Formation and Characterization of 1.5-Monolayer Self-Assembled InAs/GaAs Quantum Dots Using Postgrowth Annealing

In this paper, we report that low-density InAs/GaAs quantum dots (QDs) can be formed by postgrowth annealing the samples with 1.5-monolayer (ML) InAs coverage, which is thinner than the critical layer thickness for the Stranski-Krastanov growth. The annealing procedure was performed immediately after the deposition of the InAs layer. The effects of annealing time and annealing temperature on the dot density, dot size, and optical characteristics of the QDs were investigated. The optimum annealing conditions to obtain low-density QDs are longer than 60 s and higher than 500degC . Meanwhile, no luminescence can be observed for the wetting-layer, which may suggest that the postgrowth annealing will make the wetting layer thinner and thus reduce the effects of wetting layer on carrier relaxation and recombination. On the other hand, we observe that a decrease of the PL intensity at the annealing conditions of 60 s and 515degC , which is possibly due to the increasing surface dislocations resulted from the In adatom desorption at higher annealing temperature.     

Optical properties of Nd-doped Ca3(VO4)2 single crystal fiber

Optical absorption and photoluminescence of Ca₃(VO₄)₂ single crystal grown by a floating-zone technique and containing Nd³⁺ ions were investigated. High absorption coefficients and broadening of most absorption bands are present at 300 K, while substructures in some of the same bands can be evidenced at 12 K. Most features of measured spectra are characteristic of random occupation of more than a single Ca²⁺ site by the Nd³⁺ ion and of distortions provoked by different charge compensation mechanisms involving oxygen vacancies promotion in the crystal lattice. Nd³⁺ optical properties were studied by using the Judd-Ofelt theory to calculate the spectral parameters relevant for laser applications.     

A systematic design method to obtain broadband characteristics for singly-fed electromagnetically coupled patch antennas for circular polarization

We present a systematic, empirical design technique to obtain optimum broadband impedance, axial-ratio (AR) and gain bandwidths for a singly-fed electromagnetically coupled patch antenna for circular polarization. Our investigation has also revealed tradeoffs amongst obtainable AR, impedance bandwidth and AR bandwidth. Using two design examples at different frequency bands and for different senses of circular polarization, we have demonstrated the effectiveness of the proposed knowledge-based tuning method. We have obtained at C-band measured values of impedance bandwidth (VSWR/spl les/2) equal to 43%, 3-dB AR bandwidth of 8%, AR of less than 0.3 dB and a mean gain level of 7 dB. For the Ku-band element, a 40% impedance bandwidth and a 17.3% of 3-dB AR bandwidth have been obtained with a peak gain of 7.2 dBic.     

Proton-exchanged LiNbO/sub 3/ optical waveguides using stearic acid

Proton-exchanged optical waveguides have been fabricated in z-cut LiNbO/sub 3/ using a new proton source: stearic acid. These waveguides were characterized optically and were found to exhibit a step index profile with Delta n=0.118 measured at 0.633 mu m. The propagation losses were typically around 1.5 dB/cm, and the diffusion constant and the activation energy for the proton-exchange process were measured to be 5*10/sup 6/ mu m/sup 2//h and 69 kJ/mol, respectively.<>     

Spectral characteristics of vertical-cavity surface-emitting lasers with external optical feedback

The measurement of the effects of external optical feedback on the spectra of VCSELs (vertical-cavity surface-emitting lasers) is reported. It is surprising that VCSELs have a sensitivity to optical feedback comparable to that of conventional edge-emitting lasers such as DFBs despite their significantly different structures. This is because the extremely short cavity length of VCSELs negates the effects of their highly reflective output mirrors. As in edge-emitting lasers, VCSELs exhibit well-defined regimes of feedback effects in their spectra. Since optical isolators cannot be easily applied to VCSELs due to their array structure, these lasers may be most useful in applications which are not sensitive to the spectral qualities of the light source.<>     

Lipase‐catalyzed dynamic kinetic resolution of (R,S)‐fenoprofen thioester in isooctane

A lipase‐catalyzed enantioselective hydrolysis process under in situ racemization of the remaining (R)‐thioetser substrate with trioctylamine as the catalyst was developed for the production of (S)‐fenoprofen from (R,S)‐fenoprofen 2,2,2‐trifluoroethyl thioester in isooctane. Detailed investigations of trioctylamine concentration on the enzyme activation and the kinetic behavior of the thioester in racemization and enzymatic reactions were conducted, in which good agreement between the experimental data and theoretical results was observed. © 2002 Society of Chemical Industry     

A novel technique for profiling the lateral n- dopingconcentrations of submicron LDD MOS devices

This paper reports a simple I-V method for the first time to determine the lateral lightly-doped source/drain (S/D) profiles (n^- region) of LDD n-MOSFETs. One interesting result is the direct observation of the reverse-short-channel effect (RSCE). It is observed that S/D n^- doping profile is channel length dependent if reverse short-channel effect exists as a result of the interstitial imperfections caused by Oxide Enhanced Diffusion (OED) or S/D implant. Not only the lateral profiles for long-channel devices but also for short-channel devices can be determined. One other practical application of the present method for device drain engineering has been demonstrated with a LATID MOS device drain engineering work. It is convincible that the proposed method is well suited for the characterization and optimization of submicron and deep-submicron MOSFETs in the current ULSI technology     

Stress analysis of a cantilevered fixed partial denture with normal and reduced bone support

A two-dimensional finite element model was constructed to analyze the simulated mechanical behavior of a cantilevered fixed partial denture. The variations of the models were made by altering the degree of bone support, the number of splinted abutments, and the length of the pontics. High stress concentrations were observed around the connectors of the fixed prosthesis and the tooth closest to the cantilever. Reduced bone support increased the deflection and stress concentrations. There was reduction in displacement and stress concentration when the teeth were splinted together. To improve the prognosis of the fixed partial denture cantilever, the number of abutments should be increased and the number of pontics decreased.     

Magnetic properties of nickel filament polymer-matrix composites

The magnetic properties of polyethersulfone-matrix composites with 3-19 vol.% polycrystalline nickel filaments (0.4 (im diam) were investigated. These filaments were found to exhibit hysteresis energy loss 10800 J/m³ of nickel and coercive force 16.9 kA/m, compared to corresponding values of 4930 J/m³ and 4.7 kA/m for 2 μ.m diam polycrystalline nickel fibers, 1020 J/m³ and 0.5 kA/m for 20 μm diam polycrystalline nickel fibers, and 1280 J/m³ and 2.3 kA/m for solid polycrystalline nickel.