Time-resolved electron transport studies on InGaAs/GaAs-QWIPs

Due to the short internal response time, quantum-well infrared photodetectors (QWIPs) are interesting for high-speed applications such as heterodyne spectroscopy or laser pulse monitoring. We studied the photocurrent transients of InGaAs/GaAs-QWIPs after irradiation with infrared laser pulses of 250 fs duration. The excitation wavelength of about 9 μm matches the peak wavelength of the QWIP structure. The photocurrent transient consists of two different dynamical components, representing the fast photoionization in the quantum-wells and the slow injection current that compensates the remaining space charge. The investigations of the different components as a function of temperature and bias voltage were performed on a nanosecond time-scale. The experimental separation of the two photocurrent contributions allows us to determine the photoconductive gain. The Fourier transform of the photocurrent transient was compared with other experimental methods including heterodyne detection and microwave rectification. The quantitative agreement between these different measurement techniques is excellent.     

Volcanic monitoring for radon and chemical species in the soil and in spring water samples

Soil radon has been monitored at two fixed stations in the northern flank of Popocatepetl Volcano, a high risk volcano located 60 km SE from Mexico City. Water samples from three springs were also studied for radon as well as major and trace elements. Radon in the soil was recorded using track detectors. Radon in the water samples was evaluated using the liquid scintillation method and an Alphaguard. The major elements were determined through conventional chemical methods and trace elements using an ICP-MS equipment. Soil radon levels were low, indicating a moderate diffuse degassing through the flanks of the volcano. Groundwater radon had almost no relation with the eruptive stages. Water chemistry was stable in the reported time (2000–2002).     

In search of a noncontact underwater acoustic source

The history of studies of the photoacoustic effect in liquids and its applications to the development of a “virtual” source of underwater acoustic signals is briefly reviewed. The problem of the efficiency of the photoacoustic conversion is considered. The modes of laser generation of sound, i.e., the thermooptical, nonlinear surface, and nonlinear bulk modes of operation, are discussed taking into account the real features of the marine environment, the most important of them being the surface waves. A review of the publications concerned with the problem of a moving photoacoustic source is presented. Advances in the photoacoustic underwater remote sensing technology are described.     

High-Q whispering gallery modes in a spherical shell dielectric resonator: Characterization at microwave frequencies

We present a numerical and experimental characterization of high-Q whispering gallery resonant modes excited in a fused quartz spherical shell operated in the 18-26.5 GHz band; easy coupling and selection of modes is a most advantageous feature of the proposed structure with respect to ordinary bulk resonators. The numerical results, based on a finite element algorithm, well match experimental data, which also show the capability of our device to perform gaseous spectroscopy measurements. Received 25 October 2002 Published online 4 February 2003     

A biocatalytic route to enantioenriched, sulfanyl aldol products

The aldol products derived from sulfur- or selenium containing acceptors were prepared by kinetic resolution in the presence of antibody 84G3 with enantiomeric excesses ranging from 56 to 70%. Much higher level of enantioselectivity was obtained (enantiomeric excesses all superior to 96%) for sulfanyl aldol products derived from thiomethoxyacetone with three different acceptors.     

Phase formation and control of morphology in sputtered Cu-In alloy layers

The dependence of structural properties and surface morphology of Cu-In alloy layers on the composition and sputtering deposition sequence were investigated by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The properties of the co-sputtered alloy layers changed abruptly around the composition boundary when the Cu/In ratio reached 1/2. This can be explained by the effective heat of formation (EHF) model, which has been used to predict the sequence of phase formation for metal diffusion couples. The use of a co-sputtered alloy layer with a high In concentration was not suitable for fabricating solar cells, because the film had a very rough morphology due to large In islands formed on the CuIn₂ phase. However, it was possible to minimize this phase by In sputtering followed by co-sputtering with a Cu/In ratio of 1 (Cu-In/In/Glass). This permitted the fabrication of a homogeneous Cu-In alloy layer, which was not possible through the simple co-sputtering.     

On quasi-solution to infeasible linear complementarity problem obtained by Lemke’s method

For a linear complementarity problem with inconsistent system of constraints a notion of quasi-solution of Tschebyshev type is introduced. It’s shown that this solution can be obtained automatically by Lemke’s method if the constraint matrix of the original problem is copositive plus or belongs to the intersection of matrix classes P ₀ and Q ₀.     

Discrete limit theorems for general Dirichlet series. III

Here we prove a limit theorem in the sense of the weak convergence of probability measures in the space of meromorphic functions for a general Dirichlet series. The explicit form of the limit measure in this theorem is given. Partially supported by Lithuanian Foundation of Studies and Science     

On zeta-functions associated to certain cusp forms. I

In the paper the asymptotics for Dirichlet polynomials associated to certain cusp forms are obtained. Partially supported by Lithuanian Foundation of Studies and Science