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At the Z Facility at Sandia National Laboratories, the magnetized liner inertial fusion (MagLIF) program aims to study the inertial confinement fusion in deuterium-filled gas cells by implementing a three-step process on the fuel: premagnetization, laser preheat, and Z-pinch compression. In the laser preheat stage, the Z-Beamlet laser focuses through a thin polyimide window to enter the gas cell and heat the fusion fuel. However, it is known that the presence of the few μm thick window reduces the amount of laser energy that enters the gas and causes window material to mix into the fuel. These effects are detrimental to achieving fusion; therefore, a windowless target is desired. The Lasergate concept is designed to accomplish this by “cutting” the window and allowing the interior gas pressure to push the window material out of the beam path just before the heating laser arrives. In this work, we present the proof-of-principle experiments to evaluate a laser-cutting approach to Lasergate and explore the subsequent window and gas dynamics. Further, an experimental comparison of gas preheat with and without Lasergate gives clear indications of an energy deposition advantage using the Lasergate concept, as well as other observed and hypothesized benefits. While Lasergate was conceived with MagLIF in mind, the method is applicable to any laser or diagnostic application requiring direct line of sight to the interior of gas cell targets.

In previous years, this committee reported on the need for a US National Laser damage standard, addressing the needs of domestic industry. In 2017, a process was reported that connected the measurement of the active defect density in a small area, a, with the likely density of such defects over a larger area, A. This was presented as the basis of a Type 1, go/no-go test. 2018’s achievement, is development of process starting from a user’s requirements and flowing into test parameters. This year’s report covers the resulting test procedure that implements the test process a useful workable standard.

Dichroic coatings have been developed for high transmission at 527 nm and high reflection at 1054 nm for laser operations in the nanosecond pulse regime. The coatings consist of HfO2 and SiO2 layers deposited with e-beam evaporation, and laser-induced damage thresholds as high as 12.5 J/cm2 were measured at 532 nm with 3.5 ns pulses (22.5 degrees angle of incidence, in S-polarization). However, laser damage measurements at the single wavelength of 532 nm do not adequately characterize the laser damage resistance of these coatings, since they were designed to operate at dual wavelengths simultaneously. This became apparent after one of the coatings damaged prematurely at a lower fluence in the beam train, which inspired further investigations. To gain a more complete understanding of the laser damage resistance, results of a dual-wavelength laser damage test performed at both 532 nm and 1064 nm are presented.

Pulsed power accelerators compress electrical energy in space and time to provide versatile experimental platforms for high energy density and inertial confinement fusion science. The 80-TW “Z” pulsed power facility at Sandia National Laboratories is the largest pulsed power device in the world today. Z discharges up to 22 MJ of energy stored in its capacitor banks into a current pulse that rises in 100 ns and peaks at a current as high as 30 MA in low-inductance cylindrical targets. Considerable progress has been made over the past 15 years in the use of pulsed power as a precision scientific tool. This paper reviews developments at Sandia in inertial confinement fusion, dynamic materials science, x-ray radiation science, and pulsed power engineering, with an emphasis on progress since a previous review of research on Z in Physics of Plasmas in 2005.

The laser damage thresholds of optical coatings can degrade over time due to a variety of factors, including contamination and aging. Optical coatings deposited using electron beam evaporation are particularly susceptible to degradation due to their porous structure. In a previous study, the laser damage thresholds of optical coatings were reduced by roughly a factor of two from 2013 to 2017. The coatings in question were high reflectors for 1054 nm that contained SiO2 and HfO2 and/or TiO2 layers, and they were stored in sealed PETG containers in a class 100 cleanroom with temperature control. At the time, it was not certain whether contamination or thin film aging effects were responsible for the reduced laser damage thresholds. Therefore, to better understand the role of contamination, the coatings were recleaned and the laser damage thresholds were measured again in 2018. The results indicate that contamination played the most dominant role in reducing the laser damage thresholds of these optical coatings, even though they were stored in an environment that was presumed to be clean.

