Self-Organization and Light-Induced Chirality in Azo-Glass Material
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Azobenzene derivatives have been subject to many studies in the field of all-optical surface patterning due to their unique photoresponsive properties. Photoinduced mass movement of azobenzene enables inscription of surface relief gratings (SRG) under exposure to an interference pattern of light. Interestingly SRGs can form under single beam irradiation, where no external interference pattern is applied, and thus the structures form spontaneously. Herein, Photoinduction of spontaneous SRG (SSRG) on thin films of a disperse red 1 functionalized glass-forming compound is investigated. Glass-forming small molecules yield more homogeneous samples and it allows to mitigate the impact of the rheological effects of polymers. SSRG formation is assumed to initiate from an interference pattern formed between the incident beam and scattered beams at grazing angle by surface defects. Formation of SSRG’s takes place in three stages: nucleation, growth, and saturation. Herein, the grating formation procedure is studied by monitoring the surface topology of thin films exposed to one writing beam for various periods of time, under both linear and circular polarizations, using AFM. Even in the absence of surface defects on the initial film, irradiation produces light-induced surface defects due to the reorientation and mass movement of the azo molecules. These defects act as seeds for SSRG around which gratings gradually emerge and propagate throughout the sample. To consolidate this hypothesis, the formation of gratings was studied on samples with controlled surface roughness. Pore-shaped defects do not diffract light on top of the sample, and thus have no impact on SSRG growth, while for hill-shaped defects, the growth rate decreases sharply with defect sizes larger than the writing beam wavelength. The saturation stage has been studied using different intensities of the writing beam. The rate of grating growth increases as a function of laser intensity, but the gratings saturate at the same modulation amplitude, independent of the laser intensity Two other analogous glass-forming azobenzene derivatives were studied, and in all cases, SSRG formation was correlated with the induction of birefringence in the early stages of the irradiation. The orientation of SSRGs is influenced by the polarization of the writing beam. The direction of the gratings is determined by the optical anisotropy induced by a polarized beam. Real-time anisotropy experiments are presented to predict the grating direction. Furthermore, self-organized surface relief gratings formation under different polarizations at different incident angles was studied. Oblique irradiation of a beam polarized with a combination of p- and s-polarizations yields crossed SSRG with different pitches, forming planar chiral structures. Azimuth rotations up to 10° have been observed, and the absence of 3D chirality has been confirmed. This method thus allows generating non-reciprocal planar chiral objects by a simple, single irradiation process on a thin film of a material that can easily be processed over large areas or onto small objects.
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