Publications

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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.

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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.

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. Here, 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.

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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.

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 SiO 2 and HfO 2 and/or TiO 2 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.

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 SiO 2 and HfO 2 and/or TiO 2 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.

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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 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.

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.

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.

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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 explored 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 at 45-deg angle of incidence in P-polarization.

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.

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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. Lastly, 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.

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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. Furthermore, 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.

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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. In conclusion, 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.

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Here, 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 explored 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 at 45-deg angle of incidence in P-polarization.

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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 TiO 2 /SiO 2 layer pairs produced by means of e-beam evaporation with ion-assisted deposition, and use OptiLayer Thin Film Software to explore designs starting with TiO 2 /SiO 2 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 fs 2 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 TiO 2 /SiO 2 BBHR coatings, showing the trends with test laser pulse duration from the ns to sub-ps regimes.

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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.

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.

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