InGaAs quantum dots (QDs) embedded in tensile-strained GaAs1−xPx (x=0.0–0.45) barrier layers are grown using low-pressure metal-organic chemical-vapor deposition. Variable-temperature photoluminescence (PL) measurement demonstrates that the lowest-energy QD transition can be blueshifted up to 90nm compared with similar structures utilizing GaAs barriers. Temperature-dependent PL measurements and atomic force microscopy surface imaging show that the InGaAs QDs grown on GaAsP exhibit reduced height, which is consistent with shorter-wavelength emission. Preliminary results from broad stripe (100μm wide) diode lasers utilizing two stacks of InGaAs QDs embedded in GaAs0.82P0.18 barriers exhibit a 30% reduction in threshold current density compared with similar laser structures which have GaAs barriers.
One of the major challenges in today's computing world is energy management in portable devices and servers. Power management is essential to increase battery life. High end server systems use large clusters of machines that consume enormous amount of power. Past research has devised both software and hardware techniques to memory energy management but has overlooked the performance of applications in such environments. The result is that some of these techniques slowed down an application by 835%. In this paper, we look at software techniques for memory energy management without compromising on performance. The paper conceives of a new approach called BOS - Ballooning in the OS inspired from the VMware ESX server. The BOS approach consists of a kernel daemon which continuously monitors the accesses to memory chips and disk I/O. Based on the profiled information, the BOS daemon decides about powering down/up chips. Powering down is emulated within the kernel using mechanisms such as page migration and invisible buddy. Results indicate that chips with more allocated pages may not always be the most frequently accessed ones. A study has been done analyzing the effect of decreased memory size on disk activity and based on the study, a threshold based policy is proposed which is found to settle in the operating point for a simple applicaton. A single page migration incurs a cost of approximately 13μs and is one of the bottlenecks in the BOS approach.
Unexpected surges in request traffic (e.g., flash crowds) can exercise server-side resources such as access bandwidth, CPU, and disks in unanticipated ways. Administrators today do not have the requisite tools to fully understand the effect that flash crowds can have on server-side resources. As a result, most Web-servers today rely on significant over-provisioning, strict admission control, or alternatively use potentially expensive solutions like CDNs to provide high availability under load. A more fine-grained understanding of the performance of servers under emulated but controlled flash crowd like conditions can guide administrators to make more efficient provisioning and resource management decisions.We present the initial design of Mini-Flash Crowds (MFC) -a light-weight wide-area profiling service that reveals resource bottlenecks in a Web-server infrastructure, including access bandwidth, processing resources, and back-end data management. The MFC approach is based on a set of controlled measurements in which an increasing number of distributed clients make synchronized requests to exercise specific resources of a remote server. Using a number of wide-area experiments and controlled lab tests, we show that our approach can faithfully track the impact of request loads on different server resources. Our approach is non-intrusive and thus we can use it to actively probe numerous live Web servers. We present the results from a preliminary measurement study of resource provisioning on public Web servers.
The use of low-dimensional quantum dot (QD) active regions hold potential for realizing extremely low threshold current density lasers which are temperature insensitive l (i.e. high To, Tl)' Both of these factors are important for achieving high total power conversion efficiency, IIp, in diode lasers. Unfortunately, these unique features of the QD active layer have not been fully realized to date. Recently, experimental studies 2 . 3 have identified carrier leakage out of the QD as an underlying cause for low modal gain and high temperature sensitivity. Carriers which are thermally excited into the wetting layers surrounding the QD, lead to gain saturation at values significantly lower (-a factor of 4) than achievable if saturation occurs due to population inversion in the dot states.As a result, many stacked layers of QD material are needed to achieve typical threshold gain requirements in diode lasers. In addition, this carrier leakage process is highly sensitive to temperature. Recently, the use of a higher bandgap matrix4-6 imbedding the QD has been investigated, which may improve carrier confinement in the QD, and result in higher peak gain.QD lasers in the l.0/lm wavelength region have been reported using MOCVD growth, with performance comparable to MBE grown structures7• Much work to date has focused on wavelength extension towards 1.3 /lm using an InxGal_xAs compressively strained matrixR• Little work has been reported on shorter wavelength (A.,< 1 /lm) lasers grown by MOCVD. The use of a tensile-strained GaAsP or GaP matrix provides strain compensation for the compressively strained QDs and improves stacked structures9•1 O • In addition, the larger bandgap of GaAsP compared with GaAs or InGaAs should improve QD carrier confinement and allow wavelength tuning towards shorter wavelengths (i.e. 980nm), without introducing AI-containing compounds within the active region. Here, we report on the optical and structural properties of Ino.5Gao.5As (nominal composition) QDs grown on GaAs l _xP x for x=O-OAS. We find the QD ground state emission wavelength can be varied over a span of 100nm by adjusting the P content of the barrier layer. Thus, these structures have potential for achieving high performance laser structures in the shorter wavelength (A.,
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