References 1. M. E. Frink, J. W. Arenberg, and D. W. Mordant, "Temporary Laser Damage Threshold Enhancement by Laser Conditioning of Antireflection-Coated Glass", Applied Physics Letters 51(6), (1987). 2. J. E. Swain, W. H. Lowdermilk, D. Milam, "Raising the Surface Damage Threshold of Neutral Solution Processed BK-7 by Pulse Laser Irradiation", Laser Induced Damage in Optical Materials, NBS Special Publication 669, 1982. 3. James M. Rowe, Laser-Assisted Deposition Coatings, AFWL-TR-83-34, March 1983. 4. J. Swain, S. Stokowski, D. Milam, and F. Rainer, "Improving the Bulk Laser Damage Resistance of Potassium Dihydrogen Phosphate Crystals by Pulsed Laser Irradiation", Applied Physics Letters 40(4), February 15, 1982. 5. Tsutomu Shimizu, Noriaki Itoh, Noriaki Matsumami, "Laser-induced Re-emission of Silicon Atoms Implanted into Quartz," J. Appl. Phys. 64(7), (1988). COMMENTS Question: You did not mention the damage level you observed on these coatings. Answer: Question: Answer: Question: Answer: Question: Answer: Yes, they are slightly less than the damage levels that we get with our porous single layer coating. That is higher than KDP or LAP substrates and a little lower than fused silica at whatever wavelength you wish to take. Were any damage tests done on the two layer structures. What were the results? I am sorry, perhaps you are not familiar with the damage figures that we get. At 350 nanometers, for example, at 0.6th of a nanosecond we are in this sort of 6 to 8 joules range and one is six at one nanosecond about 15, at 10 nanoseconds about 30-35. This was with a combination of zirconium oxide and silicon? Yes, and the figures I have quoted actually were done on the sample on fused silica. As I say, on KDP substrate damage is before the coatings, so we were not able to get a figure. I am assuming that the damage thresholds that we get on silica, fused silica substrates will be the same as we get on KDP. Can you say anything about the stability of these coatings say over a day, week, month, year? Yes, we have, as you are aware, the porosity is such that we have a high surface area and the coatings do absorb material.. We have found that the absorption when they are exposed to atmospheric type conditions is no problem, but under a vacuum they are quite severe. We solved this problem at Livermore by washing the coatings frequently with an alcohol spray. We just spray the coating down and let the liquid drain down and that in effect renews the material back to pristine conditions. MANUSCRIPT NOT RECEIVED TWO LAYER BROADBAND AR COATING* Ian M. Thomas Lawrence Livermore National Laboratory P.O. Box 5508, L-490 Livermore, California 94550 ABSTRACT A two layer broadband AR coating has been developed specifically for use with harmonic converter crystals where surfaces must transmit light of two different wavelengths. The coating consists of a methyl silicone or sol-siloxane layer with refractive index 1.4 overcoated with porous silica of index 1.22. Less than 0.5% reflection at 1064 nm and 532 nm or at 532 nm and 355 nm is obtained on KDP and even lower reflections with LAP. The laser damage threshold at 1064 nm is better than that of the bare substrate surface. *Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48. MANUSCRIPT NOT RECEIVED SCANNING TUNNELING MICROSCOPY STUDY OF THE Wigbert Siekhaus, Thomas Beebe, and Markus Schildbach ABSTRACT Despite many years of study, the details of the interaction of high power laser pulses with surfaces of insulating, semiconducting and conducting materials are not understood. Though it has been shown that optically visible surface irregularities reduce the surface damage threshold, little is known about the effect of the laser beam (at or below the damage threshold) on submicroscopic irregularities. Scanning tunneling microscopy can determine the three dimensional physical structure and the electronic structure of surfaces with atomic resolution and is, therefore, uniquely suitable to investigate the effect of irregularities in physical and electronic surface structure on damage phenomena. This paper reports our preliminary STM observations of the effect of pulsed laser irradiation of surfaces. The scanning