  Metasurface optical elements enable wavefront control and polarization
manipulation with subwavelength resolution. Metasurfaces made of linearly
birefringent meta-atoms such as rectangular nano-posts 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.
