TEM modes in parallel-line transmission structures

A transmission structure of many parallel conductors is capable of propagating many distinct quasi-TEM modes. A field theory of such modes is developed for a theoretically infinite number of very thin conductors. These possess infinitely many distinct propagation modes, which are the eigenfunctions of a pair of integral operators with Green's function kernels. The modes depend strongly on the structure cross-sectional shape and size, only slightly on the number and placement of individual conductors. The theory is illustrated by a structure comprising many thin parallel wires on a dielectric substrate backed by a ground plane, whose modal functions and velocity distributions are used to solve a simple crosstalk problem     

Raman Scattering in AlN Crystals Grown by Sublimation on SiC and AlN Seeds

Semiconductors - The Raman-scattering technique is used to analyze the structural quality of bulk AlN crystals grown by sublimation on SiC and AlN seeds. Growth on SiC seeds is conducted with...     

Effects of thermal annealing on the photophysical properties of bisfluorene-cored dendrimer films

Annealing is widely used in the processing of organic semiconductors, and can modify their film morphology and photophysical properties. A study of the effect of annealing on films made from a blue emitting bisfluorene-cored dendrimer is reported. Annealing causes a 15 nm blue-shift in the photoluminescence (PL) spectrum and an 11 nm blue-shift in the amplified spontaneous emission (ASE) spectrum. It causes the PL efficiency to decrease only slightly from 0.92 to 0.83. The radiative decay rate of 1.3 × 10⁹ s^?1, the ASE threshold of 1.5 × 10¹⁸ cm^?3 and the singlet–singlet exciton annihilation rate of 5.5 × 10^?10 cm³ s^?1 are unaffected by annealing. The results indicate a scope for colour adjustment of dendrimer light-emitting diodes and lasers without affecting their efficiencies. Investigation by spectroscopic ellipsometry shows that on annealing, the films become anisotropic, with larger values of the refractive index and extinction coefficient observed for light polarised in the plane of the film than the corresponding out-of-plane values in the absorption region of the bisfluorene core. This anisotropy indicates a preferential in-plane orientation of bisfluorene cores upon annealing.     

Simulation of solar cells with quantum wells and comparison with conventional solar cells

The efficiency of photoconversion in solar cells based on GaAs with InGaAs quantum wells under the AM 1.5 conditions for various levels of base doping has been simulated using the software package Sim-Windows. The results obtained are compared with the efficiency of photoconversion in conventional solar cells. It is shown that solar cells with quantum wells can exhibit a fairly high efficiency of photoconversion in comparison with the photoconversion efficiency of conventional solar cells under the following conditions: (i) the lifetimes of for charge carriers in the quantum wells are longer than those in the barrier material and (ii) the level of doping of the base is not very high. It is established that the maximum efficiency of photoconversion in conventional solar cells is higher than the photoconversion efficiency in solar cells with quantum wells. This efficiency is attained at high doping levels in the base (～3 × 10¹⁸ cm^?3 at the parameters used in calculations). This is related to a more intense radiative recombination and also to specific features of screening and charge transport in solar cells with quantum wells at high doping levels. It is shown that, at fairly large values for the degree of concentration of incident radiation, the values for the photoconversion efficiency in solar cells with quantum wells for the low and high levels of doping of the base come closer to each other.

     

Information Processing With Pure Spin Currents in Silicon: Spin Injection, Extraction, Manipulation, and Detection

We demonstrate that information can be transmitted and processed with pure spin currents in silicon. Fe/Al₂O₃ tunnel barrier contacts are used to produce significant electron spin polarization in the silicon, generating a spin current which flows outside of the charge current path. The spin orientation of this pure spin current is controlled in one of three ways: 1) by switching the magnetization of the Fe contact; 2) by changing the polarity of the bias on the Fe/Al₂O₃ "injector" contact, which enables the generation of either majority or minority spin populations in the Si, providing a way to electrically manipulate the injected spin orientation without changing the magnetization of the contact itself; and 3) by inducing spin precession through the application of a small perpendicular magnetic field. Spin polarization by electrical extraction is as effective as that achieved by the more common electrical spin injection. The output characteristics of a planar silicon three-terminal device are very similar to those of nonvolatile giant magnetoresistance metal spin-valve structures.     

