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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

B-spline algebraic diagrammatic construction: Application to photoionization cross-sections and high-order harmonic generation

We present the first implementation of the ab initio many-body Green's function method, algebraic diagrammatic construction (ADC), in the B-spline single-electron basis. B-spline versions of the first order [ADC(1)] and second order [ADC(2)] schemes for the polarization propagator are developed and applied to the ab initio calculation of static (photoionization cross-sections) and dynamic (high-order harmonic generation spectra) quantities. We show that the cross-section features that pose a challenge for the Gaussian basis calculations, such as Cooper minima and high-energy tails, are found to be reproduced by the B-spline ADC in a very good agreement with the experiment. We also present the first dynamic B-spline ADC results, showing that the effect of the Cooper minimum on the high-order harmonic generation spectrum of Ar is correctly predicted by the time-dependent ADC calculation in the B-spline basis. The present development paves the way for the application of the B-spline ADC to both energy- and time-resolved theoretical studies of many-electron phenomena in atoms, molecules, and clusters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Monitoring microstructural evolution in irradiated steel with second harmonic generation

Material damage in structural components is driven by microstructural evolution that occurs at low length scales and begins early in component life. In metals, these microstructural features are known to cause measurable changes in the acoustic nonlinearity parameter. Physically, the interaction of a monochromatic ultrasonic wave with microstructural features such as dislocations, precipitates, and vacancies, generates a second harmonic wave that is proportional to the acoustic nonlinearity parameter. These nonlinear ultrasonic techniques thus have the capability to evaluate initial material damage, particularly before crack initiation and propagation occur. This paper discusses how the nonlinear ultrasonic technique of second harmonic generation can be used as a nondestructive evaluation tool to monitor microstructural changes in steel, focusing on characterizing neutron radiation embrittlement in nuclear reactor pressure vessel steels. Current experimental evidence and analytical models linking microstructural evolution with changes in the acoustic nonlinearity parameter are summarized.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Continuous third harmonic generation in a terahertz driven modulated nanowire

We consider the possibility of observing continuous third-harmonic generation using a strongly driven, single-band one-dimensional metal. In the absence of scattering, the quantum efficiency of frequency tripling for such a system can be as high as 93%. Combining the Floquet quasi-energy spectrum with the Keldysh Green's function technique, we derive a semiclassical master equation for a one-dimensional band of strongly and rapidly driven electrons in the presence of weak scattering by phonons. The power absorbed from the driving field is continuously dissipated by phonon modes, leading to a quasi-equilibrium in the electron distribution. We use the Kronig-Penney model with varying effective mass to establish the growth parameters of an InAs/InP nanowire near optimal for third harmonic generation at terahertz frequency range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ultrashort pulse chirp measurement via transverse second-harmonic generation in strontium barium niobate crystal

Pulse compression in dispersive strontium barium niobate crystal with a random size and distribution of the anti-parallel orientated nonlinear domains is observed via transverse second harmonic generation. The dependence of the transverse width of the second harmonic trace along the propagation direction allows for the determination of the initial chirp and duration of pulses in the femtosecond regime. This technique permits a real-time analysis of the pulse evolution and facilitates fast in-situ correction of pulse chirp acquired in the propagation through an optical system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

On square-integrability of solutions of the stationary Schrödinger equation for the quantum harmonic oscillator in two dimensional constant curvature spaces

In this work, one provides a justification of the condition that is usually imposed on the parameters of the hypergeometric equation, related to the solutions of the stationary Schrödinger equation for the harmonic oscillator in two-dimensional constant curvature spaces, in order to determine the solutions which are square-integrable. One proves that in case of negative curvature, it is a necessary condition of square integrability and in case of positive curvature, a necessary condition of regularity. The proof is based on the analytic continuation formulas for the hypergeometric function. It is observed also that the same is true in case of a slightly more general potential than the one for harmonic oscillator.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Direct optical detection of current induced spin accumulation in metals by magnetization-induced second harmonic generation

Strong spin-orbit coupling in non-magnetic heavy metals has been shown to lead to large spin currents flowing transverse to a charge current in such a metal wire. This in turn leads to the buildup of a net spin accumulation at the lateral surfaces of the wire. Spin-orbit torque effects enable the use of the accumulated spins to exert useful magnetic torques on adjacent magnetic layers in spintronic devices. We report the direct detection of spin accumulation at the free surface of nonmagnetic metal films using magnetization-induced optical surface second harmonic generation. The technique is applied to probe the current induced surface spin accumulation in various heavy metals such as Pt, β-Ta, and Au with high sensitivity. The sensitivity of the technique enables us to measure the time dynamics on a sub-ns time scale of the spin accumulation arising from a short current pulse. The ability of optical surface second harmonic generation to probe interfaces suggests that this technique will also be useful for studying the dynamics of spin accumulation and transport across interfaces between non-magnetic and ferromagnetic materials, where spin-orbit torque effects are of considerable interest.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

