concerns multiple irradiation - A subject that has seen a lot of continuing interest at this series of meetings on one hand relating to accumulation of damage (measured absorption) thus reducing the damage threshold and increasing the extent of damage, or on the other hand conditioning (reducing absorption by bleaching, annealing, etc.) thus damage thresholds. At this meeting we heard a number of papers that dealt with this subject in a more quantitative way, i.e. measuring the increase of absorption in materials as a result of repeated irradiation at high intensities and in measuring the decreased absorption as a result of multiirradiation at lower intensities. Another area that has been of continuing interest is the search for a means to monitor pre-damage as an indicator in an effort to better understand the operative mechanisms relating to damage. As relates to material property measurements talks were given that showed encouraging results, for example, in monitoring the production of free carriers by multi-photon processes arising from defects. This was accomplished by monitoring the heating of selected sites on surfaces using classical optical techniques. A suggestion by Maldutis was that increases in absorption caused by multiple pulse effects can lead to thermal self-focusing. This in turn can lead to a reduction in damage threshold of sites subjected to multiple pulse irradiation. In Maldutis' paper, one interesting confirmation of our understanding is that color centers accumulate with numbers of pulses and that the accumulation corresponds exactly with the reduced damage threshold. He showed by the measurement of transient increase in temperature and computer assisted deconvolution that the size of absorption and size of damage have a fairly good correlation. This year the materials and measurements saw an emphasis on ultra-violet radiation correlated with the study of accumulative damage. One of the specific U.V. developments this year was a new process for producing one meter sized optics, for example calcium fluoride, reported by Harshaw. A slab process using single crystal orientations looks to be scalable to very large sizes. Progress on the new nonlinear material L-arginine phosphate is an important new development. It is an exciting new material that has been known for some time in the literature and which may offer some distinctive advantage in second harmonic generation and optical parametric oscillators. The results presented at this meeting shows that this material can also exhibit greatly improved laser-induced damage resistance compared to materials that are more traditionally used in these applications. The material is simply grown in very large sizes using very inexpensive techniques. The emergence of this second harmonic generating material in parametric oscillators will receive more attention in years to come. Surfaces and Mirrors Numerous papers presented at this year's symposium covering several areas of historical interest. Real advances were reported as well as some good rules of thumb that can be applied to laser induced damage. One area of particular interest related to the polishing, grinding, or treatment of optical surfaces. We heard Professor Namba report on ultra precision grinding at sub-micron accuracies indicating that he is continuing the development of new machining and fabrication techniques based upon his early work with float polishing. Sputter etching of substrates prior to coating was reported as an excellent means of removing contamination. In addition, we heard continued reports on the beneficial aspects of laser treatment of fused silica surfaces in reducing surface absorption. Papers again reported impurities that where retained by the surfaces from the grinding compounds, mostly ceria and titania. In Desorption by lasers as a substrate cleaning technique and improved adhesion of coatings (surface activation) where covered in several papers. Most notably were the beneficial or deleterious aspects of either normal water or heavy water on surfaces. apparent violation of conventional wisdom that water is always bad, it was shown that one can actually increase the damage threshold of material by exposing it to different amounts of water. Thin film coatings were proposed for smoothing surfaces prior to the deposition of mirror-like surfaces. New techniques employing cavity ring down for measuring very high reflectivities were presented. These are increasingly necessary in the expanding and important field of free electron laser optics. Optical design techniques using multi-faceted or grazing angle of incidence for reflecting surfaces in the XUV region where proposed and showed great promise as a means of overcoming or at least alleviating short wave length damage concerns! A particularly important area reported on at this meeting was the cleaning of surfaces in-situ using laser radiation. There is an obvious benefit to any in-situ technique as it offers the possibilities of being coupled with other process variables that improve film deposition such as laser assisted film deposition. Although laser cleaning can improve bad surfaces and increase laser damage threshold surfaces, if you have surfaces which exhibit nearly intrinsic properties, laser cleaning does not improve the damage threshold measurably. A study of the importance of scatter by particulates was reported. This area has been largely neglected at this meeting because there has not been a convenient theory to explain or correlate scatter and damage. This is another case where experimental data is not available or where scatter from poor surfaces masks that from particulates. Thin Films As is usually the case, the contributions in the thin film area were the greatest in number at the meeting, being roughly one-third. Presentations this year reflect similar interest as discussed in other categories of the meeting proper. Multi-pulse damage, the performance of materials in adverse environments and in particular short-wave lengths. In fact, considerable progress has been made in the damage resistance of coated elements from 148 nanometers to 308 nanometers. Several design tricks to improve the damage resistance in coated optics were presented. Keith Lewis discussed the use of distributed Bragg reflectors as a potential mechanism for improving the damage resistance of multilayers at the expense of some optical performance. This was an extension of his previously reported attempts to place the intense field within one of the materials and not at the boundaries where one might expect adsorption or other weaknesses to be present. Another paper, which reported improvements in damage resistance, concerned the use of gradual or changing refractive index interfaces by the simultaneous deposition of two coating materials. The same ruse of co-evaporation was utilized in ion sputtering of mixed oxides to produce a stress reduction in the films. Performance of these were rather nicely correlated with the deposition process. The most From this work on ion sputtering one noted, with great pleasure, still another new deposition technique which promises, at least at this stage, to afford a truly significant advance in thin film damage resistance. The technique generally falls under the category of plasma assisted chemical vapor deposition. The author indicated that he was borrowing from fiber optics processors, by applying the particular technique of microwave excitation of a plasma which deposits material