Measuring the Topological Aberration of Optical Vortices

When a high-order optical vortex reflects at an interface, it splits into a wavelength-scale constellation of unit vortices, whose geometry depends on the angular derivatives of the reflection coefficient. The effect, known as topological aberration (TA) [1], stems from the same physical mechanism as the so-called Goos-Hänchen and Imbert-Fedorov beam shifts [2]. Although conceptually clear, measuring TA is an intricate task, given the magnitude of the singularity splitting and the practical impossibility of preparing a high-order vortex to begin with [3]. Here, we propose a novel method to measure topological aberrations using real (imperfect) vortices, based on comparing the orbital angular momentum (OAM) spectrum of the input and reflected fields. We demonstrate the method experimentally using a second-order optical vortex and a Kretschmann-Raether device [4] to enhance topological aberrations, leading to the first experimental observation of the phenomenon.