4. CONCLUSION This study of two different aspects of the laser-induced damage has led to several insights and proposals for new experiments. The model of avalanche ionization that we have described above is much simpler in form than that of Epifanov et al, 5 but it preserves the essential physics such as the importance of electron-phonon coupling. Because of its simplicity, it is possible to explore the dependence of avalanche ionization on intensity, frequency, electron-phonon coupling strengths etc. in a straight-forward manner. The problem of understanding multiple-pulse laser damage has divided the damage-community for years. The work we have described above presents a method not only for attacking the problem but also for comparing multiple-pulse damage measurements made on different materials and carried out by different experimental groups. Instead of comparing the damage thresholds directly, a comparison of the response functions would, firstly indicate whether the same relaxation processes are at work in the two materials and, secondly, permit an extrapolation to other repetition frequencies. Furthermore, a set of response functions for different materials would provide a good starting point for modelling multiple-pulse damage. REFERENCES 1. M. Sparks, D.L. Mills, R. Warren, T. Holstein, L.J. Sham, E. Loh, Jr. and D. F. King, Phy. Rev. B 24, 3519(1981) ༡. T.W. Walker, A.H. Gunther and P. Nielsen, IEEE J. Quantum Electron. QE17, 2053 (1981) 3. A.A. Manekov and A.U. Prokhorov, Sov. Phys. Uspekhi 22, 104 (1986) A.S. Epifanov, A.A. Manenkov and A.M. Prokhorov, Sov. Phys. JETP 43, 377 (1976) 5. N. Blomebergen, IEEE J. Quantum Electron. Vol.QE10, 375 (1974) 6. M.R. Lange, J.K. McIver and A.H. Gunther, Thin Solid Films 125,143 (1985) 7. V.M. Kenkre and J.K. McIver, "Theory of Electron Avalanche in Laser-Induced Damage In Solids", University of New Mexico preprint; see also V. M. Kenkre, J.K. McIver, and V.I. Kovanis, Bull. Am. Phys. Soc. 32, FQ 11(1987) 8. V.M. Kenkre," A Theoretical Approach to Laser-Induced Damage Under Multiple Pulses", University of New Mexico preprint 9. L. D. Merkle, M. Bass, R.T. Swimm, Opt. Eng. 22, 405 (1983), 25, 196 (1986) 10. L. D. Merkle, N. Koumvakalis and M. Bass, J. Appl. Phys 55, 772 (1984), 52, 2957(1986). 11. R.M. Wood, S.K. Sharma and P. Waite, NBS (US) Spec. Pub. 669, 44 (1984) 12. S.K. Balitskas and E.K. Maldutis, Sov. J. Quant. Electronics 11, 541 (1981) 13. R.M. O'Connell, A.B. Romberger, A.A. Shaffer, T.T. Saito, T.F. Deaton, and K.E. Seigenthaler,J. Opt. Soc. of Am. B 1, 853 (1984) 14. Y.K. Jhee, M. F. Becker and R.M. Walser, J. Opt. Soc. of Am. B 2, 1626 (1985) MANUSCRIPT NOT RECEIVED VARIATION, VARIABILITY AND DIFFERENCES IN THE MEASUREMENT OF LASER-INDUCED DAMAGE THRESHOLDS R.M. Wood and R.J. Chad GEC Hirst Research Centre ABSTRACT The published literature on the subject of laser induced damage is full of well measured values which disagree with what other workers have found. This paper attempts to distinguish between the variations (described by physical laws and historical/statistical laws) and differences (values measured on similar samples at different laboratories). Examples will be presented of the three types of variance taken from recent measurements. In particular the variability of the damage threshold value versus percent of sites damaged curves will be presented for a series of coated substrates. MANUSCRIPT NOT RECEIVED THE CONSEQUENCE OF DOPING OPTICAL MATERIALS WITH D20 J.B. Franck, J.O. Porteus, L. F. Johnson, J.M. Pentony, and W.N Faith Physics Division, Research Department Naval Weapons Center China Lake, CA 93555-6001 and H. Angus Macleod ABSTRACT In general, water is ubiquitous in nature. In the production of optical thin films the point in the coating process at which the water is incorporated into the film often is unknown. In an attempt to produce a unique sample set for laser-induced desorption experiments, optical thin films were grown in the presence of copious quantities of D20. The results indicate that the uptake of water in optical thin films may not take place in the coating process as might be expected. Preliminary findings indicate that the laserdamage threshold can be improved significantly using this D2O-doping process. Manuscript Received 1. Some Studies of Thin Film Distributed Bragg Reflectors K L Lewis, I T Muirhead*, A