Light-induced spiral mass transport in azo-polymer films under vortex-beam illumination

TLDR

Light‑induced mass migration in azobenzene polymers under non‑uniform illumination produces surface relief patterns, yet the underlying mechanisms remain poorly understood. The study proposes a model explaining the phenomenon via surface‑mediated interference of longitudinal and transverse optical field components. The mechanism involves surface‑mediated interference between longitudinal and transverse components of the optical field in vortex beams, leading to spiral reliefs. Spiral relief patterns form on azo‑polymer films when illuminated with Laguerre–Gauss vortex beams, with their geometry depending on topological charge and wavefront handedness, a surprising result that could enable optical nanolithography and imaging.

Abstract

When an azobenzene-containing polymer film is exposed to non-uniform illumination, a light-induced mass migration process may be induced, leading to the formation of relief patterns on the polymer-free surface. Despite many years of research effort, several aspects of this phenomenon remain poorly understood. Here we report the appearance of spiral-shaped relief patterns on the polymer film under the illumination of focused Laguerre–Gauss beams with helical wavefronts and an optical vortex at their axis. The induced spiral reliefs are sensitive to the vortex topological charge and to the wavefront handedness. These findings are unexpected because the doughnut-shaped intensity profile of Laguerre–Gauss beams contains no information about the wavefront handedness. We propose a model that explains the main features of this phenomenon through the surface-mediated interference of the longitudinal and transverse components of the optical field. These results may find applications in optical nanolithography and optical-field nanoimaging. Non-uniform light beams can create patterns in azo-polymer films by inducing mass transport, yet the process is not well understood. Using optical vortex beams, Ambrosioet al. observe the formation of spiral patterns that are surprisingly sensitive to the optical phase, which they explain with a new model.