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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Cascaded two-photon spectroscopy of Yb atoms with a transportable effusive atomic beam apparatus

We present a transportable effusive atomic beam apparatus for cascaded two-photon spectroscopy of the dipole-forbidden transition (6s{sup 2} {sup 1}S{sub 0}{r_reversible} 6s7s {sup 1}S{sub 0}) of Yb atoms. An ohmic-heating effusive oven is designed to have a reservoir volume of 1.6 cm{sup 3} and a high degree of atomic beam collimation angle of 30 mrad. The new atomic beam apparatus allows us to detect the spontaneously cascaded two-photons from the 6s7s{sup 1}S{sub 0} state via the intercombination 6s6p{sup 3}P{sub 1} state with a high signal-to-noise ratio even at the temperature of 340 Degree-Sign C. This is made possible in our apparatus because of the enhanced atomic beam flux and superior detection solid angle.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Delay time calculation for dual-wavelength quantum cascade lasers

In this paper, we calculate the turn-on delay (t{sub th}) and buildup (Δt) times of a midinfrared quantum cascade laser operating simultaneously on two laser lines having a common upper level. The approach is based on the four-level rate equations model describing the variation of the electron number in the states and the photon number present within the cavity. We obtain simple analytical formulae for the turn-on delay and buildup times that determine the delay times and numerically apply our results to both the single and bimode states of a quantum cascade laser, in addition the effects of current injection on t{sub th} and Δt are explored.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Cascaded target normal sheath acceleration

A cascaded target normal sheath acceleration (TNSA) scheme is proposed to simultaneously increase energy and improve energy spread of a laser-produced mono-energetic proton beam. An optimum condition that uses the maximum sheath field to accelerate the center of the proton beam is theoretically found and verified by two-dimensional particle-in-cell simulations. An initial 10 MeV proton beam is accelerated to 21 MeV with energy spread decreased from 5% to 2% under the optimum condition during the process of the cascaded TNSA. The scheme opens a way to scale proton energy lineally with laser energy.

43 PARTICLE ACCELERATORS↗
DOE OSTI2013

Generation of picosecond pulses and frequency combs in actively mode locked external ring cavity quantum cascade lasers

We propose a robust and reliable method of active mode locking of mid-infrared quantum cascade lasers and develop its theoretical description. Its key element is the use of an external ring cavity, which circumvents fundamental issues undermining the stability of mode locking in quantum cascade lasers. We show that active mode locking can give rise to the generation of picosecond pulses and phase-locked frequency combs containing thousands of the ring cavity modes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Eight-band k·p modeling of InAs/InGaAsSb type-II W-design quantum well structures for interband cascade lasers emitting in a broad range of mid infrared

Band structure properties of the type-II W-design AlSb/InAs/GaIn(As)Sb/InAs/AlSb quantum wells have been investigated theoretically in a systematic manner and with respect to their use in the active region of interband cascade laser for a broad range of emission in mid infrared between below 3 to beyond 10 μm. Eight-band k·p approach has been utilized to calculate the electronic subbands. The fundamental optical transition energy and the corresponding oscillator strength have been determined in function of the thickness of InAs and GaIn(As)Sb layers and the composition of the latter. There have been considered active structures on two types of relevant substrates, GaSb and InAs, introducing slightly modified strain conditions. Additionally, the effect of external electric field has been taken into account to simulate the conditions occurring in the operational devices. The results show that introducing arsenic as fourth element into the valence band well of the type-II W-design system, and then altering its composition, can efficiently enhance the transition oscillator strength and allow additionally increasing the emission wavelength, which makes this solution prospective for improved performance and long wavelength interband cascade lasers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Magnetotransport in very long wave infrared quantum cascade detectors: Analyzing the current with and without illumination

Current measurements of current have been performed on a very long wave infrared quantum cascade detector under magnetic field under both dark and light conditions. The analysis of dark current as a function of temperature highlights three regimes of transport. Under illumination, the model developed is in agreement with the oscillatory component of the experimental magnetophotocurrent. It allows to identify the key points controlling the electronic transport: crucial role of extraction, location of ionized impurities and scattering mechanisms involved in the structure. This work is valuable for the future conception of high-performance quantum cascade detectors in the infrared range.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Design of cascaded low cost solar cell with CuO substrate

