Recent progress on cooled avalanche photodiodes for single photon detection

Abstract

Recent results on the properties of cooled avalanche photodiodes for single photon detection are presented. Results from Hamamatsu silicon photodiodes, originally developed as radiation-hard photodetectors for high energy physics experiments, are extremely encouraging. Gains of approximately 10,000 can be achieved with the APD operating in proportional mode. Together with a low noise amplifier they allow photon counting with extremely high efficiency and very low noise making cold APDs almost ideal single photon detectors. Operation of APDs in Geiger mode is also reported, together with measurements of detection efficiency and noise as function of operating voltage. Prospects and hopes for future work are briefly summarized.     

Effect of Laser Linewidths on Nonlinear Optical Processes Exploiting Electromagnetically Induced Transparency

Abstract

We investigate the effects of laser linewidths on the efficiency of sum-frequency generation when electromagnetically induced transparency is exploited in the manner suggested by Harris and his co-workers, which makes use of pump lasers and an intense coupling laser. The linewidths are assumed to be due to phase diffusion. Analytic expressions are obtained which are an excellent approximation when the coupling laser is strong. It is found that increasing the coupling-laser linewidth strongly increases the opacity of the medium; a first consideration is therefore to minimize this linewidth. The relative efficiency of sum-frequency generation is measured by the ratio R of the absolute value of the third-order susceptibility, χ₃, to the imaginary part of the first order susceptibility, χ₁. For large intensities of the coupling field, we show that |χ₃| is independent of the laser linewidths, whereas it increases the value of Im(χ₁).     

Effect of Co Addition on the Microstructure, Martensitic Transformation and Shape Memory Behavior of Fe-Mn-Si Alloys

The effect of Co addition has been studied in Fe-30Mn-6Si-xCo (x = 0 to 9 wt pct) shape memory alloys in terms of their microstructure, martensitic transformation and shape recovery. Microstructural investigations reveal that in Fe-Mn-Si-Co alloys, the microstructure remains single-phase austenite (??) up to 5 pct Co and beyond that becomes two-phase comprising ?? and off-stoichiometric (Fe,Co)₅Mn₃Si₂ intermetallic ??-phases. The forward ??-?? martensite transformation start temperature (M _S) decreases with the addition of Co up to 5 pct, and alloys containing more than 5 pct Co, show slightly higher M _S possibly on account of two-phase microstructure. Unlike M _S, the ??-?? reverse transformation start temperature (A _S) has been found to remain almost unaltered by Co addition. In general, addition of Co to Fe-Mn-Si alloys deteriorates shape recovery due to decreasing resistance to plastic yielding concomitant with the formation of stress induced ?? martensite. However, there is an improvement in shape recovery beyond 5 pct Co addition, possibly due to the strengthening effect arising from the presence of (Fe,Co)₅Mn₃Si₂ precipitates within the two-phase microstructure and due to higher amount of stress induced ?? martensite.     

Selective oxidation of benzaldehyde by molecular oxygen over molybdovanadophosphoric acid supported MCM-41

A series of molybdovanadophosphoric acid (MVPA) supported on mesoporous silica was synthesized by an incipient wetness impregnation method. The catalysts were characterized by nitrogen adsorption?Cdesorption, X-ray powder diffraction, Fourier-Transform Infra red spectroscopy (FT-IR), UV?CVis Diffused reflectance spectroscopy (UV?CVis DRS), Temperature programmed reduction (TPR) and ³¹P MAS Nuclear magnetic resonance(NMR) study. The characterization data reveals the incorporation of vanadium in phosphomolybdic acid and retention of intact Keggin ion on the support. The catalytic activities were evaluated for oxidation of benzaldehyde using molecular oxygen as oxidant as the new green reaction system. Among all the promoted catalysts, 50wt% molybdovanadophosphoric acid supported on MCM-41 exhibits highest catalytic activity in oxidation of benzaldehyde, giving 95% conversion. Other oxidants like H₂O₂ and tert-butyl hydrogen peroxide (TBHP) were also tested for benzaldehyde oxidation reaction.     

Study of oxygen permeability and flame retardancy properties of biodegradable polymethylmethacrylate/starch composites

Polymethylmethacrylate/starch (PMMA/starch) composites were prepared by emulsion polymerization method using potassium persulphate (KPS) as the radical initiator. The interaction of starch with polymethylmethacrylate was evidenced by ultraviolet–visible (UV–visible) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The structure of PMMA/starch composite was studied by X‐ray diffraction (XRD) study and Scanning Electron Microscopy (SEM). The thermal properties of starch, PMMA/starch composites were compared with the virgin PMMA sample. The PMMA/starch composites were found to be flame retardant material from the measurement of limiting oxygen index (LOI) of samples. The biodegradability of the PMMA/starch composites were studied by active sludge water and it was found that degradability of composites increases with increasing time interval. The oxygen permeability of PMMA/starch composites was substantially decreased as compared to virgin PMMA. The reduction of oxygen permeability of the PMMA/starch composites may enable the materials applicable in packaging industries. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Rapid cellular genesis and apoptosis: Effects of exercise in the adult rat.

Long-term aerobic exercise improves cognition in both human and nonhuman animals and induces plastic changes in the central nervous system (CNS), including neurogenesis and angiogenesis. However, the early and immediate effects of exercise on the CNS have not been adequately explored. There is some evidence to suggest that exercise is initially challenging to the nervous system and that the plastic changes commonly associated with chronic exercise may result as adaptations to this challenge. The current experiment assessed levels of apoptosis, angiogenesis, and neurogenesis during the first week of an exercise regimen in the adult rat. The results indicate that exercise rapidly induces these processes in the hippocampus and cerebellum. The temporal pattern of these events suggests that voluntary exercise in the adult rat rapidly and transiently induces apoptosis, followed by angiogenesis. Neurogenesis is an immediate and independent consequence of exercise in the hippocampus that may require the additional metabolic support supplied by angiogenesis. This is the first report of CNS neuronal apoptosis as a consequence of exercise in the adult rat and suggests that this process is a potential mediator of rapid exercise-induced plasticity. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Three-level Atom in a Squeezed Vacuum I

Abstract

We consider a three-level atom in any of its possible configurations, ladder, lambda or vee, which interacts with two classical, resonant, monochromatic electromagnetic fields. We suppose the levels     

Entropy squeezing for a two-level atom

Abstract

The usual definition of squeezing, based on the Heisenberg uncertainty principle, measures uncertainty in terms of the standard deviation. It can run into difficulties when applied to squeezing in the two-level atom. An alternative definition of squeezing is presented for this system, based on information entropy theory, which overcomes the disadvantages of the definition based on the Heisenberg uncertainty relation. The utility of this definition is illustrated by examining squeezing in the information entropy of a two-level atom in the Jaynes-Cummings model, and in resonance fluorescence.