Key Words: Sol-gel coatings, High laser damage coatings, HR laser coatings, AR laser coatings.

Transcription

1 The sol-gel process is a versatile procedure used for the preparation of a large variety of oxide products, including optical coatings. Investigations into coatings for laser systems have been ongoing for nearly 30 years. Earlier work suffered from inconsistant damage threshold levels and expensive processing costs. Recent improvements, however, have resulted in the process being particularly successful in the preparation of quarterwave porous silica antireflection (AR) coatings for use in high power fusion laser systems. The laser damage thresholds are two to three times higher than coatings prepared by other methods. More recently, investigations into quarterwave, high-low index, multilayer, high-reflective (HR) coatings have resulted in products which, while not as superior as their AR equivalents, have compared favorably with competitive materials. The process is particularly suitable for large substrates and has the potential of economically producing the large number of optical coatings required for the next generation of fusion lasers. Key Words: Sol-gel coatings, High laser damage coatings, HR laser coatings, AR laser coatings. 1. The Sol-Gel Process-Introduction The sol-gel process is a versatile procedure for the preparation of coatings, powders, fibers or monoliths from liquid precursor materials. Most of the products are oxides and, in many cases, the liquid precursor is a solution of one or more metal organic compounds. These can be readily converted to the relevant oxides by reaction with water followed by heat. Alternatively, a liquid colloidal oxide suspension can be used and this requires only liquid evaporation for product fabrication. The term "sol-gel" is used because processing initially involves the preparation of a solution or suspension (the Sol) and this goes through a gelling step (the Gel) during subsequent fabrication. Investigations into all aspects of the sol-gel process have been ongoing for many years and the field is now well established. For a general overview the reader is referred to the book by Brinker and Scherer1) entitled "Sol-Gel Science". There are also a large number of hardbound conference proceedings exemplified by the "Better Ceramics Through Chemistry" series now up to six volumes2). Recently a new journal, "The Journal of Sol-Gel Science and Technology" was started. Only the coating aspect of sol-gel processing will be considered in this review-and emphasis will be placed on optical coatings prepared for use on high power laser components. addition to good optical performance these coatings must have a high laser damage *University of California,Lawrence Livermore National Laboratory(P.O.Box 5508,L-483 Livermore,CA ). In

2 threshold. This is a severe test and eliminates many products that otherwise perform very well. 2. Sol-Gel Coatings General The liquid systems in the sol-gel process allow liquid fabrication procedures. This is particularly useful for coating applications where simple dip, spin or meniscus coating methods can be quicker and cheaper than any of the more conventional vacuum processes. Two quite different methods have evolved for the preparation of optical coatings, the difference being in the type of liquid starting material. In one method the coating is formed by the application of a precursor solution to a substrate with subsequent conversion of the precursor to an oxide on the surface (solution method). This conversion normally requires water and heat. The second method involves the application of a colloidal suspension of an oxide to a substrate with only the evaporation of the suspending liquid being required (colloidal method). Coatings prepared by the two methods are quite different. The first gives hard, dense, abrasion resistant products that are usually quite stressed due to shrinkage during oxide conversion. Soft, porous products which are stress-free are normally obtained in the second method because the oxide has already been formed prior to application. Most of the early investigations into sol-gel optical coatings involved the use of the solution method and this was also the case with the first investigations into laser coatings. These types of coating, while normally having adequate optical performance, suffered from inconsistent and usually low laser damage threshold. Later work on coatings prepared by the colloidal method was more successful and some products are now in use on several high power fusion lasers. Both the solution and colloidal methods are described in more detail in the following sections, but first there follows a brief section on coating methods. 3. Coating Methods Three types of liquid coating methods, spin, dip and meniscus, are normally used to apply sol-gel solutions3). These are illustrated in Fig. 1. Spin and dip coating are common and widely used. Dip coating is preferred for large samples of irregular shape or having curved surfaces. A disadvantage is that comparatively large quantities of liquid are required even though none is wasted. Spinning is excellent for small, round, flat or gently curved samples. It is quick but wasteful even though not much liquid is required.

