John Curtis Bellum

Reactive ion-assisted deposition of e-beam evaporated titanium for high refractive index TiO2 layers and laser damage resistant, broad bandwidth, high-reflection coatings.

High-reflection coatings with broad bandwidth can be achieved by pairing a low refractive index material, such as SiO2, with a high refractive index material, such as TiO2. To achieve high refractive index, low absorption TiO2 films, we optimized the reactive, ion-assisted deposition process (O2 levels, deposition rate, and ion beam settings) using e-beam evaporated Ti. TiO2 high-index layers were then paired with SiO2 low-index layers in a quarter-wave-type coating to achieve a broader high-reflection bandwidth compared to the same coating composed of HfO2/SiO2 layer pairs. However, the improved bandwidth exhibited by the TiO2/SiO2 coating is associated with lower laser damage threshold. To improve the laser damage resistance of the TiO2/SiO2 coating, we also created four coatings where HfO2 replaced some of the outer TiO2 layers. We present the laser damage results of these coatings to understand the trade-offs between good laser damage resistance and high-reflection bandwidth using TiO2 and HfO2.

Comparisons between laser damage and optical electric field behaviors for hafnia/silica antireflection coatings

We compare designs and laser-induced damage thresholds (LIDTs) of hafnia/silica antireflection (AR) coatings for 1054 nm or dual 527 nm/1054 nm wavelengths and 0° to 45° angles of incidence (AOIs). For a 527 nm/1054 nm, 0° AOI AR coating, LIDTs from three runs arbitrarily selected over three years are ∼20 J/cm2 or higher at 1054 nm and <10 J/cm2 at 527 nm. Calculated optical electric field intensities within the coating show two intensity peaks for 527 nm but not for 1054 nm, correlating with the lower (higher) LIDTs at 527 nm (1054 nm). For 1054 nm AR coatings at 45° and 32° AOIs and S and P polarizations (Spol and Ppol), LIDTs are high for Spol (>35 J/cm2) but not as high for Ppol (>30 J/cm2 at 32° AOI; ∼15 J/cm2 at 45° AOI). Field intensities show that Ppol discontinuities at media interfaces correlate with the lower Ppol LIDTs at these AOIs. For Side 1 and Side 2 dual 527 nm/1054 nm AR coatings of a diagnostic beam splitter at 22.5° AOI, Spol and Ppol LIDTs (>10 J/cm2 at 527 nm; >35 J/cm2 at 1054 nm) are consistent with Spol and Ppol intensity behaviors.

Meeting thin film design and production challenges for laser damage resistant optical coatings at the Sandia Large Optics Coating Operation

Sandias Large Optics Coating Operation provides laser damage resistant optical coatings on meter-class optics required for the ZBacklighter Terawatt and Petawatt lasers. Deposition is by electron beam evaporation in a 2.3 m × 2.3 m × 1.8 m temperature controlled vacuum chamber. Ion assisted deposition (IAD) is optional. Coating types range from antireflection (AR) to high reflection (HR) at S and P polarizations for angle of incidence (AOI) from 0° to 47°. This paper reports progress in meeting challenges in design and deposition of these high laser induced damage threshold (LIDT) coatings. Numerous LIDT tests (NIF-MEL protocol, 3.5 ns laser pulses at 1064 nm and 532 nm) on the coatings confirm that they are robust against laser damage. Typical LIDTs are: at 1064 nm, 45° AOI, Ppol, 79 J/cm2 (IAD 32 layer HR coating) and 73 J/cm2 (non-IAD 32 layer HR coating); at 1064 nm, 32° AOI, 82 J/cm2 (Ppol) and 55 J/cm2 (Spol ) (non-IAD 32 layer HR coating); and at 532 nm, Ppol, 16 J/cm2 (25° AOI) and 19 J/cm2 (45° AOI) (IAD 50 layer HR coating). The demands of meeting challenging spectral, AOI and LIDT performances are highlighted by an HR coating required to provide R > 99.6% reflectivity in Ppol and Spol over AOIs from 24° to 47° within ~ 1% bandwidth at both 527 nm and 1054 nm. Another issue is coating surface roughness. For IAD of HR coatings, elevating the chamber temperature to ~ 120 °C and turning the ion beam off during the pause in deposition between layers reduce the coating surface roughness compared to runs at lower temperatures with the ion beam on continuously. Atomic force microscopy and optical profilometry confirm the reduced surface roughness for these IAD coatings, and tests show that their LIDTs remain high.