HELIOS—A laboratory based on high-order harmonic generation of extreme ultraviolet photons for time-resolved spectroscopy

In this paper, we present the HELIOS (High Energy Laser Induced Overtone Source) laboratory, an in-house high-order harmonic generation facility which generates extreme ultraviolet (XUV) photon pulses in the range of 15-70 eV with monochromatized XUV pulse lengths below 35 fs. HELIOS is a source for time-resolved pump-probe/two-color spectroscopy in the sub-50 fs range, which can be operated at 5 kHz or 10 kHz. An optical parametric amplifier is available for pump-probe experiments with wavelengths ranging from 240 nm to 20 000 nm. The produced XUV radiation is monochromatized by a grating in the so-called off-plane mount. Together with overall design parameters, first monochromatized spectra are shown with an intensity of 2 ⋅ 10{sup 10} photons/s (at 5 kHz) in the 29th harmonic, after the monochromator. The XUV pulse duration is measured to be <25 fs after monochromatization.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Nanostructure induced changes in lifetime and enhanced second-harmonic response of organic-plasmonic hybrids

In this letter we show that the optical response of organic nanofibers, grown from functionalized para-quaterphenylene molecules, can be controlled by forming organic-plasmonic hybrid systems. The interaction between nanofibers and supporting regular arrays of nanostructures leads to a strongly enhanced second harmonic response. At the same time, the fluorescence lifetime of the nanofibers is reduced from 0.32 ns for unstructured gold films to 0.22 ns for gold nanosquare arrays, demonstrating efficient organic–plasmonic interaction. To study the origin of these effects, we applied two-photon laser scanning microscopy and fluorescence lifetime imaging microscopy. These findings provide an effective approach for plasmon-enhanced second-harmonic generation at the nanoscale, which is attractive for nanophotonic circuitry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Surface and interface states of Bi{sub 2}Se{sub 3} thin films investigated by optical second-harmonic generation and terahertz emission

We investigate the surface and interface states of Bi{sub 2}Se{sub 3} thin films by using the second-harmonic generation technique. Distinct from the surface of bulk crystals, the film surface and interface show the isotropic azimuth dependence of second-harmonic intensity, which is attributed to the formation of randomly oriented domains on the in-plane. Based on the nonlinear susceptibility deduced from the model fitting, we determine that the surface band bending induced in a space charge region occurs more strongly at the film interface facing the Al{sub 2}O{sub 3} substrate or capping layer compared with the interface facing the air. We demonstrate that distinct behavior of the terahertz electric field emitted from the samples can provide further information about the surface electronic state of Bi{sub 2}Se{sub 3}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Conversion of the optical orbital angular momentum in a plasmon-assisted second-harmonic generation

We experimentally demonstrate the plasmon-assisted second-harmonic generation of an optical orbital angular momentum (OAM) beam. Because of the shape resonance, the plasmons in a periodic array of rectangular metal holes greatly enhance the nonlinear optical conversion of an OAM state. The OAM conservation (i.e., 2l{sub 1} = l{sub 2} with l{sub 1} and l{sub 2} being the OAM numbers of the fundamental and second-harmonic waves, respectively) holds well under our experimental configuration. Our results provide a potential way to realize nonlinear optical manipulation of an OAM mode in a nano-photonic device.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Dynamic embedding of effective harmonic normal mode vibrations in all-atomistic energy gap fluctuations: Case study of light harvesting 2 complex

Environmental effects in excitation energy transfer have mostly been modeled by baths of harmonic oscillators, but to what extent such modeling provides a reliable description of actual interactions between molecular systems and environments remains an open issue. Here, we address this issue by investigating fluctuations in the excitation energies of the light harvesting 2 complex using a realistic all-atomistic simulation of the potential energy surface. Our analyses reveal that molecular motions exhibit significant anharmonic features, even for underdamped intramolecular vibrations. In particular, we find that the anharmonicity contributes to the broadening of spectral densities and substantial overlaps between neighboring peaks, which complicates the meaning of mode frequencies constituting a bath model. Thus, we develop a strategy to construct a minimally underdamped harmonic bath that has a clear connection to all-atomistic dynamics by utilizing actual normal modes of molecules but optimizing their frequencies such that the resulting bath model can best reproduce the all-atomistic simulation results. By subtracting the underdamped contribution from the entire fluctuations, we also show that identifying a residual spectral density representing all other contributions with overdamped behavior is possible. We find that this can be fitted well with a well-established analytic form of a spectral density function or, alternatively, modeled as explicit time dependent fluctuations with muti-exponential or power law type correlation functions. We provide an assessment and the implications of these possibilities. The approach presented here can also serve as a general strategy to construct a simplified bath model that can effectively represent the underlying all-atomistic bath dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

An implementation of a high-order generalized finite difference method for solving the time-harmonic cold plasma wave equation in toroidal geometry

