Formation of half-period surface relief gratings in azobenzene containing polymer films
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Formation of half‑period surface relief gratings in azobenzene containing polymer films Joachim Jelken1 · Carsten Henkel1 · Svetlana Santer1 Received: 30 March 2020 / Accepted: 3 August 2020 © The Author(s) 2020
Abstract We study the peculiar response of photo-sensitive polymer films irradiated with a certain type of interference pattern where one interfering beam is S-polarized, while the second one is P-polarized. The polymer film, although in a glassy state, deforms following the local polarization distribution of the incident light, and a surface relief grating (SRG) appears whose period is half the optical one. All other types of interference patterns result in the matching of both periods. The topographical response is triggered by the alignment of photo-responsive azobenzene containing polymer side chains orthogonal to the local electrical field, resulting in a bulk birefringence grating (BBG). We investigate the process of dual grating formation (SRG and BBG) in a polymer film utilizing a dedicated set-up that combines probe beam diffraction and atomic force microscopy (AFM) measurements, and permits acquiring in situ and in real-time information about changes in local topography and birefringence. We find that the SRG maxima appear at the positions of linearly polarized light (tilted by 45° relative to the grating vector), causing the formation of the half-period topography. This permits to inscribe symmetric and asymmetric topography gratings with sub-wavelength period, while changing only slightly the polarization of one of the interfering beams. We demonstrate an easy generation of sawtooth profiles (blazed gratings) with adjustable shape. With these results, we have taken a significant step in understanding the photo-induced deformation of azo-polymer films.
1 Introduction Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00340-020-07500-w) contains supplementary material, which is available to authorized users. * Svetlana Santer santer@uni‑potsdam.de 1
Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany
Polymer films with embedded azobenzene chromophores have attracted broad interest due to their ability to convert optical energy into mechanical deformations of large magnitude (up to few hundred nanometers) [1–3]. One of the peculiar examples here is the formation of a surface relief grating (SRG) in thin polymer films irradiated with a holographic
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interference pattern (IP) of visible light [4–7]. Although the polymer film is in a solid, glassy state, it deforms following the intensity and/or polarization pattern of the incident light [8]. The mechanism of the SRG inscription involves internal, opto-mechanical stresses, which are quite high (100 MPa up to ~ 1 GPa) to deform a material of Young modulus up to several GPa [3, 9–12]. The appearance of internal stresses is initiated by photo-induced isomerization cycles between the trans and cis states of th
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