Sheng Lan

Design of impurity band-based photonic crystal waveguides and delay lines for ultrashort optical pulses

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.

Leveraging deep photonic band gaps in photonic crystal impurity bands

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.

Optical characterization of photonic crystal delay lines based on one-dimensional coupled defects

We performed optical characterizations of optical delay lines based on photonic crystal waveguides. The delay lines were composed of cylindrical air holes in silicon-on-silicon-dioxide ridge waveguides, defects were periodically created by means of increasing the separation of two neighboring air holes, and the structure was designed to have flat-transmittance impurity band. We clearly observed an impurity band with a bandwidth of ~30 nm and a maximum transmittance of ~0.5 at an operating wavelength of ~1.55 microm . A 600-fs delay was confirmed with a 20-microm -long delay line. A good agreement was confirmed between the experimental results and the calculations.

Broadband waveguide intersections with low cross talk in photonic crystal circuits.

We propose a new mechanism for constructing waveguide intersections with broad bandwidth and low cross talk in photonic crystal (PC) circuits. The intersections are created by combination of coupled-cavity wave-guides (CCWs) with conventional line-defect waveguides. This mechanism utilizes the strong dependence of the defect coupling on the field pattern in the defects and the alignment of the defects (i.e., the coupling angle) in CCWs. By properly designing the defect mode, we demonstrate through numerical simulation the establishment of such a waveguide intersection in one of the most useful PCs, which is based on a two-dimensional triangular lattice of air holes made in a dielectric material. The transmission of a 500-fs pulse at ~1.3 microm is simulated by use of the finite-difference time-domain method, showing negligible distortion and low cross talk.

Design and fabrication of impurity band-based photonic crystal waveguides for optical delay lines

We have designed, fabricated, and characterized efficient optical delay lines for ultrashort pulses based on photonic crystal waveguides formed by coupled defects. The basic structure consists of one-dimensional cylindrical air holes formed in silicon on silicon dioxide ridge waveguides. The defects are created by intentionally increasing the separation of two neighboring air holes. Through proper design, quasiflat impurity bands located at ∼1.55 μm can be successfully obtained. In addition, the wavelength tunability of the impurity bands was clearly demonstrated. Good agreement was found between the experimental observations and numerical simulations.

Coupling of defect pairs and generation of dynamical band gaps in the impurity bands of nonlinear photonic crystals for all-optical switching

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.

Two-Dimensional In0.4Ga0.6As/GaAs Quantum Dot Superlattices Realized by Self-Organized Epitaxial Growth

We report on the realization of two-dimensional (2D) In0.4Ga0.6As/GaAs quantum dot superlattices (QDSLs) by self-organized 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.

Transmission properties of coupled-cavity waveguides based on two-dimensional photonic crystals with a triangular lattice of air holes

Using numerical simulations, we systematically investigated the transmission properties of coupled-cavity waveguides (CCWs) formed in two-dimensional photonic-crystal dielectric slabs with a triangular array of air holes. We place emphasis on achieving a quasi-flat impurity band in such CCWs, which is important for the perfect transmission of ultrashort optical pulses. We show that the quasi-flat impurity band can be obtained by controlling the ratio of the air-hole radius to the lattice constant in the triangular lattice. As an example, we demonstrate the perfect transmission of a 500-fs-wide optical pulse through a CCW with a quasi-flat impurity band, indicating the possibility of application to high-speed all-optical communication systems with a maximum bit rate of approximately terabits per second.

Self-Organization of High-Density III–V Quantum Dots on High-Index Substrates

Ordered In0.4Ga0.6As quantum dots (QDs), with the density of 109 cm-2–1011 cm-2, are fabricated on GaAs(311)B substrates by atomic-hydrogen assisted molecular beam epitaxy. The density and the dot size are mainly changed by the growth temperature between 460°C and 520°C. In sharp contrast to InAs, where we can hardly observe any ordering, and coalescence or merging of dots occurs beyond a thickness of 4 monolayers, the high-density In0.4Ga0.6As QDs do not coalesce even when the QDs are in contact with each other. The inhomogeneuos distribution of In and Ga in the QDs which are In rich at the surface is found to be responsible for this unusual behavior by reflection high energy electron diffraction studies. The atomic force microscope images show that a surface coverage of nearly 100% can be achieved. It implies the existence of lateral coupling between QDs, which is strongly supported by the photoluminscence measurements.

Formation of extended states in disordered two-dimensional In0.4Ga0.6As/GaAs(311)B quantum dot superlattices

Formation of extended states or minibands in two-dimensional (2D) In0.4Ga0.6As/GaAs(311)B quantum dot superlattices (QDSLs) is directly demonstrated in time-resolved photoluminescence measurements. At a low excitation density of 1 W/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.