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

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

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

Effects of rf Current on Spin Transfer Torque Induced Dynamics

The impact of radiofrequency (rf) currents on the direct current (dc) driven switching dynamics in current-perpendicular-to-plane nanoscale spin valves is demonstrated. The rf currents dramatically alter the dc driven free layer magnetization reversal dynamics as well as the dc switching level. This occurs when the frequency of the rf current is tuned to a frequency range around the dc driven magnetization precession frequencies. For these frequencies, interactions between the dc driven precession and the injected rf induce frequency locking and frequency pulling effects that lead to a measurable dependence of the critical switching current on the frequency of the injected rf. Based on macrospin simulations, including dc as well as rf spin torque currents, we explain the origin of the observed effects.Comment: 5 pages, 4 figure

Low-Voltage-Threshold Microlasers

We have reduced the voltage required for threshold in vertical cavity surface emitting lasers (VCSEL) to 1.7 V [l], the lowest yet reported for a CW-operating VCSEL [2,3]. Room-temperature current threshold was 3 mA pulsed, 4 mA CW. This advance in VCSEL technology leads to manageable heat dissipation for high packing densities. It was achieved in a structure which can be further optimized for high wallplug efficiency and high powers. Furthermore the thickness of the molecular beam epitaxially (MBE) grown portion of the structure was reduced by about 1.5 μm compared to conventional VCSELs, resulting in decreased MBE costs, significantly shallower processing depths and easier integration of VCSELs with transistors or other electronics. The (resistance x area) products of our VCSELs are nearly as low as those reported for high-power edge-emitting lasers. MBE was used to grown-doped Al_(0.15)Ga_(0.85)As/GaAs bottom mirror layers, the active region containing 3 GaAs quantum wells, and a 1-μm-thick p-doped top contact layer. 12 pairs of alternating SiO_2/Si_3N_4 layers formed a high-reflectivity mirror which completed the laser cavity. The reactive sputter-deposited mirrors produce reflectivities of 98.3% for 9.5 pairs [3]. Individual laser elements were defined by ion milling of mesas through the p-n junction, followed by planarization with SiO_2 to define the current path. Then, Au-Zn p-contacts were deposited around the mesa tops and alloyed for current injection. A final ion-milling step was used to isolate individual contacts. In this way microlasers with diameters ranging from 7.5-25 μm were fabricated and measured