Single-mode condition for silicon rib waveguides with trapezoidal cross-section

Single-mode condition for silicon rib waveguides with trapezoidal cross-section was obtained using a numerical method based on imaginary-distance beam propagation method with non-uniform discretization. Both quasi-transverse-electric and quasi-transverse-magnetic modes were investigated. Simulated single-mode condition is given by a modified equation. Comparison with reported results shows that the Marcatili's method is in a better agreement with our results. (C) 2003 Elsevier B.V. All rights reserved.

Multimode interference 3-dB coupler in silicon-on-insulator based on silicon rib waveguides with trapezoidal cross section

A 3-dB multimode interference optical coupler based on rib waveguides with trapezoidal cross section was designed and fabricated on silicon-on-insulator wafer. Potassium hydroxide (KOH) anisotropic chemical etching of silicon was used to fabricate the waveguides to obtain smooth interface. A modified finite-difference beam propagation method was used to simulate the multimode rib waveguide with slope interfaces. The rms roughness of etching interface is as small as 1.49 nm. The propagation loss of the waveguide is 1.3 dB/cm at wavelength of 1.55 mum. The fabricated 3-dB coupler has a good uniformity of 0.2 dB.

Simulation of polarization effects in AlGaN/GaN heterojunction

A method for introducing polarization effects in the simulation of GaN-based heterojunction devices is proposed. A delta doping layer is inserted at the interface of heterojunction and the ionized donors or acceptors act as polarization induced fixed charges. Thus polarization effects can be taken into account in a traditional device simulator. Ga-face and N-face single AlGaN/GaN heterostructures are simulated, and the simulation results show that carrier confinement takes place only in the former structure while not in the latter one. The sheet density of free electrons at the interface of Ga-face AlGaN/GaN increases with the Al composition and the thickness of AlGaN. The consistence of simulation results with the experiments and calculations reported elsewhere shows that this method can effectively introduce polarization effects in the simulation of GaN-based heterojunction devices.

Effect of spontaneous and piezoelectric polarization on intersubband transition in AlxGa1-xN-GaN quantum well

A self-consistent solution of conduction band profile and subband energies for AlxGa1-xN-GaN quantum well is presented by solving the Schrodinger and Poisson equations. A new method is introduced to deal with the accumulation of the immobile charges at the AlxGa1-xN-GaN interface caused by spontaneous and piezoelectric polarization in the process of solving the Poisson equation. The effect of spontaneous and piezoelectric polarization is taken into account in the calculation. It also includes the effect of exchange-correlation to the one electron potential on the Coulomb interaction. Our analysis is based on the one electron effective-mass approximation and charge conservation condition. Based on this model, the electron wave functions and the conduction band structure are derived. We calculate the intersubband transition wavelength lambda(21) for different Al molar fraction of barrier and thickness of well. The calculated result can fit to the experimental data well. The dependence of the absorption coefficient a on the well width and the doping density is also investigated theoretically. (C) 2004 American Vacuum Society.

Development of cross-hatch grid morphology and its effect on ordering growth of quantum dots

We investigate the development of cross-hatch grid surface morphology in growing mismatched layers and its effect on ordering growth of quantum dots (QDs). For a 60degrees dislocation (MD), the effective part in strain relaxation is the part with the Burgers vector parallel to the film/substrate interface within its b(edge) component; so the surface stress over a MD is asymmetric. When the strained layer is relatively thin, the surface morphology is cross-hatch grid with asymmetric ridges and valleys. When the strained layer is relatively thick, the ridges become nearly symmetrical, and the dislocations and the ridges inclined-aligned. In the following growth of InAs, QDs prefer to nucleate on top of the ridges. By selecting ultra-thin In0.15Ga0.85As layer (50nm) and controlling the QDs layer at just formed QDs, we obtained ordered InAs QDs. (C) 2004 Elsevier B.V. All rights reserved.

Enhanced Pockels effect in GaN/AlxGa1-xN superlattice measured by polarization-maintaining fiber Mach-Zehnder interferometer

Six-period 4 nm GaN/10 nm AlxGa1-xN superlattices with different Al mole fractions x were prepared on (0001) sapphire substrates by low-temperature metal-organic chemical vapor deposition. The linear electro-optic (Pockels) effect was studied by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55 mu m. The measured electro-optic coefficients, gamma(13)=5.60 +/- 0.18 pm/V, gamma(33)=19.24 +/- 1.21 pm/V (for sample 1, x=0.3), and gamma(13)=3.09 +/- 0.48 pm/V, gamma(33)=8.94 +/- 0.36 pm/V (for sample 2, x=0.1), respectively, are about ten times larger than those of GaN bulk material. The enhancement effect in GaN/AlxGa1-xN superlattice can be attributed to the large built-in field at the interfaces, depending on the mole fraction of Al. (C) 2007 American Institute of Physics.

