We propose a ref lection-type all-optical switch for ultrashort pulses based on a single asymmetrically confined photonic crystal (PC) defect. By changing the absorption of the defect rather than its refractive index, we obtain modulation of ref lectivity with a large extinction ratio. In addition, the control light can be strongly localized in the defect region and completely absorbed by optimization of the absorption of the defect. More importantly, the significant broadening of the defect mode ensures perfect ref lection of ultrashort pulses. Finite-difference time-domain simulation of the switching operation of a three-dimensional PC structure for a 500-fs optical pulse is performed. Switching energy of a few femtojoules is achieved.

We present a detailed comparison of the optical properties of a quantum well (QW) and a two-dimensional (2D) quantum dot superlattice (QDSL) made of In0:4Ga0:6As on GaAs(311)B substrates under the same growth conditions. The lowest electron to hole transition energy in strained In0:4Ga0:6As/GaAs(311)B QWs is calculated. While good agreement between the calculated transition energy and the measured photoluminescence (PL) peak energy is found in the QW, the PL peak energy of the 2D QDSL significantly shifts to a lower energy with respect to the calculated transition energy for a QW with the same nominal thickness. Based on the linewidth broadening induced by increasing excitation density and temperature, the exciton-exciton and exciton-phonon interaction strengths in the 2D QDSL are found to be much larger than those in the QW. As the exciton-phonon scattering is sufficiently strong, a transition of the exciton motion from coherent to incoherent occurs.

We investigate the propagation of optical solitons through nonlinear photonic crystal (PC) waveguide bends. Our studies focus on the waveguide bends in two-dimensional PC slabs which are widely used for the manipulation of photons. It is found that optical solitons are completely destroyed when trying to pass through the conventional waveguide bend. With appropriate modifications to the bend structure, however, perfect transmission of optical solitons can be realized. The criteria for the design of waveguide bends with low reflection loss are also generalized.slan@stu.edu.cn

We investigate the coupling of two single defects in two-dimensional photonic crystals (PCs) with the same frequency but different field distributions. The defect pair like this is generally present in PCs as a combination of a reduced-size defect and an increased-size defect. In spite of the significant difference in field distribution, quasiflat impurity bands suitable for the transmission of ultrashort pulses can be achieved by properly choosing defect pairs. More importantly, the coupled cavity waveguide constructed with defect pairs offers an opportunity to establish a periodic modulation of defect modes with a control light. The dynamical band gap generated by the periodic modulation of defect modes suggests a high-efficiency all-optical switching operation in nonlinear PCs.

We investigate the transmission of ultrashort pulses through impurity band-based photonic crystal waveguides. It is found that in the general case the transmission behavior depends strongly on the pulse width with respect to the resonance linewidth in impurity bands. By controlling the configuration of the waveguides, quasiflat impurity bands can be obtained in which the dependence of transmission on pulse width is very weak. As long as the pulse width is much narrower than the bandwidth, pulses can transmit through the quasiflat impurity bands with negligible distortion and attenuation. The conditions necessary for achieving quasiflat impurity bands are derived by examining waveguides of different configurations and properties. The mechanism responsible for the formation of quasiflat impurity bands is revealed from the discussion of the symmetry of single defect and their coupling.