3 Meniscus coating is a recent development that is particularly good for large, flat substrates. It involves contact of a horizontal moving substrate with a meniscus of coating fluid flowing around a horizontal tube. It is well described in papers by Britten4) and Belleville and Floch5). It has the great advantage of requiring only a small amount of fluid, none of which is wasted. All three of these methods are used to coat the components of the Nova and Beamlet lasers at the Lawrence Livermore National Laboratory. Large dip coating tanks of 200 liter capacity are used for flat debris shields and curved lenses up to 80 cm in diameter. The round debris shields and 40 cm ~ 40 cm square ones are also meniscus coated. Spin coating is used mainly for KDP harmonic converter crystals which are either 27 ~ 27 cm square or right triangles with a 27 cm side. 4. Solution Method - Single Layer AR Coatings Following on work first reported by McCollister and Boling6) on graded index AR coatings in general, Mukerjee and Lowdermilk7) prepared After heating the porosity was increased by etching in dilute HF solution. Under the right conditions, a graded index coating was obtained with broadband AR properties. Damage thresholds varied widely but levels up to 9 J/cm2 at 355 nm and 1-ns pulse were obtained. Wilder9) used a similar process to prepare porous silica coatings for use in the UV. These were thinner than those of Yoldas and were not graded but acted as conventional quarterwave AR coatings. Damage thresholds at 355 nm were in the 6-9 J/cm2 range for 1-ns pulses. While the optical properties of all these coatings were generally satisfactory they had two major disadvantages. The laser damage thresholds were inconsistent and different samples gave highly varying damage figures under apparently identical preparative conditions and they involved heat treatments to ca. 500 Ž As the demand at that time was mainly for AR coatings on expensive fusion laser optics up to 1 meter in diameter, the heat treatment and property variation became a risk. Porous AR coatings described later gave higher damage thresholds with no heat treatment and this work was materials specifically for laser use. These were therefore discontinued. made by coating silica substrates with sol-gel solutions containing Na2O-SiO2 and Na2O- B2O3-SiO2 precursors, heating to form the respective glasses and then phase separating these glasses by further heat treatment. The soluble phase was leached out in NH4F-HNO3 leaving a porous residual phase consisting mainly of silica and having excellent broadband AR properties. Damage thresholds up to 20 J/cm2 at 1.06,am and 1-ns pulse were reported. A similar and simpler leaching process was used by Yoldas and Partlow8) in 1984 involving only silica. A slightly porous silica coating was prepared on a silica substrate from a solution of hydrolyzed ethyl silicate precursor solution. 5. Solution Method - Multilayer AR and HR Coatings Multilayer sol-gel coatings prepared from solution were first described by Schroeder10) in 1962 and commercial products were produced by his employer, Schott Glass, about that time. Since then there have been many investigations in this field. The first work directed towards laser application was described by Sokolovall) and colleagues in 1967 and there followed a series of at least a dozen papers describing their work up to about The investigations involved the use of TiO2, ZrO2, HfO2 and ThO2 both individually and mixed as the high index compo-

4 nents and SiO2 as the low index component of conventional quarterwave stacks designed for AR, HR, polarizer and beam splitters. Soluble alkoxide or chloride precursors were used and these required water and heat for conversion to were obtained. Damage threshold data was sparse and in some cases crazing of the coatings due to stress was reported. This work apparently ceased about 1980 as publications from these authors stopped. It is possible that low damage thresholds and the need to refinish optical component surfaces, at some expense, when coatings failed was the cause of this demise. oxides, e. g.: 6. Colloidal Method -AR Such variables as coating conditions11-13), coating stress14), scatter15), and damage threshold16,17) were investigated. Components up to 70 cm in diameter were spun coated at rpm and heat treatments up to 900 Ž were used for maximum densification and abrasion resistance18). In certain cases more than 20 layers were applied and reflections greater than 99% AR coatings prepared from colloidal silica particles were first described in a series of patents by Moulton19) beginning in The colloidal silica was prepared from sodium silicate and acid and modifications included the use of hydrolyzed ethyl silicate solution as a binder for the particles to improve abrasion resistance. Graded index coatings were also described. However no performance data was given and, of course, at that time there was no laser use. In 1984, Thomas20) prepared similar coatings using colloidal silica prepared by the base catalyzed hydrolysis of high purity (doubly distilled) ethyl silicate in ethanol The suspension was applied to fused silica or borosilicate substrates up to 80 cm in diameter

5 All samples had at least 480 shots at 10 Hz Fig. 3 Transmission curves of fused silica substrate uncoated and coated with sol-gel AR coating. by dip or spin coating at room temperature. No cure was required and the coating was ready for use immediately after the ethanol had evaporated. As the system was anhydrous, AR coatings on water and heat sensitive substrates, such as KDP harmonic converter crystals, could easily be applied. The process is illustrated in Fig. 2. The coating consists of a loose agglomeration of approximately 20 nm diameter spherical particles and has an overall refractive index of about It was thus an ideal quarterwave AR coating for substrates with indices in the 1.5 range. A transmission curve of a coating on both sides of a fused silica substrate (index 1.46) is shown in Fig. 3. Similar curves were obtained on borosilicate (index 1.51) and KDP (index 1.49) substrates. The high purity ethyl silicate starting material and rigorously clean processing conditions gave a high purity product which no doubt contributed to the very high laser damage threshold. Representative damage thresholds are shown in Table I. The high threshold and ease of application allowed immediate practical application to the optics in high power fusion lasers, the first being the Nova at LLNL. Application on other lasers in France, England and the U. S. soon followed. One unexpected advantage of this system was that the lack of abrasion resistance allowed easy removal without the necessity of refinishing the substrate. This is in contrast to coatings prepared by the solution method and became extremely important when it was found that there was a greater than expected need to re-coat the optics in large fusion lasers. In the Nova laser at LLNL, for example, debris shields are removed and recoated every two weeks. Development of this coating has continued not only at LLNL but particularly in France. At CEL-V, the home of the Phebus fusion laser, Floch and Belleville21) have been able to improve the abrasion resistance without detriment to other properties by exposure of the coated optic to ammonia-water vapors for several hours. It is thought that this treatment causes siloxane bonds to be formed between the surface hydroxyl groups of adjacent particles thus increasing the interparticle bond strength. Coatings could be cleaned by drag wiping in the normal manner but readily removed when necessary by a very mild chemical etch.

