Papers by Christopher Stolz
Journal of Applied Physics, Jul 27, 2021
Laser-induced damage in coating materials with a high index of refraction, such as hafnia, limits... more Laser-induced damage in coating materials with a high index of refraction, such as hafnia, limits the performance of high power and high energy laser systems. Understanding the underlying physics responsible for laser damage holds the key for developing damage-resistant optical films. Previous studies have reported a substantial difference in laser damage onset for hafnia films produced by different deposition methods, yet the underlying mechanisms for the observed difference remain elusive. We combined laser damage testing with analytical characterizations and theoretical simulations to investigate the response of hafnia films produced by electron (e-) beam evaporation vs ion beam sputtering (IBS) methods upon UV ns-laser exposure. We found that e-beam produced hafnia films were overall more damage resistant; in addition, we observed a polarization anisotropy associated with the onset of damage in the e-beam films, while this effect was absent in the latter films. The observed differences can be attributed to the stark contrast in the pressure inside the pores inherent in both films. The high pressure inside the IBS-induced nanobubbles has been shown to reduce the threshold for laser-induced plasma breakdown leading to film damage. The polarization effects in the e-beam coatings can be related to the asymmetric electric field intensification induced by the columnar void structure. Our findings provide a fundamental basis for developing strategies to produce laser damage-resistant coatings for UV pulsed laser applications.
Optical Engineering, Oct 24, 2016
The response of a potential candidate protective capping layer (SiO 2 or Al 2 O 3 ) to laser expo... more The response of a potential candidate protective capping layer (SiO 2 or Al 2 O 3 ) to laser exposure of 1ω (1053 nm) to high-reflector silica-hafnia multilayer coatings in the presence of variously shaped Ti particles is investigated by combining laser damage testing and numerical modeling. Each sample is exposed to a single oblique angle (45 deg) laser shot (p-polarization, ∼10 J∕cm 2 , 14 ns) in the presence of spherically or irregularly shaped Ti particles on the surface. The two capping layers show markedly different responses. For the spherical particles, the Al 2 O 3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. The SiO 2 capping layer is only mildly modified by a shallow depression, likely due to plasma erosion. The different response of the capping layer is attributed to the large difference in the thermal expansion coefficient of the materials, with that of the Al 2 O 3 about 15 times greater than that of the SiO 2 layer. For the irregular particles, the Al 2 O 3 capping layer displays minimal to no damage while the SiO 2 capping layer is significantly damaged. The difference is due to the disparity in mechanical strength with Al 2 O 3 possessing approximately 10 times higher fracture toughness.
Optical Interference Coatings
Electron-beam deposition processes are being optimized to improve the laser damage resistance of ... more Electron-beam deposition processes are being optimized to improve the laser damage resistance of large aperture (up to 0.34 m2) optical coatings for the National Ignition Facility (NIF). One modification over previous deposition process is the replacement of hafnia as a starting material with hafnium for source ejection reduction.1 This modification has led to a significant reduction (~10 times) in the nodular defect density. Although considerable effort has been spent on studying nodular defects, unfortunately the damage threshold of coatings can also be limited by a multitude of different defect types, i.e. damage sites are also present in nodule-free regions of large aperture optics.
Bulletin of the American Physical Society, 2017
State University, Fort Collins, Co 80525 -Ion beam sputtering growth of amorphous oxides coupled ... more State University, Fort Collins, Co 80525 -Ion beam sputtering growth of amorphous oxides coupled with processing using Ar ion assist is termed planarization. This process was introduced by our team reducing defect cross-section on pre-patterned substrates by up to 90% and increasing laser damage performance by 10x The effects of SiO 2 planarization processing on the laser damage resistance of single, bilayers, and multilayer coatings of HfO 2 and SiO 2 is investigated at pulse durations of 9ps and 220ps. Planarized samples experience a large increase in absorption loss at 1m wavelength, which is reduced after in-air annealing suggesting presence of oxygen point defects. It is shown the laser damage threshold reduces with planarized SiO 2 layers directly implemented compared to control samples at both pulse durations. In-air annealing instead shows a recovery of the laser damage threshold.
