110 IEEE JOURNAL OF QUANTUM ELECTRONICS, MARCH 1975 REFERENCES zyxwvutsrq obvious that a material with low intrinsic losses will be suffi- with a modulated region smaller than the guide width,” zyxwvutsrq IEEE zyxwvuts J. D. P. Bortfeld, “Analysis of heterojunction optical waveguides Quantum Electuon.,vol. QE-IO, pp. 551-556, July 1974. H. P. Kleinknecht and A. E. Widmer, “(Ga,AI)P optical waveguide modulators fabricated bv liauid-uhase eoitaxv.” J. ADDL Phvs.. voi. 45, pp. 3453-3459, iug.’1974. zyxwvutsrq I -I L . zyxwvutsrqponmlkjihgfed ~, [3] F. K. Reinhart, D. F. Nelson, and J. McKenna, “Electro-optic and waveeuide urouerties of reverse-biased gallium uhosohide 0-n iunc- tionsy’Phyk Rev., vol. 177, pp. 1208-l-221, Jan. 19-69. [4] D. H. Nelson and E. H. Nelson, “Electro-optic and piezoelectric coefficients and refractive index of gallium phosphide,” J. Appl. Phys., vol. 39, pp. 3337-3343, June 1969. - . NaCl Surface Reaction in Chemical-Laser Devices ALAN B. PETERSEN AND CURT WITTIG Abstract-Pronounced’absorptions in the 8-17-~m region are observed on surfaces of NaCl that are used as Brewster windows in chemical lasersthatderivetheir excitation fromthe CS2/02 reaction.Surface reaction involves species that are formed during the chemical andphoto- chemical processes that are active in the system. Implications regarding other reactivesystems(usingeitherchemical or electrical excitation) are also discussed. In this correspondence, we wish to communicate several ob- servations regarding the reactivity of NaCl which is used as window material in pulsed chemical lasers. The implications of theseobservationsare far-reaching and the principles in- volved may be applied to other materials and to a variety of situations in which these materials are used as an interface be- tween the normal atmosphere and some active nonequilibrium gaseous system. There have been numerous investigations into the losses that are present in various materials (single and poly- crystalline) that are suitable for devices that operate at infrared frequencies [ I]-[ 31. Most of thiseffort has been directed toward determining the intrinsic losses of these materials and preparing samples that are sufficiently free from impurity absorptions so that intrinsic losses dominate. One of the wide- spread uses of these materials is with high-power IR lasers, and in this application it is imperative that the material be free of even the smallest amount of absorption in order to avoid either serious damage to the component or undesirable thermally induced distortion of the laser beam. On the other hand, the gaseous environment that existsin most high-power IR gas lasers is in a state of pronounced non- equilibriumwith a variety of atomic, freeradical, and meta- stable species present. Thus the inside surface of any material that acts as aninterface(internalmirror, Brewster window, etc.) between the active laser medium and the outside world will be subjected to a rather harsh test of its inertness. Since it is difficult to physically remove theinterface region to the extentthat surface reaction is entirely suppressed,it is not the Joint Services Electronics Program, monitored by the Air Force Manuscript receivedOctober 1, 1974. Thisworkwassupported by Office of Scientific Research, under Grant Contract E44620-71-C-0067, and by the Research Corporation. versity of Southern California, University Park, Los Angeles, Calif. The authors are with the Department of Electrical Engineering, Uni- 90007. ciently chemically inert for use in situations where active species are present. In general, the reactions that take place at the surface of any candidate material deserve prime considera- tion in the evaluation of the material as a suitable candidate for use in infrared devices. In our own work with infrared chemical lasers, we have ob- served that NaCl Brewster windows [4] that are used in pulsed chemical lasers are seriously affected after only short-term use. Details of the laser systems employed have been described elsewhere [ 51 , Briefly, mixtures containing CS2 and 02 as the chemically active species are photolyzed in the presence of a large enough excess of He to maintain approximately isother- mal conditions (AT < 100 K). In some experiments, CQ2 or N 2 0 is also present in the gas mix. A most graphic display of the phenomenon is shown in Fig. 1. The spectrophotometer trace shows the presence of promi- nent absorptions in the 8-17-pm region of the IR. The trans- mission of the NaCl at visible wavelengths is almost unaffected and a casual observation of the NaCl shows no surface change at all. When the surface is viewed at nearly grazing incidence, a bit of interference is observed indicating a surface layer of roughly 10-1 00-nm thickness. The general effect of absorption in the 8-17-pm range is quite repeatable. The details of the spectra change from one set of conditions to the next, but the effect is easily measured under nearly all experimental condi- tions [6]. In many cases, laser emission from CQ2 or NzO was completely quenched by these absorptions, since strong broad absorptions often occurred near the characteristic laser fre- quencies of these molecules. In no instance were absorptions observed in the region 2.5-8.0 pm, where many powerful molecular lasers operate. The surface changes that are observed are due to the interac- tion of the surface with the active species that are generated, either directly or indirectly, by the flash photolysis. There is no detectable surface reaction when the gases are stored in the cell without photolyzing the mixture or when the cell is evacu- ated and the photolysis flash is firedrepeatedly. All experi- ments are carried out under approximately isothermal condi- tions since theconcentration of buffer gas far exceeds the concentration of the reactive components. The transient ac- WAVELENGTH zyxwv (prn)- 13 15 20 %L.--L! 10 zyxwv O IZQO zyxwvut c 1000 ENERGY BOO (cm-’) 600