Low-frequency noise characteristics of GaN-based UV photodiodes with AlN/GaN buffer layers prepared on Si substrates

Nitride-based metal–semiconductor–metal ultraviolet (UV) photodetectors prepared on Si (1 1 1) substrate with stacked buffer layers were proposed and prepared. With 5 V applied bias, it was found that dark current of the fabricated device was only 7.95×10⁻¹² A. With an applied bias of 10 V, it was found that peak responsivity was 0.06 A/W, corresponding to quantum efficiency of 21.2% while UV/visible rejection ratio was 244. With 5 V applied bias, it was found that noise equivalent power, NEP and detectivity, D*, of our detector were 1.70×10⁻¹³ W and 1.18×10¹³ cm Hz^0.5 W⁻¹, respectively.     

Performance of a wavelength-tunable erbium-doped fiber laser using a Sagnac interferometer

We experimentally demonstrate a wavelength-tunable erbium-doped fiber laser that is composed of a ring cavity and a single-mode fiber Sagnac interferometer in a new and simple arrangement. We find that the fiber laser output wavelength is tunable by adjusting the filter effect of the Sagnac fiber loop through a fiber polarization controller set there. The quasi-single-wavelength continuously tunable laser outputs could be achieved within some wavelength range. The multi-wavelength laser outputs could also be observed under some appropriate settings of the polarization controller. A theoretical demonstration of the wavelength tunability about the transmission-type Sagnac loop filter has also been achieved using the Jones calculus theory.     

Measurements of long-term changes in atmospheric OCS (carbonyl sulfide) from infrared solar observations

Multi-decade atmospheric OCS (carbonyl sulfide) infrared measurements have been analyzed with the goal of quantifying long-term changes and evaluating the consistency of the infrared atmospheric OCS remote-sensing measurement record. Solar-viewing grating spectrometer measurements recorded in April 1951 at the Jungfraujoch station (46.5°N latitude, 8.0°E longitude, 3.58 km altitude) show evidence for absorption by lines of the strong ν₃ band of OCS at 2062 cm⁻¹. The observation predates the earliest previously reported OCS atmosphere remote-sensing measurement by two decades. More recent infrared ground-based measurements of OCS have been obtained primarily with high-resolution solar-viewing Fourier transform spectrometers (FTSs). Long-term trends derived from this record span more than two decades and show OCS columns that have remained constant or have decreased slightly with time since the Mt. Pinatubo eruption, though retrievals assuming different versions of public spectroscopic databases have been impacted by OCS ν₃ band line intensity differences of ∼10%. The lower stratospheric OCS trend has been inferred assuming spectroscopic parameters from the high-resolution transmission (HITRAN) 2004 database. Volume mixing ratio (VMR) profiles measured near 30°N latitude with high-resolution solar-viewing FTSs operating in the solar occultation mode over a 22 years time span were combined. Atmospheric Trace MOlecucle Spectroscopy (ATMOS) version 3 FTS measurements in 1985 and 1994 were used with Atmospheric Chemistry Experiment (ACE) measurements during 2004-2007. Trends were calculated by referencing the measured OCS VMRs to those of the long-lived constituent N₂O to account for variations in the dynamic history of the sampled airmasses. Means and 1-sigma standard deviations of VMRs (in ppbv, or 10⁻⁹ per unit air volume) averaged over 30-100 hPa from measurements at 25-35°N latitude are 0.334±0.089 ppbv from 1985 (ATMOS Spacelab 3 measurements), 0.297±0.094 ppbv from 1994 ATLAS 3 measurements, 0.326±0.074 ppbv from ACE 2004 measurements, 0.305±0.096 ppbv from ACE 2005 measurements, 0.328±0.074 from ACE 2006 measurements, and 0.305±0.090 ppbv from ACE measurements through August 2007. Assuming these parameters, we conclude that there has been no statistically significant trend in lower stratospheric OCS over the measurement time span. We discuss past measurement sets, quantify the impact of changes in infrared spectroscopic parameters on atmospheric retrievals and trend measurements, and discuss OCS spectroscopic uncertainties of the current ν₃ band parameters in public atmospheric databases.     