A proposal for low cross-talk square-lattice photonic crystal waveguide intersection utilizing the symmetry of waveguide modes

We propose an approach to construct waveguide intersections with broad bandwidth and low cross-talk for square-lattice photonic crystals. by utilizing a vanishing overlap of the propagation modes in the waveguides created by defects which support dipole-like defect modes. The finite-difference time-domain method is used to simulate the waveguide intersection created in the two-dimensional square-lattice photonic crystals. Over a bandwidth of 30 nm with the center wavelength at 1300 nm, transmission efficiency above 90% is obtained with cross-talk below -30 dB. Especially, we demonstrate the transmission of a 500-fs pulse at 1.3 Am through the intersection, and the pulse after transmission shows very little distortion while the cross-talk remains at low level meantime. (c) 2006 Elsevier B.V. All rights reserved.

Geometry parameter optimization of large cross-sectional S-shaped bent rib waveguides for silicon based optical switches

地址: Chinese Acad Sci, Inst Semicond, State Key Lab Integrated Optoelect, Beijing 100083, Peoples R China

Electro-optic coefficients of Si0.75Ge0.25/Si/Si0.5Ge0.5 asymmetric superlattice measured by polarization-maintaining fiber-optic Mach-Zehnder interferometer

Ten-period 5.5 nm Si0.75Ge0.25/10.3 nm Si/2.5 nm Si0.5Ge0.5 trilayer asymmetric superlattice was prepared on Si (001) substrate by ultrahigh vacuum chemical vapor deposition at 500 degrees C. The stability of Mach-Zehnder interferometer was improved by utilizing polarization-maintaining fibers. According to the electro-optic responses of the superlattice with the light polarization along [110] and [-110], respectively, both electro-optic coefficients gamma(13) and gamma(63) of such asymmetric superlattice were measured. gamma(13) and gamma(63) are 2.4x10(-11) and 1.3x10(-11) cm/V, respectively, with the incident light wavelength at 1.55 mu m. (c) 2006 American Institute of Physics.

Spectral dependence of spin photocurrent and current-induced spin polarization in an InGaAs/InAlAs two-dimensional electron gas

The converse effects of spin photocurrent and current induced spin polarization are experimentally demonstrated in a two-dimensional electron gas system with Rashba spin splitting. Their consistency with the strength of the Rashba coupling as measured for the same system from beating of the Shubnikov-de Haas oscillations reveals a unified picture for the spin photocurrent, current-induced spin-polarization, and spin-orbit coupling. In addition, the observed spectral inversion of the spin photocurrent indicates a system with dominating structure inversion asymmetry.

Polarization insensitive gain medium with hybrid strained quantum well

A polarization insensitive gain medium for optical amplifiers has been fabricated. The active layer is a structure with alternate tensile and compressive strain quantum wells. The waveguide is made into a taper with angled facets. In the experiment we found that the structure can suppress the lasing and decrease the polarization sensitivity. The gain imbalance between transverse electric and transverse magnetic gains is small, and 0.1 dB is obtained at a driving current of 100 mA. The full-width at half-maximum of amplified spontaneous emission is 40 nm within large current. (C) 2002 Elsevier Science Ltd. All rights reserved.

Impact of polarization on quasi-two-dimensional exciton and barrier-width dependence of the exciton associated transition in wurtzite III-V nitride quantum wells

Excitonic states in AlxGa1-xN/GaN quantum wells (QWs) are studied within the framework of effective-mass theory. Spontaneous and piezoelectric polarizations are included and their impact on the excitonic states and optical properties are studied. We witnessed a significant blue shift in transition energy when the barrier width decreases and we attributed this to the redistribution of the built-in electric field between well layers and barrier layers. For the exciton the binding energies, we found in narrow QWs that there exists a critical value for barrier width, which demarcates the borderline for quantum confinement effect and the quantum confined Stark effect. Exciton and free carrier radiative lifetimes are estimated by simple argumentation. The calculated results suggest that there are efficient non-radiative mechanisms in narrow barrier QWs. (C) 2002 Elsevier Science Ltd. All rights reserved.

Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III-V nitride quantum wells

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times. (C) 2001 American Institute of Physics.

Fabrication of 4 x 4 tapered MMI coupler with large cross section

MMI coupler with large cross section has low coupling loss between the device and fiber. However, large chip area is required. Recently proposed N x N tapered MMI coupler shows a substantial reduction in device geometry. No such kind of devices with N > 2 has yet been realized up to now. The authors have demonstrated a 4 x 4 parabolically tapered MMI coupler with large cross section that can match the SM fiber in silicon-on-insulator (SOI) technology. The device exhibits a minimum uniformity of 0.36 dB and excess loss of 3.7 dB, It represents a key component for realization of MMI-based silicon integrated optical circuit technology.

Circular polarization of excitonic luminescence in CdTe quantum wells with excess electrons of different densities

The circular polarization of excitonic luminescence is studied in CdTe/Cd1-xMgxTe quantum wells with excess electrons of low density in an external magnetic field. It is observed that the circular polarization of X and X- emissions has opposite signs and is influenced by the excess electron density. If the electron density is relatively high so that the emission intensity of the negatively charged excitons X- is much stronger than that of the neutral excitons X, a stronger circular polarization degree of both X and X- emissions is observed. We find that the circular polarization of both X- and X emissions is caused by the spin polarization of the excess electrons due to the electron-spin-dependent nature of the formation of X-. If the electron density is relatively low and the emission intensity of X- is comparable to that of X, the circular polarization degree of X and X- emissions is considerably smaller. This fact is interpreted as due to a depolarization of the excess electron spins, which is induced by the spin relaxation of X-.