1.55 µm AlGaInAs/InP sampled grating laser diodes for mode-locking at THz frequencies

We report mode locking in lasers integrated with semiconductor optical amplifiers, using either conventional or phase shifted sampled grating distributed Bragg reflectors(DBRs). For a conventional sampled grating with a continuous grating coupling coefficient of ~80 cm-1, mode-locking was observed at a fundamental frequency of 628 GHz and second harmonic of 1.20 THz. The peak output power was up to 142 mW. In the phase shifted sampled grating design, the grating is present along the entire length of the reflector with π-phase shift steps within each sampled section. The effective coupling coefficient is therefore increased substantially. Although the continuous grating coupling coefficient for the phase shifted gratings was reduced to ~23 cm-1 because of a different fabrication technology, the lasers demonstrated mode locking at fundamental repetition frequencies of 620 GHz and 1 THz, with a much lower level of amplified spontaneous emission seen in the output spectra than from conventional sampled grating devices. Although high pulse reproducibility and controllability over a wide operation range was seen for both types of grating, the π-phase-shifted gratings already demonstrate fundamental mode-locking to 1 THz. The integrated semiconductor optical amplifier makes sampled grating DBR lasers ideal pump sources for generating THz signals through photomixing

A Hybrid Approach to Case Teaching

We structure case based teaching using a combination of computer-support technology and in-class activities. We create and design an on-line learning tool, Case Study On-line (CSOL) that enables instructor to structure the delivery of teaching resources and set up assignment tasks in the system. The tool enables tasks to be set in a way that they can be completed sequentially, and learning resources can be accessed only when required by students. Upon submission of their assignments, students can promptly receive instructors explanations to the tasks. Follow up activities take place in class, with the objective to promote active learning. The tool was adopted over two semesters in an undergraduate economics course and surveys were conducted among the students to evaluate the effectiveness of the tool. The results of the survey were very positive, providing support to our belief that a system of well-designed case teaching delivery tool, together with classroom activities, is able to increase the level of students engagement and interest. The paper helps to address some concerns about teaching methods in undergraduate economics courses.Technologies-based Instruction, In-class Activities, Scaffolding, Active-based Learning

Redox regulation of non-photosynthetic plant metabolism

Reduction-oxidation (redox) regulation in metabolic pathways is governed by oxidoreductases. In autotrophic tissues, there are two major systems namely ferredoxin-thioredoxin (Fdx-TRX) system and NADPH-thioredoxin reductase-thioredoxin (NTR-TRX) system. Many reports have indicated the significance of both systems in regulating broad metabolic processes in chloroplast, while the redox regulation in non-photosynthetic plant metabolism remains unclear. In the present study, the impact of these thiol-redox systems on non-photosynthetic metabolism was investigated by analyzing (i) the role of mitochondrial TRX (TRXh2 and TRXo1) in Arabidopsis plants, and (ii) the role of plastidial NADPH-thioredoxin reductase C (NTRC) in heterotrophic tomato fruits. To analyze the role of mitochondrial TRXs, two T-DNA insertion lines, trxh2.1 and trxo1.1, and a cross double mutant, trxh2.1trxo1.1, were used for further assays. In the single and double mutants, the expression of corresponding genes decreased by over 95% compared to wild-type level. The mutant lines showed comparable growth phenotype to the wild type, even though there was a small decrease in leaf size depending on the light conditions. Nevertheless, the trxh2.1trxo1.1 line tended to accumulate more soluble sugars and sugar phosphates at the end of day, suggestive of enhancement in photosynthetic processes, while the levels of several phosphate intermediates, such as phosphoenopyruvate (PEP) and adenine nucleotides (ATP and ADP), decreased in the trxh2.1trxo1.1 line. This further implied that jointly decreasing the expression of TRXh2 and TRXo1 might affect energy metabolism. Metabolite profiling of mutants lines harvested at the end of night revealed an overall decrease in the metabolites of Gly decarboxylation and tricarboxylic acid cycle, while this change diminished in the samples from the end of day. This indicated that down regulating both mitochondrial TRXs affected metabolic processes in mitochondrion as well as peroxisome. Furthermore, the trxh2.1trxo1.1 line showed a better photosynthetic performance compared to the wild type either in the condition of high CO2 concentration or fluctuating light situation. This implied the potential of TRXh2 and TRXo1 in affecting extra-mitochondrial processes, such as photosynthesis. In addition to its role in chloroplasts, NTRC is also present in non-photosynthetic plastids. To investigate the role of NTRC, a NTRC RNA interference (NTRC-RNAi) construct controlled by a fruit-specific promoter was introduced into tomato plants, which eventually led to a 60%-80% decrease in NTRC expression level in transgenic tomato fruits during their development. The NTRC-RNAi lines tended to generate smaller and lighter fruits with less accumulation of dry matters, compared to the wild type. At an early developmental stage, the accumulation of transient starch was greatly reduced in response to NTRC down-regulation, which subsequently resulted in a significant decrease in ripe fruits. This mainly attributed to the inhibition on the redox-activation of two key enzymes in starch biosynthesis, namely ADP-Glc pyrophosphatase (AGPase) and soluble starch synthase. Furthermore, down-regulation of NTRC perturbed the redox poise of NAD(P)(H), which led to a large increase in the reductive states of NAD(H) and NADP(H). Through performing metabolomics, it was found that NTRC down-regulation led to a general decrease in the level of sugars, while the levels of amino acids and organic acids were mainly increased, implying the potential of NTRC in regulating osmotic balance. These changes in metabolite level are anticipated to influence the taste and flavor of tomato fruits. Overall, this indicates that NTRC serves as a central component in modulating carbon metabolism and redox balance, which ultimately affects fruit growth as well as quality. In summary, these results indicate that in leaves mitochondrial TRXs are involved in interorganellar cross-talk, affecting photosynthetic processes in the chloroplast, while plastidial NTRC in important to regulate metabolism and redox-balance in heterotrophic fruits, affecting fruit size and quality

A dual-grating InGaAsP/InP DFB laser integrated with an SOA for THz generation

We report a dual-mode semiconductor laser that has two gratings with different periods below and above the active layer. A semiconductor optical amplifier (SOA), which is integrated with the dual-mode laser, plays an important role in balancing the optical power and reducing the linewidths of the emission modes. A stable two mode emission with the 13.92-nm spacing can be obtained over a wide range of distributed feedback and SOA injection currents. Compared with other types of dual-mode lasers, our device has the advantages of simple structure, compact size, and low fabrication cost