Publication11

We demonstrate GaAs - based 0.98-μm multiple-quantum-well (MQW) tunneling injection lasers with ultrahigh-modulation bandwidths. Electrons are injected into the active region via tunneling, leading to a "cold" carrier distribution in the quantum wells (QW's). The tunneling time (∼2 ps) measured by time resolved differential transmission spectroscopy agrees with the capture time extracted form the electrical impedance measurement. The tunneling barrier prevents electrons from going over the active region into the opposite cladding layer. The carrier escape time in tunneling injection lasers is larger than that in conventional QW lasers. Enhanced differential gain, minimized gain compression and improved high frequency performance have been achieved. The -3-dB modulation bandwidth is 48 GHz and the maximum intrinsic modulation bandwidth is as high as 98 GHz.

The microenvironmental dependence of excited state prototropism of 1-naphthol and the corresponding changes in its fluorescence emission is utilized to monitor the acyl chain melting phase transition behavior of liposome membrane made from human erythrocyte lipids. A sharp increase in the ratio of neutral/anionic form fluorescence intensity is noticed at the phase transition temperature (19 °C). This provides a convenient method for obtaining phase transition temperature in lipid membranes. The membrane modifying effect of cholesterol on the erythrocyte liposome is successfully sensed by 1-naphthol fluorescence.

100BASE-T2 is a new physical-layer specification for IEEE 802.3 LANs operating at 100 Mb/s ("Fast Ethernet"). It enables users of the prevailing 10BASE-T Ethernet LAN technology to upgrade their networks from 10 to 100 Mb/s performance while retaining an existing voice-grade cabling infrastructure. 100BASE-T2 transceivers will operate over two pairs in unshielded twisted-pair cables corresponding to EIA/TIA category 3 (UTP-3), as minimally required for 10BASE-T. In a four-pair UTP-3 cable, simultaneous operation of two 100BASE-T2 links, or one 100BASE-T2 and one 10BASE-T link, is permitted. Since voice-grade cables exhibit more signal attenuation and significantly higher crosstalk coupling between adjacent pairs than data-grade cables, sophisticated digital signal processing techniques are needed to achieve reliable duplex 100 Mb/s transmission over two pairs. The 100BASE-T2 standard defines dual-duplex baseband transmission at a modulation rate of 25 Mbaud. During each modulation interval, a four-bit data nibble or Ethernet-specific control information is encoded into a pair of quinary signals. These signals are transmitted simultaneously on the two wire pairs in both signaling directions. In the receivers, adaptive digital filters are required for echo and NEXT cancellation, equalization, and interference suppression.e.

We report on the successful demonstration of a functionally complete set of logic gates based on resonant interband tunneling diodes (RITD's) with a maximum operating frequency in excess of 12 GHz. At this high frequency of operation, the power dissipation is remarkably low - on the order of 0.5 mW per gate. The circuits for all gates, AND, OR, XOR, and INV, shared the same layout geometry, consisting of two Schottky diodes and three RITD's. Logical functionality was determined solely by varying the relative areas of the devices.

In the title compound, C31H26N2O, the pyrazoline ring is in a distorted sofa conformation, the cyclohexanone ring of the tetralone moiety is in a half-chair conformation and the other aromatic rings are planar.

The title compound, [Cu(NCS)2(C17H21N5)], displays a distorted square-pyramidal coordination geometry, where the basal plane is defined by the tridentate ligand and by one of the thiocyanate ions. The apical position is occupied by the other thiocyanate ion.