For many years the main focus of R and D in solar cells was the development of high-efficiency solar convertors. However with solar technology beginning to be a part of national grids and stand-alone power supplies for variety of individual customers, the emphasis has changed, namely, the cost per kilowatt- hour (kW-hr) started to be an important figure of merit. Although Si does dominate the market of solar convertors, this material has total cost of kilowatt-hour much higher than what the power grid is providing presently to customers. It is well known that the cost of raw semiconductor material is a major factor in formulation of the final cost of a solar cell. That motivated us to search and design a novel solar cell using cheap materials. The new p-i-n solar cell consists of hetero-structure cascade of materials with step by step decreasing energy gap. Since the lattice constant of these three materials do differ not more than 2%, the more expensive epitaxial fabrication methods can be used as well. It should be emphasized that designed solar cell is not a cascade of three solar cells connected in series. Our market study shows that Si solar panel which costs $250–400 / m{sup 2} leads to a cost of $0.12–0.30 / kW-hr. To the contrary, CuO based solar cells with Cadmium compounds on top, would cost $100 / m{sup 2}. This will allow the novel solar cell to produce electricity at a cost of $0.06–0.08 / kW-hr.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Long-infrared InAs-based quantum cascade lasers operating at 291 K (λ=19 μm) with metal-metal resonators

We demonstrate quantum cascade lasers in the InAs/AlSb material system emitting at wavelengths of λ = 19 μm and λ = 21 μm. The maximum operating temperatures are 291 K and 250 K, and the threshold current densities at 78 K are as low as 0.6 kA/cm{sup 2} and 1.3 kA/cm{sup 2} for the lasers emitting at λ = 19 μm and λ = 21 μm, respectively. These values represent the best performance to date for quantum cascade lasers operating above λ = 16 μm. Although the devices employ metal-metal waveguide geometries, the diffraction effects which typically hinder the output beam of THz devices are not observed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A high carrier injection terahertz quantum cascade laser based on indirectly pumped scheme

A Terahertz quantum cascade laser with a rather high injection coupling strength based on an indirectly pumped scheme is designed and experimentally implemented. To effectively suppress leakage current, the chosen quantum cascade module of the device is based on a five-well GaAs/Al{sub 0.25}Ga{sub 0.75}As structure. The device lases up to 151 K with a lasing frequency of 2.67 THz. This study shows that the effect of higher energy states in carrier transport and the long-range tunnel coupling between states that belong to non-neighbouring modules have to be considered in quantum design of structures with a narrow injector barrier. Moreover, the effect of interface roughness scattering between the lasing states on threshold current is crucial.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Cascading and local-field effects in non-linear optics revisited: A quantum-field picture based on exchange of photons

The semi-classical theory of radiation-matter coupling misses local-field effects that may alter the pulse time-ordering and cascading that leads to the generation of new signals. These are then introduced macroscopically by solving Maxwell's equations. This procedure is convenient and intuitive but ad hoc. We show that both effects emerge naturally by including coupling to quantum modes of the radiation field that are initially in the vacuum state to second order. This approach is systematic and suggests a more general class of corrections that only arise in a QED framework. In the semi-classical theory, which only includes classical field modes, the susceptibility of a collection of N non-interacting molecules is additive and scales as N. Second-order coupling to a vacuum mode generates an effective retarded interaction that leads to cascading and local field effects both of which scale as N{sup 2}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Modelling on dynamics properties of a stationary argon cascaded arc plasma flows

The gas dynamics properties of a stationary arc plasma flows are studied through the numerical simulations. A two dimensional axis-symmetric turbulent magneto-hydrodynamic plasma model is developed with the commercial code ANSYS FLUENT. The reliable κ-ε model is used to account for turbulence. In this paper, the plasma is assumed to be a fluid following Navier–Stokes equations, respecting local thermodynamic equilibrium, and described by only one temperature. Distributions of the pressure, velocity, temperature, density, and electric potential inside of thus cascaded arc are obtained for an arc current density of 10{sup 6} A/m{sup 2}. The pressure inside the arc varies from 10{sup 5} Pa to 100 Pa. The temperature at the arc axis can reach as high as 13 600 K. The electric potential drops uniformly along the axis with a magnitude of 160 V. In addition, distributions of the sonic velocity and Mach number are shown to describe supersonic behavior of thus cascaded arc, which have a good agreement with the analytical formula.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Cascading metallic gratings for broadband absorption enhancement in ultrathin plasmonic solar cells

The incorporation of plasmonic nanostructures in the thin-film solar cells (TFSCs) is a promising route to harvest light into the nanoscale active layer. However, the light trapping scheme based on the plasmonic effects intrinsically presents narrow-band resonant enhancement of light absorption. Here we demonstrate that by cascading metal nanogratings with different sizes atop the TFSCs, broadband absorption enhancement can be realized by simultaneously exciting multiple localized surface plasmon resonances and inducing strong coupling between the plasmonic modes and photonic modes. As a proof of concept, we demonstrate of 66.5% in the photocurrent in an ultrathin amorphous silicon TFSC with two-dimensional cascaded gratings over the reference cell without gratings.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Quantum dot quantum cascade infrared photodetector

