OPTICAL BISTABILITY AND PHASE-TRANSITIONS IN A DOPED PHOTONIC BAND-GAP MATERIAL

We discuss the nonlinear response of impurity two-level atoms in a pse udophotonic band gap (PBG) to an applied laser field. It is shown that in the case when the variance of resonant dipole-dipole interaction ( RDDI) is much larger than its average value and the spontaneous emissi on rate, a nonequilibrium second-order phase transition occurs when th e applied field strength parameter exceeds the variance of RDDI. This situation arises when the atomic density is low and the resonance freq uency is near the center of a wide PBG. At this threshold the system c hanges from glassy phase to ferroelectric phase. In the case when the average value of RDDI is larger than its variance and spontaneous emis sion decay rate, this phase transition becomes first order, leading to optical bistability. This situation arises when the atomic density is high or when the photon localization length within the PBG extends ov er many optical wavelengths. The influence of RDDI fluctuation on bist ability is discussed. These results suggest that disordered, impurity- doped, PBG materials may exhibit very low threshold switching properti es.