  Two-dimensional photonic circuits with high capacity are essential for a wide
range of applications in next-generation photonic information technology and
optoelectronics. Here we demonstrate a multi-channel spin-dependent photonic
device based on a twinning crystal metamaterial. The structural symmetry and
material symmetry of the twinning crystal metamaterial enable a total of 4
channels carrying different transverse spins because of the spin-momentum
locking. The orientation of the anisotropy controls the propagation direction
of each signal, and the rotation of the E-field with respect to energy flow
determines the spin characteristics during input/output coupling. Leveraging
this mechanism, the spin of an incident beam can be maintained during
propagation on-chip and then delivered back into the free space, offering a new
scheme for metamaterial-based spin-controlled nano-photonic applications.
