Two-step laterally tapered spot-size converter 1.55-/spl mu/m DFB laser diode having a high slope efficiency

We demonstrate a two-step lateral tapered 1.55-/spl mu/m spot-size converter distributed feedback laser diode (SSC DFB LD) having slope efficiencies as high as 0.457 and 0.319 mW/mA measured at 25 /spl deg/C and 85 /spl deg/C, respectively. The SSC DFB LD fabricated by using a nonselective grating process has a double core waveguide structure including a planar buried heterostructure type active waveguide and a ridge type passive waveguide. The fabricated SSC DFB LD operates at 1.553-/spl mu/m wavelength and shows a far-field pattern in horizontal and vertical directions of 7.3/spl deg/ and 11.6/spl deg/, respectively.     

A simple SNR representation method for AMC schemes of MIMO systems with ML detector

Adaptive modulation and coding (AMC) is a powerful technique to enhance the link performance by adjusting the transmission power, channel coding rates and modulation levels according to channel state information. In order to efficiently utilize the AMC scheme, an accurate signal-to-noise ratio (SNR) value is normally required for determining the AMC level. In this paper, we propose a simple method to represent the SNR values for maximum likelihood (ML) detector in multi-input multi-output (MIMO) systems. By analyzing the relation between the upper bound and the lower bound of the ML detector performance, we introduce an efficient way to determine the SNR for the ML receiver. Based on the proposed SNR representation, an AMC scheme for single antenna systems can be extended to MIMO systems with ML detector. From computer simulations, we confirm that the proposed SNR representation allows us to achieve almost the same system throughput as the optimum AMC systems in frequency selective channels with reduced complexity.     

Highly Concentrated and Conductive Reduced Graphene Oxide Nanosheets by Monovalent Cation–π Interaction: Toward Printed Electronics

A novel route to preparing highly concentrated and conductive reduced graphene oxide (RGO) in various solvents by monovalent cation–π interaction. Previously, the hydrophobic properties of high‐quality RGO containing few defects and oxygen moieties have precluded the formation of stable dispersion in various solvents. Cation–π interaction between monovalent cations, such as Na⁺ or K⁺, and six‐membered sp² carbons on graphene were achieved by simple aging process of graphene oxide (GO) nanosheets dispersed in alkali solvent. The noncovalent binding forces introduced by the cation–π interactions were evident from the chemical shift of the sp² peak in the solid ¹³C NMR spectra. Raman spectra and the I‐V characteristics demonstrated the interactions in terms of the presence of n‐type doping effect due to the adsorption of cations with high electron mobility (39 cm²/Vs). The RGO film prepared without a post‐annealing process displayed superior electrical conductivity of 97,500 S/m at a thickness of 1.7 μm. Moreover, mass production of GO paste with a concentration as high as 20 g/L was achieved by accelerating the cation–π interactions with densification process. This strategy can facilitate the development of large scalable production methods for preparing printed electronics made from high‐quality RGO nanosheets.     

A Novel Data Transmission Scheme for ATSC Terrestrial DTV Systems

In Advanced Television Systems Committee (ATSC) terrestrial digital television (DTV) systems, additional very low‐rate data can be transmitted by modulating the amplitude and polarity of the transmitter identification (TxID) signal. Although the additional data transmission scheme offers reliable transmission and has a very large coverage area, it has a limitation on the data rate. In this paper, we propose a novel additional data transmission scheme based on the TxID sequences of the ATSC DTV system and Walsh modulation. The proposed scheme not only increases the data rate significantly, but also offers a virtually identical coverage area compared to a conventional scheme.     

Stress Relief Principle of Micron‐Sized Anodes with Large Volume Variation for Practical High‐Energy Lithium‐Ion Batteries

Practical applications of high gravimetric and volumetric capacity anodes for next‐generation lithium‐ion batteries have attracted unprecedented attentions, but still faced challenges by their severe volume changes, rendering low Coulombic efficiency and fast capacity fading. Nano and void‐engineering strategies had been extensively applied to overcome the large volume fluctuations causing the continuous irreversible reactions upon cycling, but they showed intrinsic limit in fabrication of practical electrode condition. Achieving high electrode density is particularly paramount factor in terms of the commercial feasibility, which is mainly dominated by the true density and tapping density of active material. Herein, based on finite element method calculation, micron‐sized double passivation layered Si/C design is introduced with restrictive lithiation state, which can withstand the induced stress from Li insertion upon repeated cycling. Such design takes advantage in structural integrity during long‐term cycling even at high gravimetric capacity (1400 mAh g^?1). In 1 Ah pouch‐type full‐cell evaluation with high mass loading and electrode density (≈3.75 mAh cm^?2 and ≈1.65 g cm^?3), it demonstrates superior cycle stability without rapid capacity drop during 800 cycles.     

