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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 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Trajectory controllability of semilinear systems with multiple variable delays in control

In this paper, finite-dimensional dynamical control system described by semilinear differential state equation with multiple variable delays in control are considered. The concept of controllability we extend on trajectory controllability for systems with multiple point delays in control. Moreover, remarks and comments on the relationships between different concepts of controllability are presented. Finally, simple numerical example, which illustrates theoretical considerations is also given. The possible extensions are also proposed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Observation of mid-infrared intersubband absorption in non-polar m-plane AlGaN/GaN multiple quantum wells

Mid-infrared (4.20–4.84 μm) intersubband absorption in non-polar m-plane Al{sub 0.5}Ga{sub 0.5}N/GaN multiple-quantum wells is observed at room temperature. 10 period Al{sub 0.5}Ga{sub 0.5}N/GaN multiple-quantum wells were grown on free-standing m-plane GaN substrates by metalorganic chemical vapor deposition (MOCVD), and the high-quality structural and optical properties are revealed by x-ray diffraction and photoluminescence studies. Through this we have demonstrated that MOCVD grown non-polar m-plane AlGaN/GaN quantum wells are a promising material for mid-infrared intersubband devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Simulation of energy absorption spectrum in NaI crystal detector for multiple gamma energy using Monte Carlo method

The spectrum of gamma energy absorption in the NaI crystal (scintillation detector) is the interaction result of gamma photon with NaI crystal, and it’s associated with the photon gamma energy incoming to the detector. Through a simulation approach, we can perform an early observation of gamma energy absorption spectrum in a scintillator crystal detector (NaI) before the experiment conducted. In this paper, we present a simulation model result of gamma energy absorption spectrum for energy 100-700 keV (i.e. 297 keV, 400 keV and 662 keV). This simulation developed based on the concept of photon beam point source distribution and photon cross section interaction with the Monte Carlo method. Our computational code has been successfully predicting the multiple energy peaks absorption spectrum, which derived from multiple photon energy sources.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Note: Improved calibration of atomic force microscope cantilevers using multiple reference cantilevers

Overall precision of the simplified calibration method in J. E. Sader et al., Rev. Sci. Instrum. 83, 103705 (2012), Sec. III D, is dominated by the spring constant of the reference cantilever. The question arises: How does one take measurements from multiple reference cantilevers, and combine these results, to improve uncertainty of the reference cantilever’s spring constant and hence the overall precision of the method? This question is addressed in this note. Its answer enables manufacturers to specify of a single set of data for the spring constant, resonant frequency, and quality factor, from measurements on multiple reference cantilevers. With this data set, users can trivially calibrate cantilevers of the same type.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Multiple quasi-monoenergetic electron beams from laser-wakefield acceleration with spatially structured laser pulse

By adjusting the focus geometry of a spatially structured laser pulse, single, double, and treble quasi-monoenergetic electron beams were generated, respectively, in laser-wakefield acceleration. Single electron beam was produced as focusing the laser pulse to a single spot. While focusing the laser pulse to two spots that are approximately equal in energy and size and intense enough to form their own filaments, two electron beams were produced. Moreover, with a proper distance between those two focal spots, three electron beams emerged with a certain probability owing to the superposition of the diffractions of those two spots. The energy spectra of the multiple electron beams are quasi-monoenergetic, which are different from that of the large energy spread beams produced due to the longitudinal multiple-injection in the single bubble.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Raman signal enhancement by multiple beam excitation and its application for the detection of chemicals

In a typical Raman based sensor, a single laser beam is used for exciting the sample and the backscattered or forward scattered light is collected using collection optics and is analyzed by a spectrometer. We have investigated that by means of exciting the sample with multiple beams, i.e., by dividing the same input power of the single beam into two or three or more beams and exciting the sample from different angles, the Raman signal enhances significantly. Due to the presence of multiple beams passing through the same volume of the sample, an interference pattern is formed and the volume of interaction of excitation beams with the sample increases. By means of this geometry, the enhancement in the Raman signal is observed and it was found that the signal strength increases linearly with the increase in number of excitation beams. Experimental results of this scheme for excitation of the samples are reported for explosive detection at a standoff distance.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effects of quantum well growth temperature on the recombination efficiency of InGaN/GaN multiple quantum wells that emit in the green and blue spectral regions

