large optical systems and the presentations at this meeting are indicative of their increased application. Organics offer great opportunity for use including improved nonlinear materials for a variety of applications. Presentations at this meeting on this subject presage future importance of organic materials. One-on-one damage effects in dielectric materials are much better understood than m-on-1 effects. We heard about accumulative damage even for pulses as short as picoseconds. In addition, some models discussed the importance of thermal cracking and thermal explosion of particulates as an important source of damage. An analysis of major damage to a cracked large optic at the Livermore National Laboratory was given. By examining the data and the damaged sites concluded that stimulated Brillouin scattering was responsible for major cracks oriented along polarization axis. They suggested that to avoid this problem one may want to frequency chirp the optical pulses. In M.J. Soileau's review paper a very useful formula was brought forth for approximating damage threshold over a wide range of parameters. Over a wide range the damage threshold within an order of magnitude is essentially equal to 1 gigawatt per square centimeter per nanosecond, with the pulse width being scaled as the half power. 3. Acknowledgments The editors would like to acknowledge the invaluable assistance of Mr. Aaron A. Sanders and the other involved staff members of the National Institute of Standards and Technology in Boulder, Colorado, for their interest, support, and untiring efforts in the professional operation of the symposium. Particular thanks to Ms. Susie Rivera of NIST for her lead in the preparation and publication of the proceedings as well as Ms. Edit Haakinson of NIST. Thanks also go to Ms. Pat Whited of the Air Force Weapons Laboratory and Ms. Ann Mannos of NIST for conference coordination. 4. References [1] Glass, A.J.; Guenther, A.H., eds. Damage in Laser Glass, ASTM Spec. Tech. Pub. 469, ASTM, Philadelphia. PA; 1969. [2] Glass, A.J.; Guenther, A.H., eds. Damage in Laser Materials, Nat. Bur. Stand. (U.S.) Spec. Publ. 341; 1970. [3] Bloembergen, N. Fundamentals of Damage in Laser Glass, National Materials Advisory Board Publ. NMAB-271, National Academy of Sciences; 1970. [4] Glass, A.J.; Guenther, A.H., eds. Damage in Laser Materials: 1971, Nat. Bur. Stand. (U.S.) Spec. Publ. 356; 1971. [5] Bloembergen, N. High Power Infrared Laser Windows. National Materials Advisory Board Publ. NMAB-356; 1971. [6] Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical Materials: 1972, Nat. Bur. Stand. (U.S.) Spec. Publ. 372; 1972. [7] Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical Materials: 1973, Nat. Bur. Stand. (U.S.) Spec. Publ. 387; 1973. [8] Glass, A.J.; Guenther, A. H. Laser Induced Damage in Optical Materials: A Conference Report. Appl. Opt. 13 (1): 74-88; 1974. [9] Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical Materials: 1974, Nat. Bur. Stand. (U.S.) Spec. Publ. 414; 1974. [10]Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials: 6th ASTM Symposium. Appl. Opt. 14 (3): 698-715; 1975. [11]Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical Materials: 1975, Nat. Bur. Stand. (U.S.) Spec. Publ. 435; 1975. [12]Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials: 7th ASTM Symposium. Appl. Opt. 15 (6): 1510-1529; 1976. [13]Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical Materials: 1976. Nat. Bur. Stand. (U.S.) Spec. Publ. 462; 1976. [14]Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials: posium, Appl. Opt. 16 (5): 1214-1231; 1977. 8th ASTM Sym [15]Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical materials: 1977, Nat. Bur. Stand. (U.S.) Spec. Publ. 509; 1977. [16]Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials: 9th ASTM Symposium, Appl. Opt. 17 (15): 2386-2411; 1978. [17]Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials: 1978, Nat. Bur. Stand. (U.S.) Spec. Publ. 541; 1978. [18] Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical Materials: 10th ASTM Symposium, Appl. Opt. 18 (13): 2212-2229; 1979. [19] Bennett, H.E.; Glass, A.J.; Guenther, A.H.; Newnam, B. E. Laser Induced Damage in Optical Materials: 1979, Nat. Bur. Stand. (U.S.) Spec. Publ. 568; 1979. [20] Bennett, H. E.; Glass, A.J.; Guenther, A.H.; Newnam, B. E. Laser Induced Damage in Optical Materials: 11th ASTM Symposium, Appl. Opt. 19 (14): 23375-2397; 1980. [21] Bennett, H. E.; Glass. A.J.; Guenther, A. H.; Newnam, B.E. Laser Induced Damage in Optical Materials: 1980, Nat. Bur. Stand. (U.S.) Spec. Publ. 620; 1981. [22] Bennett, H. E.; Glass, A.J.; Guenther, A.H.; Newnam, B.E. Laser