Chuanbo Dong

Wideband and Low Dispersion Slow Light in Lattice-Shifted Photonic Crystal Waveguides

We propose a new type of photonic crystal waveguide structure to achieve wideband slow light with large group index and low dispersion. The waveguide is based on triangular lattice photonic crystal imposed by simply a selective altering the locations of the holes adjacent to the line defect. Keeping the group index at 31, 44, 61, 71, 94, and 132, respectively, while restricting its variation within a ±10% range, we accordingly attain an available bandwidth of 19.1, 13.0, 9.0, 7.5, 5.5, and 3.6 nm around 1550 nm. The normalized delay-bandwidth product keeps around 0.35 for all cases. Low dispersion slow light propagation is confirmed by studying the relative temporal pulsewidth spreading with the 2-D finite-difference time-domain method.

Phase shift multiplication effect of all-optical analog to electromagnetically induced transparency in two micro-cavities side coupled to a waveguide system

We propose phase shift multiplication effect of all-optical analog to electromagnetically induced transparency in two photonic crystal micro-cavities side coupled to a waveguide system through external optical pump beams. With dynamically tuning the propagation phase of the line waveguide, the phase shift of the transmission spectrum in two micro-cavities side coupled to a waveguide system is doubled along with the phase shift of the line waveguide. π-phase shift and 2π-phase shift of the transmission spectrum are obtained when the propagation phase of the line waveguide is tuned to 0.5π-phase shift and π-phase shift, respectively. All observed schemes are analyzed rigorously through finite-difference time-domain simulations and the coupled-mode formalism. These results show a new direction to the miniaturization and the low power consumption of microstructure integration photonic devices in optical communication and quantum information processing.

All-optical switching based on the electromagnetically induced transparency effect of an active photonic crystal microcavity

We report a new all-optical switching in a two-dimensional photonic crystal microcavity made of semiconductor multiple quantum wells and realized based on the electromagnetically induced transparency effect with exciton and two-exciton energy levels. We use the quantum coherence effects to achieve small absorption of the probe field, and the absorption of the probe field can be adjusted by controlling the pump field and decay rate. We turn the control field into pulses of light field so that we can regulate the efficiency of the switch. Through selecting the appropriate control light field intensity, we can obtain a switching efficiency of 85% and a switching time is 10 ps. This result can be used for the development of new types of nanoelectronic devices for realizing the switching process.

Modified Drude model for free-carrier absorption due to bound-to-continuum transition in quantum-dot semiconductor optical amplifier

Within the framework of the second-order perturbation theory, a model is presented to calculate the cross-section of the free-carrier absorption (FCA) due to phonon-assisted bound-to-continuum transition in quantum-dot semiconductor optical amplifier (QD-SOA). It is shown that the cross-section of the FCA in QD layer is much larger than that in bulk system when the photon energy is in the vicinity of the carrier binding energy. Conversely, when the photon energy is very large, the coefficients of the FCA for QD system and bulk system are on the same level of magnitude. And these conclusions agree well with the classical theoretical model and experimental results reported previously. In order to develop a simple and exact model to predict the FCA due to bound-to-continuum transition, a modified Drude model is proposed. The modified model contains the carriers confinement energy between the quantum dot layer and the barrier layer. The results show that the absorption cross-section calculated from first pri...

Reduction of Pure Dephasing Rates of Excitons by Population Decay in Quantum-Dot Semiconductor Optical Amplifiers

Population decay (real transition) and pure dephasing (virtual transition) determine the dephasing rates of excitons in quantum-dot (QD) semiconductor optical amplifiers. In previous theoretical studies, population-decay and pure dephasing are usually handled separately. In this paper, the theory of exciton dephasing in an InAs/GaAs QD semiconductor optical amplifier is presented to elucidate the combination of population decay and pure dephasing and determine the total dephasing rate. The proposed model extends the classical independent boson model to include the effects of real transition. Results show that population decay decreases the pure dephasing rate and imply that the former is directly coupled to pure dephasing despite their differences. Our theoretical results are consistent with the reported experimental phenomena.

THEORETICAL STUDY OF TRANSMISSION SPECTRUM IN CAVITY-WAVEGUIDE SIDE-COUPLED SYSTEMS

Starting with one-cavity side-coupled with waveguide system, we theoretically study on transmission spectrum for a two-cavity side-coupled with waveguide system based on coupled mode theory by using light-tracing method. Interesting effects called all-optical analog to EIT appear in some cases. 3D finite-difference time-domain (FDTD) method provides optical modes distributions at different wavelengths, including the EIT-like wavelength. Influences of different parameters on the narrow EIT-like peak such as cavity quality factors Q, frequency difference detuning Δω, and phase difference detuning between cavities ΔΦ are discussed. Compared with previous studies, this method is easier to understand and can be used as a model for both standing cavities like line-defect photonic crystal cavities and whispering gallery modes cavities like microring cavities.

Direct and indirect intraband optical absorption due to carrier transitions from discrete levels to continuum states in quantum dot system

Within the framework of perturbation theory, a model is presented to calculate the direct and indirect intraband optical absorption cross-section due to carrier transitions from the quantum dot (QD) level to the continuum states. The results indicate that the phonon-assisted transition process dominates the total intraband optical transition process for large photon energies, and that the amplitude of the absorption cross-section for the indirect process is temperature dependent, while it is dependent upon the QD size for the direct process. Our theoretical model may prove useful for improving the performance of optoelectronic devices based on QD lasers or amplifiers.