InGaN-based light emitting diodes and multiple quantum wells designed to emit in the green spectral region exhibit, in general, lower internal quantum efficiencies than their blue-emitting counter parts, a phenomenon referred to as the “green gap.” One of the main differences between green-emitting and blue-emitting samples is that the quantum well growth temperature is lower for structures designed to emit at longer wavelengths, in order to reduce the effects of In desorption. In this paper, we report on the impact of the quantum well growth temperature on the optical properties of InGaN/GaN multiple quantum wells designed to emit at 460 nm and 530 nm. It was found that for both sets of samples increasing the temperature at which the InGaN quantum well was grown, while maintaining the same indium composition, led to an increase in the internal quantum efficiency measured at 300 K. These increases in internal quantum efficiency are shown to be due reductions in the non-radiative recombination rate which we attribute to reductions in point defect incorporation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Multiple copies of orbital angular momentum states through second-harmonic generation in a two-dimensional periodically poled LiTaO{sub 3} crystal

We experimentally demonstrate multiple copies of optical orbital angular momentum (OAM) states through quasi-phase-matched (QPM) second-harmonic (SH) generation in a 2D periodically poled LiTaO{sub 3} (PPLT) crystal. Since the QPM condition is satisfied by involving different reciprocal vectors in the 2D PPLT crystal, collinear and noncollinear SH beams carrying OAMs of l{sub 2} are simultaneously generated by the input fundamental beam with an OAM of l{sub 1}. The OAM conservation law (i.e., l{sub 2} = 2l{sub 1}) holds well in the experiment, which can tolerate certain phase-mismatch between the interacting waves. Our results provide an efficient way to obtain multiple copies of the wavelength-converted OAM states, which can be used to enhance the capacity in optical communications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Dynamic control of polarization-inverted modes in three-dimensionally trapped multiple nanogaps

We propose a guiding principle for the dynamic control of polarization-inverted modes in multiple nanogaps for unconventional optical transitions of molecules at arbitrary three-dimensional spatial positions. Based on our developed self-consistent theory for the optical assembly of nanoparticles (NPs), we clarified that spherical silver NPs can be optically trapped and aligned in the light-propagating direction via longitudinally polarized light; they form a rod-like nano-composite with multiple nanogaps. During trapping, there is a possibility that an additional irradiation of linearly polarized far-field light may excite the bonding and anti-bonding dark plasmon modes with low radiative decay rate of several meV via cancellation of inverted polarization. Our finding reveals that not only the steep change in the enhanced intensity of light field but also the phase inversion of light field between the dynamically formed nanogaps will pave the way to the highly sensitive sensors for molecules, the unconventional chemical reactions, and so on.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Accurate molecular dynamics and nuclear quantum effects at low cost by multiple steps in real and imaginary time: Using density functional theory to accelerate wavefunction methods

The development and implementation of increasingly accurate methods for electronic structure calculations mean that, for many atomistic simulation problems, treating light nuclei as classical particles is now one of the most serious approximations. Even though recent developments have significantly reduced the overhead for modeling the quantum nature of the nuclei, the cost is still prohibitive when combined with advanced electronic structure methods. Here we present how multiple time step integrators can be combined with ring-polymer contraction techniques (effectively, multiple time stepping in imaginary time) to reduce virtually to zero the overhead of modelling nuclear quantum effects, while describing inter-atomic forces at high levels of electronic structure theory. This is demonstrated for a combination of MP2 and semi-local DFT applied to the Zundel cation. The approach can be seamlessly combined with other methods to reduce the computational cost of path integral calculations, such as high-order factorizations of the Boltzmann operator or generalized Langevin equation thermostats.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

A comparison of the optical properties of InGaN/GaN multiple quantum well structures grown with and without Si-doped InGaN prelayers

In this paper, we report on a detailed spectroscopic study of the optical properties of InGaN/GaN multiple quantum well structures, both with and without a Si-doped InGaN prelayer. In photoluminescence and photoluminescence excitation spectroscopy, a 2nd emission band, occurring at a higher energy, was identified in the spectrum of the multiple quantum well structure containing the InGaN prelayer, originating from the first quantum well in the stack. Band structure calculations revealed that a reduction in the resultant electric field occurred in the quantum well immediately adjacent to the InGaN prelayer, therefore leading to a reduction in the strength of the quantum confined Stark effect in this quantum well. The partial suppression of the quantum confined Stark effect in this quantum well led to a modified (higher) emission energy and increased radiative recombination rate. Therefore, we ascribed the origin of the high energy emission band to recombination from the 1st quantum well in the structure. Study of the temperature dependent recombination dynamics of both samples showed that the decay time measured across the spectrum was strongly influenced by the 1st quantum well in the stack (in the sample containing the prelayer) leading to a shorter average room temperature lifetime in this sample. The room temperature internal quantum efficiency of the prelayer containing sample was found to be higher than the reference sample (36% compared to 25%) which was thus attributed to the faster radiative recombination rate of the 1st quantum well providing a recombination pathway that is more competitive with non-radiative recombination processes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Mutually unbiased product bases for multiple qudits

