Light mediated emergence of surface patterns in azopolymers at low temperatures

TL;DR

This study investigates how lowering temperature affects the early stages of surface pattern formation in azopolymer thin films, revealing diffusion-driven mechanisms and the emergence of voids at very low temperatures.

Contribution

It introduces a new experimental approach to observe initial pattern formation steps and demonstrates the role of diffusion and temperature in surface relief grating development.

Findings

Pattern sub-structures grow with temperature, following diffusion behavior.

Lower temperatures slow pattern formation, enabling study of early mechanisms.

Void formation occurs at junctions at very low temperatures.

Abstract

Polymer thin films doped with azobenzene molecules do have the ability to organize themselves in spontaneous surface relief gratings (SRG) under irradiation with a single polarized beam. To shed some light in this still unexplained phenomenon, we use a new method that permits us to access experimentally the very first steps of the pattern formation process. Decreasing the temperature, we slow down the formation and organization of patterns, due to the large increase of the viscosity and relaxation time of the azopolymer. As a result decreasing the temperature allows us to access and study much shorter time scales,in the physical mechanisms underlying the pattern formation, than previously reported. We find that the patterns organize themselves in sub-structures which size increase with the temperature, following the diffusion coefficient evolution of the material. That result suggests that the pattern formation and organization is mainly governed by diffusive processes, in agreement with some theories of the SRG formation. Decreasing further the temperature we observe the emergence of small voids located at the junction of the sub-structures.