HfO{sub 2}/SiO{sub 2} Brewster`s angle polarizers are being developed at LLNL for the National Ig... more HfO{sub 2}/SiO{sub 2} Brewster`s angle polarizers are being developed at LLNL for the National Ignition Facility. Damage threshold studies using a 3-ns pulse length 1064-nm laser have revealed a number of different damage morphologies such as nodular ejection pits, plasma scalds, flat bottom pits, and overcoat delaminations. Of these laser damage morphologies, delaminations have the most negative impact on the multilayer stability. By selecting the proper SiO{sub 2} overcoat thickness, the delamination morphology is eliminated without significantly modifying the spectra characteristics of the coating and the functional damage threshold is increased by 2-4x. A model of the thermal mechanical response of the overcoats is presented for various SiO{sub 2} overcoat thicknesses. The overcoat thickness influences the electric-field profile resulting in different thermal gradients between the outer SiO{sub 2} and HfO{sub 2} layers. This modeling effort attempts to understand the relation be...
Journal of Applied Physics, 2020
High intensity and high energy laser facilities place increasing demands on optical components, r... more High intensity and high energy laser facilities place increasing demands on optical components, requiring large surface area optics with exacting specifications. Petawatt lasers are high energy, short-pulse laser systems generally based on chirped-pulse amplification, where an initial low energy short pulse is stretched, amplified, and then recompressed to produce fs to ps high-power laser pulses. In such petawatt lasers, the highest demands are placed on the final optics, including gratings which compress the pulses and mirrors which direct and focus the final high-power beams. The limiting factor in these optical components is generally laser-induced damage. Designing and fabricating these optical components to meet reflection, dispersion, and other requirements while meeting laser-induced damage requirements is the primary challenge discussed in this tutorial. We will introduce the reader to the technical challenges and tradeoffs required to produce mirrors for petawatt lasers an...
Optical Engineering, 2018
The thin film damage competition series at the Boulder Damage Symposium provides an opportunity t... more The thin film damage competition series at the Boulder Damage Symposium provides an opportunity to observe general trends in laser damage behavior between different coating types (high reflector, antireflector, polarizer, and Fabry-Perot filter), wavelength ranges (193 to 1064 nm), and pulse length ranges (40 fs to 18 ns). Additionally, the impact of deposition process, coating material, cleaning process, and layer count can be studied within a single year or more broadly across the history of this competition. Although there are instances where participants attempted to isolate a single variable to better understand its impact on laser resistance, this series of competitions isolates the variable of the damage testing service and protocol for a wide variety of participants to enable the observation of general trends. In total, 275 samples from 58 different participants have been tested at four different laser damage testing facilities over the last 10 years. Hafnia was clearly the best high refractive index material except for ultraviolet (UV) applications, although a wide range of high refractive index materials performed well. The best deposition process varied significantly between the different competitions. The best deposition process was dependent on the coating type, wavelength, and pulse duration. For 1064-nm coatings with nanosecond scale pulse lengths, e-beam coatings tended to be the best performers. For short-pulse length NIR mirrors and nanosecond pulse length UV mirrors, densified coating processes, which all involved sputtering of the target material, were the best performers. For UV antireflector (AR) coatings and excimer mirrors, both tested at nanosecond pulse lengths, they tended to favor very low energetic deposition methods yielding soft coatings, such as sol gel dip coating for the AR and resistive heating of fluorides for the excimer mirrors. Finally, cleaning method and layer count had a less obvious correlation with laser resistance over the history of this thin film damage competition series.
Optical Engineering, 2016
Laser-induced damage of intrinsic and extrinsic defects by picosecond pulses on multilayer dielec... more Laser-induced damage of intrinsic and extrinsic defects by picosecond pulses on multilayer dielectric coatings for petawatt-class lasers,"
Optical Engineering, 2016
The response of a potential candidate protective capping layer (SiO 2 or Al 2 O 3 ) to laser expo... more The response of a potential candidate protective capping layer (SiO 2 or Al 2 O 3 ) to laser exposure of 1ω (1053 nm) to high-reflector silica-hafnia multilayer coatings in the presence of variously shaped Ti particles is investigated by combining laser damage testing and numerical modeling. Each sample is exposed to a single oblique angle (45 deg) laser shot (p-polarization, ∼10 J∕cm 2 , 14 ns) in the presence of spherically or irregularly shaped Ti particles on the surface. The two capping layers show markedly different responses. For the spherical particles, the Al 2 O 3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. The SiO 2 capping layer is only mildly modified by a shallow depression, likely due to plasma erosion. The different response of the capping layer is attributed to the large difference in the thermal expansion coefficient of the materials, with that of the Al 2 O 3 about 15 times greater than that of the SiO 2 layer. For the irregular particles, the Al 2 O 3 capping layer displays minimal to no damage while the SiO 2 capping layer is significantly damaged. The difference is due to the disparity in mechanical strength with Al 2 O 3 possessing approximately 10 times higher fracture toughness.
