High fluence laser damage precursors and their mitigation in fused silica J. Bude,* P. Miller, S. Baxamusa, N. Shen, T. Laurence, W. Steele, T. Suratwala, L. Wong, W. Carr, D. Cross, and M. Monticelli Physical and Life Sciences Directorate and National Ignition Facility, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California, 94550, USA * [email protected]

Abstract: The use of any optical material is limited at high fluences by laser-induced damage to optical surfaces. In many optical materials, the damage results from a series of sources which initiate at a large range of fluences and intensities. Much progress has been made recently eliminating silica surface damage due to fracture-related precursors at relatively low fluences (i.e., less than 10 J/cm2, when damaged by 355 nm, 5 ns pulses). At higher fluence, most materials are limited by other classes of damage precursors which exhibit a strong threshold behavior and high areal density (>105 cm−2); we refer to these collectively as high fluence precursors. Here, we show that a variety of nominally transparent materials in trace quantities can act as surface damage precursors. We show that by minimizing the presence of precipitates during chemical processing, we can reduce damage density in silica at high fluence by more than 100 times while shifting the fluence onset of observable damage by about 7 J/cm2. A better understanding of the complex chemistry and physics of cleaning, rinsing, and drying will likely lead to even further improvements in the damage performance of silica and potentially other optical materials. © 2014 Optical Society of America OCIS codes: (160.6030) Silica; (160.2750) Glass and other amorphous materials; (140.3330) Laser damage.

References and links 1. 2.

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12. J. Neauport, L. Lamaignere, H. Bercegol, F. Pilon, and J. C. Birolleau, “Polishing-induced contamination of fused silica optics and laser induced damage density at 351 nm,” Opt. Express 13(25), 10163–10171 (2005). 13. P. E. Miller, T. I. Suratwala, J. D. Bude, N. Shen, W. A. Steele, T. A. Laurence, M. D. Feit, and L. L. Wong, “Methods for globally treating silica optics to reduce optical damage,” US8313662 (2012). 14. C. A. Haynam, P. J. Wegner, J. M. Auerbach, M. W. Bowers, S. N. Dixit, G. V. Erbert, G. M. Heestand, M. A. Henesian, M. R. Hermann, K. S. Jancaitis, K. R. Manes, C. D. Marshall, N. C. Mehta, J. Menapace, E. Moses, J. R. Murray, M. C. Nostrand, C. D. Orth, R. Patterson, R. A. Sacks, M. J. Shaw, M. Spaeth, S. B. Sutton, W. H. Williams, C. C. Widmayer, R. K. White, S. T. Yang, and B. M. Van Wonterghem, “National Ignition Facility laser performance status,” Appl. Opt. 46(16), 3276–3303 (2007). 15. T. A. Laurence, J. D. Bude, S. Ly, N. Shen, and M. D. 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Rao, and S. Fu, “Improve the laser damage resistance of fused silica by wet surface cleaning and optimized hf etch process,” in Pacific Rim Laser Damage 2013: Optical Materials for High Power Lasers, J. Shao, T. Jitsuno, and W. Rudolph, eds. (2013). 25. X. Gao, G. Feng, J. Han, and L. Zhai, “Investigation of laser-induced damage by various initiators on the subsurface of fused silica,” Opt. Express 20(20), 22095–22101 (2012). 26. X. Gao, G. Feng, J. Han, N. Chen, C. Tang, and S. Zhou, “Investigation of laser-induced damage by nanoabsorbers at the surface of fused silica,” Appl. Opt. 51(13), 2463–2468 (2012). 27. K. Bien-Aimé, C. Belin, L. Gallais, P. Grua, E. Fargin, J. Néauport, and I. Tovena-Pecault, “Impact of storage induced outgassing organic contamination on laser induced damage of silica optics at 351 nm,” Opt. Express 17(21), 18703–18713 (2009). 28. K. Bien-Aimé, J. Néauport, I. Tovena-Pecault, E. Fargin, C. Labrugère, C. Belin, and M. Couzi, “Laser induced damage of fused silica polished optics due to a droplet forming organic contaminant,” Appl. Opt. 48(12), 2228– 2235 (2009). 29. C. W. Carr, M. D. Feit, M. C. Nostrand, and J. J. Adams, “Techniques for qualitative and quantitative measurement of aspects of laser-induced damage important for laser beam propagation,” Meas. Sci. Technol. 17(7), 1958–1962 (2006). 30. C. W. Carr, J. B. Trenholme, and M. L. Spaeth, “Effect of temporal pulse shape on optical damage,” Appl. Phys. Lett. 90(4), 041110 (2007). 31. N. Shen, P. E. Miller, J. D. Bude, T. A. Laurence, T. I. Suratwala, W. A. Steele, M. D. Feit, and L. L. Wong, “Thermal annealing of laser damage precursors on fused silica surfaces,” Opt. Eng. 51(12), 121817 (2012). 32. S. T. Yang, M. J. Matthews, S. Elhadj, V. G. Draggoo, and S. E. Bisson, “Thermal transport in CO2 laser irradiated fused silica: In situ measurements and analysis,” J. Appl. Phys. 106(10), 103106 (2009). 33. R. M. Vignes, T. F. Soules, J. S. Stolken, R. R. Settgast, S. Elhadj, and M. J. Matthews, “Thermomechanical modeling of laser-induced structural relaxation and deformation of glass: Volume changes in fused silica at high temperatures,” J. Am. Ceram. Soc. 96(1), 137–145 (2013). 34. A. Torres, V. Courtney, A. M. Caen, F. Vietor, A. Moran, and H. L. Kelly, “Annual water quality report 2012” (San Francisco Public Utilities Commission, San Francisco, 2012).

