Digitized subwavelength surface structure on silicon platform for wavelength-/polarization-/charge-diverse optical vortex generation

Abstract

Optical vortices carrying orbital angular momentum (OAM) have recently attracted increasing interest for providing an additional degree of freedom for capacity scaling in optical communications. The optical vortex generator is an essential component to facilitate OAM-enabled optical communications. Traditional devices face challenges of limited compactness, narrow bandwidth and first-order OAM modes. Here, using the direct-binary search (DBS) optimization algorithm, we design, fabricate and demonstrate a digitized subwavelength surface structure on silicon platform for wavelength-/polarization-/charge-diverse optical vortex generation. It features an ultra-compact footprint (~3.6×3.6 μm 2 ) and ultra-wide bandwidth (1480-1630 nm), supporting two polarizations and high-order OAM modes (OAM +1 , OAM -1 , OAM +2 , OAM -2 ) with high purity of ~90%. The mode crosstalk matrix is measured in the experiment with favorable performance. When generating x-pol. OAM +1 , x-pol. OAM -1 , y-pol. OAM +1 and y-pol. OAM -1 , the crosstalk of the worst case is less than -14 dB. When generating OAM +1 , OAM -1 , OAM +2 and OAM -2 , the crosstalk between any two OAM modes is less than -10 dB, and the lowest crosstalk is about -17 dB. The wavelength-/polarization-/charge-diverse optical vortex generator enables the full access of multiple physical dimensions (wavelength, polarization, space) of lightwaves. The demonstrations may open up new perspectives for chip-scale solutions to multi-dimensional multiplexing optical communications.