SPIE Proceedings, 2006
Proceedings of SPIE follow an e-First publication model, with papers published first online and t... more Proceedings of SPIE follow an e-First publication model, with papers published first online and then in print and on CD-ROM. Papers are published as they are submitted and meet publication criteria. A unique, consistent, permanent citation identifier (CID) number is assigned to each article at the time of the first publication. Utilization of CIDs allows articles to be fully citable as soon they are published online, and connects the same identifier to all online, print, and electronic versions of the publication. SPIE uses a six-digit CID article numbering system in which: • The first four digits correspond to the SPIE volume number. • The last two digits indicate publication order within the volume using a Base 36 numbering system employing both numerals and letters. These two-number sets start with 00,
The Mercury Project: A High Average Power, Gas-Cooled Laser with Frequency Conversion and Wavefront Correction
Advanced Solid-State Photonics, 2006
The Mercury laser operated continuously for several hours at 55J and 10Hz with fourteen 4×6 cm2 Y... more The Mercury laser operated continuously for several hours at 55J and 10Hz with fourteen 4×6 cm2 Yb:S-FAP amplifier slabs pumped by eight 100kW diode arrays. Average power frequency conversion employing YCOB yielded 50% conversion efficiency.
Searching for Optimal Mitigation Geometry for Multilayer High Reflector Coatings
Optical Interference Coatings, 2010
... reflector coatings S. Roger Qiua*, Justin E. Wolfea, Anthony M. Monterrosab, Michael D. Feita... more ... reflector coatings S. Roger Qiua*, Justin E. Wolfea, Anthony M. Monterrosab, Michael D. Feita, Thomas V. Pistorc, Christopher J. Stolza ... [2] FY Genin, CJ Stolz, Morphologies of laser induced damage in hafnia-silica multilayer mirror and polarizer coatings, Proc. ...
SPIE Proceedings, 2005
In operational laser systems, it is often difficult to keep optical components completely free of... more In operational laser systems, it is often difficult to keep optical components completely free of foreign material. We have investigated the performance of high damage threshold 1.053 µm high reflectors in the presence of surface contaminants. We have looked at the impact of stainless steel, aluminum, Azurlite®, dust, cotton fibers and polyester fibers on the performance of the mirrors under laser irradiation. The first four contaminants were deposited in sizes ranging from 30 microns to 150 microns. The fibers included lengths ranging to several millimeters. The testing was done at either a single fluence in the range of 6 J/cm 2 to 24 J/cm 2 , or a ramped sequence of shots starting at 1 J/cm 2 . We will present data showing the onset of damage, the type of damage, and the propensity to damage growth in the fluence range studied.
Physics of Plasmas, 2014
The scope of the NIC was the planning and preparation for and the execution of the first 3 yr of ... more The scope of the NIC was the planning and preparation for and the execution of the first 3 yr of ignition experiments (through the end of September 2012) as well as the development, fielding, qualification, and integration of the wide range of capabilities required for ignition. Besides the operation and optimization of the use of NIF, these capabilities included over 50 optical, x-ray, and nuclear diagnostic systems, target fabrication facilities, experimental platforms, and a wide range of NIF facility infrastructure. The goal of ignition experiments on the NIF is to achieve, for the first time, ignition and thermonuclear burn in the laboratory via inertial confinement fusion and to develop a platform for ignition and high energy density applications on the NIF. The goal of the NIC was to develop and integrate all of the capabilities required for a precision ignition campaign and, if possible, to demonstrate ignition and gain by the end of FY12. The goal of achieving ignition can be divided into three main challenges. The first challenge is defining specifications for the target, laser, and diagnostics with the understanding that not all ignition physics is fully understood and not all material properties are known. The second challenge is designing experiments to systematically remove these uncertainties. The third challenge is translating these experimental results into metrics designed to determine how well the experimental implosions have performed relative to expectations and requirements and to advance those metrics toward the conditions required for ignition. This paper summarizes the approach taken to address these challenges, along with the progress achieved to date and the challenges that remain. At project completion in 2009, NIF lacked almost all the diagnostics and infrastructure required for ignition experiments. About half of the 3 yr period covered in this review was taken up by the effort required to install and performance qualify the equipment and experimental platforms needed for ignition experiments. Ignition on the NIF is a grand challenge undertaking and the results presented here represent a snapshot in time on the path toward that goal. The path forward presented at the end of this review summarizes plans for the Ignition Campaign on the NIF, which were adopted at the end of 2012, as well as some of the key results obtained since the end of the NIC. V