1. Introduction Surface laser damage ultimately limits the performance of UV optics for high fluence laser applications. Despite continued improvements in damage performance, the surface damage threshold of most optical materials is still far below the intrinsic bulk limit of the material [1– #205323 - $15.00 USD (C) 2014 OSA

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8]. Laser damage is due to near-surface defects (damage precursors) which can absorb sub band-gap light. Recent work has shown that when a damage precursor gets hot enough, the bulk material begins to absorb; temperature-activated absorption in the bulk then leads to the generation of a destructive absorption front followed by molten material ejection, fracture and optic failure [9, 10]. In high quality silica at low fluences (105 cm−2) of laser surface damage precursors remains. This becomes more important for high average fluences (>10 J/cm2), especially given contrast on the laser beam. As shown in Fig. 1 the damage behavior of such optics exhibits a strongly increasing density with fluence which typically saturates at a damage density of ≈105 cm−2. Here, both large and small beam damage testing has been used to cover as large a fluence range as possible. The large beam tests use a 5 ns flat-in-time (FIT) pulse, while the small beam test employs a 3 ns Gaussian (GS) pulse. These pulses are equivalent when pulse shape scaled, so the data from the two methods can be reasonably combined (see notes in Sections 2.4 and 2.5 for details). Aside from their threshold-like behavior, these high fluence precursors have several characteristics that are distinct from damage precursors associated with fracture surfaces. First, the resultant damage is typically distributed relatively uniformly across the surface of the silica on length scales over 1 mm. Secondly, unlike fractures, which are readily observed by scanning electron microscopy (SEM) or even optical microscopy (OM), high fluence damage precursors have yet to be associated with a readily observable surface feature prior to damage initiation. This has led some workers to refer to this set of precursors as “invisible” precursors [2]. The existence of such high-fluence precursors has frustrated efforts to produce optics for use at very high fluences and to extend the life of optics used at more modest fluences in applications exhibiting spatial or temporal contrast and modulation [14].