M Pitt, A G Cullis and G M Williams RSRE, Malvern, Worcs WR14 3PS, UK * OCLI, Dunfermline, Fife KY11 5FR, UK The Distributed Bragg Reflector (DBR) is of interest for a range of optical applications. This work is concerned with a study of the problems influencing the fabrication of such structures using molecular beam techniques. Many of the issues involved are concerned with the achievement of stable structures that do not shift under temperature cycling or laser irradiation. These centre around the fundamental properties of the coating materials selected, the degree of perfection of the films, and the control of microstructure and interface interdiffusion. Results have been obtained which show the effect of varying the thickness of the reflecting interfaces on the bandwidth and intensity of the fundamental reflection band. The degree of interface perfection in such structures has been examined using cross-sectional transmission electron microscopy, and correlated with the results of depth profiling X-ray photoelectron spectroscopy studies. Introduction The relationship between laser damage threshold and electric field distribution in thin film coatings has been the subject of many studies reported in the literature. Evidence has been obtained which suggests that coating designs which avoid high values of electric field intensity within layers, or at interfaces, tend to have significantly improved damage thresholds. In antireflection designs, the most vulnerable interface is generally that between the coating and substrate, since this is the likely site for incorporation of impurities. Apfel et al [1] found that the addition of a silica barrier layer between a glass substrate and first layer of a four-layer AR coating improved the average 1.06 m damage threshold. Previously, Newman et al [2] had studied the influence of electric field distribution on the damage resistance of thin fims of between 1/4 and 51/4 in thickness at 1.06 μm. For high index materials such as Ti02, the damage thresholds for odd 1/4 thicknesses were greater than for even A/4 thicknesses, as expected from calculated field values. This was further developed in a subsequent study of 1.06μm reflectors [3], where increases in damage threshold were obtained by using non-quarter wave thicknesses for the top few layers of a 1/4 stack. The designs were developed to minimise the standing wave field in the high index layer, which also served to reduce the field values at the interfaces between the successive layers. Reflectors incorporating 11 /4 layers of TiO2/Si02 were found to have thresholds of about 1J/cm2 for 30ps pulses at 1.06 μm, whilst those with non-quarter wave layers added to the top of the reflector were able to resist twice these fluence levels. The optimum thicknesses for the top layers were derived by Apfel [4]. Similar improvements in damage threshold were found at other wavelengths. For example Newnam et al [5] showed the significant increase in damage resistance at 248nm (8 nsec pulses) possible in scandia/magnesium fluoride reflectors, with thresholds increased from about 3 J/cm2 to 5 J/cm2. A discussion was also presented highlighting the role that laser pulse width may have in determining the degree of enhancement in damage threshold, and the role that coating defects would have in masking any such effect. Carniglia et al [6] assessed the enhancement possible at 355nm using such suppressed electric field designs in scandia/magnesium fluoride, reporting a 40% increase in threshold compared with the basic HR stack. However a variant of the suppressed field design, in which the thickness ratio of the top two HL pairs was reversed, performed no better than the standard design, since the peak electric field in the high index scandia layer was equivalent to that in the standard case. The role of interfaces in such designs was not fully explored. In many fabrication processes it is not possible to guarantee a perfect interface, that is, one that is free from impurity species. Furthermore, due to the high coating temperatures frequently employed for oxide and fluoride materials, the possibility of interdiffusion effects between adjacent layers is increased, with the ensuing formation of hetero-species. Such effects can be explored by depth profiling techniques, providing that the specific analytical technique used is |