General Lossless Polarization and Phase Transformation Using Bilayer Metasurfaces

Metasurface optical elements enable wavefront control and polarization manipulation with subwavelength resolution. Metasurfaces made of linearly birefringent meta‐atoms such as rectangular nanoposts are commonly used to control phase and polarization, but a single layer of these meta‐atoms cannot implement transformations with elliptically polarized or circularly polarized eigenstates (i.e., chiral transformations). Here, it is shown that two cascaded metasurface layers comprising linearly birefringent meta‐atoms provide sufficiently many degrees of freedom to implement arbitrary unitary phase and polarization transformations. A systematic design method for such metastructures is described and used to design a bifocal metalens that cannot be implemented by a single layer of birefringent meta‐atoms. The presented design and implementation techniques introduce a systematic approach for realizing the most general form of lossless polarization and phase transformations using metasurfaces with high efficiency.