Theoretical design for 120 GHz semiconductor junction circulators: effects of different numbers of terms in the Green's function

A theoretical analysis of the predicted performance and field distributions for semiconductor junction circulators is based on Bosma's Green's function approach which involves a summation of infinite series in the mathematical derivation. The effects are considered of taking a different number of terms in the series, looking at three different circulators centred at 120 GHz. The narrowband semiconductor circulators show a similar effect to the ferrite junction circulator because the field distributions inside the semiconductor discs can be approximated by only considering the dominant modes. However, there is no dominant mode in the broadband circulator and the higher order modes play an important role in the operation of this circulator. In order to obtain a precise representation of the circulation effects inside both narrowband and broadband circulators, at least up to the 6th term, or 18 terms in all, are required to be added in the summation of Green's functions.     

CMOS retinal prosthesis with on-chip electrode impedance measurement

A retinal prosthesis device with built-in self-test capability is proposed and demonstrated. The measurement of electrode impedance as a self-test is achieved without increasing the chip area by employing analogue multiplexers to allow the electrodes to be used for both stimulation of retinal cells and measurement of impedance. Measurement is performed using a four-terminal method to ensure good accuracy. A prototype stimulus chip with 16/spl times/16 channels is fabricated using standard 0.6 /spl mu/m CMOS technology, and is demonstrated to provide self-test functionality with error of as little as 0.05% in the frequency range of 100 Hz to 100 kHz.     

Generalized multiuser orthogonal space-division multiplexing

This paper addresses the problem of performing orthogonal space-division multiplexing (OSDM) for downlink, point-to-multipoint communications when multiple antennas are utilized at the base station (BS) and (optionally) all mobile stations (MS). Based on a closed-form antenna weight solution for single-user multiple-input multiple-output communications in the presence of other receiver points, we devise an iterative algorithm that finds the multiuser antenna weights for OSDM in downlink or broadcast channels. Upon convergence, each mobile user will receive only the desired activated spatial modes with no cochannel interference. Necessary and sufficient conditions for the existence of OSDM among the number of mobile users, the number of transmit antennas at the BS, and the number of receive antennas at the MS, are also derived. The assumption for the proposed method is that the BS knows the channels for all MS's and that the channel dynamics are quasi-stationary.     

Numerical investigation of self-heating effects of oxide-confined vertical-cavity surface-emitting lasers

We present a comprehensive numerical model to simulate self-heating effects of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) under continuous-wave operation. The model self-consistently accounts for the close interaction between optical, electrical, and thermal processes in VCSELs. In particular, hot carriers and nonequilibrium optical phonons in the quantum wells are modeled by solving a carrier energy balance equation and an optical phonon rate equation. Our numerical simulations reveal that they are responsible for aggravated thermal rollovers in VCSELs' L-I characteristics. Detailed comparisons are made and good agreement is obtained between simulations and experiments for the L-I-V and lasing wavelength characteristics of VCSELs with varying oxide aperture size. Various mechanisms that result in the L-I thermal rollover behavior are also investigated with the aid of simulations.     

Fast minimization methods for solving constrained total-variation superresolution image reconstruction

In this paper, we study the problem of reconstructing a high-resolution image from several decimated, blurred and noisy low-resolution versions of the high-resolution image. The problem can be formulated as a combination of the total variation (TV) inpainting model and the superresolution image reconstruction model. The main purpose of this paper is to develop an inexact alternating direction method for solving such constrained TV image reconstruction problem. Experimental results are given to show that the proposed algorithm is effective and efficient.     

Injectable Collagen–Genipin Gel for the Treatment of Spinal Cord Injury: In Vitro Studies

Spinal cord injury (SCI) often results in a cavitary lesion, contained within the dura, which involves only a portion of the cord. Injectable biopolymers are an attractive treatment option for SCI to re‐establish cell migratory pathways within the lesion while minimizing the collateral damage attendant to an open surgical procedure. In this study we evaluate a thermoresponsive soluble collagen gel incorporating genipin, an amine reactive covalent cross‐linker with low cytotoxicity and fluorogenic attributes. Unlike previous studies, genipin is being investigated as an in situ covalent cross‐linker that will continue to act on the gel after injection. Physical characterization studies show that the addition of genipin provides control over the mechanical and degradative behavior of the gel, to meet design specifications of an injectable material for neural tissues. Additionally, an improved in situ assay to predict the extent of cross‐linking reaction is investigated. Encapsulation of mesenchymal stem cells (MSCs) in collagen–genipin gels show the gels support cell viability and proliferation, and thus serve as a cell delivery vehicle. Neural stem cells are found to be more sensitive to genipin, with respect to toxicity, as compared to MSCs. From our studies, 0.25‐0.5 mM is an appropriate genipin concentration to use for an in situ forming scaffold capable of delivering cells and therapeutic agents.     