Optical Engineering, Dec 18, 2018
The thin film damage competition series at the Boulder Damage Symposium provides an opportunity t... more The thin film damage competition series at the Boulder Damage Symposium provides an opportunity to observe general trends in laser damage behavior between different coating types (high reflector, antireflector, polarizer, and Fabry-Perot filter), wavelength ranges (193 to 1064 nm), and pulse length ranges (40 fs to 18 ns). Additionally, the impact of deposition process, coating material, cleaning process, and layer count can be studied within a single year or more broadly across the history of this competition. Although there are instances where participants attempted to isolate a single variable to better understand its impact on laser resistance, this series of competitions isolates the variable of the damage testing service and protocol for a wide variety of participants to enable the observation of general trends. In total, 275 samples from 58 different participants have been tested at four different laser damage testing facilities over the last 10 years. Hafnia was clearly the best high refractive index material except for ultraviolet (UV) applications, although a wide range of high refractive index materials performed well. The best deposition process varied significantly between the different competitions. The best deposition process was dependent on the coating type, wavelength, and pulse duration. For 1064-nm coatings with nanosecond scale pulse lengths, e-beam coatings tended to be the best performers. For short-pulse length NIR mirrors and nanosecond pulse length UV mirrors, densified coating processes, which all involved sputtering of the target material, were the best performers. For UV antireflector (AR) coatings and excimer mirrors, both tested at nanosecond pulse lengths, they tended to favor very low energetic deposition methods yielding soft coatings, such as sol gel dip coating for the AR and resistive heating of fluorides for the excimer mirrors. Finally, cleaning method and layer count had a less obvious correlation with laser resistance over the history of this thin film damage competition series.
SPIE Proceedings, 1997
This is a preprint of a paper intended for publication in a journal or proceedings. Since changes... more This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.
Laser-Induced Damage in Optical Materials: 2002 and 7th International Workshop on Laser Beam and Optics Characterization, 2003
Transport mirrors within the National Ignition Facility, a 192-beam 4-MJ fusion laser at 1053 nm,... more Transport mirrors within the National Ignition Facility, a 192-beam 4-MJ fusion laser at 1053 nm, will be exposed to backscattered light from plasmas created from fusion targets and backlighters. This backscattered light covers the UV and visible spectrum from 351-600 nm. The transport mirror BK7 substrates will be intentionally solarized to absorb >95% of the backscattered light to prevent damage to the metallic mechanical support hardware. Solarization has minimal impact on the 351-and 1053-nm laser-induced damage threshold or the reflected wavefront of the multilayer hafnia silica coating. Radiation sources of various energies were examined for BK7 darkening efficiency within the UV and visible region with 1.1 MeV gamma rays from a Cobalt 60 source ultimately being selected. Finally, bleaching rates were measured at elevated temperatures to generate a model for predicting the lifetime at ambient conditions (20°C), before solarized BK7 substrates exceed 5% transmission in the UV and visible region. Over a 30-mm thickness, BK7 glass will bleach in 10 years to 5% transmission at 600 nm, the most transmissive wavelengths over the 351-600 nm regions.
SPIE Proceedings, 1996
Hafnia-silica multilayer mirrors and polarizers were deposited by e-beam evaporation onto BK7 gla... more Hafnia-silica multilayer mirrors and polarizers were deposited by e-beam evaporation onto BK7 glass substrates. The mirrors and polarizers were coated for operation at a wavelength of 1053 nm at 45° and at Brewster's angle (56°), respectively. They were tested with a single 3-ns laser pulse. The morphology of the laser-induced damage was characterized by optical and scanning electron microscopy. Four distinct damage morphologies were found: pits, flat bottom pits, scalds, and delaminates. The pits and flat bottom pits (< 30 µm in diameter) were detected at lower fluences (as low as 5 J/cm 2). The pits seemed to result from ejection of nodular defects by causing local enhancement of the electric field. Scalds and delaminates could be observed at higher fluences (above 13 J/cm 2) and seemed to result from the formation of plasmas on the surface. These damage types often originated at pits and were typically less than 300 µm in diameter; their size increased almost linearly with fluence. Finally, the effects of the damage on the characteristics of the beam (reflectivity degradation and phase modulations) were measured.
Uploads
Papers by Christopher Stolz