GaAs-based quantum dot vertical-cavity surface-emitting lasers and microcavity light emitting diodes

textDue to quantum dots’ atom-like density of states, quantum dot lasers have been expected to exhibit ultralow threshold current, temperature-insensitive operation, high modulation speed due to narrow spectral linewidth, high material gain and high differential gain. Quantum dot optoelectronic devices have advanced rapidly since the advent of self-organized QD growth technique. To obtain temperature-insensitive low threshold QD lasers, ground state lasing is highly desirable, the devices should operate close to transparency and far below the saturation, and thermal population of the upper energy levels needs to be suppressed. Three stack InAs/GaAs QD edge-emitting lasers are fabricated and characterized with either 500 Å or 300 Å GaAs spacer thickness. For 500 Å spaced QD lasers, a combination of wide energy separation of 95 meV and relatively high internal efficiency of 74% leads to low threshold operation and high characteristic temperature To of 126 K beyond room temperature. In semiconductor vertical-cavity surface-emitting lasers (VCSELs), extremely high reflectivities are required for both top and bottom mirrors. Critical VCSEL design issues include alignment between the cavity resonance and the optical gain peak, lateral optical confinement, high contrast-ratio DBR mirrors. Ground state lasing of a 1.07 mm oxide-confined InGaAs/GaAs QD VCSEL is demonstrated using intracavity contacts and low loss cavity design, with the room temperature lasing threshold of ~ 700 mA and ~ 270 mA for 10 mm and 2 mm devices, respectively. The much lower experimental cavity Q is shown due to the excessive distributed loss in the upper dielectric mirror. In planar microcavity light-emitting diodes (MCLEDs) enhanced efficiency and narrower spectral linewidth are achieved through modifying the optical mode structure. To fully take advantage of the enhanced mode coupling provided by the microcavity, it is important to electronically confine the carriers to small optical mode volume, which can be achieved in apertured QD-MCLEDs. Apertured QD-MCLEDs are first demonstrated showing the efficiency enhancement with reduced mode size. The highest efficiency of 16 % for apertured QD-MCLED is achieved by designing resonance tuning at low temperature of 160 K.Electrical and Computer Engineerin

License Plate Detection with Shallow and Deep CNNs in Complex Environments

License plate detection is a challenging problem due to the large visual variations in complex environments, such as motion blur, occlusion, and lighting changes. An advanced discriminative model is needed to accurately segment license plates from the backgrounds. However, effective models for the problem tend to be computationally prohibitive. To address these two conflicting challenges, we propose to detect license plate based on two CNNs, a shallow CNN and a deep CNN. The shallow CNN is used to quickly remove most of the background regions to reduce the computation cost, and the deep CNN is used to detect license plate in the remaining regions. These two CNNs are trained end to end and are complementary to each other to guarantee the detection precision with low computation cost. Experimental results show that the proposed method is promising for license plate detection

Adaptation of Prokaryotic Toxins for Negative Selection and Cloning-Independent Markerless Mutagenesis in Streptococcus Species

ABSTRACT The Streptococcus mutans genetic system offers a variety of strategies to rapidly engineer targeted chromosomal mutations. Previously, we reported the first S. mutans negative selection system that functions in a wild-type background. This system utilizes induced sensitivity to the toxic amino acid analog p-chlorophenylalanine (4-CP) as a negative selection mechanism and was developed for counterselection-based cloning-independent markerless mutagenesis (CIMM). While we have employed this system extensively for our ongoing genetic studies, we have encountered a couple limitations with the system, mainly its narrow host range and the requirement for selection on a toxic substrate. Here, we report the development of a new negative selection system that addresses both limitations, while still retaining the utility of the previous 4-CP-based markerless mutagenesis system. We placed a variety of toxin-encoding genes under the control of the xylose-inducible gene expression cassette (Xyl-S) and found the Fst-sm and ParE toxins to be suitable candidates for inducible negative selection. We combined the inducible toxins with an antibiotic resistance gene to create several different counterselection cassettes. The most broadly useful of these contained a wild-type fst-sm open reading frame transcriptionally fused to a point mutant form of the Xyl-S expression system, which we subsequently named IFDC4. IFDC4 was shown to exhibit exceptionally low background resistance, with 3- to 4-log reductions in cell number observed when plating on xylose-supplemented medium. IFDC4 also functioned similarly in multiple strains of S. mutans as well as with Streptococcus gordonii and Streptococcus sanguinis. We performed CIMM with IFDC4 and successfully engineered a variety of different types of markerless mutations in all three species. The counterselection strategy described here provides a template approach that should be adaptable for the creation of similar counterselection systems in many other bacteria. IMPORTANCE Multiple medically significant Streptococcus species, such as S. mutans, have highly sophisticated genetic systems available, largely as a consequence of their amenability to genetic manipulation via natural competence. Despite this, few options are available for the creation of markerless mutations in streptococci, especially within wild-type strains. Markerless mutagenesis is a critical tool for genetic studies, as it allows the user to explore many fundamental questions that are not easily addressable using marked mutagenesis. Here, we describe a new approach for streptococcal markerless mutagenesis that offers a variety of advantages over the current approach, which employs induced sensitivity to the toxic substrate 4-CP. The approach employed here should be readily adaptable for the creation of similar markerless mutagenesis systems in other organisms

LytTR Regulatory Systems: A potential new class of prokaryotic sensory system.

The most commonly studied prokaryotic sensory signal transduction systems include the one-component systems, phosphosignaling systems, extracytoplasmic function (ECF) sigma factor systems, and the various types of second messenger systems. Recently, we described the regulatory role of two separate sensory systems in Streptococcus mutans that jointly control bacteriocin gene expression, natural competence development, as well as a cell death pathway, yet they do not function via any of the currently recognized signal transduction paradigms. These systems, which we refer to as LytTR Regulatory Systems (LRS), minimally consist of two proteins, a transcription regulator from the LytTR Family and a transmembrane protein inhibitor of this transcription regulator. Here, we provide evidence suggesting that LRS are a unique uncharacterized class of prokaryotic sensory system. LRS exist in a basal inactive state. However, when LRS membrane inhibitor proteins are inactivated, an autoregulatory positive feedback loop is triggered due to LRS regulator protein interactions with direct repeat sequences located just upstream of the -35 sequences of LRS operon promoters. Uncharacterized LRS operons are widely encoded by a vast array of Gram positive and Gram negative bacteria as well as some archaea. These operons also contain unique direct repeat sequences immediately upstream of their operon promoters indicating that positive feedback autoregulation is a globally conserved feature of LRS. Despite the surprisingly widespread occurrence of LRS operons, the only characterized examples are those of S. mutans. Therefore, the current study provides a useful roadmap to investigate LRS function in the numerous other LRS-encoding organisms