Damage Density (cm-2)

106 105 104 103 102

from large beam from small beam 0

20

40

60 80 100 120 140 160 Fluence (J/cm2)

Fig. 1. Damage density as a function of fluence for typical AMP-etched silica optics. The AMP etch suppresses damage at low fluence, but another set of damage precursors remain. These high fluence precursors exhibit a sharp turn-on threshold with saturated damage densities exceeding 105 cm−2. These precursors are a particular problem in high fluence lasers as they

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limit the lifetime of optics exposed to high fluences. Adapted from [15]. Large and small beam protocols described in Sections 2.4 and 2.5.

Laser-induced damage of silica remains a surface phenomenon and damage is rarely observed in the bulk. This suggests that we have yet to approach the intrinsic damage threshold of the material itself (>150 J/cm2 for 3 ns GS pulses [16]) and that high-fluence damage has an extrinsic source. The number of damage sites on an optic that has been carefully etched using the AMP protocol (see Fig. 1) tends to plateau at high fluence [15]. This suggests that the number of precursors interrogated by the optical beam is finite, as would be expected from an extrinsic source. The inability to reliably correlate a surface feature, using SEM or OM, with subsequent high fluence damage suggests that high fluence damage precursors must be substantially less than a micron in size. The minimum size of a damage site at initiation suggests an approximate scale length for precursor size: under short pulse illumination (100 J/cm2 [15] – in fact, at a density of 105 cm−2, 100 nm size precursors would occupy a 0.001% of the surface. This work suggests that it may be possible to create highly damage-resistant silica surfaces. In fact, it is known that for small regions on the silica surface, IR-laser heating can remove precursors [20] and controllably create damage resistant regions [21–23]. This evidence supports the notion that high density, discrete, nanoscale absorbers are deposited on the silica surface, most likely during wet chemical processes, and that they can be removed by a surface heat treatment. Further evidence exists with respect to etching: the damage performance of an etched optic is a strong function of processing conditions [8, 24]. Of particular significance in this regard was the empirical observation that the presence of hexafluorosilicate salts is associated with damage. Recent work has shown that sub-micron particles of optically absorbing species such as metals [25, 26] and organic liquids [27, 28] can act as laser damage precursors. In this work, we demonstrate that a variety of common ionic solids, even those that exhibit no bulk absorptivity at the wavelengths of interest, absorb incident light and given sufficient fluence can initiate optical damage on silica surfaces. The presence of such materials is consistent with impurities which can be introduced during etching, cleaning, rinsing and related processing steps typically associated with precision optics handling. The size scale over which such materials can initiate damage ranges from deposits that are readily apparent to the unaided eye to structures that are too small to be detected reliably by optical or electron microscopy (≈500 nm wide x 50 nm high). Moreover, it appears that small scale, highly adherent deposits can be introduced even under processing conditions that are typically considered ultra-pure. These results suggest new directions for investigating the laser-matter interactions which govern damage. 2. Experimental methods and materials 2.1 Silica sample preparation All samples were prepared from 50 mm (2 inch) round fused silica optics obtained from Sydor Optics (Rochester, NY) or CVI-Melles Griot (Rochester, NY), which were polished using conventional finishing techniques. Prior to etching, each optic was cleaned in a 45 °C aqueous solution containing 40% concentrated nitric acid and 10% hydrogen peroxide for 45 minutes. Following leaching, each optic was thoroughly spray rinsed in deionized (DI) water