We report on progress for increasing the laser-induced damage threshold of dichroic beam combiner coatings for high transmission at 527 nm and high reflection at 1054 nm (22.5° angle of incidence, S-polarization). The initial coating consisted of HfO2 and SiO2 layers deposited with electron beam evaporation, and the laser-induced damage threshold was 7 J/cm2 at 532 nm with 3.5 ns pulses. This study introduces different coating strategies that were utilized to increase the laser damage threshold of this coating to 12.5 J/cm2.

This paper reports on the fundamental idea behind a US National Committee, The Optics and Electro-Optics Standards Council (OEOSC) Task Force (TF) 7, proposal for a so-called Type 1 laser damage test procedure. A Type 1 test is designed to give a simple binary, pass or fail, result. Such tests are intended for the transactional type of damage testing typical of acceptance and quality control testing. As such is it intended for bulk of certification of optics for the ability to survive a given fluence, useful for manufacturers of optics and their customers, the system builders. At the root of the proposed method is the probability that an optic of area A will have R or less damage occurrences with a user specified probability P at test fluence Φ. This assessment is made by a survey of area and the observation of n events. The paper presents the derivation of probability of N or less damage sites on A given n events observed in area a. The paper concludes with the remaining steps to development of a useful test procedure based on the idea presented.

Optical coatings deposited using electron beam evaporation are subject to aging effects that change the spectral characteristics of the optical coating. The aim of this study was to determine whether aging effects can also negatively impact the laser damage resistance of an optical coating. Maintaining high resistance to laser damage is particularly important for the performance of high fluence laser systems. In 2013, we deposited different high reflection coatings for 1054 nm containing HfO2/TiO2/SiO2 layers. For this study, we re-measured the laser damage thresholds of these coatings at 3.5 ns to determine if aging effects cause the laser damage threshold to decline, and to compare whether HfO2 or TiO2 is superior in terms of long-term laser damage resistance.

We report the design, fabrication, and characterization of miniaturized, flush-mounted Langmuir probe arrays for RF diagnosis of plasmas in the HF to microwave range of frequencies. We developed probes of radii≥125 µm by electroless nickel metallization of ultrasonically drilled through-substrate vias. Planar arrays with as many as 25 probes spaced 1.6 mm apart (39 probes/cm2) in Pyrex, silicon carbide, and alumina substrates were produced. The sensor system was built to have a frequency response between 2 MHz and 3 GHz, and a probe impedance greater than or within close range of the plasma sheath impedance for plasma densities≥1016 m−3. We characterized a self-biasing nickel probe part of a 2×2 array with alumina substrate using a high-density magnetized helicon plasma source; we found that the measurement of the plasma potential from the MEMS probe compares well with independent measurements using a hot emissive probe and an ion sensitive probe. The sensor technology can be used to monitor plasma-based manufacturing systems, plasma-based energy generation systems, and as on-board plasma diagnostics in spacecraft including nanosatellites.

The laser damage thresholds of optical coatings can degrade over time due to a variety of factors, including contamination and aging. Optical coatings deposited using electron beam evaporation are particularly susceptible to degradation due to their porous structure. In a previous study, the laser damage thresholds of optical coatings were reduced by roughly a factor of 2 from the years 2013 to 2017. The coatings in question were high reflectors for 1054 nm that contained SiO2 and HfO2 and/or TiO2 layers, and they were stored in sealed PETG containers in a class 100 clean room with temperature control. At the time, it was not certain whether contamination or thin-film aging effects were responsible for the reduced laser damage thresholds. Therefore, to better understand the role of contamination, the coatings were recleaned and the laser damage thresholds were measured again in 2018. The results indicate that the contamination played the most dominant role in reducing the laser damage thresholds of these optical coatings, even though they were stored in an environment that was presumed to be clean.

