All-optical diodes based on photonic crystal molecules consisting of nonlinear defect pairs

We investigate the unidirectional transmission behavior of photonic crystal (PC) molecules consisting of nonlinear defect pairs by use of the coupled mode theory and the finite-difference time-domain technique. As compared with the all-optical diodes based on single asymmetrically confined PC defects (or PC atoms), a significant enhancement in the transmission contrast is achieved. A qualitative comparison of the dynamical response to external excitation is carried out for nonlinear PC atoms and molecules and the physical origins responsible for the enhancement of transmission contrast are clarified. In addition, the tolerance of the resulting optical diodes against the fabrication error is discussed. Furthermore, it is revealed that the figure of merit, which is defined as the product of the threshold transmission and the transmission contrast, can be greatly improved by dropping the use of air defects. Some abnormal and intriguing transmission behaviors are observed in the PC molecules without using air defects. (c) 2006 American Institute of Physics.