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which typically had a measured resistivity of >17.7 MΩ-cm. Spray rinsing was performed using a custom-built spray chamber that delivered a flat, fan-shaped high-velocity DI water stream by means of a set of stainless steel nozzles (McMaster Carr, Santa Fe Springs CA). The optic was continuously translated through the sprayers using a pneumatically-driven linear actuator. The basic acid etching process used in this work, AMP, is described in detail in the literature [8]. Briefly, optics were etched in a high purity buffered oxide etchant (Columbus Chemical Industries, Columbus, WI) to the desired etch depth, submerged in an ultrasonically agitated DI water bath, and spray rinsed as described above. Samples were then allowed to dry vertically in a Class 100 clean room. 2.2 Salt deposition A variety of salts were deposited on the surfaces of select samples in a Class 100 environment by either aerosol or direct aqueous deposition. Salts were deposited as an aerosol by delivering a solution of desired concentration through the inner bore of a pneumatic concentric nebulizer (Meinhard TR30-A3) using pressurized air. The aerosol droplets landed and dried on a vertically-mounted optic approximately six inches from the nozzle. A second series of salts was deposited onto freshly cleaned and etched substrates by directly depositing 25μl aliquots of the aqueous solution onto horizontally-mounted silica surfaces using a micropipette. All solutions deposited by this method were prepared using salts of >99.99% purity in DI water. All salts were deposited from solutions prepared at a nominal concentration of 50μg/ml except CaF2 which utilized a concentration of 8μg/ml. 2.3 Photoluminescence (PL) spectroscopy Fluorescence lifetime imaging was performed as described in a previous work [4]. We adapted high sensitivity confocal fluorescence lifetime microscopy for characterizing luminescence near fused silica surfaces. A pulsed laser (LDH-P-C-405B, Picoquant, Berlin) was focused using an objective (20X M Plan Apo, Mitutoyo) onto a fused silica sample. Scattering and luminescence excited by the laser were collected by the same objective and focused onto a confocal pinhole (250 μm). The scattering channel and PL channel, filtered using a 430 nm longpass filter (Omega, Brattleboro, Vermont), are measured using avalanche photodiodes (APD) from Micro Photon Devices (PDM 50CT). Each detected photon was time-stamped with its absolute arrival time (100 ns resolution) and its arrival time relative to the laser pulse (150 ps for the 400 nm laser) using a time-to-digital converter (TimeHarp 200, Picoquant, Berlin). We measured the spectrum of the PL using a monochromator/spectrometer before the photon counting APD. The wavelength is scanned during acquisition to obtain the lifetime as a function of wavelength. Using a luminosity standard (Model 63355, Oriel-Newport, Stratford, Connecticut), the spectral profile of detection efficiency was calibrated. 2.4 Large-area laser damage tests Large beam tests were used to determine the cumulative damage density as a function of fluence. The laser system and data analysis technique used for the large beam test is described in detail elsewhere [29]. Briefly, the exit surface of a sample was illuminated with a 5 ns flatin-time pulse of 355 nm light using an approximately 1 cm diameter beam. A spatiallyregistered CCD camera was used to produce a spatial fluence distribution of the laser. The sample was then inspected with an optical microscope and damage sites were mapped to the fluence distribution of the laser using an automated image analysis program routine. Typical mean shot fluences were between 25 and 40 J/cm2. With the given aperture size, this method is able to practically measure damage densities down to about 0.1 cm−2. Because of the low density of damage sites at low fluences ( 30 cm2 of illuminated area were performed at nominal fluences of 15 J/cm2 and 23 J/cm2. No damage sites were produced in the 15 J/cm2 shots and only one was produced in the 23 J/cm2 shots. It is known that in silica, damage thresholds scale with about τ0.45, where τ is the pulse length, so that short pulses have lower thresholds than longer pulses [30]. Also, as indicated above, this scaling makes a 3 ns GS equivalent to a 5 ns FIT. 2.5 Small-area laser damage tests The details of the small-area laser damage tests have been described in detail elsewhere [31]. Briefly, the system consists of a Coherent Infinity Q-switched Nd:YAG laser operating at 355 nm. The temporal profile of the laser pulse was Gaussian with full width of half max (FWHM) pulse duration of about 3 ns GS. The laser pulse energy and its spatial profile were monitored by picking off a fraction of the beam and recording it using a power meter andcharge coupled device (CCD) camera. The beam has a 1/e2 diameter of ≈80 μm. An imaging microscope was used to observe the sample under laser irradiation. These tests were performed in one of two modes: R/1, in which the fluence was incrementally ramped with 5J cm−2 steps while focused on a single area until damage is registered on the imaging CCD, or S/1, in which a single region sees only a single fluence. 2.6 IR laser surface mitigation A 10.6 μm, 10 W CO2 laser was used to create an array of heated disks on the surface. The pulse length of the laser was three minutes and the beam width (1/e2) was 2 mm. It is estimated that that the surface temperature reaches about 2000 K based on accurate thermal models of IR-heated silica [32]. Surface profiles were measured using a white light interferometric microscope. The surface heating creates a very shallow pit 100nm deep as the result of thermally-induced densification [33]. Laser damage along the surface was measured using the R/1 test procedure described above. 2.7 Atomic force microscopy The surface morphology of the samples was characterized using the atomic force microscopy (Digital Instrument Dimension 3100). The AFM sits on a vibration isolation table and is operated in a temperature and air-flow controlled room. High aspect ratio silicon tips with small tip radius (Veeco OTESPAW) were used to ensure the accuracy of the AFM results and to minimize any tip convolution of the shape measured. Fresh AFM tips were used for each sample, and the sharpness of each tip used in the study was characterized against a titanium roughness sample. A height calibration standard was used periodically to ensure the accuracy and reproducibility of the measurements. To estimate the density of surface features, 50 mm x 50 mm scans with roughly 10 nm lateral resolution were obtained at several randomly selected regions for each sample. 3. Results and discussion State-of-the-art DI water production systems supply a host of industries, including the semiconductor business, for which cleanliness is paramount. Gross particle filters remove micron-sized impurities, while ion exchange resins adsorb ions from the feed water and UV illumination is used to decompose organic material. Such systems can routinely produce water having a resistivity of 18 MΩ-cm, which corresponds to a total ionic contamination level of a few parts per billion. Some of the most important ionic contaminants found in DI water are the common ions such as K+, Na+, Mg2+ and Ca2+. Interestingly, when combined with common anions such as Cl- or F-, many of the resultant bulk salts can be made into high transparency UV optics. Yet, as we show below, when deposited onto the surface of silica, each of these salts was found to absorb sufficient light to initiate a damage event. Figure 2(a) shows a clean, etched silica #205323 - $15.00 USD (C) 2014 OSA

