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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Amplification of terahertz radiation on transitions between Wannier - Stark ladders in weak-barrier superlattices

The experimental and numerical studies of the transport properties and wave functions of electrons in GaAs/Al{sub x}Ga{sub 1-x}As superlattices with weak barriers in a strong electric field demonstrate the possibility of terahertz lasing on transitions between Wannier - Stark ladders, which can be tuned by applying voltage. The calculated gain can be as high as 500 cm{sup -1}, which exceeds several times the values characteristic of modern THz quantum cascade lasers. Thus, one would expect lasing to occur in a simple n{sup +} - superlattice - n{sup +} stripe structure. Such lasers, due to the possibility of wide tuning and simplicity of the superlattices they are based on, could compete with cascade lasers. (terahertz radiation)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Dual-wavelength laser diodes based on epitaxially stacked heterostructures

AlGaAs/GaAs double laser heterostructures containing two different active regions have been grown by metalorganic vapour phase epitaxy in a single growth process, and their properties have been studied. A typical slope of their power - current curves is 2.3 W A{sup -1}, which is almost twice that for a laser diode (1.2 W A{sup -1}). We demonstrate lasing of the emitter regions of the dual-wavelength lasers in the range 800 - 815 nm, with a separation between the peaks of {approx}7 nm. The observed effect of pump current on the lasing wavelength of the active regions in the dual-wavelength and single lasers indicates that the emitter region farther away from the heat sink is more sensitive to pump current variations. (lasers)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

808-nm laser diode bars based on epitaxially stacked double heterostructures

We have fabricated and investigated linear arrays of single laser diodes (LDs) and epitaxially stacked double LDs based on AlGaAs/GaAs heterostructures emitting in the 808-nm range. The power - current characteristic of the double-LD bars has a slope of 2.18 W A{sup -1}, which is almost twice that of the single-LD bars (1.16 W A{sup -1}). The voltage drop across the former bars is also larger. At a pump current of 60 A, the output power of 5-mm-long arrays of LDs based on epitaxially stacked double heterostructures is 100 W under quasi-cw pumping, which is a factor of 1.8 above that of the single-LD bars under identical conditions. (lasers)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Leaky-wave semiconductor laser with improved energetic characteristics and very narrow dirrectional pattern

A leaky-wave semiconductor laser diode has been developed based on the InGaAs/GaAs/InGaP heterostructure. This design made it possible to obtain a high radiation output in a narrow angular range (about 1{sup 0}-2{sup 0}) with an energy of 170 {mu}J in a laser with a cavity length of 0.8 mm and a stripe contact width of 360 {mu}m, pumped by a single current pulse with an amplitude of 88 A and width of 5 {mu}s. (lasers)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Threshold characteristics of semiconductor lasers under conditions of violation of electroneutrality in quantum wells

The threshold characteristics of semiconductor lasers are studied theoretically when the electroneutrality in quantum wells is violated. It is shown that even with the infinitely large threshold concentration of the charge carriers of one sign in the wells, the minimum threshold concentration of the carriers of the opposite sign is nonzero. It is found that in InGaAs/GaAs/AlGaAs heterostructures emitting near the wavelength 1.044 {mu}m, in a wide range of values of the electron concentration in the wells the threshold concentrations of free electrons and holes in the waveguide region are small, the contribution of the recombination current in the waveguide region to the total threshold current is negligible and in the case of a single quantum well, the threshold current density is virtually constant, i.e., the violation of electroneutrality in the InGaAs/GaAs/AlGaAs structures with a single quantum well has almost no effect on the threshold current. In the structures with two or three wells the violation of electroneutrality manifests itself much stronger and can lead to either a decrease or an increase in the threshold current. (semiconductor lasers. physics and technology)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effect of thermal neutrons on emission characteristics of InGaAs/AlGaAs heterolasers

It is studied how the threshold current of InGaAs/AlGaAs quantum-well injection heterolasers emitting near the wavelength λ = 0.7 μm changes under irradiation by thermal neutrons. It is found that the threshold pump current decreases at small doses (10{sup -2} neutron cm{sup -2}), while doses exceeding 6 × 10{sup 7} neutron cm{sup -2} cause an increase in this current and degradation of the structure. It is found that the main reasons for an increase in the threshold current at high irradiation doses are the nuclear reactions of the {sub 49}In{sup 115} (n, γ) → {sub 49}In{sup 116} type and the β-decay of the {sub 49}In{sup 116} isotope, which results in the appearance of {sub 50}Sn{sup 116} atoms. (lasers)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Ultranarrow-waveguide AlGaAs/GaAs/InGaAs lasers

