beam over the test optic surface where it was found that thin-film AR coated optics damaged at intensities in the 2 to 5 MW/cm2 range with surface temperatures over 250C during the long-duration exposures. Significantly, none of the 10 RAR nano-textured fused silica optics tested could be damaged up to the maximum system intensity of 15.5 MW/cm2, and surface temperatures remained low. YAG optics tested during the final cycle exhibited a similar result with RAR nano-textured surfaces surviving intensities over 3 times higher than thin-film AR coated surfaces. This result was correlated with PCI measurements that also show zero-added surface absorption for the RAR nano-textured YAG optics.
An investigation into the potential for increased laser damage resistance was made for a random distribution and
regular array of nanometer scale surface relief structures integrated with dielectric thin-films to create 355nm
wavelength selective high reflectors (HR). First, Random Anti-Reflection (RAR) nanostructures were fabricated in
a thick silica cap layer deposited on top of a conventional 30-layer HR stack designed as 45 degree turning mirrors.
Surface absorption scans of these RAR nano-texture enhanced HR stacks showed a slight decrease in absorption
with no impact on performance, however standardized pulsed laser damage threshold testing found no improvement
in damage resistance over non-textured silica-cap HR stacks. Second, polarization and wavelength selective nanostructure
resonant (NSR) array filters designed to be embedded within thick high damage resistance silica layers
were modeled using rigorous coupled wave analysis. Prototypes were fabricated of one NSR design consisting of a
low-aspect ratio grating defined in a fused silica substrate with a single thin layer of hafnium oxide over-coat. The
performance of NSR prototypes was limited due to multiple fabrication and testing issues. Initial 355nm
wavelength, 5ns pulse, s-on-1 laser damage testing yielded a damage threshold in the 3 to 4 J/cm2 range, comparable
to that obtained for the multi-layer HR stacks. Despite these modest early results, it appears that with further
fabrication improvements, nano-structure array resonators embedded within silica layers could yield significant
increases in the laser damage resistance of HR optics.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.