Fabrication of Multiscale Gradient Polymer Patterns by Direct Molding and Spatially Controlled Reflow

Size variations of pattern spacing as well as gradient control of the as‐formed polymeric pattern via a spatially controlled reflow process are presented. Micro‐ and nanopatterns of polymethyl methacrylate (PMMA) in the form of line‐and‐space strips are first generated by capillary force lithography (CFL), and the residual layers are removed by ashing process. Subsequently, the exposed PMMA strips underwent a controlled reflow process above the glass transition temperature (T_g) while heating single or both sides of the substrate either in parallel to the line pattern (parallel reflow) or perpendicular to the line pattern (perpendicular reflow). As a result of this controlled reflow, a linear or a parabolic profile of pattern spacing is achieved depending on the heating mode. Furthermore, multiscale gradient patterns are formed with the spacing ranging from 98 nm to 4.23 μm (a difference of two orders of magnitude) in a single patterned layer using the original micropattern of 16 μm width and 8 μm spacing. In order to explain reflow behaviors, a simple theoretical model relating the normalized pattern width to the polymer viscosity is derived based on a leveling kinetics of polymer melt. Also, gradient PMMA channels are fabricated and bonded to a glass substrate, which are used to flow a liquid inside the channels by capillarity‐driven flow.     

Rate-Adaptive MAC Protocol for Wireless Multicast Over OFDMA-Based MANETs

Unlike in unicast transmissions, a feedback channel has not been implemented for multicast transmission over Mobile ad hoc networks (MANET), due to the increase in the overhead with increasing group member size. As a result, rate-adaptation for wireless multicast has not been considered either. In this paper, a novel rate-adaptive Medium Access Control protocol is proposed to allow dynamic rate-adaptation for wireless multicast transmission over MANETs by utilizing the orthogonality of the subcarriers in an Orthogonal Frequency-Division Multiplexing system. In the proposed rate-adaptation method, each member station is assigned a unique subcarrier and by using this subcarrier, its preferred data rate in the current channel condition is reported to the multicast source. Due to their orthogonality, the feedbacked packets simultaneously transmitted by the group members can be distinguished at the source. Therefore, the source chooses the most appropriate data rate for all member stations. By using this method, the data rate for wireless multicast is able to be adaptively changed, so that the overall network performance is improved.     

Performance Analysis of a Discrete‐Time Two‐Phase Queueing System

This paper introduces the modeling and analysis of a discrete‐time, two‐phase queueing system for both exhaustive batch service and gated batch service. Packets arrive at the system according to a Bernoulli process and receive batch service in the first phase and individual services in the second phase. We derive the probability generating function (PGF) of the system size and show that it is decomposed into two PGFs, one of which is the PGF of the system size in the standard discrete‐time Geo/G/1 queue without vacations. We also present the PGF of the sojourn time. Based on these PGFs, we present useful performance measures, such as the mean number of packets in the system and the mean sojourn time of a packet.     

Chiral Stereoisomer Engineering of Electron Transporting Materials for Efficient and Stable Perovskite Solar Cells

A series of chiral stereoisomers of electron transporting materials with two chiral substituents is rationally designed and synthesized, and the influence of stereoisomerism on their physical and electronic properties is investigated to demonstrate highly efficient and stable perovskite solar cells (PSCs). Compared to mesomeric naphthalene diimide (NDI) derivatives, which have heterochiral side groups with centrosymmetric molecular packing of symmetric‐shaped conformers in the crystalline state, enantiomeric NDI derivatives have homochiral side groups that exhibit non‐centrosymmetric molecular packing of asymmetric‐shaped conformers in the crystalline state and exhibit better solution processability based on one order of magnitude higher solubility. A similar trend is observed in different rylene diimide stereoisomers based on larger semiconducting core perylene diimide. The PSCs based on NDI enantiomers with good film‐forming ability and a very high lowest phase transition temperature (T_lowest) of 321 °C exhibit a high and uniform average power conversion efficiency (PCE) of 19.067 ± 0.654%. These PSCs also have a high temporal device stability, with less than 10% degradation of the PCE at 100 °C for 1000 h without encapsulation. Therefore, chiral stereoisomer engineering of charge transporting materials is a potential approach to achieve high solution processability, excellent performance, and significant temporal stability in organic electronic devices.