We have designed, fabricated and studied ultranarrow-waveguide heterostructure lasers emitting in the spectral range 1000 – 1100 nm. The lasers have been characterised by current – voltage, light – current, far-field intensity distribution and internal optical loss measurements. The ultranarrow-waveguide lasers have been shown to have a threshold current density of ∼75 A cm{sup −2}, internal quantum efficiency near 100 % and internal optical loss near the lasing threshold under 1 cm{sup −1}, which corresponds to the level of standard heterostructures. We have demonstrated the possibility of obtaining up to 5 W of output power in continuous mode and up to 30 W in pulsed mode, with a beam convergence (FWHM) of 17.8°. The slope of the internal optical loss as a function of pump current for the ultranarrow-waveguide lasers can be markedly lower than that in lasers with a standard design, but internal quantum efficiency drops to 40 % with increasing pump current. The use of barrier layers in ultranarrow-waveguide lasers makes it possible to substantially reduce the drop in internal quantum efficiency. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Investigation of an electron-beam pumped VECSEL based on an InGaAs/AlGaAs heterostructure

We report the results of a study of an e-beam pumped vertical-external-cavity surface-emitting laser (VECSEL) based on an InGaAs/AlGaAs heterostructure. Metalorganic chemical vapour deposition (MOCVD) is employed to grow two structures of different design, which contain 10 quantum wells (QWs) and a built-in distributed Bragg reflector (DBR) mirror. Under repetitively pulsed electron-beam excitation (50 Hz, 250 ns), a peak output power of 5.5 W is achieved at a wavelength of 2.5 W and an output power of 2.5 W at 1.013 μm with a total convergence angle no larger than 20 mrad. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Continuous-wave laser diodes based on epitaxially integrated InGaAs/AlGaAs/GaAs heterostructures

We report the results of the development and investigation of the main characteristics of integrated laser emitters for the spectral range from 1040 to 1080 nm. These devices are fabricated using epitaxially integrated InGaAs/AlGaAs/GaAs heterostructures with one or two emitting regions, and measurements are performed in pulsed, quasi-continuous, and continuous-wave pump regimes. It is found that along with the obvious advantage of integrated emitters − an increase in the output optical power, they also have a significant limitation, which consists in increasing the amount of released heat. Despite this, it is shown that such integrated double laser diodes operate efficiently in the continuous-wave generation regime (P{sub max} ∼ 6 W), demonstrating a 1.7-fold increase in the differential quantum efficiency as compared to single laser diodes. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Leaky wave in high-power AlGaAs/InGaAs/GaAs semiconductor lasers

Lasers based on AlGaAs/InGaAs/GaAs heterostructures operating in the spectral range of 1.0 − 1.1 μm are investigated in order to optimise cladding layers. The effect of the thickness and composition of the cladding layers on the leakage of radiation from the laser waveguide is analysed. It is shown that for cladding thicknesses of 0.86 − 1.24 μm, it almost does not affect the output optical power. The effect of the crystal length and reflectivity of the laser mirrors on the leaky wave is demonstrated. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Lossless backward second-harmonic generation of extremely narrow subdiffractive beams in two-dimensional photonic crystals

We report efficient second harmonic generation using extremely narrow beams (with diameter of the order of the wavelength) that propagate in the self-collimation (or nondiffractive) regime in a two-dimensional photonic crystal. We design and numerically test an AlGaAs photonic crystal membrane, where both fundamental and generated second harmonic beams propagate without diffraction and without out-of-plane losses. The characteristics of the generated second harmonic that is mainly propagating in the backward direction and the conversion efficiency of the proposed scheme are obtained by nonlinear finite-difference time-domain numerical simulations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Optical precursors with tunneling-induced transparency in asymmetric quantum wells

A scheme for separating optical precursors from a square-modulated laser pulse through an asymmetric double Al{sub x}Ga{sub 1-x}As/GaAs quantum-well structure via resonant tunneling is proposed. Destructive interference inhibits linear absorption, and a tunneling-induced transparency (TIT) window appears with normal dispersion, which delays the main pulse; then optical precursors are obtained. Due to resonant tunneling, constructive interference for nonlinear susceptibility is created. The enhanced dispersion in a narrow TIT window is about one order of magnitude larger than that of the linear case. In this case, the main pulse is much delayed and the precursor signals are easier to obtain. Moreover, the main pulse builds up due to the gain introduced by the enhanced cross-nonlinearity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Structural and optical changes induced by incorporation of antimony into InAs/GaAs(001) quantum dots

