Bulk and integrated acousto-optic spectrometers for radio astronomy

The development of sensitive heterodyne receivers (front end) in the centimeter and millimeter range, and the construction of sensitive RF spectrometers (back end) enable the spectral lines of interstellar molecules to be detected and identified. A technique was developed which combines acoustic bending of a collimated coherent light beam by a Bragg cell followed by detection by a sensitive array of photodetectors (thus forming an RF acousto-optic spectrometer (AOS). An AOS has wide bandwidth, large number of channels, and high resolution, and is compact, lightweight, and energy efficient. The thrust of receiver development is towards high frequency heterodyne systems, particularly in the millimeter, submillimeter, far infrared, and 10 micron spectral ranges

Acousto-optic spectrometer for radio astronomy

A prototype acousto-optic spectrometer which uses a discrete bulk acoustic wave Itek Bragg cell, 5 mW Helium Neon laser, and a 1024 element Reticon charge coupled photodiode array is described. The analog signals from the photodiode array are digitized, added, and stored in a very high speed custom built multiplexer board which allows synchronous detection of weak signals to be performed. The experiment is controlled and the data are displayed and stored with an LSI-2 microcomputer system with dual floppy discs. The performance of the prototype acousto-optic spectrometer obtained from initial tests is reported

Formation of Low Threshold Voltage Microlasers

Vertical cavity surface emitting lasers (VCSELs) with threshold voltages of 1.7V have been fabricated. The resistance-area product in these new vertical cavity lasers is comparable to that of edge-emitting lasers, and threshold currents as low as 3 mA have been measured. Molecular beam epitaxy was used to grow n-type mirrors, a quantum well active region, and a heavily Be-doped p-contact. After contact definition and alloying, passive high-reflectivity mirrors were deposited by reactive sputter deposition of SiO2/Si3N4 to complete the laser cavity

Reconstrucción Del Discurso Pedagógico Desde El Saber Como Expresión Del Orden Establecido

El discurso docente evidencia debilidades en cuanto al uso adecuado de competencias comunicativas que desde lo pragmático, cognoscitivo, lingüístico y tecnológico contribuyan a la consolidación del aprendizaje significativo. Por tanto, el referido estudio tiene como propósito generar un modelo teórico del discurso pedagógico desde el saber como expresión del poder impuesto y del orden establecido por la sociedad para la implementación de competencias comunicativas de tipo oral y escrito en la universidad. Teóricamente, estará sustentado en la Semiótica de Saussure, la Acción Comunicativa de Habermas y el Análisis del Discurso de Van Dijk. Bajo el enfoque interpretativo, la naturaleza se inscribe en el paradigma cualitativo, nivel descriptivo, con modalidad de campo, pues los datos se tomarán desde el escenario donde interactúan los sujetos informantes, quienes serán seis docentes de tres universidades en Venezuela. La vía metodal será la Teoría Fundamentada, siendo la inducción la ruta para la construcción de conocimiento

Metastable states influence on the magnetic behavior of the triangular lattice: Application to the spin-chain compound Ca3Co2O6

It is known that the spin-chain compound Ca3Co2O6 exhibits very interesting plateaus in the magnetization as a function of the magnetic field at low temperatures. The origin of them is still controversial. In this paper we study the thermal behavior of this compound with a single-flip Monte Carlo simulation on a triangular lattice and demonstrate the decisive influence of metastable states in the splitting of the ferrimagnetic 1/3 plateau below 10 K. We consider the [Co2O6]n chains as giant magnetic moments described by large Ising spins on planar clusters with open boundary conditions. With this simple frozen-moment model we obtain stepped magnetization curves which agree quite well with the experimental results for different sweeping rates. We describe particularly the out-of-equilibrium states that split the low-temperature 1/3 plateau into three steps. They relax thermally to the 1/3 plateau, which has long-range order at the equilibrium. Such states are further analyzed with snapshots unveiling a domain-wall structure that is responsible for the observed behavior of the 1/3 plateau. A comparison is also given of the exact results in small triangular clusters with our Monte Carlo results, providing further support for our thermal description of this compound.Comment: 8 pages, 11 figures, submitted to PR

Room-Temperature Continuous-Wave Vertical-Cavity Single-Quantum-Well Microlaser Diodes

Room-temperature continuous and pulsed lasing of vertical-cavity, single-quantum-well, surface-emitting microlasers is achieved at ~983nm. The active Ga[sub][0-8]In[sub][0-2]As single quantum well is 100 [angstroms] thick. These microlasers have the smallest gain medium volumes among lasers ever built. The entire laser structure is grown by molecular beam epitaxy and the microlasers are formed by chemically assisted ion-beam etching. The microlasers are 3-50-μm across. The minimum threshold currents are 1.1 mA (pulsed) and 1.5 mA (CW)

Low-Threshold Electrically Pumps Vertical-Cavity Surface-Emitting Microlasers

Vertical-cavity electrically driven lasers with three GaInAs quantum wells and diameters of several μm exhibit room-temperature pulsed current thresholds as low as 1.3mA with 958 nm output wavelength

Developing Online Sense of Community: Graduate Students\u27 Experiences and Perceptions

Building a sense of community (SoC) is an important process in the success of distance education and students’ retention. However, developing a community in online learning environments is not an easy task. The purpose of this research study is to explore perceptions of graduate students on SoC and learning after using different collaborative activities with diverse forms of interaction (text, audio, and video) in an online educational research course. Quantitative data from two surveys and qualitative data from individual interviews were collected. Findings indicate that multimodal and scaffolding interactive activities help to support connectedness and learning, and therefore foster online graduate students’ sense of community. The results of this study add to the literature with regard to instructional strategies used to support the development of online sense of community