Induced Damage in Optical Materials: 12th ASTM Symposium, Appl. Opt. 20 (17): 3003-3019; 1981. [23] Bennett, H. E.; Guenther, A.H.; Milam, D.; Newnam, B. E. Laser Induced Damage in Optical Materials: 1981, Nat. Bur. Stand. (U.S.) Spec. Publ. 638; 1983. [24] Bennett, H. E.; Guenther, A.H.; Milam, D.; Newnam, B.E. Laser Induced Damage in Optical Materials: 13th ASTM Symposium, Appl. Opt. 22 (20): 3276-3296; 1983. [25] Bennett, H. E.; Guenther, A.H.; Milam, D.; Newnam, B. E. Laser Induced Damage in Optical Materials: 1982, Nat. Bur. Stand. (U.S.) Spec. Publ. 669; 1984. [26] Bennett, H. E.; Guenther, A.H.; Milam, D.; Newnam, B. E. Laser Induced Damage in Optical Materials: 14th ASTM Symposium, Appl. Opt. 23 (21): 3782-3795; 1984. [27] Bennett, H. E.; Guenther, A.H.; Milam, D.; Newnam, B. E. Laser Induced Damage in Optical Materials: 1983, Nat. Bur. Stand. (U.S.) Spec. Publ. 688; 1985. [28] Bennett, H. E.; Guenther, A.H.; Milam, D.; Newnam, B.E. Laser Induced Damage in Optical Materials: 15th ASTM Symposium, Appl. Opt. 25 (2): 258-275; 1986. [29] Bennett, H. E.; Guenther, A.H.; Milam, D.; Newnam, B.E. Laser Induced Damage in Optical Materials: 1984, Nat. Bur. Stand. (U.S.) Spec. Publ. 272; 1986. [30] Bennett, H.E.; Guenther, A.H.; Milam, D.; Newnam, B. E. Optical Materials: 16th ASTM Symposium, Appl. Opt. 26 Laser Induced Damage in (5): 813-827; 1987. [31] Bennett, H.E.; Guenther, A.H.; Milam, D.; Newnam, B.E. Laser Induced Damage in Optical Materials: 1985, Nat. Bur. Stand. (U.S.) Spec. Publ. 746; 1987. [32] Bennett, H. E.; Guenther, A.H.; Milam, D.; Newnam, B. E. Laser Induced Damage in Optical Materials: 1986, NIST (U.S.) Spec. Publ. 752; 1987. Manuscript Received Multiple Shot Intrinsic Bulk Damage in KBr at 532 nm R. Thomas Casper, Scott C. Jones and Peter Braunlich Department of Physics Evidence is presented in support of a cumulative damage mechanism for multiple shot. intrinsic bulk laser damage in KBr at 532 nm. The major feature of the model, suggested by Manenkov et al., is the build-up of internal stresses in the interaction volume. These stresses most likely result from an accumulation of photochemically produced stable point defects, such as F-centers, and then serve to increase the rate of further accumulation. It is also found that thermal stress due to the temperature rise during the laser pulse must be considered. This plays a particularly important role at low temperatures where, otherwise, the efficiency of F-center production is very small. This model exhibits the observed temperature and flux dependences for multiple shot damage and is compared with experimental data for temperatures from 50 to 300 K. Key Words: alkali-halide; defect accumulation; expansion; F-center; 532 nm; internal stress; KBr; multiple shot damage; thermal stress; yield stress. 1. Introduction It is of great practical importance to understand the response of optical materials exposed to repetitive laser pulses. At power levels far below that required for single pulse intrinsic damage these materials experience irreversible modifications when subjected to a series of laser pulses [1-6]. The aim of this research is to answer some of the questions concerning this phenomenon. Recent work on NaCl [7-9], KBr [10-13] and KI [14,15] lends strong support to the theory that single shot damage in these materials at 532 nm results from the production of free electrons by a multiphoton absorption process (four photons for NaCl and KBr and three photons for KI) followed by heating as the free electrons absorb additional energy from the laser pulse. These experiments showed that peak temperatures approaching the melting point can be reached with no indication of damage. This suggests that melting is the mode of failure under these conditions. Shen, et al. [16] have since been able to provide additional evidence for the free electron heating mechanism. In contrast to the temperature increases of several hundred degrees in the experiments just mentioned, at the photon fluxes used in the current multiple shot experiments the peak temperature rise is less than ten degrees. It is clear that two different modes of failure are responsible for the behavior observed in the single and multiple shot experiments. This investigation pursues the idea that the failure of alkali halides due to multiple laser pulses results from the accumulation of microscopic structural changes in the crystal lattice. Perhaps the most easily produced stable primary defect is the F-H pair, or Frenkel defect. It is suggested here that these F-H pairs play a major role in the cumulative effects leading to catastrophic optical failure in alkali halides. |