Linear and nonlinear record high optical birefringence in anisotropic van der Waals crystals

Multilayered van der Waals (vdW) materials are semiconductors composed of atomically thin crystal layers, held together by weak vdW forces. They offer unique crystal structures and electronic properties, distinct from conventional semiconductors, making them a promising platform for linear and nonlinear optics. In this context, the large refractive indexes given by highly polarizable transition metals, combined with excitonic resonances and unconventional crystalline structures, provides a toolbox for exploring non-linear physics and strong light–matter interactions with unprecedented opportunities for nanoscale optics. Recent reports highlight novel vdW materials, particularly PdPSe, a pentagonal crystal with strong nonlinear responses, and As₂S₃, a record high birefringence crystal, as favorable candidates to engineer nonlinear responses and miniaturization of optical components, owing to the combination of high refractive index and strong optical anisotropy of the underlying crystal structures. While still in its infancy, research on vdW materials promise a florid ground for fundamental studies, bridging the gap between material science and nanoscale optics.