Photonic band gaps can be generated in the impurity bands of photonic crystals formed by periodically placed defects. Even slight periodic modulation of the properties of these defects can open up very deep band gaps in the impurity bands. This phenomenon originates from the concentration of electromagnetic field at the defect regions, making electromagnetic wave extremely sensitive to the small changes of the defects. A dynamical photonic band gap in the impurity band created by a control light, provides a mechanism for constructing high-efficiency optical switches.

We investigate the response of photonic crystal (PC) atoms with Kerr nonlinearity to the excitation of ultrashort optical pulses. Surprisingly, it is found that PC atoms with sharp resonant peaks in frequency spectrum (or with large quality factors) can respond to ultrashort pulses very rapidly when Kerr nonlinearity is introduced. More interestingly, the response time under high-power densities can be even shorter than the duration of the pump pulse. It is revealed that the nonlinearity induced broadening of dynamical frequency spectrum is responsible for the ultrafast response time, implying the possibility of ultrafast all-optical switching.

We report on the realization of two-dimensional (2D) In0:4Ga0:6As/GaAs quantum dot superlattices (QDSLs) by selforganized epitaxial growth. The conditions for the formation of extended states or minibands are analyzed by treating QD arrays as disordered systems. Ordered quantum dot (QD) arrays are fabricated on GaAs (311)B substrates. High density and small size are achieved by decreasing the growth temperature. A large red shift of the photoluminescence (PL) peak energy and a dramatic narrowing of the linewidth are found when the dots become smaller and closer. The exciton coherence length in the high-density ordered QD array is confirmed to be much larger than the QD diameter by PL decay time measurements and by using beryllium impurities as scattering centers. As a comparison, the incoherent exciton motion dominated by nonresonant tunneling is discussed. The transition from coherent to incoherent, including the intermediate state, and the localization of excitons are demonstrated by various mechanisms.

We investigate the dynamics of nonlinear photonic crystal sPCd atoms by use of numerical simulations in a pump-probe scheme based on a finite-difference time-domain technique. A two-dimensional nonlinear PC atom with multimode is intentionally employed for the numerical simulations. The use of the multimode PC atom makes it possible that the pump and probe waves are set at different frequencies, ensuring the clear identification of the dynamics of the nonlinear PC atom. The physical model for the dynamics of nonlinear PC atoms has been established and very good agreement between the analytical and simulation results has been achieved.

Formation of extended states or minibands in two-dimensional (2D)In0.4Ga0.6As/GaAs (311) Bquantum dot superlattices (QDSLs) is directly demonstrated in time-resolved photoluminescence measurements. At a low excitation density of 1W/cm2, photoluminescence transients with; 15 ps rise time and; 25 ps decay time are observed. Both rise and decay times are found to increase with increasing excitation density. The excitons in 2D QDSLs exhibit different relaxation and recombination behaviors as compared to those in quantum wells and quantum dots. A physical model treating 2D QDSLs as disordered systems containing localized and extended states can successfully interpret all of the experimental observations.