Pure Dielectric Waveguides Enable Compact, Ultrabroadband Wave Plates

The ability to manipulate polarization of light on a chip is of fundamental importance for many applications. Here, we demonstrate an ultrabroadband quarter-wave plate for silicon photonics based on pure dielectric waveguides. The concept of birefringence is used to introduce a phase lag of π/2 between two orthogonally polarized modes in nanophotonic waveguides, such that a quasi-linearly polarized mode is converted into a quasi-circularly polarized mode, which is accompanied with a longitudinal optical vortex component due to the spin-orbit interaction. Our device is ultracompact (2.3 μm) with low insertion losses (around 0.05 dB), and it allows for an ultrabroad operation bandwidth of 280 nm around 1.55 μm. We also propose a half-wave plate structure, which can either serve as a chiral converter for quasi-circularly polarized modes or a polarization rotator for quasi-linearly polarized modes. These results may find important applications in many fields, such as integrated quantum computing and polarization handling on a chip.