Using numerical methods, we study pulse propagation near the band edge
of a one-dimensional photonic band gap material with a spatial gradia
tion in the linear refractive index, together with a nonlinear medium
response. We find that such a structure can result in unidirectional p
ulse propagation. That is, the field will be transmitted for, say, a l
eft-to-right direction of propagation, while for right-to-left nearly
complete reflection occurs. This behavior constitutes the operational
mechanism for a passive optical diode.