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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 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Acousto-optic modulation of a photonic crystal nanocavity with Lamb waves in microwave K band

Integrating nanoscale electromechanical transducers and nanophotonic devices potentially can enable acousto-optic devices to reach unprecedented high frequencies and modulation efficiency. Here, we demonstrate acousto-optic modulation of a photonic crystal nanocavity using Lamb waves with frequency up to 19 GHz, reaching the microwave K band. The devices are fabricated in suspended aluminum nitride membrane. Excitation of acoustic waves is achieved with interdigital transducers with period as small as 300 nm. Confining both acoustic wave and optical wave within the thickness of the membrane leads to improved acousto-optic modulation efficiency in these devices than that obtained in previous surface acoustic wave devices. Our system demonstrates a scalable optomechanical platform where strong acousto-optic coupling between cavity-confined photons and high frequency traveling phonons can be explored.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Modulation of electron-acoustic waves in a plasma with kappa distribution

In the present work, employing a one dimensional model of an unmagnetized collisionless plasma consisting of a cold electron fluid, hot electrons obeying κ velocity distribution, and stationary ions, we study the amplitude modulation of an electron-acoustic waves by use of the conventional reductive perturbation method. Employing the field equations of such a plasma, we obtained the nonlinear Schrödinger equation as the evolution equation. Seeking a harmonic wave solution with progressive wave amplitude to the evolution equation, as opposed to the plasma with vortex distribution, the amplitude wave assumes a shock wave type of solution. Finally, the modulational stability of the wave is studied and it is observed that the wave is modulationally stable for all admissible wave numbers.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Refractive index modulation of Sb{sub 70}Te{sub 30} phase-change thin films by multiple femtosecond laser pulses

In this study, the controllable effective refractive index modulation of Sb{sub 70}Te{sub 30} phase-change thin films between amorphous and crystalline states was achieved experimentally by multiple femtosecond laser pulses. The modulation mechanism was analyzed comprehensively by a spectral ellipsometer measurement, surface morphology observation, and two-temperature model calculations. We numerically demonstrate the application of the optically modulated refractive index of the phase-change thin films in a precisely adjustable color display. These results may provide further insights into ultrafast phase-transition mechanics and are useful in the design of programmable photonic and opto-electrical devices based on phase-change memory materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Large eddy simulations of a transcritical round jet submitted to transverse acoustic modulation

This article reports numerical computations of a turbulent round jet of transcritical fluid (low temperature nitrogen injected under high pressure conditions) surrounded by the same fluid at rest under supercritical conditions (high temperature and high pressure) and submitted to transverse acoustic modulations. The numerical framework relies on large eddy simulation in combination with a real-gas description of thermodynamics and transport properties. A stationary acoustic field is obtained by modulating the normal acoustic velocity at the lateral boundaries of the computational domain. This study specifically focuses on the interaction of the jet with the acoustic field to investigate how the round transcritical jet changes its shape and mixes with the surrounding fluid. Different modulation amplitudes and frequencies are used to sweep a range of conditions. When the acoustic field is established in the domain, the jet length is notably reduced and the jet is flattened in the spanwise direction. Two regimes of oscillation are identified: for low Strouhal numbers a large amplitude motion is observed, while for higher Strouhal numbers the jet oscillates with a small amplitude around the injector axis. The minimum length is obtained for a Strouhal number of 0.3 and the jet length increases with increasing Strouhal numbers after reaching this minimum value. The mechanism of spanwise deformation is shown to be linked with dynamical effects resulting from reduction of the pressure in the transverse direction in relation with increased velocities on the two sides of the jet. A propagative wave is then introduced in the domain leading to similar effects on the jet, except that a bending is also observed in the acoustic propagation direction. A kinematic model, combining hydrodynamic and acoustic contributions, is derived in a second stage to represent the motion of the jet centerline. This model captures details of the numerical simulations quite well. These various results can serve to interpret observations made on more complex flow configurations such as coaxial jets or jet flames formed by coaxial injectors.

42 ENGINEERING↗
DOE OSTI2024

Wavelength modulation laser-induced fluorescence for plasma characterization

Laser-Induced Fluorescence (LIF) spectroscopy is an essential tool for probing ion and atom velocity distribution functions (VDFs) in complex plasmas. VDFs carry information about the kinetic properties of species that is critical for plasma characterization. Accurate interpretation of these functions is challenging due to factors such as multicomponent distributions, broadening effects, and background emissions. Our research investigates the use of Wavelength Modulation (WM) LIF to enhance the sensitivity of VDF measurements. Unlike standard Amplitude Modulation (AM) methods, WM–LIF measures the derivative of the LIF signal. This approach makes variations in VDF shape more pronounced. VDF measurements with WM–LIF were investigated with both numerical modeling and experimental measurements. The developed model enables the generation of both WM and AM signals, facilitating comparative analysis of fitting outcomes. Experiments were conducted in a weakly collisional argon plasma with magnetized electrons and non-magnetized ions. Measurements of the argon ion VDFs employed a narrow-band tunable diode laser, which scanned the 4p 4 D 7/2 –3d 4 F 9/2 transition centered at 664.553 nm in vacuum. A lock-in amplifier detected the second harmonic WM signal, which was generated by modulating the laser wavelength with an externally controlled piezo-driven mirror of the diode laser. Finally, our findings indicate that the WM–LIF signal is more sensitive to fitting parameters, allowing for better identification of VDF parameters such as the number of distribution components, their temperatures, and velocities. In addition, WM–LIF can serve as an independent method to verify AM measurements and is particularly beneficial in environments with substantial light noise or background emissions, such as those involving thermionic cathodes and reflective surfaces.

