There exists a large variety of physical probes to examine various features of surface structure. In order to gain a better understanding of what constitutes a "good" surface for high power laser use, very detailed characterization of surface conditions must be carried out. At a metal surface, for example, the damage threshold is seen to be sensitive to surface features on a scale of a few nm. This sensitivity is thought to be due to the influence of surface roughness on electron motion within the surface. A surface layer rich in defects or impurities would be expected to have a similar effect on the damage propensity of the material. The specific way that fabrication affects the surface layer, the detailed characterization of the surface layer, on the scale of electron motion as well as on the optical scale, and the consequent influence of surface properties or damage all need to be examined more carefully. might be profitable, for example, to prepare a set of surface samples of controlled variation, by polishing with various grit sizes, for different periods of time, and so producing varying deposits of microfracture. These samples could then be polished to produce a smooth surface with varying degrees of subsurface structure. After testing, the smooth polished layer could be removed by etching to exposure the subsurface making it amendable to microscopic and other types of investigation. Alternately ground surfaces could be overcoated and a similar investigation carried out. This question of subsurface structure influences is probably the most significant area remaining to be addressed. It In the case of multilayer films, mechanical and thermal contact between film layers and at the substrate are also properties of overriding importance. The present preference for sputtered films over evaporated films may be a consequence of the substrate cleaning inherent in the sputtering process, and the better adhesion of sputtered films. Evaporated film techniques may be amendable to improvement in this regard. It was reported at this conference that alkali halides grown by reactive atmosphere processing exhibit much higher damage thresholds than commercially available material. This improvement is amenattributed to higher purity of the material. It was also reported that in RAP-grown material, not only were higher damage levels obtained, but that the relative levels of KC and NaCl were reversed. This strongly suggests that impurities and defects play an important role mediating the onset of avalanche breakdown. It might be expected that currently quoted "intrinsic" damage strengths are actually amenable to improvement. There is still a need for developing a better understanding of damage processes, especially avalanche breakdown, from a detailed, solid-state theoretic point of view. The use of average electron properties is questionable, particularly in low-mobility materials, where the mean free path may be of the order of atomic dimensions. Materials development and improvement continues to progress. The area of IR window materials is currently getting the major emphasis. There is substantial room for improvement in this area, as, for example, in the reduction of void concentration in ZnSe, the reduction of impurity absorption in a variety of materials, and the aforementioned improvement in alkali halide manufacture. We can look forward to continuing development of materials for the UV and vacuum uv. Here the stateof-the-art is relatively primitive. The special problems of the uv, arising from the high probability of impurity ionization, the effect of two- and three-photon absorption, and the possibility of color center formation, remain to be enumerated fully and analyzed. Clearly, all of the problems of multilayer dielectric films and metal surfaces will carry over into the uv spectrum, with added complications. As the understanding of various damage-related phenomena improves, and the capability of identifying relevant material parameters emerges, it becomes possible to formulate figures of merit for optical materials. An example of this is the analysis of thermal distortion in this proceedings. There is need for the identification, measurement, and tabulation of these figures of merit, for optical materials for high power lasers. The availability of this kind of information is, in fact, the ultimate goal of this symposium. There is a growing appreciation of the importance of damage processes in designing large laser systems. Total system considerations must be taken into account in such a design. The interplay of material properties (as described by the figures of merit mentioned above), structural specifications, systems operating characteristics (beam quality, pulse length), and environmental factors comprise what we call synergistic effects. A material may exhibit a certain damage threshold under single pulse exposure, but respond quite differently when incorporated into a multi-pulse or CW system. The effect of flash-lamp irradiation on damage resistance can be important in pulsed solid-state lasers. Failure of one element may lead to the progressive failure of other, more expensive elements. There is evidence that when surface damage occurs, material may be splattered onto other parts of the surface, or even redeposited on adjacent surfaces, leading to damage on subsequent exposure. We must continue to emphasize that the ultimate use of laser materials is in laser systems, not solely in damage tests, and that the real environment