Coherence loss and recovery of an electron spin coupled inhomogeneously to a one-dimensional interacting spin bath: An adaptive time-dependent density-matrix renormalization group study

Coherence evolution and echo effect of an electron spin, which is coupled inhomogeneously to an interacting one-dimensional finite spin bath via hyperfine-type interaction, are studied using the adaptive time-dependent density-matrix renormalization group method. It is found that the interplay of the coupling inhomogeneity and the transverse intrabath interactions results in two qualitatively different coherence evolutions, namely, a coherence-preserving evolution characterized by periodic oscillation and a complete decoherence evolution. Correspondingly, the echo effects induced by an electron-spin flip at time tau exhibit stable recoherence pulse sequence for the periodic evolution and a single peak at root 2 tau for the decoherence evolution, respectively. With the diagonal intrabath interaction included, the specific feature of the periodic regime is kept, while the root 2 tau-type echo effect in the decoherence regime is significantly affected. To render the experimental verifications possible, the Hahn echo envelope as a function of tau is calculated, which eliminates the inhomogeneous broadening effect and serves for the identification of the different status of the dynamic coherence evolution, periodic versus decoherence.

Mode coupling and vertical radiation loss for whispering-gallery modes in 3-D microcavities

The characteristics of whispering-gallery modes (WGMs) in 3-D cylindrical, square, and triangular microcavities with vertical optical confinement of semiconductors are numerically investigated by the finite-difference time-domain (FDTD) technique. For a microcylinder with a vertical refractive index 3.17/3.4/3.17 and a center layer thickness 0.2 mu m, Q-factors of transverse electric (TE) WGMs around wavelength 1550 nm are smaller than 10(3), as the radius R < 4 mu m and reach the orders of 10(4) and 10(6) as R = 5 and 6 mu m, respectively. However, the Q-factor of transverse magnetic (TM) WGMs at wavelength 1.659 mu m reaches 7.5 x 10(5) as R = 1 mu m. The mode coupling between the WGMs and vertical radiation modes in the cladding layer results in vertical radiation loss for the WGMs. In the microcylinder, the mode wavelength of TM WGM is larger than the cutoff wavelength of the vertical radiation mode with the same mode numbers, so TM WGMs cannot couple with the vertical radiation mode and have high Q-factor. In contrast, TE WGMs can couple with the corresponding vertical radiation mode in the 3-D microcylinder as R < 5 mu m. However, the mode wavelength of the TE WGM approaches (is larger than) the cutoff wavelength of the corresponding radiation modes at R = 5 mu m (6 mu m), so TE WGMs have high Q-factors in such microcylinders too. The results show that a critical lateral size is required for obtaining high, Q-factor TE WGMs in the 3-D microcylinder. For 3-D square and triangular microcavities, we also find that the Q-factor of TM WGM is larger than that of TE WGM.

Confinement factor and absorption loss of AlInGaN based laser diodes emitting from ultraviolet to green

Confinement factor and absorption loss of AlInGaN based multiquantum well laser diodes (LDs) were investigated by numerical simulation based on a two-dimensional waveguide model. The simulation results indicate that an increased ridge height of the waveguide structure can enhance the lateral optical confinement and reduce the threshold current. For 405 nm violet LDs, the effects of p-AlGaN cladding layer composition and thickness on confinement factor and absorption loss were analyzed. The experimental results are in good agreement with the simulation analysis. Compared to violet LD, the confinement factors of 450 nm blue LD and 530 nm green LD were much lower. Using InGaN as waveguide layers that has higher refractive index than GaN will effectively enhance the optical confinement for blue and green LDs. The LDs based on nonpolar substrate allow for thick well layers and will increase the confinement factor several times. Furthermore, the confinement factor is less sensitive to alloys composition of waveguide and cladding layers, being an advantage especially important for ultraviolet and green LDs.

Optical waveguide propagation loss measurement using multiple reflections method

Optical waveguide propagation loss measurement method based on optical multiple reflections detection is presented in this paper. By using a precision reflectometer, uncertain influence on waveguide propagation loss measurement caused by fiber-waveguide coupling can be eliminated effectively and the waveguide net propagation loss can be measured accurately. To demonstrate this, the propagation loss of a Silicon-on-Insulator (SOI) rib waveguide fabricated by RIE is measured with the obtained value being 4.3 dB/cm. This method provides a non-destructive means for evaluating waveguide propagation loss. (c) 2005 Elsevier B.V. All rights reserved.

Rearrangeable nonblocking SOI waveguide thermooptic 4x4 switch matrix with low insertion loss and fast response

A rearrangeable nonblocking 4 x 4 thermooptic silicon-on-insulator waveguide switch matrix at 1.55-mu m integrated spot size converters is designed and fabricated for the first time. The insertion losses and polarization-dependent losses of the four channels are less than 10 and 0.8 dB, respectively. The extinction ratios are larger than 20 dB. The response times are 4.6 mu s for rising edge and 1.9 mu s for failing edge.

A silicon-on-insulator-based thermo-optic waveguide switch with low insertion loss and fast response

A silicon-on-insulator-based thermo-optic waveguide switch integrated with spot size converters is designed and fabricated by inductively coupled plasma reactive ion etching. The device shows good characteristics, including low, insertion loss of 8 +/- 1 dB for wavelength 1530-1580 nm and fast response times of 4.6 As for rising edge and 1.9 mu s for failing edge. The extinction ratios of the two channels are 19.1 and 18 dB, respectively.

