Unidirectional excitation of waveguide mode by optical spin-orbit couplings with on-chip nanoantenna array

Gratings are commonly used as waveguide couplers because they can provide extra momentum to assist the incident light beam to match the wave vector in the waveguides. Here, we propose two kinds of grating-like nanoantenna arrays which not only satisfies the wavevector matching condition, but also induces the conversion from the spin angular momentum to orbital angular momentum. We realize the polarization-dependent unidirectional propagations of TE0-like and TE1-like modes in a silicon-on-insulator waveguide with on-chip nanoantenna array. In addition, the maximum contrast ratio between the forward and backward transmittances is higher than 30 dB and the unidirectional optical power propagation with large coupled mode purity can be achieved. The method and the results presented in this paper might provide practical guidelines for the further development of polarization sorters, switchers, quantum information and processing.