On the Inscription of Period and Half-Period Surface Relief Gratings in Azobenzene-Functionalized Polymers Anna Sobolewska and Andrzej Miniewicz* Institute of Physical and Theoretical Chemistry, Wroclaw UniVersity of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ReceiVed: January 3, 2008; In Final Form: February 14, 2008 Laser-light-induced surface relief grating inscription was carried out in the newly synthesized azobenzene- functionalized poly(amide-imide)s having the same main- and side-chain structures but different substituents in the azobenzene groups. The gratings were inscribed employing the two-wave mixing technique with linearly polarized laser beams. Three different polarization configurations were used: s-s, p-p, and s-p. The relatively deep surface relief gratings of period Λ were formed for the case of s-s and p-p polarizations, whereas the s-p inscription resulted in the half-period grating (Λ/2) with the weak surface modulation. The origin of the formation of Λ/2 structure for s-p configuration results from the interference between zeroth- and first-order beams scattered on the polarization refractive index grating and having the same polarization. On the basis of this idea, we presented the simple kinetic model predicting and modeling the half-period grating formation with its temporal evolution. The proposed model is consistent with the experimental findings. 1. Introduction Surface relief gratings (SRGs), holographically inscribed on amorphous azopolymer films, have been extensively investigated 1-18 since the first reports announced independently by two research groups of Rochon and co-workers 19 and Tripathy et al. 20 in 1995. They were fabricated in azopolymers using two interfering coherent laser beams, without any pre- and/or postprocessing procedure, and were inscribed at room temperature, i.e., well below the glass transition temperature (T g ) of these polymers. 4,5 The basic phenomenon underlying the SRGs formation in azobenzene-functionalized polymers is the ability of azobenzene chromophores to undergo many successive reversible trans- cis-trans photoisomerization cycles which for linearly polarized light results in the permanent molecular reorientation of trans molecules perpendicular to the polarization plane. The chro- mophore reorientation is linked with a macroscopic polymer chain migration which is observed as a free surface modulation. The large modulation depth surface relief gratings were suc- cessfully inscribed on azo-functionalized polymer thin films. 2,6,18 Because of the hundreds of nanometers deep surface modulation, the very high light diffraction efficiency could be observed in gratings inscribed in azopolymers. During holographic recording, prior to the surface relief grating formation, the amplitude and the phase gratings arise in azopolymers because of the volu- metric absorption coefficient and refractive index modulations, respectively. 3,6,13,21-23 The light diffraction efficiencies on these last two gratings are usually small when compared to the diffraction efficiency related to the surface relief grating. A lot of work devoted to the SRGs characterization has been done. 2,24-28 Scientists considered possible mechanisms of the surface relief gratings formation and pointed out their potential applications. Main models explaining the origin of the surface relief gratings formation under spatially sinusoidal illumination are listed below: (1) The free volume model proposed by Barrett and co- workers 2,24 in which the driving force responsible for the mass transport was assumed to arise from pressure gradients induced by photoisomerization of azobenzene groups. Resulting vis- coelastic flow of the material from the high-pressure to the low- pressure areas leads to the sinusoidal SRGs and was modeled using the Navier-Stokes equation. 24 (2) The field gradient force model proposed by Tripathy et al. 6,25 is based on the forces originating from the optically induced electric field gradient. The movement of polymer chains depends on the spatial variation of the material susceptibility (due to light-induced birefringence and dichroism), the optical field, and the field gradient along the grating wave vector K. This model nicely explains the light polarization dependence of the surface relief formation. 6 (3) The asymmetric diffusion model proposed by Lefin and co-workers 26,27 relates SRG formation with the creation of concentration gradients. The essential feature of the model is that dye-molecules undergo a 1-D random walk along their photoexcitation direction, thus inducing a net flux of molecules out of illuminated regions toward the darker regions. (4) The mean-field theory proposed by Pedersen 28 assumes that chromophores are subjected to anisotropic intermolecular interactions. The mean field of oriented by light chromophores tend to align other chromophores in the same direction and cause an attractive force between side-by-side chromophores oriented in the same direction causing their order and aggregation. This mechanism properly describes the surface relief formation in side-chain azobenzene liquid crystalline polymers, which show an in-phase SRGs; it means that surface profile maxima are coincident with the maxima of the light intensity pattern. It should be pointed out that such behavior is in contrast to amorphous polymers, where the reverse effect is observed; i.e., the light intensity maxima correspond to surface minima. In this paper we report on a surface relief gratings inscription for three different types of linear polarization configurations: s-s, p-p, and s-p using a two-wave mixing setup. The * Corresponding author: phone (048) 71-320-35-00, fax (048) 71-320- 33-64, e-mail andrzej.miniewicz@pwr.wroc.pl. 4526 J. Phys. Chem. B 2008, 112, 4526-4535 10.1021/jp800048a CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008