An improved TM method for full two-port calibration of the asymmetric test fixtures

Three known standards, including at least one transmission standard, are normally required for the full two-port calibration of test fixtures. Based on the triple-through method, a new general-purpose calibration procedure using only one known reflection standard is proposed in this paper. The experimental results show that our method call provide a simple and accurate approach to fall two-port calibration of the asymmetric test fixtures. (c) 2005 Wiley Periodicals, Inc.

Metal catalysis-free, direction-controlled planar growth of single-crystalline alpha-Si3N4 nanowires on Si(100) substrate

Single-crystalline alpha-Si3N4 nanowires are controlled to grow perpendicular to the wet-etched trenches in the SiO0.94 film on the plane of the Si substrate without metal catalysis. A detailed characterization is carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photoluminescence at 600 nm from alpha-Si3N4 nanowires is attributed to the recombination at the defect state formed by the Si dangling bond N3 equivalent to Si-center dot. The growth mechanism is considered to be related to the catalysis and nitridation of SiO nanoclusters preferably re-deposited around the inner corner of the trenches, as well as faster Si diffusion along the slanting side walls of the trenches. This simple direction-controlled growth method is compatible with the CMOS process, and could facilitate the fabrication of alpha-Si3N4 nanoelectronic or nanophotonic devices on the Si platform.

Strong in-plane optical anisotropy of asymmetric (001) quantum wells

It is well known that asymmetry in the (001) direction can induce in-plane optical anisotropy (IPOA) in (001) quantum wells (QWs). In this letter, asymmetry is introduced in (001) GaAs/AlGaAs QWs by inserting 1 ML (monolayer) of InAs or AlAs at interfaces. Strong IPOA, which is comparable to that in the InGaAs/InP QWs with no common atom, is observed in the asymmetric GaAs/AlGaAs QW by reflectance difference spectroscopy. (C) 2006 American Institute of Physics.

Mach-Zehnder interferometers with asymmetric modulation arms in applications of high speed silicon-on-insulator based optical switches

Modulation arms with different widths are introduced to Mach-Zehnder interferometers (MZIs) to obtain improved performance. Theoretical analysis and numerical simulation have shown that when the widths of the two arms are properly designed to achieve an inherent m pi/2 (m is an odd integer) optical phase difference between the arms, the asymmetric MZI presents higher modulation speed. Furthermore, the carrier-absorption induced divergence of insertion losses in silicon-on-insulator (SOI) based MZI optical switches can be obviously improved.

Study on in-plane optical anisotropy of Si0.75Ge0.25/Si/Si0.5Ge0.5 asymmetric superlattice by reflectance difference spectroscopy - art. no. 071908

Si0.75Ge0.25/Si/Si0.5Ge0.5 trilayer asymmetric superlattices were prepared on Si (001) substrate by ultrahigh vacuum chemical vapor deposition at 500 degrees C. The nonlinear optical response caused by inherent asymmetric interfaces in this structure predicted by theories was verified by in-plane optical anisotropy in (001) plane measured via reflectance difference spectroscopy. The results show Si0.75Ge0.25/Si/Si0.5Ge0.5 asymmetric superlattice is optically biaxial and the two optical eigen axes in (001) plane are along the directions [110] and [-110], respectively. Reflectance difference response between the above two eigen axes can be influenced by the width of the trilayers and reaches as large as similar to 10(-4)-10(-3) in 15-period 2.7 nm-Si0.75Ge0.25/8 nm-Si/1.3 nm-Si0.5Ge0.5 superlattice when the normal incident light wavelength is in the range of 500-1100 nm, which is quite remarkable because the optical anisotropy does not exist in bulk Si.

InxGa1-xAs/AlyGa1-yAs/AlzGa1-zAs asymmetric step quantum-well middle wavelength infrared detectors

InxGa1-xAs/AlyGa1-yAs/AlzGa1-zAs asymmetric step quantum-well middle wavelength (3-5 mum) infrared detectors are fabricated. The components display photovoltaic-type photocurrent response as well as the bias-controlled modulation of the peak wavelength of the main response, which is ascribed to the Stark shifts of the intersubband transitions from the local ground states to the extended first excited states in the quantum wells, at the 3-5.3 mum infrared atmospheric transmission window. The blackbody detectivity (D-bb*) of the detectors reaches to about 1.0x10(10) cm Hz(1/2)/W at 77 K under bias of +/-7 V. By expanding the electron wave function in terms of normalized plane wave basis within the framework of the effective-mass envelope-function theory, the linear Stark effects of the intersubband transitions between the ground and first excited states in the asymmetric step well are calculated. The obtained results agree well with the corresponding experimental measurements. (C) 2001 American Institute of Physics.

