  Nonreciprocal electromagnetic devices play an important role in modern
optical and microwave technologies. Conventional methods for realizing such
systems are incompatible with integrated circuits. With recent advances in
integrated photonics, the need for efficient on-chip magnetless nonreciprocal
devices is more urgent than ever. This paper leverages space-time engineered
asymmetric photonic bandgaps to generate optical isolation. It is shown that a
properly designed space-time modulated slab is highly reflective/transparent
for opposite directions of propagation. The proposed method requires a low
modulation frequency, is magnetless and can achieve very high isolation levels.
Experimental proof of concept at microwave frequencies is provided.