Received 22 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 5 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005839 | OPTICS EXPRESS 5844

surface onto which an aerosol of an 85 mM NaCl aqueous solution was sprayed. The drying of the impinging droplets left behind many small NaCl crystals, ranging in size from about 5 microns to the sub-micron level. Despite the fact that NaCl is not a bulk absorber of UV light (band gap of 8.5 eV), these crystals exhibited PL when excited with a 488 nm excitation source. Figure 2(b) shows the PL of these crystals. A broad distribution of PL lifetimes from 40 ps to 5 ns and a broad emission spectrum are observed, the same as seen from precursors which have previously been associated with facture surfaces [4]. It is important to note the similarity in the PL characteristics despite the varying material composition [4]. After illuminating the surface at a fluence of 25 J/cm2, 5ns FIT, using the large area damage test apparatus, each of the NaCl crystals was either ejected from the surface or generated a catastrophic damage event on the substrate as shown in Fig. 2(c). An SEM image of a typical damage site resulting from laser exposure of the salt is seen in Fig. 2(d); the site appears similar to typical laser damage sites in fused silica.

Fig. 2. (a) Optical micrograph of NaCl crystals left by aerosol deposition. The precipitates congregate in a pattern reflecting the impinging droplets due to the lack of bulk flow at these length scales. (b) Fast photoluminescence (PL) of the same region. The PL behavior is similar to that of defect precursors associated with fracture surfaces and show that the precipitated crystals have optical activity. (c) The same region after the large area laser test. All the damage sites are associated with the presence of a NaCl crystal prior to the shot. (d) Typical damage site morphology. The scale bars for (a)-(c) are 100 μm; the scale bar for (d) is 20 μm.

