Inverse designed free-form chiral metasurfaces

In this work, we proposed a modified inverse design algorithm that perform the adjoint-based optimization involving all components of the Jones matrix. In the modified algorithm, the polarization basis vectors constructing the Jones matrix has been expanded from linearly basis vectors to arbitrary basis vectors covering the entire Poincaré sphere. By further incorporating Max-min multi-wavelength optimization, we developed high extinction ratio, broadband, and high efficiency freeform chiral metasurface. The optimized two-dimensional chiral metasurface shown a flat-top bandwidth efficiency of over 98% within a 50nm broadband range. We also designed three chiral metasurfaces with
$\psi=0$
, and
$\chi=45,30$
, and 20, representing ellipticity parameters, and these metasurfaces achieved polarization dichroism up to 80% within the broadband range of 800-1000nm. Moreover, we further realized the polarization conversion for arbitrary non-orthogonal basis vectors across a broad bandwidth, and the polarization conversion efficiency reached 95%. Our work may promote the development of high performance compact polarization optical elements for advanced polarization and chirality related applications.