We designed and produced optical coatings for broad bandwidth high reflection (BBHR) of femtosecond (fs) pulses for high energy petawatt (PW) lasers. These BBHR coatings consist of TiO2/SiO2 and/or HfO2/SiO2 layer pairs formed by reactive E-beam evaporation with ion-assisted deposition in Sandia’s Large Optics Coating Facility. Specifications for the HR band and center wavelength of the coatings are for 45° angle of incidence (AOI), P polarization (Ppol), with use of the coatings at different AOIs and in humid or dry/vacuum environments providing corresponding different HR center wavelengths and spectral widths. These coatings must provide high laserinduced damage threshold (LIDT) to handle the PW fluences, and also low group delay dispersion (GDD) to reflect fs pulses without distortion of their temporal profiles. We present results of LIDT and GDD measurements on these coatings. The LIDT tests are at 45° or 65° AOI, Ppol in a dry environment with 100 fs laser pulses of 800 nm line center for BBHR coatings whose HR band line centers are near 800 nm. A GDD measurement for one of the BBHR coatings whose design HR center wavelength is near 900 nm shows reasonably low and smoothly varying GDD over the HR band. Our investigations include BBHR coatings designed for 45° AOI, Ppol with HR bands centered at 800 nm in dry or vacuum environments, and featuring three options: all TiO2/SiO2 layer pairs; all HfO2/SiO2 layer pairs; and TiO2/SiO2 inner layer pairs with 5 outer HfO2/SiO2 layer pairs. LIDT tests of these coatings with 100 fs, 800 nm line center pulses in their use environment show that replacing a few outer TiO2 layers of TiO2/SiO2 BBHR coatings with HfO2 leads to ~ 80% higher LIDT with only minor loss of HR bandwidth.

We have designed and reported on a dichroic beam combiner coating consisting of HfO2/SiO2 layer pairs to provide high transmission at 527 nm and high reflection at 1054 nm for 22.5° angle of incidence (AOI) in S polarization (Spol). The laser-induced damage threshold (LIDT) of this first coating at the use AOI and polarization with 3.5 nanosecond (ns) pulses at 532 nm is 7 J/cm2, and only marginally adequate for our beam combining application. In this paper, we describe the use of a combination of Al2O3 and HfO2 high index layers to modify the first as well as a second dichroic coating in two different ways, which results in a higher LIDT of 10 J/cm2 for 3.5 ns pulses at 532 nm and 22.5° AOI, Spol for the second dichroic coating, and in the same 7 J/cm2 LIDT for the first dichroic coating.

Preventing contamination is vital to achieving high laser-induced damage thresholds in optical coatings. The importance of removing contamination from optical substrates has led to the development of many specialized cleaning processes, including the application of solvents, acids, mild detergents, and abrasives. To further enhance contamination removal, the substrate may be treated with ion cleaning just prior to depositing the optical coating. Ion cleaning is attractive thanks to the convenience of providing in-situ treatment to optical substrates, and also avoiding the hassle of managing hazardous chemicals or applying mechanical force to scrub off detergents and other cleaning agents. In this study, we compare the effectiveness of ion cleaning for increasing the laser-induced damage thresholds of high reflection (527 nm and 1054 nm) and antireflection (527 nm) coatings. Ion cleaning was performed using a radio frequency ion source with argon and oxygen. The coatings investigated were deposited with layers of HfO 2 and SiO 2 in an e-beam evaporation system, and are designed to withstand nanosecond pulses from a kJ-class laser.

X-ray diffraction measurements to characterize phase transitions of dynamically compressed high-Z matter at Mbar pressures require both sufficient photon energy and fluence to create data with high fidelity in a single shot. Large-scale laser systems can be used to generate x-ray sources above 10 keV utilizing line radiation of mid-Z elements. However, the laser-to-x-ray energy conversion efficiency at these energies is low, and thermal x-rays or hot electrons result in unwanted background. We employ polycapillary x-ray lenses in powder x-ray diffraction measurements using solid target x-ray emission from either the Z-Beamlet long-pulse or the Z-Petawatt (ZPW) short-pulse laser systems at Sandia National Laboratories. Polycapillary lenses allow for a 100-fold fluence increase compared to a conventional pinhole aperture while simultaneously reducing the background significantly. This enables diffraction measurements up to 16 keV at the few-photon signal level as well as diffraction experiments with ZPW at full intensity.

