A method for analyzing the coupling of 3-D tapered rib waveguides with fibers

The fields in 3-dimensional tapered waveguides are unstable compared with the fields in the straight waveguides. In the case of waveguide-to-fiber coupling and fiber-to-waveguide coupling, a sequence of short straight waveguides has been modeled to approximate the 3-dimensional tapered waveguide; and the unstable incident and reflected fields, as well as their derivatives, were determined by the beam propagation method(BPM). Then free space radiation mode(FSRM) was employed to calculate the reflected and transmitted powers. Analysis results of the coupling of fiber with silicon-on-insulator(SOI) tapered rib waveguides showed the feasibility of the method.

Photo-capacitance response of internal tunnelling coupling in quantum-dot-imbedded heterostructures under selective photo-excitation

Under selective photo-excitation, the capacitance response of internal tunnelling coupling in quantum-dots-imbedded heterostructures is studied to clarify the electronic states and the number densities of electrons filling in the quantum dots (QDs). The random nature for both optical transitions and the filling in a QD assembly makes highly resolved capacitance peaks appear in the C-V characteristic after turning off the photo-excitation.

Electrical and optical coupling in an electroabsorption modulator integrated with a DFB laser

Electrical and optical coupling in an electroabsorption (EA) modulator integrated with a distributed feedback (DFB) laser have been investigated. The integrated device is treated as a three-port optoelectronic device with two electrical ports and one optical output port. The scattering parameters of this three-port device have been measured in the designed experiment. The measured results indicate that there exists the electrical coupling between the DFB laser and EA modulator of the integrated light source whenever the current applied to the laser section is below or above the threshold current, and the optical coupling will have stronger influence on the frequency responses than the electrical coupling when the bias current is above the threshold. A small-signal equivalent circuit model for the integrated device is established considering both the electrical and internal optical coupling. Experiments show that the equivalent circuit model is reasonable and the determined element values are correct. Based on the measurement and modeling, the influences of the electrical and optical coupling on the high-frequency responses are investigated and the effective measure to eliminate the additional modulation in the DFB laser are discussed.

Weak antilocalization and beating pattern in an InGaAs/InAlAs quantum well

We have observed the weak antilocalization (WAL) and beating SdH oscillation through magnetotransport measurements performed on a heavily delta-doped In0.52Al0.48As/In0.53Ga0.47As/In0.5Al0.48As single quantum well in an applied magnetic field up to 13 T and a temperature at 1.5 K. Both effects are caused by the strong Rashba spin-orbit (SO) coupling due to high structure inversion asymmetry (SIA). The Rashba SO coupling constant alpha and zerotield spin splitting Delta(0) are estimated and the obtained values are consistent from different analysis for this sample. (c) 2007 Elsevier Ltd. All rights reserved.

Spin Hall effect of excitons with spin-orbit coupling

The center-of-mass motion of a quasi-two-dimensional exciton with spin-orbit coupling (SOC) in the presence of a perpendicular electric field is calculated by perturbation theory. The results indicate that a quasi-two-dimensional exciton with SOC can exhibit the spin Hall effect (SHE), which is similar to two-dimensional electrons and holes. A likely way to establish exciton SHE in experiments and a possible phase transition from dark to bright state driven by SOC are suggested. (c) 2007 American Institute of Physics.

Mode coupling between first- and second-order whispering-gallery modes in coupled microdisks

The mode characteristics for two coupled microdisks are investigated by the finite-difference time-domain technique. In the two coupled micodisks, mode coupling between the same order whispering-gallery modes (WGMs) results in coupled WGMs with split mode wavelengths. The numerical results show that the split mode wavelengths of the coupled first- and second-order WGMs can have a crossing point in some cases, which can induce anticrossing mode coupling between them and greatly reduce the mode Q factor of the coupled first-order WGMs. The time variation of mode field pattern shows the transformation between the coupled first- and second-order WGMs. (C) 2007 Optical Society of America

Weak anti-localization in InAlAs/InGaAs/InAlAs high mobility two-dimensional electron gas systems

Magneto-transport measurements have been carried out on three heavily Si delta-doped In-0.52 Al-0.48 As/In-0.53 Ga-0.47 As/In-0.52 A(10.48) As single quantum well samples in which two subbands were occupied by electrons. The weak anti-localization (WAL) has been found in such high electron mobility systems. The strong Rashba spin-orbit (SO) coupling is due to the high structure inversion asymmetry (SIA) of the quantum wells. Since the WAL theory model is so complicated in fitting our experimental results, we obtained the Rashba SO coupling constant alpha and the zero-field spin splitting Delta(0) by an approximate approach. The results are consistent with that obtained by the Shubnikov-de Haas (SdH) oscillation analysis. The WAL effect in high electron mobility system suggests that finding a useful approach for deducing alpha and Delta(0) is important in designing future spintronics devices that utilize the Rashba SO coupling.