We investigate the interplay between mutual unbiasedness and product bases for multiple qudits of possibly different dimensions. A product state of such a system is shown to be mutually unbiased to a product basis only if each of its factors is mutually unbiased to all the states which occur in the corresponding factors of the product basis. This result implies both a tight limit on the number of mutually unbiased product bases which the system can support and a complete classification of mutually unbiased product bases for multiple qubits or qutrits. In addition, only maximally entangled states can be mutually unbiased to a maximal set of mutually unbiased product bases.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Multiple surface plasmons in an unbounded quantum plasma half-space

The propagation of surface plasmons on a quantum plasma half-space in the absence of any external confinement is investigated. By means of the Quantum Hydrodynamic Model in the electrostatic limit, it is found that the equilibrium density profile is a smooth continuous function which, in the linear regime, supports multiple non-normal surface modes. Defining a spectrum function and using a cutting condition, the dispersion relations of these modes and their relevance for realistic dynamics are computed. It is found that the multiple surface plasmons present a significant red-shift with respect to the case of fully bounded quantum plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Optimization schemes for efficient multiple exciton generation and extraction in colloidal quantum dots

Multiple exciton generation (MEG) is a process in which more than one electron hole pair is generated per absorbed photon. It allows us to increase the efficiency of solar energy harvesting. Experimental studies have shown the multiple exciton generation yield of 1.2 in isolated colloidal quantum dots. However real photoelectric devices require the extraction of electron hole pairs to electric contacts. We provide a systematic study of the corresponding quantum coherent processes including extraction and injection and show that a proper design of extraction and injection rates enhances the yield significantly up to values around 1.6.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Multiple exciton generation in chiral carbon nanotubes: Density functional theory based computation

We use a Boltzmann transport equation (BE) to study time evolution of a photo-excited state in a nanoparticle including phonon-mediated exciton relaxation and the multiple exciton generation (MEG) processes, such as exciton-to-biexciton multiplication and biexciton-to-exciton recombination. BE collision integrals are computed using Kadanoff-Baym-Keldysh many-body perturbation theory based on density functional theory simulations, including exciton effects. We compute internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon in the course of the relaxation. We apply this approach to chiral single-wall carbon nanotubes (SWCNTs), such as (6,2) and (6,5). We predict efficient MEG in the (6,2) and (6,5) SWCNTs within the solar spectrum range starting at the 2Eg energy threshold and with QE reaching ~1.6 at about 3Eg, where Eg is the electronic gap.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Linear and nonlinear frequency- and time-domain spectroscopy with multiple frequency combs

Two techniques that employ equally spaced trains of optical pulses to map an optical high frequency into a low frequency modulation of the signal that can be detected in real time are compared. The development of phase-stable optical frequency combs has opened up new avenues to metrology and spectroscopy. The ability to generate a series of frequency spikes with precisely controlled separation permits a fast, highly accurate sampling of the material response. Recently, pairs of frequency combs with slightly different repetition rates have been utilized to down-convert material susceptibilities from the optical to microwave regime where they can be recorded in real time. We show how this one-dimensional dual comb technique can be extended to multiple dimensions by using several combs. We demonstrate how nonlinear susceptibilities can be quickly acquired using this technique. In a second class of techniques, sequences of ultrafast mode locked laser pulses are used to recover pathways of interactions contributing to nonlinear susceptibilities by using a photo-acoustic modulation varying along the sequences. We show that these techniques can be viewed as a time-domain analog of the multiple frequency comb scheme.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Cherenkov detector analysis for implosions with multiple nuclear reactions

Nuclear reactions that produce γ rays occur in inertial fusion implosions and are commonly measured with Cherenkov detectors. Typically a detector is primarily sensitive to a single reaction, but in some implosions, multiple fusion reactions can occur and are combined in the data. Here we discuss an analysis technique using multiple thresholded detectors to reproduce the individual burn histories from reactions like DT and HT fusion, which is applicable to separated-reactant mix experiments. Finally, requirements for this technique and resulting analysis uncertainties are quantified using synthetic data.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Using multiple x-ray emission images of inertially confined implosions to identify spatial variations and estimate confinement volumes (invited)

We describe two methods to analyze multiple x-ray images of a small, self-emitting object, and we apply these methods to the stagnating hotspots in inertial confinement fusion experiments. The first method, the common integrated profile, can be employed to assess and quantify spatial variations in opacity. It is both a simple assessment of consistency and a sophisticated measurement of variations in a region that is otherwise difficult to observe. Second, we present a method to estimate volumes of highly asymmetric objects using multiple images of x-ray emission. The method is based on image intensities and does not require any explicit assumption of symmetry

47 OTHER INSTRUMENTATION↗