A high-order physics-informed meshless finite difference numerical technique is introduced for solving the time-harmonic cold plasma wave equation in toroidal geometries, presenting a novel application of the generalized finite difference (GFD) method to plasma wave simulations. The algorithm employs an irregular distribution of computational points, with local point density informed by the shortest wavelength derived from the cold plasma dispersion relation. Numerical stability and robustness are addressed using regularization techniques. The algorithm, implemented for two spatial dimensions, solves for the wave electric field and is demonstrated to achieve convergence rates of $\mathcal{O}$($\mathcal{h}$ $\mathcal{P}$ )⁠. Verification tests reproduce plane wave solutions, and example simulations of ion cyclotron resonance heating and electron cyclotron resonance heating demonstrate its capability, approaching realistic tokamak plasma scenarios. This work contributes to laying a foundation for the GFD method to be used in more sophisticated, optimized, and physically realistic full-wave simulations in time-harmonic plasma wave research.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Soft x-ray high-harmonic generation in an anti-resonant hollow core fiber driven by a 3 μ m ultrafast laser

High-harmonic upconversion driven by a mid-infrared femtosecond laser can generate coherent soft x-ray beams in a tabletop-scale setup. Here, we report on a compact ytterbium-pumped optical parametric chirped pulse amplifier (OPCPA) laser system seeded by an all-fiber front-end and employing periodically poled lithium niobate (PPLN) nonlinear media operated near the pulse fluence limits of current commercially available PPLN crystals. The OPCPA delivers 3 µm wavelength pulses with 775 µJ energy at 1 kHz repetition rate, with transform-limited 120 fs pulse duration, diffraction-limited beam quality, and ultrahigh 0.33% rms energy stability over >18 h. Using this laser, we generate soft x-ray high harmonics (HHG) in argon gas by focusing into a low-loss, high-pressure gas-filled anti-resonant hollow core fiber (ARHCF), generating coherent light at photon energies up to the argon L-edge (250 eV) and carbon K-edge (284 eV), with high beam quality and ∼1% rms energy stability. This work demonstrates soft x-ray HHG in a high-efficiency guided-wave phase matched geometry, overcoming the high losses inherent to mid-IR propagation in unstructured waveguides, or the short interaction lengths of gas cells or jets. The ARHCF can operate in the long term without damage and with the repetition rate, stability, and robustness required for demanding applications in spectromicroscopy and imaging. Finally, we discuss routes for further optimizing the soft x-ray HHG flux by driving He at higher laser intensities using either the signal (1.5 μm) or idler wavelengths (3 μm).

Femtosecond lasers↗
DOE OSTI2014

Tunable coherent soft X-ray source based on the generation of high-order harmonic of femtosecond laser radiation in gas-filled capillaries

We have carried out experimental and theoretical investigations of a tunable coherent soft X-ray radiation source in the 30 – 52 nm wavelength range based on the generation of high-order harmonics of femtosecond laser radiation propagating in a dielectric xenon-filled capillary. The long path of laser pulse propagation through the capillary permits tuning the generated harmonic wavelengths to almost completely span the range under consideration. (interaction of radiation with matter)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Nonlinear harmonic mixing in an InGaAs/InGaP/GaAs laser on a germanium substrate

An InGaAs/InGaP/GaAs laser with quantum wells, grown on a lightly doped germanium substrate, is experimentally investigated. Generation on the second and sum harmonics is observed in this composite cavity laser, which confirms the possibility of efficient difference-harmonic generation. (lasers)

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Extracavity and external cavity second-harmonic generation in a periodically poled silica fibre

We have studied second-harmonic generation (SHG) of a cw single-frequency ytterbium-doped fibre laser, using a periodically poled silica fibre as a nonlinear medium for frequency conversion. All-fibre external cavity SHG has been investigated for the first time. A twofold increase in second-harmonic power in a fibre ring cavity has been demonstrated and possibilities of further optimising the fibre scheme have been analysed. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Modification of the spectrum of high harmonics by a weak vacuum ultraviolet field

Within the framework of the time-dependent effective range theory, we have calculated the spectra of high harmonics in a field containing an intense low-frequency component and a weak (perturbative) high-frequency component with a photon energy exceeding the ionisation potential of the atomic target. We have presented a quantum-mechanical substantiation of the quasi-classical theory [A.V. Flegel et al. Quantum Electron., 47, 222 (2017)], which describes additional generation channels of high harmonics induced by a weak high-frequency field. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Generation of the second harmonic in ridge waveguides formed in periodically poled lithium niobate

Generation of the second harmonic of semiconductor laser radiation in the IR range (980 – 1140 nm) in ridge waveguides with a large refractive index step, formed in periodically poled nonlinear LiNbO{sub 3} crystals, is demonstrated. Ridge waveguides have been prepared by precise cutting using a diamond microsaw. Spectral dependences of the SHG power and intensity distributions in the near field of second harmonic and pump radiations have been measured. The results obtained indicate good prospects of designing tunable laser radiation sources in the visible range due to the SHG in ridge waveguides formed in periodically poled nonlinear crystals under semiconductor laser pumping. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