GaN/AlN Multiple Quantum Wells and Nitride-Based Waveguide Structures for Ultrafast All-Optical Switch Utilizing Intersubband Transition

Intersubband transition （ISBT） in multiple quantum wells （MQW） has drawn much attention for ultrafast optoelectronic devices owing to its wide wavelength-tunability and extremely fast energy relaxation process. Recently, the extension of ISBT wavelength to near-infrared wavelength, especially 1.55 μm, is of particular interest because such wavelength is vital for the development of ultrafast photonic devices for silica-fiber-based optical-communication networks. Among various materials proposed for intersubband transition at 1.55 μm, GaN/AlN MQW structures are promising due to their large conduction band offset of approximately 2 eV. Furthermore, the large electron effective mass and the large LO phonon energy in nitrides make their intersubband relaxation extremely fast in the order of sub-picoseconds. This makes intersubband transition in nitrides immensely interesting for the development of ultrafast photonic devices operating at a bit rate higher than 1 Tb/s.//The intersubband transition at 1.55 μm and shorter wavelengths have been achieved by molecular beam epitaxy （MBE） with the shortest wavelength of 1.08 μm. On the other hand, growth by metalorganic vapor phase epitaxy （MOVPE） has not yielded satisfactory results as the shortest ISBT wavelength reported is merely 2.4 μm. The demonstration of 1.55-μm ISBT by MOVPE, however, is still attractive since much better crystalline quality for device fabrication can be achieved. Moreover, MOVPE also has another advantage over MBE in industrial point of view. Indeed, the ultrafast optical switching utilizing intersubband transition has been demonstrated by MBE-grown GaN ridge waveguide structure with a bit rate higher than 1 Tb/s. However, such device requires optical-pulse switching energy higher than 10pJ/μm2 to utilize the saturable intersubband absorption, which is still too large for the applications in conventional optical communication networks. Reduction of the switching energy is therefore another important issue for the intersubband transition devices. In order to reduce the switching energy, not only the waveguide fabrication process, but also the epitaxial growth technique and the device structure have to be improved.//In this dissertation, the GaN/AlN multiple quantum wells and nitride-based waveguide structures are studied and fabricated for the applications of ultrafast all-optical switch utilizing intersubband transition. The ultrafast intersubband transition device is realized by using AlN waveguide structure with GaN/AlN quantum wells. This AlN-waveguide-based intersubband transition device can operate in the optical communication wavelength range, covering 1.3 μm, the shortest wavelength ever demonstrated for the intersubband transition devices.//In order to perform epitaxial growth of such structure with high quality, MOVPE is more preferable than MBE because the AlN layer can be grown with much better quality by the MOVPE. However, since the MOVPE growth of GaN/AlN MQW for the 1.55-μm ISBT is very difficult, the AlN waveguide structure was fabricated with a combination of both MOVPE and MBE growth techniques: MOVPE growth for AlN buffer layer and MBE re-growth for GaN/AlN multiple quantum wells. With this combination, the high quality waveguide with intersubband absorption in a wavelength range of 1.3-1.55 μm is achieved.//In addition to the improvement in the epitaxial growth technique, this dissertation also discusses on the problems in growing the waveguide structure of both MOVPE and MBE. Moreover, the design and fabrication of nitride-based waveguide structures are studied in details to improve the waveguide quality. The high-optical-confinement waveguide structures are proposed and successfully fabricated for the first time thanks to the successful demonstration of epitaxial growth and the improvement of fabrication process. Additionally, a new waveguide characterization method using the supercontinuum light source is also proposed and demonstrated. With this new characterization method, not only are the direct measurements of intersubband absorption in waveguides realized, but the problems in waveguide quality of the MBE-grown waveguide are also revealed. This provides very useful information for the improvement of fabrication process, especially the epitaxial growth process. The achievements in each area of epitaxial growth, waveguide fabrication process, and characterization, have made contributions to the improvement of waveguide characteristic, leading to the successful demonstration of the first AlN-waveguide-based intersubband transition devices with high performance.報告番号: 甲21171 ; 学位授与年月日: 2006-03-23 ; 学位の種別: 課程博士 ; 学位の種類: 博士(工学) ; 学位記番号: 博工第6261号 ; 研究科・専攻: 工学系研究科電子工学専

Consciosusness in Cognitive Architectures. A Principled Analysis of RCS, Soar and ACT-R

This report analyses the aplicability of the principles of consciousness developed in the ASys project to three of the most relevant cognitive architectures. This is done in relation to their aplicability to build integrated control systems and studying their support for general mechanisms of real-time consciousness.\ud To analyse these architectures the ASys Framework is employed. This is a conceptual framework based on an extension for cognitive autonomous systems of the General Systems Theory (GST).\ud A general qualitative evaluation criteria for cognitive architectures is established based upon: a) requirements for a cognitive architecture, b) the theoretical framework based on the GST and c) core design principles for integrated cognitive conscious control systems

A 24-GHz SiGe Phased-Array Receiver—LO Phase-Shifting Approach

A local-oscillator phase-shifting approach is introduced to implement a fully integrated 24-GHz phased-array receiver using an SiGe technology. Sixteen phases of the local oscillator are generated in one oscillator core, resulting in a raw beam-forming accuracy of 4 bits. These phases are distributed to all eight receiving paths of the array by a symmetric network. The appropriate phase for each path is selected using high-frequency analog multiplexers. The raw beam-steering resolution of the array is better than 10 [degrees] for a forward-looking angle, while the array spatial selectivity, without any amplitude correction, is better than 20 dB. The overall gain of the array is 61 dB, while the array improves the input signal-to-noise ratio by 9 dB

Computing infrastructure issues in distributed communications systems : a survey of operating system transport system architectures

