Near-unity efficiency optical vortex generation in van der Waals materials

TL;DR

This paper demonstrates near-unity efficiency in optical vortex generation using van der Waals crystals and Bessel beams, surpassing previous theoretical limits and enabling compact, fabrication-free vortex devices.

Contribution

The study introduces a method employing Bessel beams and vdW crystals to achieve high-efficiency optical vortex generation, with experimental validation of efficiency up to 0.82.

Findings

Achieved up to 0.82 conversion efficiency in hBN crystals.

Bessel beams outperform Gaussian beams in vortex generation efficiency.

Conversion efficiency depends on the numerical aperture of the lens.

Abstract

Optical spin-orbit coupling provides a promising, fabrication-free route for developing ultra-compact optical vortex generators. However, the conversion efficiency has been theoretically limited to 0.5. Here, we demonstrate enhanced vortex generation efficiency by employing a Bessel beam as the input and propagating it through van der Waals (vdW) crystals. The large birefringence of vdW crystals and the single transverse wave vector of a Bessel beam allow a near unity spin-orbit conversion efficiency and a topological charge transition of $\ell \rightarrow \ell + 2$. Through combined analytical and experimental investigations, we demonstrate a conversion efficiency of up to 0.82 in hexagonal boron nitride (hBN) crystals with a thickness of $27.4\,\mu\mathrm{m}$. The higher efficiency of Bessel input beams over Gaussian beams is attributed to their distinct transverse wave vector distribution of constituent plane wave components. Furthermore, we demonstrate the dependence of conversion efficiency on the numerical aperture (NA) of the objective lens, which is in good alignment with theoretical predictions. These demonstrations provide a fabrication-free route to highly efficient optical vortex generation via microscale vdW materials platforms.