This behavior is not unique to NaCl. Aliquots of dilute aqueous solutions of MgF2, KDP, and CaF2 were deposited on freshly etched silica samples. As the samples dried, randomly oriented crystals of the solvated salt precipitated on the silica surface. As noted above, bulk specimens of each of these salts are used routinely to fabricate high-transparency UV optics. Small area R/1 and S/1 tests were performed by focusing the laser spot on individual particles. Two characteristic fluences are shown in Fig. 3: the cleaning threshold, as determined by the R/1 test, is the fluence at which the precipitate is ejected from the surface without causing damage to the underlying silica substrate, while the damage threshold, as determined by the S/1 test, is the fluence at which damage occurred to the silica substrate. The data set marked “control” refers to test done on the clean surface without salt doping for comparison. Here, the R/1 threshold was about 40J/cm2; the salts always damage at much lower fluences than the clean surface. The material ejection and damage recorded here show that all of these precipitated crystals absorb incident light at photon energies (355 nm = 3.5 eV) less than half the energy of the bulk band gap Eg. As shown in Fig. 3, the band gap Eg for all three materials is greater than 8 eV. The difference between the optical properties of the

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bulk materials and the precipitates probed here is the environment in which the crystals are formed. High quality optical crystals are typically grown under carefully controlled conditions and the resulting bulk crystals are transparent in the UV. When dissolved solutes in solution begin to nucleate rapidly along a drying front, they may form defective structures. Other ionic compounds precipitated on silica also produced damage precursors, including KBr (damaged at 17 J/cm2), NaBr (13 J/cm2) and NaF (22 J/cm2) as indicated. This behavior is not limited only to inorganic ionic solids: we also found that sucrose, an organic material, when deposited on silica, produced damage at 27 J/cm2 analogously to the inorganic salts. While previous work has shown that sub-micron particles of optically absorbing materials such as organic liquids can serve as damage precursors [27, 28], the results here show that even nominally transparent materials can absorb light and initiate a damage event. We find that the way a particle is formed governs its laser damage behavior more than its chemical composition and bulk optical properties. Consequently, we expect any impurities precipitated from solution during common processing operations such as cleaning, rinsing and subsequent drying have the potential to form precipitates that can act as high fluence precursors. To test whether DI water itself can harbor impurities that form damage precursors, a freshly etched and dried silica surface was mounted vertically and sprayed with an aerosol of DI water. Less than 50 μL of DI water was dispensed onto the surface. While there was little volume transferred to the sample, as the water quickly evaporated, any trace level solutes carried by water precipitated directly on the surface. Under the conditions of this experiment there was no opportunity for solutes to be removed from the surface by gravityinduced bulk flow.

Fig. 3. Cleaning threshold (R/1 protocol) and damage threshold (S/1 protocol) for various salts as measured by small area laser damage testing. The number above each salt is the band gap of the bulk material. Despite the fact that in bulk these materials are transparent at the test wavelength (355 nm = 3.5 eV), they exhibit near-metallic like absorption and can act as high fluence precursors when precipitated on silica surfaces.

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Received 22 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 5 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005839 | OPTICS EXPRESS 5846

Fig. 4. (a) Optical micrograph of the surface of a sample that has been subjected to an aerosol deposition of DI water. (b) The same region after large area damage testing showing many typical damage sites. These sites are not associated with any visible feature in panel (a). (c) Cumulative damage density as a function of fluence after aerosol deposition of DI water. For reference, a series of samples prepared without the aerosol deposition is also shown. (d) AFM image of nanoscale precipitates, highlighted with blue circles, found on the surface after aerosol deposition of DI water. The nanoscale features are discrete, sub-micron in lateral extent, and range between 10 and 100 nm in height.

