Encircling exceptional points in a Riemann sphere for efficient asymmetric polarization-locked devices

Abstract: Dynamically encircling exceptional points (EPs) in two-dimensional Hamiltonian parameter space has enabled intriguing chiral dynamics in which the final state of the system depends on the encircling direction. Here, we show that full Hamiltonian parameter space can be described in a Riemann sphere, and those points on the parameter space boundary with the eigenstates of the system Hamiltonian being convergent, converge to the north vertex. We present that encircling one EP on the Riemann sphere leads to chiral response, and a continuous encircling trajectory passing through the north vertex can realize near-unity asymmetrical transmission. An asymmetric polarization-locked devices are designed by mapping the encircling path onto the L-shaped silicon waveguides. We experimentally demonstrate near 100% asymmetrical polarization conversion efficiency between TE and TM modes with the mode crosstalk below -20 dB at 1550 nm. Our results bring the study of EP-associated dynamics into the realm of highly-efficient asymmetrical polarization conversion and power up new application opportunities for EP physics.