Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 60Defining ɛ pi the pre-irradiation light dose and ɛ int the interfering beam light dose wecan describe the photodeformation obtained in the pre-irradiation experiments of Figure3.2(c) as:a(ɛ int , ɛ pi ) = a(ɛ int , 0) − a pl (ɛ pi ) (3.3)pastel-00527388, version 1 - 19 Oct 2010where a pl (ɛ pi ) is the plastic deformation induced by the pre-irradiation. The deformationamplitude a(ɛ int , ɛ pi ) obtained under exposure to the interfering beam dose ɛ int aftera pre-irradiation dose ɛ pi is equal to the photodeformation amplitude a(ɛ int , ɛ pi = 0)obtained under exposure to the interference pattern without pre-irradiation diminishedof the plastic photoexpansion a pl (ɛ pi ) due to the pre-irradiation dose ɛ pi . By takinginto account only the saturation deformation amplitudes observed in Figure 3.2(c)we obtain the curve in Figure 3.2(d) relative to the matrix photoexpansion, whichrepresents:a(ɛ int = ∞, ɛ pi ) = a(∞, 0) − a pl (ɛ pi ) (3.4)This allows to deduce the saturation plastic deformation amplitude:a pl (ɛ pi ) = a(∞, 0) − a(∞, ɛ pi ) (3.5)which can be simply rewritten:a pl (ɛ) = a(∞, 0) − a(ɛ) (3.6)The experimental variation of a pl (ɛ) is plotted in Figure 3.3(a). By using eq. 3.2, wecan easily obtain the amplitude of the elastic part of the photodeformation:a el (ɛ) = a(ɛ) − a pl (ɛ) (3.7)In Figure 3.3(b-c) we show the variation of a el (ɛ) deduced from the experimental dataobtained with two different interference power densities, 4 mW/mm 2 and 1.5 mW/mm 2respectively.The behavior of a pl (ɛ) and a el (ɛ) indicates that the plasticity of the photodeformationgrows with the irradiation dose, reaching a maximum amplitude of about 4 nm atɛ pi ≃ 10 mJ. On the other hand, the elastic deformation amplitude exhibits a transientfeature for low doses and beyond reaches a plateau.We note that the amplitude of a el (ɛ) at saturation is slightly larger for 4 mW/mm 2irradiation power density (b) than for 1.5 mW/mm 2 irradiation power density (c). Thisis coherent with the stress-strain model for an elastic deformation. However, we are