must be considered in evaluating the material. It seems clear that MIL-SPEC scratch and dig tests are of limited utility in specification of highly polished surfaces. The development of new surface specification standards, based on light scattering or FECO interferometry, is highly recommended, and would be an appropriate undertaking of the ASTM. Finally, looking ahead to next year's symposium, we must seek to open up the discussion to new topics. The symposium attracts people interested in the interaction of intense light with optical materials, the development of new materials, and the design of high power laser systems. We are just beginning to emphasize system considerations. We have not addressed ourselves to the problems of fiber optics at all, to date. Our discussions of electro-optic materials have been very limited. These are a few of the directions in which the symposium might grow. The co-chairmen solicit and will welcome suggestions for new topic areas to be considered in the damage symposium in coming years. 4. Acknowledgement We would like to acknowledge the invaluable assistance of Dr. Harold S. Boyne, Mrs. Pauline W. Smith, Mrs. Marjorie L. Wilson and Mrs. Florence M. Indorf of the National Bureau of Standards in Boulder, Colorado for their interest, support, and untiring efforts in the operation of this Symposium and in the preparation and publication of the proceedings. Also, we would like to take special note of Mrs Normal Lear and Mrs. Kay Jentsch, who added pleasurably to the conference operation particularly during the discussion periods following the formal presentations. The continued success of the Damage Symposia would not have been possible without their support. [1] "Damage in Laser Glass", A. J. Glass and A. H. Guenther, Editors, ASTM Special Technical Publication 469, ASTM, Philadelphia, PA (1969). [2] "Damage in Laser Materials", A. J. Glass and A. H. Guenther, Editors, NBS Special Publication 341, U. S. Government Printing Office, Washington, D.C. (1970). "Fundamentals of Damage in Laser Glass", N. Bloembergen, National Materials Advisory Board Publication NMA B-271, National Academy of Sciences, Washington, D.C. (1970). [3] [4] "Damage in Laser Materials: 1971", A. J. Glass and A. H. Guenther, Editors, NBS Special Publication 356, U. S. Government Printing Office, Washington, D.C. (1971). [5] "High Power Infrared Laser Windows", N. Bloembergen, National Materials Advisory Board Publication NMAB-292, National Academy of Sciences, Washington, D.C. (1972). Proceedings of the Conference on High Power Infrared Laser Window Materials, (October 1971). C. S. Sahagian and C. A. Pitha, Editors, Special Report No. 127, Air Force Cambridge Research Laboratories, (1971). [6] [7] [8] [9] "Laser Induced Damage in Optical Materials: Editors, NBS Special Publication 372, U. S. Washington, D.C. (1972). 1972", A. J. Glass and A. H. Guenther, Government Printing Office, "Laser Induced Damage of Optical Elements, A Status Report", A. J. Glass and A. H. Guenther, Applied Optics 12, pp. 637-649 (1973). "Laser Induced Damage in Optical Materials: 1973", A. J. Glass and A. H. Guenther, Editors, NBS Special Publication 387, U. S. Government Printing Office, Washington, D.C. (1973). [10] "Laser Induced Damage to Optical Materials, 1973: a Conference Report", A. J. Glass and A. H. Guenther, Applied Optics, 13, pp. 74-88 (1974). A. J. Glass A. H. Guenther 0.1 Opening Remarks Alexander J. Glass Lawrence Livermore Laboratory Livermore, California 94550 Speaking for Art Guenther and myself, I'd like to welcome you all to the Sixth Annual Symposium on Damage in Laser Materials. The National Bureau of Standards has again provided us with these superb accommodations, and has carried the burden of the preparation of the proceedings for this symposium over the years, and I want to reiterate the thanks that Haynes Lee has expressed to the Bureau, to the ASTM, and to ONR for their support in helping us carry out the work of the Symposium. The organization of the meeting this year is the same as it has been in previous years, in that we have tried to group papers roughly according to subjects. We have a session this morning on damage and its consequences in glass systems, then we have a session that is devoted to basic theoretical investigations, and another on damage to crystals, particularly in nonlinear optical materials. most of the day will be spent on a variety of papers on coatings, windows, and mirrors. Tomorrow, I think it is clear to everybody there are two fairly well delineated applications areas represented in these papers. One of them is short pulse, high peak power, essentially single shot lasers, mostly Neodymium glass for laser fusion. The other is high average power CW lasers for military and industrial application, which are primarily infrared lasers. In both areas very large systems are on line, or coming on line, depending on where you work. The topics that are being discussed here are not of purely academic interest. In the design of large systems it turns out that it is damage considerations that ultimately limit performance and dictate the design, and in many cases, determine the cost of the system. The cost sometimes becomes infinite, which means the system is infeasible. It is also true that because of the fact that the damage and the damage phenomena are the limiting factors, that real advances in the damage area lead to real benefits in terms of cost reduction, as well as in terms of improved system