  A novel concept for the design of nonlinear optical diodes is proposed which
uses the multistability of coupled nonlinear microcavities and the dependence
of switching thresholds on the direction of incidence. A typical example of
such diode can be created by combining two mirror symmetric microcavities where
modes of the opposite parity dominate. It is shown that a strong nonreciprocal
behavior can be achieved together with a negligible insertion loss. To describe
the dynamical properties of such systems, a model based on the coupled mode
theory is developed, and a possible implementation in the form of multilayered
structures is considered.