Mode analysis and Q-factor enhancement due to mode coupling in rectangular resonators

Modes in rectangular resonators are analyzed and classified according to symmetry properties, and quality factor (Q-factor) enhancement due to mode coupling is observed. In the analysis, mode numbers p and q are used to denote the number of wave nodes in the direction of two orthogonal sides. The even and odd mode numbers correspond to symmetric and antisymmetric field distribution relative to the midlines of sides, respectively. Thus, the modes in a rectangle resonator can be divided into four classes according to the parity of p and q. Mode coupling between modes of different classes is forbidden; however, anti-crossing mode coupling between the modes in the same class exists and results in new modes due to the combination of the coupled modes. One of the combined modes has very low power loss and high Q-factor based on far-field emission of the analytical field distribution, which agrees well with the numerical results of the finite-difference time-domain (FDTD) simulation. Both the analytical and FDTD results show that the Q-factors of the high Q-factor combined modes are over one order larger than those of the original modes. Furthermore, the general condition required to achieve high-Q modes in the rectangular resonator is given based on the analytical solution.

Mesoscopic Fano effect modulated by Rashba spin-orbit coupling and external magnetic field

By employing non-equilibrium Green's function method, the mesoscopic Fano effect modulated by Rashba spin-orbit (SO) coupling and external magnetic field has been elucidated for electron transport through a hybrid system composed of a quantum dot (QD) and an Aharonov-Bohm (AB) ring. The results show that the orientation of the Fano line shape is modulated by the Rashba spin-orbit interaction k(R)L variation, which reveals that the Fano parameter q will be extended to a complex number, although the system maintains time-reversal symmetry (TRS) under the Rashba SO interaction. Furthermore, it is shown that the modulation of the external magnetic field, which is applied not only inside the frame, but also on the QD, leads to the Fano resonance split due to Zeeman effect, which indicates that the hybrid is an ideal candidate for the spin readout device. (C) 2007 Elsevier B.V All rights reserved.

Finite-difference time-domain analysis of deformed square cavity filters with a traveling-wave-like filtering response by mode coupling

An add-drop filter based on a perfect square resonator can realize a maximum of only 25% power dropping because the confined modes are standing-wave modes. By means of mode coupling between two modes with inverse symmetry properties, a traveling-wave-like filtering response is obtained in a two-dimensional single square cavity filter with cut or circular corners by finite-difference time-domain simulation. The optimized deformation parameters for an add-drop filter can be accurately predicted as the overlapping point of the two coupling modes in an isolated deformed square cavity. More than 80% power dropping can be obtained in a deformed square cavity filter with a side length of 3.01 mu m. The free spectral region is decided by the mode spacing between modes, with the sum of the mode indices differing by 1. (c) 2007 Optical Society of America.

Silicon bench for passive coupling and packaging of high-frequency optoelectronic devices

Coupling and packaging have become decisive factors in the final performance and cost of high-frequency optoelectronic devices. Here, we report the design and successful fabrication of a silicon bench that integrates a V-groove and high-frequency coplanar waveguide (CPW) on the same high-resistivity silicon wafer as an effective optoelectronic packaging solution.

Strong room-temperature ferromagnetism in Cu-implanted nonpolar GaN films

This work was supported by the Natural Science Foundation of China (Grant No. 60876068) and The Project sponsored by SRF for ROCS (Grant No. 08Y1010000), SEM

Charge-density and spin-density excitations in a two-dimensional electron gas with Rashba spin-orbit coupling

We study theoretically the charge-density and spin-density excitations in a two-dimensional electron gas in the presence of a perpendicular magnetic field and a Rashba type spin-orbit coupling. The dispersion and the corresponding intensity of excitations in the vicinity of cyclotron resonance frequency are calculated within the framework of random phase approximation. The dependence of excitation dispersion on various system parameters, i.e., the Rashba spin-orbit interaction strength, the electron density, the Zeeman spin splitting, and the Coulomb interaction strength is investigated.

Rashba spin-orbit coupling in InSb nanowires under transverse electric field

We investigate the Rashba spin-orbit coupling brought by transverse electric field in InSb nanowires. In small k(z) (k(z) is the wave vector along the wire direction) range, the Rashba spin-orbit splitting energy has a linear relationship with k(z), so we can define a Rashba coefficient similarly to the quantum well case. We deduce some empirical formulas of the spin-orbit splitting energy and Rashba coefficient, and compare them with the effective-mass calculating results. It is interesting to find that the Rashba spin-orbit splitting energy decreases as k(z) increases when k(z) is large due to the k(z)-quadratic term in the band energy. The Rashba coefficient increases with increasing electric field, and shows a saturating trend when the electric field is large. As the radius increases, the Rashba coefficient increases at first, then decreases. The effects of magnetic fields along different directions are discussed. The case where the magnetic field is along the wire direction or the electric field direction are similar. The spin state in an energy band changes smoothly as k(z) changes. The case where the magnetic field is perpendicular to the wire direction and the electric field direction is quite different from the above two cases, the k(z)-positive and negative parts of the energy bands are not symmetrical, and the energy bands with different spins cross at a k(z)-nonzero point, where the spin splitting energy and the effective g factor are zero.