Broadband Complete Polarization Control via Inverse-Designed Photonic Crystal Slabs

Polarization is a crucial characteristic of electromagnetic fields, and the ability to fully control it has many useful applications. While novel nanophotonic devices have been designed to achieve unprecedented capability to manipulate light on demand, their usage in the complete control of polarization states for the transmitted light has been relatively limited, and traditional design methods always produce devices with narrow operation bandwidths. In this work, a two-phase topology optimization strategy is proposed in conjunction with adjoint method to inverse design photonic crystal slabs capable of complete polarization control. It successfully produces devices operating over a broad bandwidth that is significantly larger than the current state-of-the-art designs, and their performances are also robust to material loss. This is also find that the C2v symmetry of the structure can regularize the problem, so that less simulation time and faster convergence can be obtained without compromising performance. This study demonstrates the power of the inverse design method, which can be further applied to achieve more complex polarization control and beyond. Broadband complete polarization control is achieved by inverse designed photonic crystal slabs. Through a two-phase adjoint-based topology optimization, an efficient complete polarization control device with a state-of-art broad operational bandwidth is generated. The polarization state of the output light can cover the meridian of the Poincare sphere by rotating the input linearly polarized light at normal incidence. image