Energy and Emission Characteristics of Superlattice Quantum-Cascade Structures

Energy levels, wave functions, and matrix elements of optical dipole transitions have been numerically calculated for superlattice quantum-cascade structures. An expression for the gain has been obtained with regard to the complete set of energy levels in different models of spectral-line broadening. A universal relation between the gain and emission spectra for unipolar laser structures has been proposed. The effect of spectral broadening on the shape of emission spectra is estimated. The electroluminescence spectra are compared with the calculated spontaneous recombination spectra and good agreement between the results is shown

Laser Diodes with a Wide Flat Modal Gain Spectrum in the (1–3)- μm Range

The gain spectra of multilayer quantum-well heterostructures on the basis of GaInAs/GaInPAs and GaInAsSb/AlGaAsSb compounds have been analyzed. The possibility of implementing of wide and almost flat spectrum of modal gain in the near IR range upon inhomogeneous excitation of quantum wells in such compounds is shown

Optoelectronic properties and characteristics of doping superlattices

Optical and electric properties of doping superlattices, or n-i-p-i crystals, can be varied in a wide range under excitation and through the choice of the thicknesses and doping of the crystal layers. Some basic results concerned the transformation of the electron energy spectrum of doping superlattices are summarized. Parameters and characteristics of doping superlattices related to optoelectronics devices, such as photodetectors, laser diodes, and optical modulators, are presented

Nonlinear optical processes in doped semiconductor superlattices

For doped semiconductor superlattices we have analyzed the effects of absorption saturation and change in the refractive index depending on the level of their excitation and structure parameters. The calculations were carried out with account for the tails of the density of states and screening of the electrostatic potential. It is shown that doped superlattices may display "shading," i.e., the increase in the absorption coefficient at a fixed frequency with increase in light intensity. For δ-doped superlattices a stronger nonmonotonicity of the change in the refractive index with increase in the excitation level in comparison with typical structures can manifest itself

Asymmetric multiple-quantum-well heterostructure laser systems: conception, performance, and characteristics

A new type of laser diodes and amplifiers based on asymmetric quantum-well heterostructures having active layers of different thickness and/or compositions has been considered. Bistable switching and regimes of regular radiation pulsation at two or three remote wavelengths in the range of 790 to 850 nm in the GaAs-AlGaAs bi- and triple-quantum-well heterostructures are described. Influence of non-linear processes including gain suppression due to carrier heating on lasing regimes has been examined. Transformation of gain bands for TE and TM modes in dependence on the pump current has been studied for asymmetric four-quantum-well structures. The interval of tuning amplification wavelengths in the system reaches up to 70 nm. Keywords: quantum-well laser diodes, non-linear effects, bistability, regular pulsation, gain spectra

Tunable photoluminescence spectra of doped semiconductor superlattices

Taking into account the density state tails appearing due to fluctuations of impurity concentrations, the spontaneous emission spectra of doped semiconductor superlattices are calculated. In the framework of the model developed, the explanation of the experimentally observed longwave edge and the shift of the photoluminescence spectra with increase in the excitation level and temperature is given. The role of the defects formed on α-irradiation is discussed, and the lifetime of current carriers is evaluated depending on the design parameters and excitation conditions of the GaAs doped superlattices

Self-consistent calculations of the energy and tunable emission spectra of doping superlattices

Based on the developed method of self-consistent calculations the results of description of tunable optical spectra versus temperature and excitation level for the GaSb doping superlattices of different design are presented. Account of the appearing tails of the density of states allows describing the long-wavelength edges and shape transformation in the spectra of absorption, gain, and luminescence and peculiarities in the optical transitions characteristics. Possible laser diode structures with n-i-p-i crystals in the active region are suggested including ordinary and δ-doped superlattices. Effects of tunable lasing are examined and ways for control of the radiation wavelength are discussed

Observation of exotic meson production in the reaction π−p→η′π−p \pi^{-} p \to \eta^{\prime} \pi^- p at 18 GeV/c

An amplitude analysis of an exclusive sample of 5765 events from the reaction $\pi^{-} p \to \eta^{\prime} \pi^- p$ at 18 GeV/c is described. The $\eta^{\prime} \pi^-$ production is dominated by natural parity exchange and by three partial waves: those with $J^{PC} = 1^{-+}, 2^{++},$ and $4^{++}$. A mass-dependent analysis of the partial-wave amplitudes indicates the production of the $a_2(1320)$ meson as well as the $a_4(2040)$ meson, observed for the first time decaying to $\eta^{\prime}\pi^-$. The dominant, exotic (non-$q\bar{q})$ $1^{-+}$ partial wave is shown to be resonant with a mass of $1.597 \pm 0.010^{+0.045}_{-0.010}$ GeV/c^2 and a width of $0.340 \pm 0.040 \pm 0.050$ GeV/c^2 . This exotic state, the $\pi_1(1600)$, is produced with a $t$ dependence which is different from that of the $a_2(1320)$ meson, indicating differences between the production mechanisms for the two states.Comment: 5 pages with 4 figure

Transcriptional landscape of the human and fly genomes: Nonlinear and multifunctional modular model of transcriptomes

Regions of the genome not coding for proteins or not involved in cis-acting regulatory activities are frequently viewed as lacking in functional value. However, a number of recent large-scale studies have revealed significant regulated transcription of unannotated portions of a variety of plant and animal genomes, allowing a new appreciation of the widespread transcription of large portions of the genome. High-resolution mapping of the sites of transcription of the human and fly genomes has provided an alternative picture of the extent and organization of transcription and has offered insights for biological functions of some of the newly identified unannotated transcripts. Considerable portions of the unannotated transcription observed are developmental or cell-type-specific parts of protein-coding transcripts, often serving as novel, alternative 5′ transcriptional start sites. These distal 5′ portions are often situated at significant distances from the annotated gene and alternatively join with or ignore portions of other intervening genes to comprise novel unannotated protein-coding transcripts. These data support an interlaced model of the genome in which many regions serve multifunctional purposes and are highly modular in their utilization. This model illustrates the underappreciated organizational complexity of the genome and one of the functional roles of transcription from unannotated portions of the genome. Copyright 2006, Cold Spring Harbor Laboratory Press © 2006 Cold Spring Harbor Laboratory Press

Partial-wave analysis of the eta pi+ pi- system produced in the reaction pi-p --> eta pi+ pi- n at 18 GeV/c

A partial-wave analysis of 9082 eta pi+ pi- n events produced in the reaction pi- p --> eta pi+ pi- n at 18.3 GeV/c has been carried out using data from experiment 852 at Brookhaven National Laboratory. The data are dominated by J^{PC} = 0^{-+} partial waves consistent with observation of the eta(1295) and the eta(1440). The mass and width of the eta(1295) were determined to be 1282 +- 5 MeV and 66 +- 13 Mev respectively while the eta(1440) was observed with a mass of 1404 +- 6 MeV and width of 80 +- 21 MeV. Other partial waves of importance include the 1++ and the 1+- waves. Results of the partial wave analysis are combined with results of other experiments to estimate f1(1285) branching fractions. These values are considerably different from current values determined without the aid of amplitude analyses.Comment: 22 pages, 8 figure