performance. Because of the fact that these are real problems that we are dealing with and real systems, you will find, in general that the subject seems to move ahead fairly quickly, and leaves behind it a number of fundamental investigations left undone. I think that it is typical of materials science and of applied science in general, that you learn how to deal with a problem long before you understand it, from the basis of first principles. The climate of applied science that exists in most places today is such that a lot of fascinating physics problems are going to be left behind. It is one of the minor regrets, but a necessary characteristic of this field, that we have to skip along the surface rather lightly, with the hope that someday, if people have sufficient motivation and there is a sufficiently hospitable climate to do so, some of these fascinating problems in materials science and in the interaction of high power light waves with quantum systems can be reviewed and dug into more deeply. We take a very broad view of damage in the symposium. Many of the things we talk about don't really have to do with the catastrophic failure of materials but only, like small scale self focusing, with a reduction in the performance of the system. Along this line, we have a working definition of damage as being "whatever turns you off". If you look at the schedule, you will find that there is nothing in it particularly associated with ultraviolet materials or ultraviolet systems. That is, not to say that damage problems don't exist in those systems. Even though ultraviolet lasers, excimer lasers, etc. operate at moderate powers in comparison to the IR and near visible systems, they are already damage limited. You even find people doing the calorimetry on ultraviolet lasers in terms of the damage to the coated elements. We know something about the damage mechanisms in the ultraviolet. We know that if one goes to short wavelengths one begins to get into circumstances in which two-photon absorption can become important in many materials. At the moment, my impression is that in the ultraviolet systems, extrinsic features such as impurities and imperfections in coatings are still dominant. We can look forward, however, as ultraviolet laser technology advances, to see more investigations in that area. We also have not, in any of the Damage Symposiums, addressed ourselves to synergistic problems that arise due to the influence of the environment on the elements of the system. This environmental influence alters the damage characteristics. Certainly, for military or industrial systems, one has a hostile environment consisting of dirt, sea water, or high altitude. A mirror or a coated surface which might bear up very well in the laboratory, may have an entirely different characteristic when it gets out into the working environment. The same is true of the high power, short pulse, Neodymium glass lasers for laser fusion. Ask yourself, what do 14 MeV neutrons do to reflecting surfaces, and do they retain their integrity and their reflecting properties at low absorption. I think that everyone working in laser fusion would be delighted to have that as a serious problem, but it is one that we undoubtedly will have to confront in future meetings of this kind. We would like to acknowledge the invaluable assistance of Dr. Harold S. Boyne, Mrs. Pauline W. Smith, Mrs. Marjorie L. Wilson and Mrs. Florence M. Indorf of the National Bureau of Standards in Boulder, Colorado for their interest, support, and untiring efforts in the operation of this Symposium and in the preparation and publication of the proceedings. Also, we would like to take special note of Mrs Normal Lear and Mrs. Kay Jentsch, who added pleasurably to the conference operation particularly during the discussion periods following the formal presentations. The continued success of the Damage Symposia would not have been possible without their support. [1] "Damage in Laser Glass", A. J. Glass and A. H. Guenther, Editors, ASTM Special Technical Publication 469, ASTM, Philadelphia, PA (1969). [2] "Damage in Laser Materials", A. J. Glass and A. H. Guenther, Editors, NBS Special Publication 341, U. S. Government Printing Office, Washington, D.C. (1970). "Fundamentals of Damage in Laser Glass", N. Bloembergen, National Materials Advisory Board Publication NMAB-271, National Academy of Sciences, Washington, D.C. (1970). [3] [4] "Damage in Laser Materials: [5] [6] [7] [8] [9] 1971", A. J. Glass and A. H. Guenther, Editors, NBS Special Publication 356, U. S. Government Printing Office, Washington, D.C. (1971). "High Power Infrared Laser Windows", N. Bloembergen, National Materials Advisory Board Publication NMAB-292, National Academy of Sciences, Washington, D.C. (1972). Proceedings of the Conference on High Power Infrared Laser Window Materials, (October 1971). C. S. Sahagian and C. A. Pitha, Editors, Special Report No. 127, Air Force Cambridge Research Laboratories, (1971). "Laser Induced Damage in Optical Materials: Editors, NBS Special Publication 372, U. S. Washington, D.C. (1972). 