Unraveling the nucleation and growth of spontaneous surface relief gratings

Abstract

Nucleation and growth of spontaneous surface relief gratings (SSRGs) on a Disperse Red 1 (DR1) glass-forming derivative were investigated. No interference pattern is applied and surface patterning is induced using single-beam irradiation: the gratings are self-organized. Grating growth is assumed to initiate from an interference pattern formed between the incident light beam and waves scattered at grazing angle by surface defects. However, the mechanism is not yet fully understood and there is not a comprehensive explanation of the structure formation process. 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, growth rate decreases sharply with defect sizes larger than the writing beam wavelength. 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.