Structural Characterization of Venom Toxins by Physical Methods and the Perspectives on Structure-Function Correlation of Proteins

This account describes work in our laboratories on the application of physical methods to the structural studies of various toxins during the past few years. A general review of background and the meaningful results obtained from these approaches are described. The results are compared with updated information on related subjects carried out at other laboratories. Examples of structural studies on some small toxins present abundantly in Formosan cobra (Naja naja atra), one of the indigenous toxic snakes in Taiwan, are given with the emphasis on the identification and characterization of complex toxic protein components based on near-infrared Fourier transform Raman Spectroscopy. We will also describe our initial efforts in solving the solution structures of several small proteins and peptides by means of two-dimensional nuclear magnetic resonance (2D-NMR) and computer-simulated modeling. The structural information obtained by these modem physical techniques provides the framework for unraveling the complex structure-activity relationships engendered by these biomolecules.     

Very large blue and red Stark shifts of the excitonic transition in InGaAs-InP-InAsP antisymmetric coupled quantum wells

A new strained InGaAs-InP-InAsP antisymmetric coupled quantum well (CQW) structure with both very large blue and red quantum-confined Stark shifts for the first heavy-hole-to-electron excitonic transition, E _hh1fE _e1, is studied theoretically in this paper. In the antisymmetric coupled quantum well, an antisymmetric-like pair of potential profiles between the shallow-deep conduction band profile and the deepshallow valence band profile are formed. The sub-band eigen-energies, E, and the associated envelope wave functions in the CQW structures with or without an applied electric field are calculated by the transfer-matrix method. The effect of strain on the pseudomorphic layers has been taken into account. Results indicate that the strained InGaAs-InP-InAsP antisymmetric CQW structure exhibits significant enhancement of the blue and red Stark effects in the E _hh1fE _e1 transition. The influences of various antisymmetric CQW structural parameters, such as the total well width, the individual well width, the central barrier thickness and the composition of the strained layer on the quantum-confined Stark shift, as well as the envelope wave function overlap, are studied systematically. These strong Stark effects in the antisymmetric CQW structure may have potential applications in sophisticated new electronic devices, such as optical switching devices.     

Lead Tetraacetate Oxidation on Caryachine,A Phenolic Pavine

Lead tetraacetate oxidation on caryachine, a pavine alkaloid possessing a phenolic group at the position para to the N-methine, in dichloromethane yielded four products — 10-acetoxy-O-acetyl-caryachine, 10-acetoxycaryachine, 7-acetoxy-O-acetylcaryachine and (6S, 11R, 12R)-11-acetoxy-O-acetyl-caryachine. The reaction took place via adoption of an acetoxy group at a position majorly ortho to the phenolic group, but very minorly at the desired benzylic position para to the phenolic group.     

A low cost and simple configuration of multifunctional multichannel analyzer based on a personal computer

A low cost and simple configuration of multifunctional MCA based on a personal computer is described. It can be used as a spectrum multiscaling analyzer, multi-input pulse height analyzer and dual parameter MCA. It can also be used in a master-slave ADC configuration in order to cut down the dead time of the system. All the functions can be performed by loading the corresponding programs without altering the hardware configuration. Interrupt service routines are used to handle the digital data converted by ADC. A dead time counter is also provided to record the dead time profile by measuring the ADC busy time.     

Quasi pulse-echo ultrasonic technique for flaw classification

One of the important flaw characterization tasks in the field of ultrasonic non-destructive evaluation (NDE) is to provide flaw type information by analysing the flaw responses acquired during an inspection. Here a new quasi pulse-echo ultrasonic classification technique is presented which utilizes the time separation and amplitude difference of mode-converted diffracted signals to distinguish between smooth and sharp-edged flaw geometries. Experiments with cylindrical cavities, surface-breaking fatigue cracks and slag inclusions have been used to test the practicality of this approach. All results of these tests show good consistency in the separation of smooth and sharp-edged flaws, provided that the signal-to-noise ratio is sufficient. Furthermore, the scattering feature used for classification in this method is also verified by detailed elastodynamic scattering calculations.