Solving an old puzzle: fine structure of diffraction spots from an azo-polymer surface relief grating

TL;DR

This study combines experimental and theoretical methods to analyze the diffraction patterns from azo-polymer surface relief gratings, revealing detailed insights into the diffraction spot structure and its evolution during inscription.

Contribution

It introduces a novel in situ setup for real-time measurement of diffraction efficiency and spot structure during grating inscription in photo-responsive polymers.

Findings

Diffraction efficiency follows a Bessel function over time.

Probe beam size influences diffraction pattern interpretation.

Ring-shaped features appear in diffraction spots after initial maxima.

Abstract

We report on the experimental and theoretical interpretation of the diffraction of a probe beam during inscription of a surface relief grating with an interference pattern into a photo-responsive polymer film. For this we developed a set-up allowing for the simultaneous recording of the diffraction efficiency (DE), the fine structure of the diffraction spot and the topographical changes, in situ and in real time while the film is irradiated. The time dependence of the DE, as the surface relief deepens, follows a Bessel function exhibiting maxima and minima. The size of the probe beam relative to the inscribed grating (i.e., to the size of the writing beams) matters and has to be considered for the interpretation of the DE signal. It is also at the origin of a fine structure within the diffraction spot where ring-shaped features appear once an irradiation time corresponding to the first maximum of the DE has been exceeded.