6 Hydrolyzed ethyl silicate solution has also been used to bind the particles together to increase abrasion resistance in much the same manner as that used by Moulton described earlier. Thomas22) obtained some improvement using up to 20% silicate while retaining AR properties and high damage threshold (on silica substrates). Larger quantities increased the refractive index uniformly up to approximately 1.4 thus allowing coatings to be specifically tailored for use on substrates with indices varying from Floch and Belleville23) were able to prepare a highly abrasion resistant coating by incorporating a fluorine containing polymer into the colloidal silica-hydrolyzed silica system. The high laser damage threshold was retained and there was some indication of a graded index as the transmission spectrum was broader than the standard quarterwave one. 7. Colloidal Method-HR The colloidal method has been used to prepare simple quarterwave HR multilayer stacks of alternating high and low index materials. Silica was used exclusively for the low index component, the processing being exactly the same for the single layer AR coating described earlier. For the high index suspensions, work has been carried out using TiO224), AIOOH25), ThO226), ZrO227,28), and HfO228). Colloidal suspensions of TiO2 and AlOOH were readily prepared by hydrolysis of the relevant alkoxides in much the same manner as for silica. These alkoxides were distillable and so high purity products could be produced. The other suspensions were prepared by hydrolysis of chloride or nitrate salts. These starting materials were not volatile and therefore not as readily purified as the alkoxides. Suspensions were originally prepared in water but did not have good coating properties because of the high surface tension. It was possible to replace most of the water with methanol with TiO2, AlOOH and ThO2 and all of the water with methoxyethanol with ZrO2 and HfO2. Flow properties were much improved. Spin coating was used for the initial work with the suspensions being applied to a spinning substrate in a clean room allowing minutes drying time between layers. A 24-layer sample could easily be obtained in a few hours at room temperature. Coatings were stress free because of the porosity and so there was no problem with crazing or peeling. The number of layers required for 99 + % reflection depends on the difference in index between the high and low index components. TiO2 has the highest index and is therefore the preferred material. Unfortunately while the single shot damage threshold of a single TiO2 layer was high, the multishot (10 Hz) threshold deteriorated rapidly (to about 2 J/ cm2). Work with TiO2-SiO2 systems was therefore soon discontinued as was that with ThO2- SiO2 because of radioactivity. This left AIOOH, HfO2 and ZrO2 and it will be seen from Table II that AlOOH had the highest damage threshold but unfortunately the owest index. The effect is shown in Figs. 4 and I 5 which shows transmission curves for AIOOH- SiO2 and ZrO2-SiO2 HR coatings. The advantage of higher index spread not only shows in the number of layers required but in the width of the reflection curve. Greater width allows a greater error in thickness to be tolerated during processing without losing reflectivity. Table III shows that a higher damage threshold can be obtained with the AlOOH-SiO2 expense of requiring more layers. system at the A more recent development to HR coatings prepared from colloidal suspensions has been

7 Table II Refractive indices and laser damage thresholds of single oxide coatings with and without organic polymer. Fig. 4 Transmission curve of [AlOOH-SiO2]18 HR sol-gel coating. Table III Laser damage thresholds and layers required for 99 % reflection of HR coatings with and without organic polymers. Damage thresholas at 1.06 pm with 10 ns pulse at 10 Hz the incorporation of soluble organic polymers into the high index oxide suspensions. This has a two-fold advantage. The first is that the overall refractive index is increased because part of the normal air space between particles is now filled with polymer and the second is that there is an overall increase in strength of the whole system. There is some indication that this Fig. 5 Transmission curve of [ZrO2-SiO2]14 HR solgel coating. increase in strength also increases the laser damage threshold. Not all polymers work in this system and among several required properties are the following: (a) (b) the polymer must be compatible with the oxide and not flocculate the suspension; the polymer must not soak into the underlying low index silica suspension, other-

