  We present a (nongyrotropic) metasurface magnetless specular isolator. This
device reflects as a mirror a wave incident under a specified angle in one
direction and absorbs it in the opposite direction. The metasurface is
synthesized in terms of bianisotropic susceptibility tensors, whose
nonreciprocity resides in normal components and exhibits a hybrid electric,
magneto-electric nature. The metaparticle is implemented in the form of a
U-shaped conducting structure loaded by a transistor. The operation principle
of the specular isolator is demonstrated by both full-wave simulation and
experiment, with isolation levels reaching 41 and 38 dB respectively. This
system represents the first realization of a metasurface involving
nonreciprocal normal susceptibilities and features a previously unreported type
of nonreciprocity.