Calculation of propagation loss in photonic crystal waveguides by FDTD technique and Pade approximation

The propagation losses in single-line defect waveguides in a two-dimensional (2D) square-lattice photonic crystal (PC) consisted of infinite dielectric rods and a triangular-lattice photonic crystal slab with air holes are studied by finite-difference time-domain (FDTD) technique and a Pade approximation. The decaying constant beta of the fundamental guided mode is calculated from the mode frequency, the quality factor (Q-factor) and the group velocity v(g) as beta = omega/(2Qv(g)). In the 2D square-lattice photonic crystal waveguide (PCW), the decaying rate ranged from 10(3) to 10(-4) cm(-1) can be reliably obtained from 8 x 10(3)-item FDTD output with the FDTD computing time of 0.386 ps. And at most 1 ps is required for the mode with the Q-factor of 4 x 10(11) and the decaying rate of 10(-7) cm(-1). In the triangular-lattice photonic crystal slab, a 10(4)-item FDTD output is required to obtain a reliable spectrum with the Q-factor of 2.5 x 10(8) and the decaying rate of 0.05 cm(-1). (c) 2004 Elsevier B.V. All rights reserved.

Research on improved methods of reduction of bend loss of silicon-on-insulator waveguides

Improved methods of reduction of bend loss of silicon-on-insulator waveguides were simulated and analyzed by means of effective index method (EIM) and two dimensional beam propagation method (2D-BPM). The simulation results indicate that two different methods, one of which are introducing an offset at the junction of two waveguides and the other is etching groove at the outside of bend waveguide, can decrease bend loss. And the later one is more effective. Meanwhile, experiments validate them. By etching groove, the insertion loss of bend waveguide of R = 16mm, transverse displacement 70mum was decreased 5dB. And its bend loss was almost eliminated.

Design, fabrication, and characterization of an ultracompact low-loss photonic crystal corner mirror

An ultracompact, low-loss, and broad-band corner mirror, based on photonic crystals, is investigated in this paper. Based on the theoretical analysis of the loss mechanism, the boundary layers of the photonic crystal region are revised to improve the extra losses, and the transmission characteristics are evaluated by using the 3-D finite-difference time-domain method. The device with optimized structure was fabricated on silicon-on-insulator substrate by using electron-beam lithography and inductively coupled plasma etching. The measured extra losses are about 1.1 +/- 0.4 dB per corner mirror for transverse-electronic polarization for the scanning wavelength range of 1510-1630 nm. Dimensions of the achieved PC corner mirror are less than ;7 x 7 mu m(2), which are only about one tenth of conventional wave-guide corner mirrors.

Realization of an ultracompact low-loss photonic crystal corner mirror

We report on the realization and characterization of an ultracompact, low-loss, and broadband corner mirror based on photonic crystals (PCs). By modifying the boundary layers of the PC region, extra losses of 1.1 +/- 0.4 dB per corner mirror are achieved for transverse-electronic polarization for silicon-on-insulator ridge waveguides fabricated by electron beam lithography and inductively coupled plasma etching. Dimensions of the PC corner mirror are less than 7 x 7 mu m(2), which are only about one tenth of conventional waveguide corner mirrors.

Design of Low-Loss Surface-Plasmon Quantum Cascade Lasers

National Research Projects of China 60525406 60736031 60806018 60906026 2006CB604903 2007AA03Z446 2009AA03Z403

Polarity determination for GaN thin films by electron energy-loss spectroscopy

The intensity of the N K edge in electron energy-loss spectra from a GaN thin film shows a pronounced difference when the orientation of the film approaches the (0002) and (000-2) Bragg reflections, along the polar direction. This experimental result can be interpreted by the effect associated with interference between the Bloch waves of the incident electron in the GaN crystal. The theoretical calculations indicate that, at the Bragg condition of g=0002 along the Ga-N bond direction, the thickness-averaged electron current density on the N atom plane is much higher than that at g=000 (2) over bar, with a maximum as the specimen thickness is about 0.4xi(0002) (the two-beam extinction distance). The delocalization effect on the experimental spectra is also discussed. (C) 2002 American Institute of Physics.

Numerical evaluation of energy loss rate for hot carriers in quantum wells

Using the Frohlich potential associated with realistic optical phonon modes in quantum well systems, the energy loss rates of hot electrons, holes, and electron-hole pairs are calculated, with special emphasis on the effects of carrier density, hot phonon population, quantum well width, and phonon dispersion on the hot-carrier relaxation process in quasi-two-dimensional systems. (C) 1998 Academic Press Limited.

Investigation of vertical radiation loss for whispering-gallery modes in 3-D microresonators by FDTD simulation

The vertical radiation loss of three-dimensional (3-D) microresonators is investigated by 3-D finite-difference time-domain (FDTD) simulation. The simulation shows that the vertical radiation causes an important loss in the microresonators with weak waveguiding, and result in decrease of the quality factors (Q-factors) of whispering-gallery (WG) modes. Through the simulation, we find that TM-like modes have much weaker vertical radiation loss than TE-like modes. High Q-factor TM-like modes are observed in the 3-D microresonators with weak vertical waveguiding, but the Q-factors of TE-like modes decrease greatly.

Design and Fabrication of Low-microwave loss Coplannar Waveguide and precise V groove on Silicon Substrate for Optoelectronic Packaging - art. no. 601938

Optoelectronic packaging has become a most important factor that influences the final performance and cost of the module. In this paper, low microwave loss coplanar waveguide(CPW) on high resistivity silicon(HRS) and precise V groove in silicon substrate were successfully fabricated. The microwave attenuation of the CPW made on HRS with the simple process is lower than 2 dB/cm in the frequency range of 0 similar to 26GHz, and V groove has the accuracy in micro level and smooth surface. These two techniques built a good foundation for high frequency packaging and passive coupling of the optoelectronic devices. Based on these two techniques, a simple high resistivity silicon substrate that integrated V groove and CPW for flip-chip packaging of lasers was completed. It set a good example for more complicate optoelectronic packaging.