  The polarization of optical fields is a crucial degree of freedom in the
all-optical analogue of electromagnetically induced transparency (EIT).
However, the physical origins of EIT and polarization induced phenomena have
not been well distinguished, which can lead to confusion in associated
applications such as slow light and optical/quantum storage. Here we study the
polarization effects in various optical EIT systems. We find that a
polarization mismatch between whispering gallery modes in two indirectly
coupled resonators can induce a narrow transparency window in the transmission
spectrum resembling the EIT lineshape. However, such polarization induced
transparency (PIT) is distinct from EIT: it originates from strong polarization
rotation effects and shows unidirectional feature. The coexistence of PIT and
EIT provides new routes for the manipulation of light flow in optical resonator
systems.