tunneling microscope used is a variation of Lyding's1 design, in which both the tunneling needle and the target are held by tubular piezoelectric crystals, a design, which leads to very high vibrational and thermal stability, and allows reproducible movement of the target in and out of the tunneling range by purely electronic means without the use of mechanical devices. The tunneling needle is Pt/Rh. The specimens were either cleaved in air (C) or mechanically (SiC) or electrochemically polished (au), and showed before irradiation flat areas with atomic corrugation and steps one or several atoms high. The surface disorder created by exposure to a pulsed YAG laser beam (lambda 1064 nm, 5 ns pulse duration) will be shown. 1 J.W. Lyding, S. Skala, J.S. Hubacek, R. Bockenbrough, G. Gammie, "A Variable Temperature Scanning Tunneling Microscope," APS Regional Meeting, New Orleans (1988). Manuscript Received Tarnishing Measurements of A1203 Overcoated Silver Mirrors W.D. Kimura Spectra Technology, Inc. Q.D. Appert, P.N. Arendt, V.E. Sanders, and M.L. Scott Los Alamos National Laboratory Los Alamos, New Mexico 87545 Presented are tarnishing results for silver mirrors protected using a technique, developed at Los Alamos National Laboratory, of applying a very thin overcoat (10 A) of alumina (A1203) on the bare silver surface. The s-polarization absorptance characteristics of the overcoated mirrors at 0.5145 and 1.06 μm are measured as a function of incidence angle (0-88°) using a photoacoustic calorimetry technique developed at Spectra Technology, Inc. Measurements are also performed on bare silver mirrors. When untarnished, both types of mirrors give similar results. The mirrors were exposed to room air and allowed to tarnish naturally for >120 days. It is found that the bare silver mirrors suffered severe tarnishing; whereas, the overcoated silver mirrors had a significantly greater resistance to tarnishing. Key words: A1203; alumina; glancing incidence; photoacoustic calorimetry; silver mirrors; tarnishing 1. Introduction Silver mirrors are highly reflective at visible and near infrared wavelengths, which makes them desirable for high power laser applications. Even higher reflectivity is possible by using the mirrors at glancing or grazing angles of incidence [1]. Besides the high power laser community, there is also interest in using silver mirrors for astronomical applications. One drawback with bare silver is that once exposed to room or outside air, silver sulfide quickly forms on the mirror surface. It has been found that glancing incidence operation is particularly sensitive to this tarnishing effect [2]. Typically, dielectric overcoats on the bare silver have been used to prevent this tarnishing. This overcoat can affect the mirror reflectivity properties, especially at glancing angles of incidence [3]-[4]. Dielectric coatings are also known to have lower damage thresholds than bare metals. There have been other attempts to reduce this susceptibility to tarnishing, such as using copper underlayers [5], but this has been shown to provide insufficient protection [2]. At Los Alamos National Laboratory (LANL), two of the authors (P. Arendt and M. Scott) developed the novel idea of applying a very thin overcoat of alumina (A1203) onto the bare silver mirror surface. Their initial tests of the tarnishing resistance of the alumina overcoated mirrors by exposing them to fuming ammonium sulfide showed that an alumina layer as thin as 10 A appeared to still significantly retard the tarnishing process. This paper describes tarnishing measurements performed at Spectra Technology, Inc. (STI) of alumina overcoated mirrors. The objectives of these measurements were to measure the absorptance characteristics of new bare silver and overcoated mirrors, expose the mirrors to laboratory room air (>120 days), and periodically remeasure the absorptance to detect any tarnishing. 2. Description of Mirrors Two sets of bare silver and alumina overcoated mirrors were tested. The coating parameters are the same for both sets; however, one set was coated at LANL and the other set was coated at Spectra Physics Optics Division, Mountain View, CA. The 10 A alumina (purity: 99.99%) |