  We show that, a metasurface composed of subwavelength metallic slit array
embedded in an asymmetric environment can exhibit either extraordinary optical
transmission (EOT) or extraordinary optical diffraction (EOD). By employing an
analytical model expansion method and the diffraction order chart in k-vector
space, we found that the resonance decaying pathway of the local slit cavity
mode can be tuned to either 0th or -1st diffraction order by changing the
parallel wavevector, which gives rise to enhanced 0th transmission (EOT) of the
structure for small incident angles, and enhanced -1st diffraction (EOD) for
large incident angles. Based on this appealing feature, a multifunctional
metasurface that can switch its functionality between transmission filter,
mirror and off-axis lens is demonstrated. Our findings provide a convenient way
to construct multifunctional integrated optical devices on a single planar
device.
