Generating optical vortex beams by momentum-space polarization vortices centered at bound states in the continuum

TL;DR

This paper introduces a novel method to generate optical vortex beams using polarization vortices at bound states in the continuum within photonic structures, simplifying fabrication and enabling high-order vortex generation at any wavelength.

Contribution

The paper proposes and experimentally demonstrates a momentum-space approach to generate optical vortices leveraging polarization topology at BICs, expanding topological photonics applications.

Findings

Successfully generated optical vortices using momentum-space polarization vortices.

The method simplifies fabrication and alignment of vortex beam generators.

Any even order of optical vortex can be achieved at arbitrary wavelengths.

Abstract

An optical vortex (OV) is a beam with spiral wave front and screw phase dislocation. This kind of beams is attracting rising interest in various fields. Here we theoretically proposed and experimentally realized a novel but easy approach to generate optical vortices. We leverage the inherent topological vortex structures of polarization around bound states in the continuum (BIC) in the momentum space of two dimensional periodic structures, e.g. photonic crystal slabs, to induce Pancharatnam-Berry phases to the beams. This new class of OV generators operates in the momentum space, meaning that there is no real-space center of structure. Thus, not only the fabrication but also the practical alignment would be greatly simplified. Any even order of OV, which is actually a quasi-non-diffractive high-order quasi-Bessel beam, at any desired working wavelength could be achieved in principle. The proposed approach expands the application of bound states in the continuum and topological photonics.