Enhanced Electrochromism with Rapid Growth Layer‐by‐Layer Assembly of Polyelectrolyte Complexes

In this work, a facile method to deposit fast growing electrochromic multilayer films with enhanced electrochemical properties using layer‐by‐layer (LbL) self‐assembly of complex polyelectrolyte is demonstrated. Two linear polymers, poly(acrylic acid) (PAA) and polyethylenimine (PEI), are used to formulate stable complexes under specific pH to prepare polyaniline (PANI)/PAA‐PEI multilayer films via LbL deposition. By introducing polymeric complexes as building blocks, [PANI/PAA‐PEI]_n films grow much faster compared with [PANI/PAA]_n films, which are deposited under the same condition. Unlike the compact [PANI/PAA]_n films, [PANI/PAA‐PEI]_n films exhibit porous structure that is beneficial to the electrochemical process and leads to improved electrochromic properties. An enhanced optical modulation of 30% is achieved with [PANI/PAA‐PEI]₃₀ films at 630 nm compared with the lower optical modulation of 11% measured from [PANI/PAA]₃₀ films. The switching time of [PANI/PAA‐PEI]₃₀ films is only half of that of [PANI/PAA]₃₀ films, which indicates a faster redox process. Utilizing polyelectrolyte complexes as building blocks is a promising approach to prepare fast growing LbL films for high performance electrochemical device applications.     

Demonstration of aluminum-free metamorphic InP/In0.53Ga0.47As/InP double heterojunction bipolar transistors on GaAssubstrates

We report, for the first time, the successful fabrication of aluminum-free metamorphic (MM) InP/In_0.53 Ga_0.47 As/InP double heterojunction bipolar transistors (DHBTs) on GaAs substrates with a linearly graded In_xGa_1-xP buffer grown by solid-source molecular beam epitaxy (SSMBE). Devices with 5×5 μm² emitters display a peak current gain of 40 and a common-emitter breakdown voltage (BV_CE0) higher than 9 V, a current gain cut-off frequency (f_T) of 48 GHz and a maximum oscillation frequency (f_max) of 42 GHz. A minimum noise figure of 2.9 dB and associated gain of 19.5 dB were measured at a collector current level of 2.6 mA at 2 GHz. Detailed analysis suggests that the degradation of the base-emitter heterojunction interface and the increase of bulk recombination are the most probable causes for the poorer device performance of current metamorphic HBTs compared with lattice-matched HBTs     

Efficient Computation of EXIT Functions for Nonbinary Iterative Decoding

The calculation of nonbinary extrinsic information transfer charts for the iterative decoding of concatenated index-based codes is addressed. We show that the extrinsic information at the output of a constituent a posteriori probability decoder can be calculated with very low complexity, where expensive histogram measurements are not required any more. An example for turbo trellis-coded modulation demonstrates the capabilities of the proposed approach     

A Multiwalled‐Carbon‐Nanotube‐Based Biosensor for Monitoring Microcystin‐LR in Sources of Drinking Water Supplies

A multiwalled carbon nanotube (MWCNT)‐based electrochemical biosensor is developed for monitoring microcystin‐LR (MC‐LR), a toxic cyanobacterial toxin, in sources of drinking water supplies. The biosensor electrodes are fabricated using vertically well‐aligned, dense, millimeter‐long MWCNT arrays with a narrow size distribution, grown on patterned Si substrates by water‐assisted chemical vapor deposition. High temperature thermal treatment (2500 °C) in an Ar atmosphere is used to enhance the crystallinity of the pristine materials, followed by electrochemical functionalization in alkaline solution to produce oxygen‐containing functional groups on the MWCNT surface, thus providing the anchoring sites for linking molecules that allow the immobilization of MC‐LR onto the MWCNT array electrodes. Addition of the monoclonal antibodies specific to MC‐LR in the incubation solutions offers the required sensor specificity for toxin detection. The performance of the MWCNT array biosensor is evaluated using micro‐Raman spectroscopy, including polarized Raman measurements, X‐ray photoelectron spectroscopy, cyclic voltammetry, optical microscopy, and Faradaic electrochemical impedance spectroscopy. A linear dependence of the electron‐transfer resistance on the MC‐LR concentration is observed in the range of 0.05 to 20 μg L^?1, which enables cyanotoxin monitoring well below the World Health Organization (WHO) provisional concentration limit of 1 μg L^?1 for MC‐LR in drinking water.