Metasurface-based functional optical splitter for a spatially parallelized dual-polarization coherent modulator

We propose a surface-normal dual-polarization in-phase and quadrature modulator (DP-IQM) that employs a thin dielectric metasurface (MS) layer inserted on a high-speed electro-absorptive modulator array. The metasurface provides the functionalities of all the passive components necessary for a DP-IQM, including a polarization beam splitter/combiner and an interferometric circuit, to a normal-incident beam. A dielectric metasurface composed of silicon nanoposts is designed and fabricated to experimentally demonstrate polarization and beam splitting functionalities with a phase error of less than 0.08 rad and a power imbalance of less than 0.9 dB. These small errors correspond to an error-vector magnitude of less than 7% when it is used to generate dual-polarization quaternary phase-shift-keying (DP-QPSK) or 16 quadrature amplitude modulation (DP-16QAM) signals. The surface-normal configuration of our scheme allows scaling to a large two-dimensional (2D) array to generate spatially parallelized DP coherent signals for various applications, including optical communication, computing, and sensing.