Table 2. TALYSTEP Surface Roughness [nm] of Tio, Single Layers Table 3. Summary of Surface Roughness Measurements on RIPD Thin Films Manuscript Received Measurement of Thermal Expansion Coefficients of Optical Thin Films Wu Zhouling Tang Jinfa Shi Baixuan Zhejiang University, Hangzhou, PRC be Thermal expansion coefficients of optical thin films can to Key Words: optical thin films; photothermal deflection; thermal ex- 1. Introduction Precise measurements of thermophysical properties like thermal conductivity and thermal expansion coefficient of optical thin films are very important for thermal modeling of laser-induced damage to optical coatings [1,2]. In this paper, we report our recent measurements of thermal expansion coefficients of optical thin films by means of the combination of PDOBD [3,4] and TPOBD [5-8]. The experimental results show that this method is quite verstile and very simple. 2. Experimental Procedure Based on PDOBD and TPOBD, we have built up an apparatus for the measurement of thermal expansion coefficients of optical thin films. Details of the detection system are given in figure 1. PDOBD was firstly reported by M.A.Olmstead et al [3] in 1983. The physical basis of it was shown in figure 2, and it can be briefly described as follows: when a solid sample is illuminated by an intensity-modulated light beam, the absorption of the energy will cause a thermal wave inside the sample and hence a corresponding displacement on sample surface. By probing this corresponding surface displacement with a second laser beam, one can relate its deflection to the related properties of the sample. TPOBD was firstly developed by A. C. Boccara et al [5] in 1980, and its physical basis was shown in figure 3. Defferent from PDOBD, here the reason for the optical beam deflection is the photothermal gradient index. Optical thin films are usually thermally thin when the modulation frequency is relatively low [7]. In this case, the PDOBD signal S1 [3,4] and TBOBD signal S2 [5-8] satisfy and where C1, C2---constants, decided by experimental parameters; ---thermal expansion coefficient of the sample; 1 Р ---incident power of the pump laser; R reflectance of the sample. From eq. (1) and eq. (2) we can get with ease (1) (2) where C is a constant dependent on experimental conditions and the properties of the substrate of the sample. From eg. (4) we can see that the ratio of PDOBD signal over TPOBD signal is proportional to the thermal expansion coefficient of an optical thin film under the thermally-thin-sample approximation [7]. This is in fact the basis of our measurements of thermal expansion coefficients of optical thin films. To caliberate the experimental system, we use Au film as caliberating sample, of which the thermal expansion coefficient was already known. The principle of this caliberation method is as follows: Equation (4) is also valid for caliberating sample if the substrate and Table 1 showed our measured thermal expansion coefficients of several single-layer films and their comparison with those of previous work and related bulk materials. From this table we can find: (1). Except that of the relatively unstable ZrO2 single layer film, our results are in good agrement with those previousely reported. This fact approves the applicability of the combination of PDOBD and TPOBD in the measurements of thermal expansion coefficients of optical thin films. (2). Except that of SiO2 single layer film, the thermal expansion coefficients of the thin films investigated showed apparent differences from those of the related bulk materials. A possible explanation of these differences might be changes in structures of the materials after vacuum deposition. 4. Discussion 4.1 Repeated Precision of the Experimental Setup Ten measurements of Tio, film showed an average of 2.2x10-6 deg-1 and a maximum deviation of 0.3x10-6deg-1. This corresponds to a relative error smaller than 14%. 4.2 Limits of Our Method in the Measurement of 4.2.1 Film Thickness Limit Because of the thermally-thin-samle approximation, our method is only applicable to samples with 1<<(10-100) um, which, fortunately, can be satisfied in most optical thin films. 4.2.2 Optical Absorption Limit |