Production of Optical Coatings Resistant to Damage by Petawatt Class Laser Pulses

There are a number of ultra-high intensity lasers in operation around the world that produce petawatt (PW) class pulses. The Z-Backlighter lasers at Sandia National Laboratories belong to the class of these lasers whose laser beams are large (tens of cm) in diameter and whose beam trains require large, meter-class, optics. This chapter provides an in-depth overview of the production of state-of-the-art high laser-induced damage threshold (LIDT) optical coatings for PW class laser pulses, with emphasis on depositing such coatings on meter-class optics. We begin with a review of ultra-high intensity laser pulses and the various approaches to creating them, in order to establish the context and issues relating to high LIDT optical coatings for such pulses. We next describe Sandia’s PW Z-Backlighter lasers as a specific example of the class of large-scale lasers that generate PW pulses. Then we go into details of the Sandia Large Optics Coating Operation, describing the features of the large optics coating chamber in its Class 100 clean room environment, the coating process controls, and the challenges in the production of high LIDT coatings on large dimension optical substrates. The coatings consist of hafnia/silica layer pairs deposited by electron beam evaporation with temperature control of the optical substrate and with ion assisted deposition (IAD) for some coatings as a means of mitigating stress mismatch between the coating and substrate. We continue with details of preparation of large optics for coating, including the polishing and washing and cleaning of the substrate surfaces, in ways that insure the highest LIDTs of coatings on those surfaces. We turn next to LIDT tests with nanosecond and sub-picosecond class laser pulses while emphasizing the need, when interpreting LIDT test results, to take into account the differences between the test laser pulses and the pulses of the actual PW laser system. We present a comprehensive summary of results of LIDT tests on Sandia coatings for PW pulses. Two sections of the chapter present specific coating case studies, one for designs of a high reflection (HR) coating with challenging performance specifications and one for the antireflection (AR) coatings of a diagnostic beamsplitter. The coatings are for non-normal angle

Impact of different cleaning processes on the laser damage threshold of antireflection coatings for Z-Backlighter optics at Sandia National Laboratories

Abstract. 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.

Optical damage testing at the Z-Backlighter facility at Sandia National Laboratories

To enable laser-based radiography of high energy density physics events on the Z-Accelerator[4,5] at Sandia National Laboratories, a facility known as the Z-Backlighter has been developed. Two Nd:Phosphate glass lasers are used to create x-rays and/or proton beams capable of this radiographic diagnosis: Z-Beamlet (a multi-kilojoule laser operating at 527nm in a few nanoseconds) and Z-Petawatt (a several hundred joule laser operating at 1054nm in the subpicosecond regime) [1,2]. At the energy densities used in these systems, it is necessary to use high damage threshold optical materials, some of which are poorly characterized (especially for the sub-picosecond pulse). For example, Sandia has developed a meter-class dielectric coating capability for system optics. Damage testing can be performed by external facilities for nanosecond 532nm pulses, measuring high reflector coating damage thresholds >80J/cm2 and antireflection coating damage thresholds >20J/cm2 [3]. However, available external testing capabilities do not use femtosecond/picosecond scale laser pulses. To this end, we have constructed a sub-picoseond-laser-based optical damage test system. The damage tester system also allows for testing in a vacuum vessel, which is relevant since many optics in the Z-Backlighter system are used in vacuum. This paper will present the results of laser induced damage testing performed in both atmosphere and in vacuum, with 1054nm sub-picosecond laser pulses. Optical materials/coatings discussed are: bare fused silica and protected gold used for benchmarking; BK7; Zerodur; protected silver; and dielectric optical coatings (halfnia/silica layer pairs) produced by Sandias in-house meter-class coating capability.