Singly curved or doubly curved shells, most commonly used in large span structures, are analyzed using flat plate/shell/plate-shell. Depending on their structural actions and stresses generated, sometimes they are also prestressed. If the prestressing is in a direction where the curvature of the shell is negligible or zero, the shell structure could be analyzed using surface elements (e.g. cylindrical shells prestressed longitudinally). If the prestressing is in a direction where the structural element has a curvature (e.g. cylindrical shells prestressed along the curved axis), it introduces circumferential forces and hence may introduce stress gradients in direction normal to the curved axis viz., in the thickness direction, which cannot be idealised using surface elements. Analysis of such shells using ADINA code is presented in this paper. The results show that for a prestressing cable placed somewhere within the thickness of the shell and sufficiently away from the boundaries, the variation of normal radial stress changes from tension above the cable profile to compression below the cable profile, clearly indicating the existence of radial tension. The magnitude of the radial tension is influenced by the parameters like radius, semi-central angle, rise of the shell and thickness. The radial stress varies inversely with radius and directly with rise, while change in thickness and semi-central angle do not affect its behaviour at the apex.

Nanometer sized gold and silver clusters protected with 2- mercaptobenzothiazole monolayers have been prepared and characterized by various spectroscopic methods. Optical absorption spectra show features assigned to charge-transfer excitation between the monolayer and the cluster, in addition to a red shifting and reduction of plasmon absorption. The monolayers on clusters are compared with the corresponding 2D-monolayers investigated previously. The dominant adsorbate geometries on these clusters are different. Whereas the temperature dependent dynamics are minimal for 2D- SAMs, they are significant for monolayers on 3D surfaces, which is attributed to the decreased packing density on the cluster surfaces. The thermal stability of these monolayers is high and comparable to that of alkanethiolate monolayers. The monolayers undergo irreversible structural changes upon heating, which have been studied by differential scanning calorimetry and IR spectroscopy.

The fluorescence quenching behavior of 2-naphthol was studied with aliphatic and alcohol amines (diethylamine, triethylamine, n-propylamine, n-butylamine, ethanolamine, diethanolamine, triethanolamine) as quenchers in solvents of varying polarity (cyclohexane, acetonitrile and methanol). The quenching proceeded via the formation of an emissive exciplex. The equilibrium constant for the formation of the exciplex (KE) in the first excited state was calculated from the fluorescence intensities of the monomer and exciplex intensities. All the rate constants appearing in the kinetic scheme (k1 to k6) for the two state system were then estimated with this equilibrium constant (KE) by using an approximate but essentially steady-state method based on the Ware's mechanism. Variable temperature experiments were carried out to obtain certain thermodynamic parameters like the entropy (ΔSE0), enthalpy (ΔHE0), free energy of formation (ΔGE0) and the formation constant (KE) for the exciplex. The equilibrium constant (KE) calculated from both the methods agreed very well. AM1 calculations were also done to obtain information about the ground and excited geometry of the naphthol-amine complex.

In the title compound, C18H16O4S, the pyran ring adopts a sofa conformation. The bond angles around the two planar C atoms in the ring deviate from their ideal values.

The conformational analysis of a synthetic peptide Boc-Lys(Z)-Gly-Val-NHMe has been carried out, as a model for nucleating segment in helix formation. 1H NMR studies (270 MHz) suggested that the Gly (2) NH, Val (3) NH and NHMe groups are solvent shielded. Conformational energy calculations and intramolecular hydrogen bonding constrains favour 310 helix structure for the peptide. Theoretical and spectroscopic results are consistent with the presence of a transannular 4 → 1 hydrogen bond between Lys (1) CO and NHMe with Gly (2) NH and Val (3) being sterically shielded from the solvent environment.

Au and Ag clusters of 3-nm mean diameter were prepared with 1,4- benzenedimethanethiol (BDMT) as the capping molecule. In both the monolayer- capped clusters, BDMT adsorbs in the dithiolate form in contrast to in the 2D monolayers, where the dithiolate form exists only in the case of Ag. As a result, the molecule lies flat on the Cluster surface in both Au and Ag, whereas the molecular plane is perpendicular to the surface in the 2D monolayer of Au. This difference in adsorbate structure is attributed to the evolution in the cluster surface as adsorption occurs. Transmission electron microscopy and X-ray powder diffraction suggest that no super-lattice of the clusters exists. Mass spectrometry suggests the presence of the phase transfer reagent at the surface. X-ray photoelectron spectroscopy confirms the chemical similarity of the two surfaces and suggests that the thiolate at the surface is susceptible to X-ray beam induced damage.