of high quality on the inside of a cylinder. significant result of this early work was that film adsorption as low as 107 was observed, which is at least 105 lower than what has been reported before. Initial indications of greatly improved damage resistance in the 40 to 50 joule per square centimeter range were reported. These are certainly preliminary numbers, and we are all anticipating additional varification of this approach, and also hope to learn whether or not this process can be applied to other geometrical surfaces. There is no question that thin films continues to be the area of principal concentration, because of the potential leverage for improvement and the limitations it places on system design at the present time. Most thin film coatings are still produced in the industry in a way that is not very different from the way in which they have been produced in the pre-laser days. There have been refinements and improvements perhaps in vacuum systems and procedures but few real fundamental changes. This year we saw quite an emphasis on trying to use techniques that were developed for other technologies such as the fiber optic industry and the electronics industry. Techniques like plasma assisted depositions, chemical vapor depositions, ion plating techniques, and various other techniques have been developed and refined in other technologies, where the economic drivers are much larger than they are in the optics area. Groups are now trying to exploit those advances and apply them to thin film optical coatings. The reemergence of metal coatings as important in laser systems was noted because of the advent, and finally the working, of free electron lasers. These lasers require coatings or optics that have large bandwidths. With these wideband width systems one now has to go back to materials like thin films of silver with broad spectrum reflectance to satisfy FEL requirements. FELs also have some peculiar needs in the sense that they both have cw and pulsed formats. many picosecond pulses. The output of these devices include high average powers from One of the most significant advances in the thin film area related to the ultraviolet and the vacuum ultra violet, where the development of reflectances above 85% at 58.4 and 30.4 nanometers was reported using in-situ ultra-high vacuum deposited aluminum. One of the pleasant surprises was that the optical constants are in fact more favorable than we had been led to believe by previous work. We now have better values of the optical constants for pure aluminum in this region of the spectrum than were available previously. Another area that received notice was the effective reflectance of coatings in laser systems. There is a problem of optimism on the part of designers as to the values that they hope to obtain for reflectance of coatings. There was a very interesting paper by Newnam and Saunders on the reflectance values measured by the ring down cavity technique for "typical" and the best coatings that they could obtain at 1.6 micrometers. The best they obtained was a 99.97% for titanium oxide/silicon dioxide. The gold coating they employed had an effect reflectance of 97.68%, which shows one should not be too optimistic about the anticipated reflectance of coatings. The transient effect of increased absorption by ultra violet radiation at 350 nanometers and the absorption being measured at 1 micrometer was demonstrated. important for many systems, such as free electron lasers. There was evidence of a significant recovery after the radiation stopped, but residual absorption still remained. This is Likewise, there was evidence given by the Japanese on the aging of lithium or lanthanum fluoride films after storage. It was presupposed that the lithium fluoride was actually becoming oxidized with time. The Japanese also showed that sputter etching of substrates is effective in removing impurities. They observed 2 to 3 times higher damage threshold for AR coatings on these treated surfaces. They consider sputter etch a more convenient process than the argon fluoride laser radiation treatments (also found to remove impurities). We would like to add a note of encouragement to those people in coatings research that are trying some nontraditional, nonoptical techniques for depositing coatings. There is more than 40 or 50 years of history using traditional technique, so it will take some time we believe to completely transition those new approaches to acceptable coatings for optics. But we think it is important that those techniques that have proved to work so well in other areas be examined very carefully and with some patience. Relative to that point, ion plating technology, which has been developed in the last year or two, appears to be producing moderate damage thresholds. There is some concern that chamber contaminants may be causing problems. Such problems will have to be overcome before these coatings can emerge from the average performance category. However, this is one of those technologies that is in its infancy, and should be pursued for optimization and capability! Fundamental Mechanisms As was customary, the meeting concluded with a most free-wheeling discussion on issues of fundamental mechanisms relating to the interaction of intense light with matter. This session was initiated with a review by M.J. Soileau of the 20 years of discussions and presentations relating to this field. As previously indicated and expected, one of the underlying threads throughout the conference dealt with multi-pulse phenomena. There was considerable theoretical exposition on this issue treating both high and low intensity irradiation, that is, accumulation of damage versus conditioning as annealing of damage. The relative independence of individual pulses from a thermal standpoint, depending upon their pulse width relative to their separation, materials, optical, thermal and thermomechanical properties as well as design, are all factors which effect damage. Interestingly, at this meeting there was more than the normal emphasis on the chemical aspects of laser-material interactions. This was certainly evident in discussions of laser interactions with monomeric and polymeric conjugates as well as liquid crystals. these classes of materials continues to grow in the high power community. The use of An interesting perspective was presented by Professor Manenkov of the Institute of General Physics in Moscow on the relative importance of impact ionization or of avalanche as an intrinsic process. Professor Manenkov reviewed the work of his group over the past decade or so. It has been directed primarily toward producing a detailed model of laser induced breakdown in optical material going away from the previous one or two parameter models that have been invoked to try to describe this extremely complicated process. He reviewed the incorporation of not only laser parameters into the model, but basic material properties and non-equilibrium material properties, as well as multi-photon mechanisms to describe laser-induced breakdown in transparent dialectics. He then moved on to defects, particularly as they play a role in polymeric optic materials. This is an area of considerable activity in the Soviet Union. Continuing on with the subject of organic materials, emphasis was given at this meeting to liquid crystals and polymeric materials of similar structures. These materials are starting to see greater and greater use in optical systems of all kinds, particularly |