Control of charge transport mode in the Schottky barrier by δ-doping: Calculation and experiment for Al/GaAs

The possibility of controlling the effective barrier height in Schottky diodes by introducing a δ-doped layer near the metal-semiconductor contact is considered. A decrease in the effective barrier height is caused by the increased role of carrier tunneling through the barrier. A complete quantum-mechanical numerical simulation of the effect of the δ-layer parameters (concentration and depth) on the current-voltage characteristics of modified diodes was carried out for the Schottky barrier contacts to n-GaAs. The simulation results were found to fit well the experimental characteristics of diodes produced by metal-organic chemical vapor epitaxy. The studies carried out made it possible to choose the optimal δ-layer parameters to produce low-barrier (about 0.2 eV) diodes with a reasonable nonideality factor (n≤1.5). Such structures can be employed to fabricate microwave detector diodes without bias.     

Opportunistic Beamforming over Rayleigh Channels with Partial Side Information

In recent years, diversity techniques have evolved into highly attractive technology for wireless communications in different forms. For instance, the channel fluctuations of the users in a network are exploited as multiuser diversity by scheduling the user with the best signal-to-noise ratio (SNR). When fading is slow, beamforming at a multiple antenna transmitter is used to induce artificial channel fluctuations to ensure multiuser diversity in the network. Such a beamforming scheme is called opportunistic beamforming since the transmitter uses random beamforming to artificially induce opportunism in the network [1]. Opportunism requires a large number of users in the system in order to reach the performance of the true beamforming that uses perfect channel state information (CSI). In this paper we investigate the benefit of having partial CSI at an opportunistic transmitter. In the investigation, we focus on the maximum normalized SNR scheduling where user?s feedback consists of SNR relative to its channel gain. We show that opportunism can be beneficially used to increase the average throughput of the system. Simulations support the analytical average throughput results obtained as the amount of CSI and the number of users vary.     

Performance of coherent ASK lightwave systems with finiteintermediate frequency

The impact of finite intermediate frequency (IF) on the performance of heterodyne ASK lightwave systems is examined and quantified in the presence of laser phase noise and shot noise. For negligible linewidths, it is shown that certain finite choices of IF (R _b,3R_b/2,2R_b,5R_b/2, etc.) lead to the same ideal bit-error-rate (BER) performance as infinite choices of IF. Results indicate that for negligible linewidths the worst case sensitivity penalty is 0.9 dB for proper heterodyne detection and occurs when f_IF=1.25 R_b. For nonnegligible linewidths (e.g., when ΔνT⩾0.04) the sensitivity penalty is always less than 0.9 dB for finite choices of IF. The analysis presented does lead to a closed-form signal-to-noise ratio (SNR) expression at the decision gate of the receiver which can readily be used for BER and sensitivity penalty computations. The SNR expression provided includes all the key system parameters of interest such as system bit rate (R_b), the peak IF SNR (ξ), laser linewidth (Δν), and the IF filter expansion factor (α). The findings of this work suggest that the number of channels in a multichannel heterodyne ASK lightwave system can be increased substantially by properly choosing a small value for the IF at the expense of a small penalty <1 dB. On the negative side, IF frequency stabilization becomes a more critical requirement in multichannel systems employing small values of IF     

Performance analysis of Drever-Hall laser frequency stabilizationusing a proportional+integral servo

In this paper, we derive analytical expressions for finite gain, finite bandwidth laser frequency locking systems. The common case of Drever-Hall (FM sideband) locking to a Fabry-Perot cavity is discussed, using a proportional+integral (PI) controller as an example. The effects of nonnegligible loop time-delay on stability are discussed, and an optimal design procedure for a frequency locking system is given