Tool wear detection has traditionally restricted itself to 2D study and measurement. A new technique for the measurement and visualisation of tool wear pattern has been presented in this paper. This method provides visualisation of the tool wear geometry using a pair of stereo images and generates the volume of crater wear as a new parameter for inspection. The results demonstrate that the volume of crater wear can be effectively used to characterise the tool wear. The average depth of the tool wear and surface area of the crater are also obtained as parameters for measurement. The technique provides a fast and a possible on-line method of tool wear analysis and measurement. © 1997 Elsevier Science Ltd.

Using substrate transfer processes, we have fabricated heterojunction bipolar transistors with submicrometer emitter-base and collector-base junctions, minimizing RC parasitics and increasing fmax to 500 GHz. The process also provides a microstrip wiring environment on a low-εr dielectric substrate. First design iterations of emitter-coupled-logic master-slave flipflops exhibit 48 GHz maximum clock frequency when connected as static frequency dividers. Baseband amplifiers have been demonstrated with bandwidths up to 85 GHz.

In the title complex, [Zn(C36H20N4S4)], the ZnII ion occupies a special position on an inversion centre with four-coordinate geometry. The porphyrin ring shows a wave-like conformation, with the best closest interporphyrin plane separation being 3.60 (6) Å. The two disordered thienyl groups are inclined with respect to the porphyrin plane at angles of 70 (4) and 67 (2)°.

The pyran ring adopts a distorted sofa conformation in the title compounds, 6-methyl-3-phenylthiochroman-4-one (C16H14O2S), (I), and 6-methoxy-3-phenylthiochroman-4-one (C16H14O3S), (II). The S atom substituted at the third position is attached equatorially in (II) and axially in (I). High-resolution proton NMR studies could not provide a conclusive explanation for this feature.

In this paper, a 6.7-kbps vector sum excited linear prediction (VSELP) coder with less computational complexity is presented. A very efficient VSELP codebook with nine basis vectors and a heuristic K-selection method (to reduce the search space and complexity) is constructed to obtain the stochastic codebook vector. The nine basis vectors are obtained by optimizing a set of randomly generated basis vectors. During the optimization process, we have trained the basis vectors to give the system apriori knowledge of the characteristics of the input. The coder is implemented on a TMS320C541 digital signal processor. The performance is evaluated by testing the 6.7-kbps VSELP coder with different test speech data taken from different speakers. The quality of the coder is estimated by comparing the performance of the 6.7-kbps VSELP coder with an 8-kbps VSELP speech coder based on the IS-54 standards. © 2002 Elsevier Science B.V. All rights reserved.

Time-dependent system size probabilities of a birth and death process related to the Rogers-Ramanujan continued fraction are obtained. The range for the parameter in this continued fraction is obtained to ensure the positivity of the recursively defined birth and death rates. The general behavior of the birth and death rates is described and the asymptotic behavior of the transition probabilities and the (quasi)-stationary distribution is determined. For the transient case the birth and death process can be seen as a model in queuing theory where the length of the queue encourages customers to join the queue (the more the merrier). © 1998 Academic Press.

A block equation solver for the solution of large, sparse, banded unsymmetric system of linear equations is presented in this paper. The method employs Crout variation of Gauss elimination technique for the solution. The solver ensures the efficient use of the available memory by doing block factorization and storage. It uses a skyline storage scheme which will avoid unnecessary operations on zero elements above the skyline which has found widespread use in banded symmetric solvers. A FORTRAN code with ample comments is provided. ©1997 by John Wiley & Sons, Ltd.