We present experimental evidence of Sb incorporation inside InAs/GaAs(001) quantum dots exposed to an antimony flux immediately before capping with GaAs. The Sb composition profile inside the nanostructures as measured by cross-sectional scanning tunneling and electron transmission microscopies show two differentiated regions within the quantum dots, with an Sb rich alloy at the tip of the quantum dots. Atomic force microscopy and transmission electron microscopy micrographs show increased quantum-dot height with Sb flux exposure. The evolution of the reflection high-energy electron-diffraction pattern suggests that the increased height is due to changes in the quantum-dot capping process related to the presence of segregated Sb atoms. These structural and compositional changes result in a shift of the room-temperature photoluminescence emission from 1.26 to 1.36 {mu}m accompanied by an order of magnitude increase in the room-temperature quantum-dot luminescence intensity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Electron and hole gas in modulation-doped GaAs/Al{sub 1-x}Ga{sub x}As radial heterojunctions

We perform self-consistent Schroedinger-Poisson calculations with exchange and correlation corrections to determine the electron and hole gas in a radial heterojunction formed in a GaAs/AlGaAs core-multi-shell nanowire, which is either n- or p-doped. We show that the electron and hole gases can be tuned to different localizations and symmetries inside the core as a function of the doping density/gate potential. Contrary to planar heterojunctions, conduction electrons do not form a uniform 2D electron gas (2DEG) localized at the GaAs/AlGaAs interface, but rather show a transition between an isotropic, cylindrical distribution deep in the GaAs core (low doping) and a set of six tunnel-coupled quasi-1D channels at the edges of the interface (high doping). Holes, on the other hand, are much more localized at the GaAs/AlGaAs interface. At low doping, they present an additional localization pattern with six separated 2DEGs strips. The field generated by a back-gate may easily deform the electron or hole gas, breaking the sixfold symmetry. Single 2DEGs at one interface or multiple quasi-1D channels are shown to form as a function of voltage intensity, polarity, and carrier type.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Pseudomorphic growth of InAs on misoriented GaAs for extending quantum cascade laser wavelength

The authors have studied the impact of epilayer strain on the deposition of InAs/GaAs on (100) and (111)B with 2° offset toward<2-1-1> surfaces. Consequences of a 7% lattice mismatch between these orientations in the form of three-dimensional growth are less apparent for (111)B with 2° offset toward<2-1-1> surfaces compared to (100). By exploring a range of molecular beam epitaxy process parameters for InAs/GaAs growth and utilizing scanning electron microscopy, atomic force microscopy, and Raman spectroscopy to evaluate the quality of these strained layers, the authors develop empirical models that describe the influence of the process conditions in regards to surface roughness with >92% accuracy. The smoothest InAs/GaAs samples demonstrated average surface roughness of 0.08 nm for 10 μm{sup 2} areas, albeit at very low deposition rates. The authors have found the most important process conditions to be substrate temperature and deposition rate, leading us to believe that controlling diffusion length may be the key to reducing defects in severely strained structures. InGaAs/AlGaAs quantum cascade laser structures were also produced on (111)B with 2° offset toward<2-1-1> to take advantage of the piezoelectric effect, and the modified laser transitions due to these effects were observed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural and electrical properties of InAlAs/InGaAs/InAlAs HEMT heterostructures on InP substrates with InAs inserts in quantum well

A complex study of the influence of nanoscale InAs inserts with thicknesses from 1.7 to 3.0 nm introduced into In{sub 0.53}Ga{sub 0.47}As quantum wells (QWs) on the structural and electrical properties of In{sub 0.52}Al{sub 0.48}As/In{sup 0.53}Ga{sub 0.47}As/In{sub 0.52}Al{sub 0.48}As heterostructures with one-sided δ-Si-doping has been performed. The structural quality of a combined QW was investigated by transmission electron microscopy. A correlation between the electron mobility in QW with the thickness of InAs insert and the technology of its fabrication is established. Specific features of the InP(substrate)/InAlAs(buffer) interface are investigated by transmission electron microscopy and photoluminescence spectroscopy. A relationship between the energy positions of the peak in the photoluminescence spectra in the range of photon energies 1.24 eV < ħω < 1.38 eV, which is due to the electronic transitions at the InP/InAlAs interface, and the structural features revealed in the interface region is established. It is found that an additional QW is unintentionally formed at the InP/InAlAs interface; the parameters of this QW depend on the heterostructure growth technology.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