Giant optical spin-orbit interactions in ferroelectric van der Waals waveguides

TL;DR

This paper demonstrates that highly birefringent ferroelectric van der Waals waveguides enable giant optical spin-orbit interactions, allowing for dense, chip-scale polarization control and beam steering in integrated photonics.

Contribution

The study introduces highly anisotropic vdW waveguides, particularly NbOI2, as a platform for strong optical spin-orbit interactions and nonlinearities, advancing integrated opto-spintronic technology.

Findings

Giant optical spin-splitting observed via the optical spin Hall effect.

Spatial separation of optical spin currents on sub-micrometer scales.

Empirical scaling law linking dielectric anisotropy to geometric spin-splitting.

Abstract

Optical spin-orbit interactions (SOI) link photonic spin to momentum, offering a route toward on-chip polarization control and beam steering. Nevertheless, achieving sufficient optical SOI and nonlinearities on sub-micrometer scales - a prerequisite for dense photonic integration - remains an outstanding challenge. Here, we show that highly birefringent van der Waals (vdW) waveguides provide an ideal, chip-compatible platform to address this limitation. We focus on the ferroelectric semiconductor NbOI2, which exhibits record optical nonlinearities and dielectric anisotropy. Using femtosecond optical microscopy, we image light propagation and harmonic conversion beyond the total internal reflection barrier over tens of micrometers in NbOI2 slab waveguides. We report giant optical spin-splitting through the optical spin Hall effect, which facilitates spatial separation of optical spin currents on sub-micrometer scales, in quantitative agreement with a microscopic light-matter interaction model. We further leverage optical spin-momentum locking to realize polarization-controlled waveguide steering. We generalize these observations across various vdW waveguides and empirically confirm a scaling law linking dielectric anisotropy to geometric spin-splitting. Our results establish highly anisotropic vdW waveguides as an ideal platform for densely integrated opto-spintronic technologies.