Design and laser damage properties of a dichroic beam combiner coating for 22.5° incidence and S polarization with high-transmission at 527nm and high-reflection at 1054nm

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 coatings 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.

Laser damage by ns and sub-ps pulses on hafnia/silica anti-reflection coatings on fused silica double-sided polished using zirconia or ceria and washed with or without an alumina wash step.

Sandias Large Optics Coating Operation has extensive results of laser induced damage threshold (LIDT) testing of its anti-reflection (AR) and high reflection coatings on substrates pitch polished using ceria and washed in a process that includes an alumina wash step. The purpose of the alumina wash step is to remove residual polishing compound to minimize its role in laser damage. These LIDT tests are for multi longitudinal mode, ns class pulses at 1064 nm and 532 nm (NIF-MEL protocol) and mode locked, sub-ps class pulses at 1054 nm (Sandia measurements), and show reasonably high and adequate laser damage resistance for coatings in the beam trains of Sandias Z-Backlighter terawatt and petawatt lasers. An AR coating in addition to coatings of our previous reports confirms this with LIDTs of 33.0 J/cm2 for 3.5 ns pulses and 1.8 J/cm2 for 350 fs pulses. In this paper, we investigate both ceria and zirconia in doublesided polishing (common for large flat Z-Backlighter laser optics) as they affect LIDTs of an AR coating on fused silica substrates washed with or without the alumina wash step. For these AR coated, double-sided polished surfaces, ceria polishing in general affords better resistance to laser damage than zirconia polishing and laser damage is less likely with the alumina wash step than without it. This is supported by specific results of laser damage tests with 3.5 ns, multi longitudinal mode, single shot pulses at 1064 nm and 532 nm, with 7.0 ns, single and multi longitudinal mode, single and multi shot pulses at 532 nm, and with 350 fs, mode-locked, single shot pulses at 1054 nm.

Nanosecond, 1064 nm damage thresholds for bare and anti reflection coated silica surfaces

We employed the same measurement techniques that have proven successful for bulk damage thresholds measurements to measure damage thresholds of bare silica surfaces polished using various methods and to measure damage thresholds for antireflection coated silica, again for various surface polishes. Light in a single transverse and longitudinal mode, from a Q-switched Nd:YAG laser is focused to an 8 µm spot on the front and rear surfaces of silica windows polished using ceria, alumina, or alumina/silica to find the damage threshold. We repeated the exercise for the same surfaces anti reflection coated with silica/hafnia film stacks. We used surface third harmonic generation to precisely place the focus on the surfaces. Key findings include: 1. The surface damage threshold can be made equal to the bulk damage threshold. There is a large difference in single-pulse damage thresholds of bare silica surfaces polished using ceria, alumina, and alumina followed by silica. The ceria polished samples have a statistical damage threshold ranging from 50 to 450 GW/cm2. The alumina polished surfaces damage at 200-500 GW/cm2, with half the spots damaging at the bulk threshold of 500 GW/cm2. The windows polished by alumina followed by silica damage almost universally at the bulk damage threshold of 500 GW/cm2. 2. There are strong conditioning effects for these surfaces. The ceria polished surfaces have reduced thresholds for multiple pulses. The alumina polished surfaces attain the bulk damage threshold at most locations using multiple pulse annealing. 3. The underlying polishes strongly affect the damage thresholds for the AR coatings. The alumina plus silica polished samples have the highest thresholds, with statistical variations from 150-380 GW/cm2. The alumina polished samples damage at only 50 GW/cm2, but with annealing the threshold rises to 200 GW/cm2, while the ceria polished samples damage at 50-200 GW/cm2 with no strong multiple shot effect. 4. We found there was no beam size variation of the damage threshold irradiance for the bare alumina/silica polished samples. 5. We showed that air breakdown does not limit the surface irradiance, silica breakdown does. 6. We recorded damage morphologies for the different surfaces.

Laser damage comparisons of broad-bandwidth, high-reflection optical coatings containing TiO2, Nb2O5, or Ta2O5 high-index layers

Abstract. 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.