1972", A. J. Glass and A. H. Guenther, Government Printing Office, "Laser Induced Damage of Optical Elements, A Status Report", A. J. Glass and A. H. Guenther, Applied Optics 12, pp. 637-649 (1973). "Laser Induced Damage in Optical Materials: 1973", A. J. Glass and A. H. Guenther, Editors, NBS Special Publication 387, U. S. Government Printing Office, Washington, D.C. (1973). [10] "Laser Induced Damage to Optical Materials, 1973: a Conference Report", A. J. Glass and A. H. Guenther, Applied Optics, 13, pp. 74-88 (1974). A. J. Glass A. H. Guenther 0.1 Opening Remarks Alexander J. Glass Lawrence Livermore Laboratory Livermore, California 94550 Speaking for Art Guenther and myself, I'd like to welcome you all to the Sixth Annual Symposium on Damage in Laser Materials. The National Bureau of Standards has again provided us with these superb accommodations, and has carried the burden of the preparation of the proceedings for this symposium over the years, and I want to reiterate the thanks that Haynes Lee has expressed to the Bureau, to the ASTM, and to ONR for their support in helping us carry out the work of the Symposium. The organization of the meeting this year is the same as it has been in previous years, in that we have tried to group papers roughly according to subjects. We have a session this morning on damage and its consequences in glass systems, then we have a session that is devoted to basic theoretical investigations, and another on damage to crystals, particularly in nonlinear optical materials. Tomorrow, most of the day will be spent on a variety of papers on coatings, windows, and mirrors. I think it is clear to everybody there are two fairly well delineated applications areas represented in these papers. One of them is short pulse, high peak power, essentially single shot lasers, mostly Neodymium glass for laser fusion. The other is high average power CW lasers for military and industrial application, which are primarily infrared lasers. In both areas very large systems are on line, or coming on line, depending on where you work. The topics that are being discussed here are not of purely academic interest. In the design of large systems it turns out that it is damage considerations that ultimately limit performance and dictate the design, and in many cases, determine the cost of the system. The cost sometimes becomes infinite, which means the system is infeasible. It is also true that because of the fact that the damage and the damage phenomena are the limiting factors, that real advances in the damage area lead to real benefits in terms of cost reduction, as well as in terms of improved system performance. Because of the fact that these are real problems that we are dealing with and real systems, you will find, in general that the subject seems to move ahead fairly quickly, and leaves behind it a number of fundamental investigations left undone. I think that it is typical of materials science and of applied science in general, that you learn how to deal with a problem long before you understand it, from the basis of first principles. The climate of applied science that exists in most places today is such that a lot of fascinating physics problems are going to be left behind. It is one of the minor regrets, but a necessary characteristic of this field, that we have to skip along the surface rather lightly, with the hope that someday, if people have sufficient motivation and there is a sufficiently hospitable climate to do so, some of these fascinating problems in materials science and in the interaction of high power light waves with quantum systems can be reviewed and dug into more deeply. We take a very broad view of damage in the symposium. Many of the things we talk about don't really have to do with the catastrophic failure of materials but only, like small scale self focusing, with a reduction in the performance of the system. Along this line, we have a working definition of damage as being "whatever turns you off". If you look at the schedule, you will find that there is nothing in it particularly associated with ultraviolet materials or ultraviolet systems. That is, not to say that damage problems don't exist in those systems. Even though ultraviolet lasers, excimer lasers, etc. operate at moderate powers in comparison to the IR and near visible systems, they are already damage limited. You even find people doing the calorimetry on ultraviolet lasers in terms of the damage to the coated elements. We know something about the damage mechanisms in the ultraviolet. We know that if one goes to short wavelengths one begins to get into circumstances in which two-photon absorption can become important in many materials. At the moment, my impression is that in the ultraviolet systems, extrinsic features such as impurities and imperfections in coatings are still dominant. We can look forward, however, as ultraviolet laser technology advances, to see more investigations in that area. We also have not, in any of the Damage Symposiums, addressed ourselves to synergistic problems that arise due to the influence of the environment on the elements of the system. This environmental influence alters the damage characteristics. Certainly, for military or industrial systems, one has a hostile environment consisting of dirt, sea water, or high altitude. A mirror or a coated surface which might bear up very well in the laboratory, may have an entirely different characteristic when it gets out into the working environment. The same is true of the high power, short pulse, Neodymium glass lasers for laser fusion. Ask yourself, what do 14 MeV neutrons do to reflecting surfaces, and do they retain their integrity and their reflecting properties at low absorption. I think that everyone working in laser fusion would be delighted to have that as a serious problem, but it is one that we undoubtedly will have to confront in future meetings of this kind. |