This study examined whether coating system contamination, indicated by poor base pressure, has a negative impact on the laser-induced damage threshold of HfO2/SiO2 high reflection coatings for 527 nm.

The laser damage behavior of high quality coatings under nanosecond pulse illumination is controlled by statistically distributed defects, whose physical nature and defect mechanisms are still largely unknown. Defect densities are often retrieved by modeling the fluence dependence of the damage probability measured by traditional damage test (TDT) methods, based on ‘damage’ or ‘no damage’ observations. STEREO-LID (Spatio-TEmporally REsolved Optical LaserInduced Damage) allows the determination of the damage fluence (and intensity) in a single test by identifying the initiation of damage both temporally and spatially. The advantages of this test method over the TDT are discussed. In particular, its ability to retrieve detailed defect distribution functions is demonstrated by comparison of results from HfO2 films prepared by ion-assisted electron beam evaporation, ion-beam sputtering, and atomic layer deposition.

We have designed a dichroic beam combiner coating consisting of 11 HfO2/SiO2 layer pairs deposited on a large fused silica substrate. The coating provides high transmission (HT) at 527 nm and high reflection (HR) at 1054 nm for light at 22.5° angle of incidence (AOI) in air in S polarization (Spol). The coating's design is based on layers of near half-wave optical thickness in the design space for stable HT at 527 nm, with layer modifications that provide HR at 1054 nm while preserving HT at 527 nm. Its implementation in the 527 nm/1054 nm dual wavelength beam combiner arrangement has two options, with each option requiring one or the other of the high intensity beams to be incident on the dichroic coating from within the substrate (from glass). We show that there are differences between the two options with respect to the laser-induced damage threshold (LIDT) properties of the coating, and analyze the differences in terms of the 527 nm and 1054 nm E-field intensity behaviors for air → coating and glass → coating incidence. Our E-field analysis indicates that LIDTs for air → coating incidence should be higher than for glass → coating incidence. LIDT measurements for Spol at the use AOI with ns pulses at 532 nm and 1064 nm confirm this analysis with the LIDTs for glass → coating incidence being about half those for air → coating incidence at both wavelengths. These LIDT results and the E-field analysis clearly indicate that the best beam combiner option is the one for which the high intensity 527 nm beam is incident on the coating from air and the 1054 nm high intensity beam is incident on the coating from glass.

We have designed and produced an optical coating suitable for broad bandwidth high reflection (BBHR) at 45° angle of incidence (AOI), P polarization (Ppol) of petawatt (PW) class fs laser pulses of ~ 900 nm center wavelength. We have produced such BBHR coatings consisting of TiO2/SiO2 layer pairs deposited by ion assisted e-beam evaporation using the large optics coater at Sandia National Laboratories. This paper focuses on laser-induced damage threshold (LIDT) tests of these coatings. LIDT is difficult to measure for such coatings due to the broad range of wavelengths over which they can operate. An ideal test would be in the vacuum environment of the fs-pulse PW use laser using fs pulses identical to of the PW laser. Short of this ideal testing would be tests over portions of the HR band of the BBHR coating using ns or sub-ps pulses produced by tunable lasers. Such tests could be over ~ 10 nm wide wavelength intervals whose center wavelengths could be tuned over the BBHR coating’s operational band. Alternatively, the HR band of the BBHR coating could be adjusted by means of wavelength shifts due to changing the AOI of the LIDT tests or due to absorbed moisture by the coating under ambient conditions. We conduct LIDT tests on the BBHR coatings at selected AOIs to gain insight into the coatings’ laser damage properties, and analyze how the results of the different LIDT tests compare.

Reducing contamination is essential for producing optical coatings with high resistance to laser damage. One aspect of this principle is to make every effort to limit long interruptions during the coating’s deposition. Otherwise, contamination may accumulate during the pause and become embedded in the coating after the deposition is restarted, leading to a lower laser-induced damage threshold (LIDT). However, pausing a deposition is sometimes unavoidable, despite our best efforts. For example, a sudden hardware or software glitch may require hours or even overnight to solve. In order to broaden our understanding of the role of embedded contamination on LIDT, and determine whether a coating deposited under such non-ideal circumstances could still be acceptable, this study explores how halting a deposition overnight impacts the LIDT, and whether ion cleaning can be used to mitigate any negative effects on the LIDT. The coatings investigated are a beam splitter design for high reflection at 1054 nm and high transmission at 527 nm, at 22.5° angle of incidence in S-polarization. LIDT tests were conducted in the nanosecond regime.

