Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanismsEmodintat the origin ofspeed_corrthe photo-induced deformation inefficazobenzene-containing thin filmsEmodpolsat_corr_N752field_calc_2(arbitrary units)Emodint1.511(arbitrary units)speed_corrE intS PE pol0.50.5saturation amplitudegrowth ratedeformation efficiency000 15 30 45 60 75 90phi! (deg.)pastel-00527388, version 1 - 19 Oct 2010Figure 3.13: Correlation between matter migration SRG growth and the calculatedintensity and polarization spatial modulations E int and E pol (eq. 3.27, 3.28), as a functionof the polarization axis ϕ. Calculated values are not scaled and expressed in arbitraryunits. SRG growth is described by several parameters calculated in Figure 3.9:saturation amplitude (✕), computed as the maximum SRG amplitude normalized withrespect to a fixed arbitrary value (150 nm); SRG growth rate (□) measured as the slopeof the SRG growth over the first 100 s and normalized with respect to the growth rateobserved at ϕ = 45 ◦ ; average normalized deformation efficiency ().its maximum at ϕ = 45 ◦ , extremely close to the maximum of E pol at ϕ = 47.5 ◦ , wherethe interference field is a pure polarization grating.This observation explains the asymmetry between the matter migration observed unders-polarized and p-polarized interference patterns, with respect to the symmetric behaviorobserved for 30 ◦ -60 ◦ and 15 ◦ -75 ◦ configurations. Even if complementary, s and p arenot symmetric from the point of view of E pol , since E pol = 0 in the first case whileE pol = 0.37 in the second one. In this sense, a polarization configuration symmetric tos should have a E pol = 0 as well. This could be obtained, for example, with a light fieldhaving only an ⃗ E ‖ component and no ⃗ E ⊥ component. Unfortunately this configurationwould require p-polarized interfering beams in normal incidence (θ = 0 ◦ ), hence inducingan infinite spatial period interference pattern.3.3.3 DiscussionThe experiments performed with the superimposition of a polarization and an intensitygratings of variable amplitude show that the SRG formation is mainly governed by thespatial modulation of the polarization. This is clearly seen when comparing the variationof the polarization spatial modulation with the SRG formation efficiency as a functionof ϕ (Figure 3.13).