In-plane optical anisotropy of symmetric and asymmetric (001) GaAs/Al(Ga)As superlattices and quantum wells

Two sensitive polarized spectroscopies, reflectance difference spectroscopy and photocurrent difference spectroscopy, are used to study the characteristic of the in-plane optical anisotropy in the symmetric and the asymmetric (001) GaAs/Al(Ga)As superlattices (SLs). The anisotropy spectra of the symmetric and the asymmetric SLs show significant difference: for symmetric ones, the anisotropies of the 1HH-->1E transition (1H1E) and 1L1E are dominant, and they are always approximately equal and opposite; while for asymmetric ones, the anisotropy of 1H1E is much less than that of 1L1E and 2H1E, and the anisotropy of 3H2E is very strong. The calculated anisotropy spectra within the envelope function model agree with the experimental results, and a perturbation approach is used to understand the role of the electric field and the interface potential in the anisotropy. (C) 2001 American Institute of Physics.

In-plane optical anisotropy in asymmetric Si1-xGex/Si/Si1-yGey superlattices

A trilayer asymmetric superlattice, Si/Si1-xGex/Si1-yGey, is proposed, in which the broken inversion symmetry makes the microstructure optically biaxial; in particular, inequivalent interfaces in this heterostructure may cause a polarization ratio as large as about 2.5% in the absence of an external field. The electronic structure and absorption spectra for two types of trilayer superlattice with different parameters are calculated by use of the tight-binding model; the findings indicate the importance of the carrier confinement for the anisotropy value. The effect of external electric field on the optical anisotropy for such structures has also been discussed, and a Pockels coefficient of 10-9 cm V-1 estimated.

Photoluminescence study of InxGa1-xAs/InyAl1-yAs one-side-modulation-doped asymmetric step quantum wells

Fourier transform photoluminescence measurements were carried out to investigate the optical transitions in InxGa1-xAs/InyAl1-yAs one-side-modulation-doped asymmetric step quantum wells. Samples with electron density n(s) between 0.8 and 5.3 x 10(12) cm(-2) rue studied. Strong recombination involving one to three populated electron subbands with the first heavy-hole subband is observed. Fermi edge singularity (FES) clearly can be observed for some samples. The electron subband energies in the InGaAs/InAlAs step quantum wells were calculated by a self-consistent method, taking into account strain and nonparabolicity effects and the comparison with the experimental data shows a good agreement. Our results can help improve understanding for the application of InGaAs/InAlAs step quantum wells in microelectronic and optoelectronic devices. (C) 1998 Elsevier Science Ltd. All rights reserved.

Design and Fabrication of a Photonic Crystal Channel Drop Filter Based on an Asymmetric Silicon-on-Insulator Slab

We report on the design and fabrication of a photonic crystal (PC) channel drop filter based on an asymmetric silicon-on-insulator (SOI) slab. The filter is composed of two symmetric stick-shape micro-cavities between two single-line-defect (W1) waveguides in a triangular lattice, and the phase matching condition for the filter to improve the drop efficiency is satisfied by modifying the positions and radii of the air holes around the micro-cavities. A sample is then fabricated by using electron beam lithography (EBL) and inductively coupled plasma (ICP) etching processes. The measured 0 factor of the filter is about 1140, and the drop efficiency is estimated to be 73% +/- 5% by fitting the transmission spectrum.

Optimization of biodiesel production from trap grease via acid catalysis

As a kind of waste collected from restaurants, trap grease is a chemically challenging feedstock for biodiesel production for its high free fatty acid (FFA) content. A central composite design was used to evaluate the effect of methanol quantity, acid concentration and reaction time on the synthesis of biodiesel from the trap grease with 50% free fatty acid, while the reaction temperature was selected at 95 degrees C. Using response surface methodology, a quadratic polynomial equation was obtained for ester content by multiple regression analysis. Verification experiments confirmed the validity of the predicted model. To achieve the highest ester content of crude biodiesel (89.67%), the critical values of the three variables were 35.00 (methanol-to-oil molar ratio), 11.27 wt% (catalyst concentration based on trap grease) and 4.59 h (reaction time). The crude biodiesel could be purified by a second distillation to meet the requirement of biodiesel specification of Korea.

Indole Alkaloids from the Leaves of Anthocephalus chinensis

Anthocephalusine A (1) and 3 beta-isodihydrocadambine 4-oxide (2) were isolated from the leaves of Anthocephalus chinensis (Rubiaceac), together with five known compounds. The structures were established by spectroscopic methods including 2D-NMR analyses.

Resonance-enhanced group delay times in an asymmetric single quantum barrier

It is shown that transmission and reflection group delay times in an asymmetric single quantum barrier are greatly enhanced by the transmission resonance when the energy of incident particles is larger than the height of the barrier. The resonant transmission group delay is of the order of the quasibound state lifetime in the barrier region. The reflection group delay can be either positive or negative, depending on the relative height of the potential energies on the two sides of the barrier. Its magnitude is much larger than the quasibound state lifetime. These predictions have been observed in microwave experiments. (c) 2005 Elsevier B.V. All rights reserved.