When an optical coating is damaged, deposited incorrectly, or is otherwise unsuitable, the conventional method to restore the optic often entails repolishing the optic surface, which can incur a large cost and long lead time. We propose three alternative options to repolishing, including (i) burying the unsuitable coating under another optical coating, (ii) using ion milling to etch the unsuitable coating completely from the optic surface, and then recoating the optic, and (iii) using ion milling to etch through a number of unsuitable layers, leaving the rest of the coating intact, and then recoating the layers that were etched. Repairs were made on test optics with dielectric mirror coatings according to the above three options. The mirror coatings to be repaired were quarter wave stacks of HfO 2 and SiO 2 layers for high reflection at 1054 nm at 45° incidence in P-polarization. One of the coating layers was purposely deposited incorrectly as Hf metal instead of HfO 2 to evaluate the ability of each repair method to restore the coating’s high laser-induced damage threshold (LIDT) of 64 J/cm 2 . The repaired coating with the highest resistance to laser-induced damage was achieved using repair method (ii) with an LIDT of 49 - 61 J/cm 2 .

The Z-Backlighter lasers at Sandia National Laboratories are kilojoule class, pulsed systems operating with ns pulse lengths at 527 nm and ns and sub-ps pulse lengths at 1054 nm (www.z-beamlet.sandia.gov), and are linked to the most powerful and energetic x-ray source in the world, the Z-Accelerator (http://www.sandia.gov/z-machine/). An important Z-Backlighter optic is a flat, fused silica optic measuring 32.5 cm × 32.5 cm × 1 cm with an antireflection (AR) coating on both sides. It is used as a debris shield to protect other Z-Backlighter laser optics from high-velocity particles released by the experiments conducted in the Z-Accelerator. Each experiment conducted in the Z-Accelerator releases enough debris to cloud the surface of a debris shield, which means that a debris shield cannot be used for more than one experiment. Every year, the large optics coating facility [1] at Sandia provides AR coatings for approximately 50 debris shields, in addition to AR coatings for numerous other meter-class Z-Backlighter lenses and windows. As with all Z-Backlighter optical coatings, these AR coatings must have a high laser-induced damage threshold (LIDT) in order to withstand the powerful Z-Backlighter laser fluences. Achieving a good LIDT depends not only on the coating deposition processes but also on the polishing and cleaning processes used to prepare the coated and uncoated surfaces [2]. We spend a lot of time, both before and after the coatings have been deposited, manually cleaning the optics, including the debris shields, even though they are an expendable type of optic. Therefore, in this study we have tested new cleaning methods in addition to our current method to determine their impact on the LIDT of AR coatings, and conclude whether a shorter-duration or less labor-intensive cleaning process would suffice.

Recent experiments on Sandia's Z facility have confirmed simulation predictions of dramatically reduced instability growth in solid metallic rods when thick dielectric coatings are used to mitigate density perturbations arising from an electrothermal instability. These results provide further evidence that the inherent surface roughness as a result of target fabrication is not the dominant seed for the growth of magneto-Rayleigh-Taylor instabilities in liners with carefully machined smooth surfaces, but rather electrothermal instabilities that form early in the electrical current pulse as Joule heating melts and vaporizes the liner surface. These results suggest a new technique for substantially reducing the integral magneto-Rayleigh-Taylor instability growth in magnetically driven implosions, such as cylindrical dynamic material experiments and inertial confinement fusion concepts.