Figures 4(a) and 4(b) show optical micrographs of the sample before and after a large-area damage test. There is no optical evidence of precipitates on the surface as was seen with the aerosol-deposited NaCl crystals [compare Figs. 2(a) and 4(a)]. Still, damage is readily evident on the surface after a large-area laser damage test [Fig. 4(b)]. As shown in Fig. 4(c), the damage density of the optic sprayed with DI water is at least 100 times greater than an average of ten samples prepared without the spray. This emphasizes that even a very small volume of high-quality DI water aerosolized onto and dried on the surface can introduce high fluence damage precursors. Despite the fact that they are not visible by optical microscopy, precipitates formed by the very low (parts per billion) level of impurities in DI water likely deposited damage precursors where the aerosol droplet dried. To look for evidence of such nanoscale precipitates, atomic force microscopy (AFM) was performed on the silica surface that was sprayed with DI water aerosol [see Fig. 4(d)]. The most common surface features are pits left from etching and polishing. However, careful inspection the AFM image reveals positive displacement on the surface above the background surface roughness which we interpret as precipitated particles. They are highlighted by blue circles in Fig. 4(d). The density of these features is about 105 cm−2, which is typical of the damage density measured in Fig. 4(b). The precipitates are generally tens of nanometers in height and a few hundred nanomters in diameter, a size regime accessible by AFM but not reliably detected by optical or electron microscopy. These results suggest that precipitates of a variety of common salts, independent of their bulk light properties, can be high-fluence damage precursors when present on the surface of silica optics; similar mechanisms may be expected for precipitates on other optical materials. Such precipitates need not be introduced onto the optic after processing, and in fact can be formed during the etching and rinsing operations. The curves in Fig. 5(a) show the effect of introducing fixed impurity doses of NaCl (blue), CaF2 (black), and CaCl2 (green) into an etch bath. After etching in these compromised baths, the optics were cleaned and rinsed thoroughly per standard procedures and subjected to large area laser damage testing. Samples where no impurities had been introduced into the etchant were found to have typical low

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Received 22 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 5 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005839 | OPTICS EXPRESS 5847

damage densities of ≈200 cm−2 at 40 J/cm2. As shown in Fig. 5(a), once the impurity concentrations reaches roughly 100 ppb, the damage density was found to increase by factors of more than 10. The etch bath contains substantial concentrations of anions, such as both F-, and SiF62-, which can form relatively insoluble salts in the presence of cationic dopants such as Ca2+ or Na+. Both the calcium and sodium salts were found to deposit high fluence precursors. Many different ionic salts can form high fluence precursors; similarly there are many trace elements in DI water, some of which may remain undetected. We therefore sampled impurities naturally found in water by doping DI water used for processing with small amounts of municipal water (sourced from the Hetch-Hetchy Reservoir in Yosemite National Park, California). Such experiments illustrate how effective a DI water system needs to be to achieve high damage resistance. Published data from Hetch-Hetchy indicates that the feed stream to our DI water system has a total dissolved solids composition of ≈110 mg/L and a corresponding resistivity of ≈5000 Ω-cm [34]. We performed two sets of experiments to investigate the potency of natural sources of contamination. For the first set, we etched, rinsed and dried a set of silica samples and then immersed them in baths containing varying mixtures of DI and untreated municipal water; this contamination process imitates incomplete deionization during processing. In the second set, the etchant was contaminated with a known fraction of untreated municipal water. Damage densities for these samples, as measured by large area tests, are shown in Fig. 5(b).

Fig. 5. (a) Damage density at 40 J/cm2, 5 ns FIT, as a function of impurity concentration intentionally added to the etchant. The damage density increases significantly for impurity concentrations greater than 100 ppb. Beyond an impurity concentration of 100 ppb, the damage density begins to increase strongly. (b) Damage density at 40 J/cm2 as a function of fraction untreated municipal water in various process steps.

Both sets of experiments show the profound influence that the natural contaminants in the water feeding the deionization system have on the damage density of the resultant optic. The first set shows that immersion in DI water contaminated with greater than 0.01% of untreated municipal water (corresponding to a total resistivity of roughly 17.7 MΩ-cm) is sufficient to cause a measurable increase in high fluence damage density. Water of this resistivity is close to the maximum 18.2 MΩ-cm limit (at 25 °C) imposed by the dissociation of water. The second set shows that normal rinsing operations in high-purity DI water are insufficient to compensate for an etch bath contaminated with a small fraction of untreated municipal water. To test whether there are enough or the right composition of impurities in DI water to be a dominant source of damage precursors under standard rinsing conditions, we studied the effect of DI water exposure on special heat treated samples with high damage threshold regions [21–23]. We started with AMP etched silica samples. An array of 2 mm zones was heated with a 10.6 μm IR laser to generate regions of high damage resistance on the surface. The damage resistance was measured on an array of points on a line moving through the