8 (c) wise the index will be increased; the polymer must have a high laser damage threshold. Thomas28) used polyvinyl alcohol (PVA) as the organic polymer with AlOOH, ZrO2 and HfO2. This polymer fulfills all property requirements listed above but is only soluble in water. Coating problems did arise because of the necessity to retain at least some water in the suspension. Laser damage threshold was increased and the number of layers required for 99% reflection was reduced. Floch and Belleville29) used polyvinyl pyrrolidone (PVP) as the organic polymer which is soluble in organic solvents but had a tendency to soak into the underlying layers. This latter problem was solved by the use of a UV cure which insolublizes and therefore fixes the polymer in the high index layer. Table II shows the increase in index and damage threshold of single oxide layers with polymer incorporated. Table III shows the effect on number of layers required and increase in damage threshold of HR samples with polymers incorporated. ZrO2/PVP-SiO2 and HfO2/PVP-SiO2 are now the systems of choice and, as recent designs specify 40 cm square mirrors, meniscus coating is the preferred method of application. Investigation continues in the hope that this will result in an inexpensive source of HR components. 8. Summary Porous silica AR coatings prepared by the sol-gel process continue to be used on most of the worlds high power fusion lasers. Optical performance is excellent and the laser damage threshold exceeds any alternative coating material by at least a factor of two. Multilayer porous oxide HR coatings prepared by the sol-gel process have not been as successful and are not currently used on any laser system. However, investigation continues as they represent a considerable cost savings for the large number of HR components that will be required for future fusion lasers now being designed in the United States and Europe. 9. Acknowledgments Work performed under the auspices of the U.S.Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48. References 1) C. J. Brinker and G. W. Scherer: Sol-Gel Science (Academic Press 1990). 2) Better Ceramics Through Chemistry, Materials Research Society, Symposium Proceedings, Volume 32 (1984), Volume 73 (1986), Volume 121 (1988), Volume 180 (1990), Volume 271 (1992), Volume 346 (1994). 3) I. M. Thomas: Optical Coating Fabrication in Sol-Gel Optics, Processing and Applications, edited by L.C. Klein (Kluwer Academic Publishers 1994). 4) J. A. Britten and I. M. Thomas: Mat. Res. Soc. Symp. Proc. 271 (1992) ) P. F. Belleville and H. G. Floch: SPIE Proceedings 1848 (1992) ) H. L. McCollister and N. L. Boling: U. S. Patent # (1981) assigned to Owens- Illinois, Inc. 7) S. P. Mukherjee and W. H. Lowdermilk: Appl. Opt. 21 (1982) ) B. E. Yoldas and D. P. Partlow: Appl. Opt. 23 (1984) ) J. G. Wilder: Appl. Opt. 23 (1984) ) H. Schroeder: Opt. Acta 9 (1962) ) T. N. Krylova, R. S. Sokolova and I. F. Bokhonskaya: Sov. J. Opt. Technol. 34 (1967) ) R. S. Sokolova: Sov. J. Opt. Technol. 40 (1973) ) R. S. Sokolova and I. V. Gorenkova: Sov. J. Opt. Technol. 44 (1977) ) R. S. Sokolova: Sov. J. Opt. Technol. 41 (1974) ) R. S. Sokolova, I. V. Egorenkova and N. A. Razumovskaya: Sov. J. Opt. Technol. 46 (1979) ) M. B. Svechnikov: Sov. J. Opt. Technol. 45

9 (1978) ) T. N. Krylova, R. S. Sokolova, I. F. Bokhonskaya, I. V. Egorenkova and A. Y. Kuznetsov: Sov. J. Opt. Technol. 41 (1974) ) A. K. Devdariana, A. I. Sviridova and L. Y. Kurtts: Sov. J. Opt. Technol. 37 (1970) ) H. R. Moulton: U.S. Patents 2,432,484 (1947) and 2,601,123 (1952) assigned to American Optical Co. 20) I. M. Thomas: Appl. Opt. 25 (1986) ) P. F. Belleville and H. G. Floch: Sol-gel Optics III, SPIE Proceedings 2288 (1994) p ) I. M. Thomas: Appl. Opt. 31 (1992) ) H. G. Floch and P.F. Belleville: Sol-gel Optics II, SPIE Proceedings (1992) p ) I. M. Thomas: Appl. Opt. 26 (1987) ) I. M. Thomas: Appl. Opt. 28 (1989) ) H. Floch, J. J. Priotton, J.F. Mengue and C. Cordillot: NIST Special Publication 756 (1988) ) H. Floch and J. J. Priotton: NIST Special Publication 801 (1990) ) I. M. Thomas: SPIE Proceedings 1848 (1992) ) H. G. Floch and P. F. Belleville: Optical thin film from the sol-gel process, International Symposium on Optical Intereference Coatings (Grenoble, France 1994).