Blue-green ZnSe lasers with a new type of active region

We report the results of an experimental study of molecular-beam epitaxy of ZnSe-based laser heterostructures with a new structure of the active region, which contains a fractional-monolayer CdSe recombination region in an expanded ZnSe quantum well and a waveguide based on a variably-strained, short-period superlattice are reported. Growth of a fractional-monolayer CdSe region with a nominal thickness of 2–3 ML, i.e., less than the critical thickness, on a ZnSe surface (Δa/a∼7%) leads to the formation of self-organized, pseudomorphic, CdSe-enriched islands with lateral dimensions ∼10–30 nm and density ∼2×10¹⁰ cm⁻², which serve as efficient centers of carrier localization, giving rise to effective spatial separation of defective regions and regions of radiative recombination and, as a result, a higher quantum efficiency. Laser structures for optical pumping in the (Zn, Mg) (S, Se) system with a record-low threshold power density (less than 4 kW/cm² at 300 K) and continuous-wave laser diodes in the system (Be, Mg, Zn) Se with a 2.5 to 2.8-ML-thick, fractional-monolayer CdSe active region have been obtained. The laser structures and diodes have an improved degradation resistance. Fiz. Tekh. Poluprovodn. 33, 1115–1119 (September 1999)     

T-Ray Sensing and Imaging

T-ray wavelengths are long enough to pass through dry, nonpolar objects opaque at visible wavelengths, but short enough to be manipulated by optical components to form an image. Sensing in this band potentially provides advantages in a number of areas of interest to security and defense such as screening of personnel for hidden objects and the retection of chemical and biological agents. Several private companies are developing smaller, reliable cheaper systems allowing for commercialization and this motivates us to review a number of promising applications within this paper. While there are a number of challenges to be overcome there is little doubt that T-ray technology will play a significant role in the near future for advancement of security, public health, and defense.     

Polariton condensate coherence in planar microcavities in a magnetic field

In planar GaAs microcavities in a magnetic field up to 5 T perpendicular to the structure growth plane, under conditions of resonant pulsed pumping to a point close to the inflection point of the lower dispersion curve, Zeeman splitting of the spin sublevels of the polariton condensate is observed. This is accompanied by a significant change in the degree of circular polarization and the second-order correlator g²(0). It is found that the correlator is different for the spin sublevels of the polariton condensate, split in a magnetic field. In particular, correlator measurements indicate different condensation thresholds for the spin sublevels. The correlator values initially differing in terms of the absence of a field increase, reach a maximum, and then decrease and become equal for different polarizations in a field of 5 T.     

Optimal DLC protocol configuration for realistic broadband fixed wireless access networks based on ATM

DLC protocols, scheduling, and multiplexing issues for broadband fixed wireless access networks based on ATM are discussed. The established DSA++ mac protocol has been developed at ComNets and has been widely applied within the German project ATMmobil. It has been a fundamental contribution to the etsi bran standardisation of HIPERLAN /2. The DSA++ is here modified to perform optimal within the fixed wireless access network environment. Therefore, realistic traffic as well as radio channel models for the respective scenarios are introduced. Simulation results are presented, showing the feasibility of the respective protocol stack for offering broadband multimedia services with reasonable quality of service. In addition to this advanced protocol approach, two systems are discussed which are currently under standardisation process. The IEEE 802.16 as well as the European hiperaccess systems are both proposed for fixed wireless access networks. Expected scenarios and applications are presented and basic requirements the standards have to fulfil are derived. As another aspect of access networks, their impact on the global information society is discussed.     

Area, Delay, and Power Characteristics of Standard-Cell Implementations of the AES S-Box

Cryptographic substitution boxes (S-boxes) are an integral part of modern block ciphers like the Advanced Encryption Standard (AES). There exists a rich literature devoted to the efficient implementation of cryptographic S-boxes, wherein hardware designs for FPGAs and standard cells received particular attention. In this paper we present a comprehensive study of different standard-cell implementations of the AES S-box with respect to timing (i.e. critical path), silicon area, power consumption, and combinations of these cost metrics. We examine implementations which exploit the mathematical properties of the AES S-box, constructions based on hardware look-up tables, and dedicated low-power solutions. Our results show that the timing, area, and power properties of the different S-box realizations can vary by up to almost an order of magnitude. In terms of area and area-delay product, the best choice are implementations which calculate the S-box output. On the other hand, the hardware look-up solutions are characterized by the shortest critical path. The dedicated low-power implementations do not only reduce power consumption by a large degree, but they also show good timing properties and offer the best power-delay and power-area product, respectively.     