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Received 22 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 5 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005839 | OPTICS EXPRESS 5848

heated zone. The top panels in Figs. 6(a) and 6(b) are white light interferometric images of the the surface profile and the array of damage sites. Surface heating leaves only a very shallow pit (100 nm as shown in red). Beyond this, the sample surfaces were extremely smooth as measured by AFM. The bottom panels show the relative change in R/1 damage fluence as a function of distance through the heat-treated zone. The R/1 threshold in the center of the heated zone is about 60% higher than the R/1 threshold of the surface which was 40 J/cm2 for our test conditions. Next, we rinsed several parts in DI water for a minute to several hours, allowed them to air dry, and then re-measured the damage resistance through the heated zone. As shown in Fig. 6(b), the rinse has eliminated the damage free zone, so that the whole part has a uniform damage threshold. It is salient that the damage threshold in the center of the heated zone returns to its previous state in that it suggests that the precursors reintroduced to the heated zone yield the same behavior and thresholds as the rest of the surface. Hence, precipitates must be deposited during DI rinsing or subsequent drying steps.

Fig. 6. White light interferometric images showing IR heat-treated zone (top) and R/1 damage test data (bottom) for samples (a) as-treated and (b) after heat treatment and water rinsing. The R/1 damage threshold is normalized to the surface damage threshold (near 40 J/cm2). The area between the dashed lines is the heated-treated site.

To reinforce the connection between precipitates and damage, we performed AFM (atomic force microscopy) searches for precursors which were impossible to see using optical microscopy or SEM. Figure 7 shows the relationship between the density of features found via AFM to measured damage for a broad range of sample quality. The AFM count density was collected by sampling localized bumps on the surface (positive displacement from the surface), eliminating the general surface roughness. The damage density at 40 J/cm2 is reported based on large area laser damage tests. Note, however, because of the limits of scaling, it was difficult to obtain low noise statistics. For the very best samples with damage density less than 100 cm−2 at 40 J/cm2, the AFM count density was 10 J/cm2 for pulses 5 ns in duration) on silica optical surfaces are generated as precipitates during chemical processing steps such as cleaning, etching, rinsing. We have shown that these precursors are generally present both on etched AMP samples as well unetched samples processed through cleaning and rinsing. A variety of ionic species, when precipitated on optical surfaces over a wide size scale, can lead to optical damage. These precipitates need not be composed of bulk optical absorbers at the wavelengths of interest, and in fact the damage behavior appears to be nearly independent of composition. We provide evidence that suggests that the presence of such precipitates at size scales below that which can be reliably observed by optical or even electron microscopy represent a significant barrier to the fabrication of UV optics for high fluence applications. Finally, by working to exclude reagent and process contamination and to minimize precipitation during chemical processing operations, we have demonstrated the production of silica optics with extraordinarily low damage densities which saturate about 200 cm−2 on test samples. Such low damage densities are of importance to laser systems limited by damage of silica optical elements. This work raises a series of interesting scientific questions including: a) how can salts that are non-bulk absorbers become precursors, b) how much energy must a precursor absorb to launch a destructive absorption event on the surface of an optic, c) are there similar precipitate precursors present on other optical materials, and d) how can we control or prevent nucleation and precipitation, and ultimately, damage? Because we have yet to reach the intrinsic damage limit of fused silica, it is expected that further work along these lines can result in even lower damage densities on silica surfaces. Acknowledgments The authors wish to acknowledge the fine work of John Bigelow and Ed Northcutt for the design and construction of the spray system and much of the fixturing utilized in this work. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 within the LDRD program.

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Received 22 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 5 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005839 | OPTICS EXPRESS 5851