We develop and characterize high index of refraction thin films by e-beam evaporation of Ti3O5, Ta2O5 and Nb2O5 in reactive, ion-assisted deposition processes. We then deposit broad bandwidth high reflection (HR) coatings based on quarter-wave stacks of these high index layers alternating with SiO2 low index layers. The HR band is centered at 1054 nm and designed for 45o angle of incidence. We compare the laser induced damage thresholds of these coatings in order to explore tradeoffs between their laser damage properties and HR bandwidths.

We report on the effect of deposition rate, O2 back pressure, and ion beam properties towards achieving high index of refraction and low absorption e-beam evaporated TiO2 layers for laser damage resistant, broad bandwidth, high reflection coatings.

We designed a dichroic beam combiner coating with 11 HfO2/SiO2 layer pairs and deposited it on a large substrate. It provides high transmission (HT) at 527 nm and high reflection (HR) at 1054 nm for a 22.5-deg angle of incidence (AOI), S polarization (Spol), and uses near half-wave layer thicknesses for HT at 527 nm, modified for HR at 1054 nm. The two options for the beam combiner each require that a high intensity beam be incident on the coating from within the substrate (from glass). We analyze the laser-induced damage threshold (LIDT) differences between the two options in terms of the 527- and 1054-nm E-field behaviors for air → coating and glass → coating incidences. This indicates that LIDTs should be higher for air → coating than for glass → coating incidence. LIDT tests at the use AOI, Spol with ns pulses at 532 and 1064 nm confirm this, with glass → coating LIDTs about half that of air → coating LIDTs. These results clearly indicate that the best beam combiner option is for the high intensity 527 and 1054 nm beams to be incident on the coating from air and glass, respectively.

Broad bandwidth coatings allow angle of incidence flexibility and accommodate spectral shifts due to aging and water absorption. Higher refractive index materials in optical coatings, such as TiO2, Nb2O5, and Ta2O5, can be used to achieve broader bandwidths compared to coatings that contain HfO2 high index layers. We have identified the deposition settings that lead to the highest index, lowest absorption layers of TiO2, Nb2O5, and Ta2O5, via e-beam evaporation using ion-assisted deposition. We paired these high index materials with SiO2 as the low index material to create broad bandwidth high reflection coatings centered at 1054 nm for 45 deg angle of incidence and P polarization. High reflection bandwidths as large as 231 nm were realized. Laser damage tests of these coatings using the ISO 11254 and NIF-MEL protocols are presented, which revealed that the Ta2O5/SiO2 coating exhibits the highest resistance to laser damage, at the expense of lower bandwidth compared to the TiO2/SiO2 and Nb2O5/SiO2 coatings.

We designed an optical coating based on TiO2/SiO2 layer pairs for broad bandwidth high reflection (BBHR) at 45-deg angle of incidence (AOI), P polarization of femtosecond (fs) laser pulses of 900-nm center wavelength, and produced the coatings in Sandia’s large optics coater by reactive, ion-assisted e-beam evaporation. This paper reports on laser-induced damage threshold (LIDT) tests of these coatings. The broad HR bands of BBHR coatings pose challenges to LIDT tests. An ideal test would be in a vacuum environment appropriate to a high energy, fs-pulse, petawatt-class laser, with pulses identical to its fs pulses. Short of this would be tests over portions of the HR band using nanosecond or sub-picosecond pulses produced by tunable lasers. Such tests could, e.g., sample 10-nm-wide wavelength intervals with center wavelengths tunable over the broad HR band. Alternatively, the coating’s HR band could be adjusted by means of wavelength shifts due to changing the AOI of the LIDT tests or due to the coating absorbing moisture under ambient conditions. We had LIDT tests performed on the BBHR coatings at selected AOIs to gain insight into their laser damage properties and analyze how the results of the different LIDT tests compare.

Optical coatings with the highest laser damage thresholds rely on clean conditions in the vacuum chamber during the coating deposition process. A low base pressure in the coating chamber, as well as the ability of the vacuum system to maintain the required pressure during deposition, are important aspects of limiting the amount of defects in an optical coating that could induce laser damage. Our large optics coating chamber at Sandia National Laboratories normally relies on three cryo pumps to maintain low pressures for e-beam coating processes. However, on occasion, one or more of the cryo pumps have been out of commission. In light of this circumstance, we decided to explore how deposition under compromised vacuum conditions resulting from the use of only one or two cryo pumps affects the laser-induced damage thresholds of optical coatings. The coatings of this study consist of HfO2 and SiO2 layer materials and include antireflection coatings for 527 nm at normal incidence, and high reflection coatings for 527 nm, 45° angle of incidence (AOI), in P-polarization (P-pol).