Chiral Excitonic Organic Photodiodes for Direct Detection of Circular Polarized Light

A facile route to soft matter self‐powered bulk heterojunction photodiode detectors sensitive to the circular polarization state of light is shown based on the intrinsic excitonic circular dichroism of the photoactive layer blend. As light detecting materials, enantiopure semiconducting small molecular squaraine derivates of opposite handedness are employed. Via Mueller matrix ellipsometry, the circular dichroism is proven to be of H‐type excitonic nature and not originating from mesoscopic structural ordering. Within the green spectral range, the photodiodes convert circular polarized light into a handedness‐dependent photocurrent with a maximum dissymmetry factor of ±0.1 corresponding to 5% overall efficiency for the polarization discrimination under short circuit conditions. On the basis of transfer matrix optical simulations, it is rationalized that the optical dissymmetry fully translates into a photocurrent dissymmetry for ease of device design. Thereby, the photodiode's ability to efficiently distinguish between left and right circularly polarized light without the use of external optical elements and voltage bias is demonstrated. This allows a straightforward and sustainable future design of flexible, lightweight, and compact integrated platforms for chiroptical imaging and sensing.     

Anodization for Simplified Processing and Efficient Charge Transport in Vertical Organic Field‐Effect Transistors

Vertical organic transistors are an attractive alternative to realize short channel transistors, which are required for powerful electronic devices and flexible electronic circuits operating at high frequencies. Unfortunately, the vertical device architecture comes along with an increased device fabrication complexity, limiting the potential of this technology for application. A new design of vertical organic field‐effect transistors (VOFETs) with superior electrical performance and simplified processing is reported. By using electrochemical oxidized aluminum oxide (AlO_x) as a pseudo self‐aligned charge‐blocking structure in vertical organic transistors, direct leakage current between the source and drain can be effectively suppressed, enabling VOFETs with very low off‐current levels despite the short channel length. The anodization technique is easy to apply and can be surprisingly used on both n‐type and p‐type organic semiconductor thin films with significant signs of degradation. Hence, the anodization technique enables a simplified process of high‐performance p‐type and n‐type VOFETs, paving the road toward complementary circuits made of vertical transistors.     

Testing the Interactivity Principle: Effects of Mediation, Propinquity, and Verbal and Nonverbal Modalities in Interpersonal Interaction

Early channel reliance research compared different modes of communication to assess relationships among nonverbal and verbal cues. Emerging communication technologies represent a new venue for gaining insights into the same relationships. In this article, the authors advance a principle of interactivity as a framework for decomposing some of those relationships and report an experiment in which physical proximity—whether actors are in the same place ("co-located") or interacting at a distance ("distributed")—and the availability of other nonverbal environmental, auditory, and visual information in distributed modes is varied. Results indicate that both proximity and availability of nonverbal cues affect communication processes, social judgments participants make about each other, and task performance. The authors discuss implications about gains and losses due to presence of nonverbal features.     

Coded differential space-time modulation for flat fading channels

In this paper, powerful coding techniques for differential space-time modulation (DSTM) over Rayleigh flat fading channels and noncoherent detection without channel state information at the receiver are investigated. In particular, multilevel coding, bit-interleaved coded modulation, and so-called hybrid coded modulation (HCM) are devised and compared. For improved noncoherent reception multiple-symbol differential detection (MSDD) is adapted to DSTM. In order to reduce the computational effort required for MSDD, a low-complexity version of MSDD is applied. Evaluating the ergodic channel capacity for the different schemes as appropriate performance measure, HCM with simplified MSDD is shown to offer a favorable tradeoff between complexity and achievable power efficiency. Simulation results employing turbo codes in properly designed HCM schemes confirm the predictions from information theory.     

Analysis of temperature-dependent optical gain in GaN-InGaN quantum-well structures

The temperature dependent spectral gain in InGaN-GaN multiple quantum-well structures with 10% In content is investigated. Mode gain is measured in a temperature range between 239 K and 312 K using the Hakki-Paoli technique and compared to simulations. The simulation accounts for temperature-dependent polarization dephasing, and hence homogeneous broadening, in a rigorous fashion, without any fit parameter. It is found that the evolution of the gain spectrum with temperature at different drive currents can be modeled using a temperature-independent single value for inhomogeneous broadening. The resulting compositional fluctuations are compared to structural measurements.