EFFECT OF PHOTOSTIMULATED DESORPTION PROCESSES ON OPTICAL PARAMETERS OF THIN-FILM COATINGS O. E. Sidoryuk and L. A. Skvortsov UDC 681.7.064.45 In modern quantum electronics, interference coatings with precise optical parameters are being used more and more. The stability of such parameters during use of the coatings must satisfy quite specific requirements. Therefore, investigations to determine factors affecting this stability are of special interest. Thus, for example, it is known that optical radiation changes the sorption equilibrium of thin-film coatings [I, 2]. In their turn, sorption pro- cesses occurring in the presence of the surrounding atmosphere affect the optical properties of thin films in a number of cases [3, 4]. Therefore, the question of the direct effect of radiation on optical properties of precision coatings naturally arises. In [5], Burakov and Godlevskii give results of such investigations indicating a change of the reflection coef- ficient of a laser mirror exposed to high-power radiation of a ruby laser. In the opinion of Burakov and Godlevskii [5], the reason for this is a change of the adsorption equilibrium in the coating stimulated by the laser radiation. The present paper is also devoted to a study of the effect of radiation on the optical characteristics of thin-film coatings, in particular, on the value of their absorption coef- ficients. In the considered case, the specimens that were investigated were simultaneously exposed to radiation in two spectral ranges, one of which was in the region of transmissivity of the coating material, with the other being near or in the region of its intrinsic absorp- tion. Such a situation can occur, for example, in solid-state or gas lasers when the optical coatings are affected both by the generated radiation and by short-wave pumping radiation. In this paper, we investigated both one-layer coatings made of titanium dioxide and multilayer structures made of alternating layers of titanium dioxide and silicon dioxide. All coatings were deposited onto a fused-quartz substrate by reactive thermal evaporation. The optical absorption was measured at wavelength % = 1064 nm both in the presence of UV radiation and without it. A method of modulated laser photothermal radiometry was used to measure small absorptions in the coatings [6]. The method is based on recording of the change of the value of the integrated thermal flux from the specimen during exposure to periodically repeating pulses of laser radiation. The measurement diagram is shown in Fig. i. The radiation source with working lasing wavelength % = 1064 nm, at which the absorption was measured, was a continuous Nd-activated yttrium-aluminum garnet (YAG) laser with average radiation power P = 50 W. The specimens were additionally exposed to short-wave radiation with a PRK-4 mercury lamp and also with an LGI-21 nitrogen laser (% = 337 nm) operating in a pulsed mode with pulse repetition frequency i00 Hz and average radiation power 3 mW. In a number of cases, measurements were carried out under vacuum. For this, the speci- men was placed in a chamber consisting of a quartz flask transmissive to the radiation from the sources that were used. For withdrawal of the heat emitted by the specimen, the chamber was equipped with a germanium window. The flask was evacuated with a roughing pump. During investigations of the titanium:dioxide coatings and structures based on them, it was observed that their exposure to UV radiation with wavelength less than 400 nm led to a significant increase of the absorption at the working lasing wavelength 1064 nm, i.e., to the appearance of so-called "induced" absorption. It is significant that this effect has a reversible nature. After the UV illumination was switched off, the value of the absorption is the near-IR spectral region relaxed to its initial "dark" value. Table 1 gives the characteristic relaxation times during which the induced absorption decreased approximately threefold after the UV illumination was switched off. Of interest is the fact that for the titanium dioxide coatings this time was signifi- "Polyus" Scientificand Industrial Association, Moscow. Translated from Zhurnal Priklad- noi Spektroskopii, Voi. 53, No. 4, pp. 641-644, October, 1990. Original article submitted August 4, 1989. 0021-9037/90/5304-10995 12.50 1991 Plenum Publishing Corporation 1099