When an optical coating is damaged, deposited incorrectly, or is otherwise unsuitable, the conventional method to restore the optic often entails repolishing the optic surface, which can incur a large cost and long lead time. We propose three alternative options to repolishing, including (i) burying the unsuitable coating under another optical coating, (ii) using ion milling to etch the unsuitable coating completely from the optic surface and then recoating the optic, and (iii) using ion milling to etch through a number of unsuitable layers, leaving the rest of the coating intact, and then recoating the layers that were etched. Repairs were made on test optics with dielectric mirror coatings according to the above three options. The mirror coatings to be repaired were quarter wave stacks of HfO2 and SiO2 layers for high reflection at 1054 nm at 45 deg incidence in P-polarization. One of the coating layers was purposely deposited incorrectly as Hf metal instead of HfO2 to evaluate the ability of each repair method to restore the coating’s high laser-induced damage threshold (LIDT) of 64.0 J/cm2. The repaired coating with the highest resistance to laser-induced damage was achieved using repair method (ii) with an LIDT of 49.0 to 61.0 J/cm2.

We describe an optical coating design suitable for broad bandwidth high reflection (BBHR) at 45° angle of incidence (AOI), P polarization (Ppol) of femtosecond (fs) laser pulses whose wavelengths range from 800 to 1000 nm. Our design process is guided by quarter-wave HR coating properties. The design must afford low group delay dispersion (GDD) for reflected light over the broad, 200 nm bandwidth in order to minimize temporal broadening of the fs pulses due to dispersive alteration of relative phases between their frequency components. The design should also be favorable to high laser-induced damage threshold (LIDT). We base the coating on TiO2/SiO2 layer pairs produced by means of e-beam evaporation with ion-assisted deposition, and use OptiLayer Thin Film Software to explore designs starting with TiO2/SiO2 layers having thicknesses in a reverse chirped arrangement. This approach led to a design with R > 99% from 800 to 1000 nm and GDD < 20 fs2 from 843 to 949 nm (45° AOI, Ppol). The design’s GDD behaves in a smooth way, suitable for GDD compensation techniques, and its electric field intensities show promise for high LIDTs. Reflectivity and GDD measurements for the initial test coating indicate good performance of the BBHR design. Subsequent coating runs with improved process calibration produced two coatings whose HR bands satisfactorily meet the design goals. For the sake of completeness, we summarize our previously reported transmission spectra and LIDT test results with 800 ps, 8 ps and 675 fs pulses for these two coatings, and present a table of the LIDT results we have for all of our TiO2/SiO2 BBHR coatings, showing the trends with test laser pulse duration from the ns to sub-ps regimes.

We have examined how three different cleaning processes affect the laser-induced damage threshold (LIDT) of antireflection coatings for large dimension, Z-Backlighter laser optics at Sandia National Laboratories. Laser damage thresholds were measured after the coatings were created, and again 4 months later to determine which cleaning processes were most effective. Coatings that received cleaning exhibited the highest LIDTs compared to coatings that were not cleaned. In some cases, there is nearly a twofold increase in the LIDT between the cleaned and uncleaned coatings (19.4 J/cm2 compared to 39.1 J/cm2). Higher LIDTs were realized after 4 months of aging. The most effective cleaning process involved washing the coated surface with mild detergent, and then soaking the optic in a mixture of ethyl alcohol and deionized water. Also, the laser damage results indicate that the presence of nonpropagating (NP) damage sites dominates the LIDTs of almost every optic, despite the cleaning process used. NP damage sites can be attributed to defects such as nodules in the coating or surface contamination, which suggests that pursuing further improvements to the deposition or cleaning processes are worthwhile to achieve even higher LIDTs.