We demonstrate an InAs quantum dot quantum cascade infrared photodetector operating at room temperature with a peak detection wavelength of 4.3 μm. The detector shows sensitive photoresponse for normal-incidence light, which is attributed to an intraband transition of the quantum dots and the following transfer of excited electrons on a cascade of quantum levels. The InAs quantum dots for the infrared absorption were formed by making use of self-assembled quantum dots in the Stranski–Krastanov growth mode and two-step strain-compensation design based on InAs/GaAs/InGaAs/InAlAs heterostructure, while the following extraction quantum stairs formed by LO-phonon energy are based on a strain-compensated InGaAs/InAlAs chirped superlattice. Johnson noise limited detectivities of 3.64 × 10{sup 11} and 4.83 × 10{sup 6} Jones at zero bias were obtained at 80 K and room temperature, respectively. Due to the low dark current and distinct photoresponse up to room temperature, this device can form high temperature imaging.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Scattering assisted injection based injectorless mid infrared quantum cascade laser

An injectorless five-well mid infrared quantum cascade laser is analyzed which relies on phonon scattering injection in contrast to resonant tunneling injection, which has been previously used for injectorless designs. A Monte Carlo based self-consistent electron and photon transport simulator is used to analyze the performance of the analyzed design and compare it to existing injectorless designs. The simulation results show that the analyzed design could greatly enhance the optical gain and the characteristic temperatures of injectorless quantum cascade lasers (QCLs) which have typically been hindered by low characteristic temperatures and significant temperature related performance degradation. Simulations of the analyzed device predict threshold current densities of 0.85 kA/cm{sup 2} and 1.95 kA/cm{sup 2} at 77 K and 300 K, respectively, which are comparable to the threshold current densities of conventional injector based QCLs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Comparative analysis of quantum cascade laser modeling based on density matrices and non-equilibrium Green's functions

We study the operation of an 8.5 μm quantum cascade laser based on GaInAs/AlInAs lattice matched to InP using three different simulation models based on density matrix (DM) and non-equilibrium Green's function (NEGF) formulations. The latter advanced scheme serves as a validation for the simpler DM schemes and, at the same time, provides additional insight, such as the temperatures of the sub-band carrier distributions. We find that for the particular quantum cascade laser studied here, the behavior is well described by simple quantum mechanical estimates based on Fermi's golden rule. As a consequence, the DM model, which includes second order currents, agrees well with the NEGF results. Both these simulations are in accordance with previously reported data and a second regrown device.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Double metal waveguide InGaAs/AlInAs quantum cascade lasers emitting at 24 μm

A study on far-infrared In{sub 0.53}Ga{sub 0.47}As/Al{sub 0.48}In{sub 0.52}As quantum cascade lasers operating with a double metal waveguide is presented. To increase the laser upper state lifetime, a diagonal bound-to-continuum transition scheme is used in the active region. The observed threshold current density at 50 K is 5.7 kA/cm{sup 2}, and the maximum operation temperature is 240 K. The laser emission wavelength is 24.4 μm, which is the longest wavelength in the mid-infrared quantum cascade lasers so far reported.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Regimes of external optical feedback in 5.6 μm distributed feedback mid-infrared quantum cascade lasers

External optical feedback is studied experimentally in mid-infrared quantum cascade lasers. These structures exhibit a dynamical response close to that observed in interband lasers, with threshold reduction and optical power enhancement when increasing the feedback ratio. The study of the optical spectrum proves that the laser undergoes five distinct regimes depending on the phase and amplitude of the reinjected field. These regimes are mapped in the plane of external cavity length and feedback strength, revealing unstable behavior only for a very narrow range of operation, making quantum cascade lasers much more stable than their interband counterparts.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Single mode terahertz quantum cascade amplifier

A terahertz (THz) optical amplifier based on a 2.9 THz quantum cascade laser (QCL) structure has been demonstrated. By depositing an antireflective coating on the QCL facet, the laser mirror losses are enhanced to fully suppress the lasing action, creating a THz quantum cascade (QC) amplifier. Terahertz radiation amplification has been obtained, by coupling a separate multi-mode THz QCL of the same active region design to the QC amplifier. A bare cavity gain is achieved and shows excellent agreement with the lasing spectrum from the original QCL without the antireflective coating. Furthermore, a maximum optical gain of ∼30 dB with single-mode radiation output is demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