The performance of distributed applications (such as file transfer, remote login, tele-conferencing, full-motion video, and scientific visualization) is influenced by several factors that interact in complex ways. In particular, application performance is significantly affected both by communication infrastructure factors and computing infrastructure factors. Several communication infrastructure factors include channel speed, bit-error rate, and congestion at intermediate switching nodes. Computing infrastructure factors include (among other things) both protocol processing activities (such as connection management, flow control, error detection, and retransmission) and general operating system factors (such as memory latency, CPU speed, interrupt and context switching overhead, process architecture, and message buffering). Due to a several orders of magnitude increase in network channel speed and an increase in application diversity, performance bottlenecks are shifting from the network factors to the transport system factors.This paper defines an abstraction called an "Operating System Transport System Architecture" (OSTSA) that is used to classify the major components and services in the computing infrastructure. End-to-end network protocols such as TCP, TP4, VMTP, XTP, and Delta-t typically run on general-purpose computers, where they utilize various operating system resources such as processors, virtual memory, and network controllers. The OSTSA provides services that integrate these resources to support distributed applications running on local and wide area networks.A taxonomy is presented to evaluate OSTSAs in terms of their support for protocol processing activities. We use this taxonomy to compare and contrast five general-purpose commercial and experimental operating systems including System V UNIX, BSD UNIX, the x-kernel, Choices, and Xinu

Modeling effects of L-type ca(2+) current and na(+)-ca(2+) exchanger on ca(2+) trigger flux in rabbit myocytes with realistic T-tubule geometries.

The transverse tubular system of rabbit ventricular myocytes consists of cell membrane invaginations (t-tubules) that are essential for efficient cardiac excitation-contraction coupling. In this study, we investigate how t-tubule micro-anatomy, L-type Ca(2+) channel (LCC) clustering, and allosteric activation of Na(+)/Ca(2+) exchanger by L-type Ca(2+) current affects intracellular Ca(2+) dynamics. Our model includes a realistic 3D geometry of a single t-tubule and its surrounding half-sarcomeres for rabbit ventricular myocytes. The effects of spatially distributed membrane ion-transporters (LCC, Na(+)/Ca(2+) exchanger, sarcolemmal Ca(2+) pump, and sarcolemmal Ca(2+) leak), and stationary and mobile Ca(2+) buffers (troponin C, ATP, calmodulin, and Fluo-3) are also considered. We used a coupled reaction-diffusion system to describe the spatio-temporal concentration profiles of free and buffered intracellular Ca(2+). We obtained parameters from voltage-clamp protocols of L-type Ca(2+) current and line-scan recordings of Ca(2+) concentration profiles in rabbit cells, in which the sarcoplasmic reticulum is disabled. Our model results agree with experimental measurements of global Ca(2+) transient in myocytes loaded with 50 μM Fluo-3. We found that local Ca(2+) concentrations within the cytosol and sub-sarcolemma, as well as the local trigger fluxes of Ca(2+) crossing the cell membrane, are sensitive to details of t-tubule micro-structure and membrane Ca(2+) flux distribution. The model additionally predicts that local Ca(2+) trigger fluxes are at least threefold to eightfold higher than the whole-cell Ca(2+) trigger flux. We found also that the activation of allosteric Ca(2+)-binding sites on the Na(+)/Ca(2+) exchanger could provide a mechanism for regulating global and local Ca(2+) trigger fluxes in vivo. Our studies indicate that improved structural and functional models could improve our understanding of the contributions of L-type and Na(+)/Ca(2+) exchanger fluxes to intracellular Ca(2+) dynamics

When Do Redundant Requests Reduce Latency ?

Several systems possess the flexibility to serve requests in more than one way. For instance, a distributed storage system storing multiple replicas of the data can serve a request from any of the multiple servers that store the requested data, or a computational task may be performed in a compute-cluster by any one of multiple processors. In such systems, the latency of serving the requests may potentially be reduced by sending "redundant requests": a request may be sent to more servers than needed, and it is deemed served when the requisite number of servers complete service. Such a mechanism trades off the possibility of faster execution of at least one copy of the request with the increase in the delay due to an increased load on the system. Due to this tradeoff, it is unclear when redundant requests may actually help. Several recent works empirically evaluate the latency performance of redundant requests in diverse settings. This work aims at an analytical study of the latency performance of redundant requests, with the primary goals of characterizing under what scenarios sending redundant requests will help (and under what scenarios they will not help), as well as designing optimal redundant-requesting policies. We first present a model that captures the key features of such systems. We show that when service times are i.i.d. memoryless or "heavier", and when the additional copies of already-completed jobs can be removed instantly, redundant requests reduce the average latency. On the other hand, when service times are "lighter" or when service times are memoryless and removal of jobs is not instantaneous, then not having any redundancy in the requests is optimal under high loads. Our results hold for arbitrary arrival processes.Comment: Extended version of paper presented at Allerton Conference 201

Spatial data stream multiplexing scheme for high-throughput WLANs

A novel scheme using spatial data stream multiplexing (SDSM) in the upcoming multiple-input multipleoutput (MIMO)-based IEEE 802.11n physical layer is proposed. It is shown that with SDSM, the same data rate can be achieved by using less number of transmit and receive antennas and therefore this scheme can reduce the number of antennas which results in reducing mutual coupling effects, hardware costs and implementation complexities. The maximum data rates that can be achieved using a 2 * 2 MIMO system is 270 Mbps and for a 4 * 4 MIMO system is 540 Mbps. The same data rates can be achieved using the SDSM technique which reduces the 2 * 2 MIMO system to 1 * 1 SISO system and the 4 * 4 MIMO system to a 2 * 2 MIMO system