47 OTHER INSTRUMENTATION↗
DOE OSTI2013

Amplitude and phase modulation of radiation in a travelling-wave amplifier based on a laser diode

An analytical solution (in quadratures) to the problem of propagation of quasi-monochromatic optical signal in a semiconductor amplifier under harmonic modulation of its pump current is obtained for the first time. It is shown that the modulation of the output radiation has amplitude and phase features. The relation is found between the coefficients of the amplitude and phase modulation with the effect of gain saturation taken into account. Adequacy of the results obtained is confirmed experimentally. (control of laser radiation parameters)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Stochastic resonance at a subharmonic of a periodic modulation signal in solid-state lasers

The stochastic excitation of a subharmonic of a periodic modulation signal in the intensity spectrum of a solid-state laser is experimentally studied upon modulation of the pump rate by the noise and periodic signal. The stochastic resonance (SR) is observed in the presence of bistability in the laser. The conditions for SR at a subharmonic of the periodic modulation signal are determined. (control of laser radiation parameters)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Bistability of self-modulation oscillations in an autonomous solid-state ring laser

Bistable self-modulation regimes of generation for a ring YAG : Nd chip laser with the counterpropagating waves asymmetrically coupled via backward scattering are simulated numerically. Two branches of bistable self-modulation regimes of generation are found in the domain of the parametric resonance between the selfmodulation and relaxation oscillations. The self-modulation regimes observed in earlier experiments pertain to only one of the branches. Possible reasons for such a discrepancy are considered, related to the influence of technical and natural noise on the dynamics of solid-state ring lasers. (control of laser radiation parameters)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Soliton communication lines based on spectrally efficient modulation formats

We report the results of mathematical modelling of optical-signal propagation in soliton fibre-optic communication lines (FOCLs) based on spectrally efficient signal modulation formats. We have studied the influence of spontaneous emission noise, nonlinear distortions and FOCL length on the data transmission quality. We have compared the characteristics of a received optical signal for soliton and conventional dispersion compensating FOCLs. It is shown that in the presence of strong nonlinearity long-haul soliton FOCLs provide a higher data transmission performance, as well as allow higher order modulation formats to be used as compared to conventional communication lines. In the context of a coherent data transmission, soliton FOCLs allow the use of phase modulation with many levels, thereby increasing the spectral efficiency of the communication line. (optical communication lines)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Propagation of frequency-modulated pulses in active one-dimensional photonic crystals

The propagation of frequency-modulated pulses in onedimensional photonic crystals with gain is considered. A correct expression is derived for the delay time of the pulse maximum. This expression takes into account the input pulse characteristics: duration, frequency modulation and spectrum position in the photonic band gap. The analytical results are basically in agreement with the results of numerical simulation. The influence of gain in the photonic-crystal structure is considered. It is shown that the parameters of a transmitted pulse can be controlled by changing the inputpulse frequency modulation. (nonlinear optical phenomena)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Control of the photoelectron dynamics for the effective conversion of short-pulse, frequency-modulated optical radiation into X-ray radiation

We report a theoretical investigation of high-order harmonic generation (HHG) in the ionisation of atoms by nonlinear frequency-modulated laser pulses of short duration. It is shown that the reduction in the instantaneous value of the laser pulse frequency can lead to a significant increase in the width of the plateau in the HHG spectrum. We have found optimal parameters of frequency modulation at which photons with energies of 1 keV are efficiently generated at a relatively low laser-pulse intensity. High HHG efficiency at optimal parameters is explained by the peculiarities of atomic ionisation dynamics and acceleration of photoelectrons by a frequency-modulated laser field. (extreme light fields and their applications)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Optimisation of frequency-modulated characteristics of output radiation in a lidar with Raman amplification

The reported study is aimed at increasing the power in the transmission path of a lidar with Raman amplification for longpath sensing of methane by optimising the frequency-modulated characteristics of the output radiation. The pump current of the used distributed-feedback master laser was modulated by a linearfrequency signal with simultaneous application of a non-synchronous high-frequency signal. For such a modulation regime, the Raman amplifier provided the mean output power of 2.5 W at a wavelength of 1650 nm. The spectral broadening did not significantly decrease the lidar sensitivity at long paths. (lidars)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Acousto-optical modulation of light at a doubled sound frequency

A method of acousto-optical (AO) Bragg diffraction is proposed that provides the amplitude modulation of optical radiation at a doubled acoustic frequency. The method is based on the double transmission of the light through the AO modulator made of a gyrotropic crystal and is experimentally tested by the example of the modulation of light with a wavelength of 0.63 μm, controlled by the paratellurite AO cell. (acoustooptics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Acousto-optic modulators based on a KYW crystal

Two types of acousto-optic modulators based on new acousto-optic crystals KY(WO{sub 4}){sub 2} are developed and experimentally investigated. Their characteristics are compared with those of existing acousto-optic quartz modulators. It is shown that the new modulators provide an almost 10-fold efficiency gain, which eliminates the need for water cooling of such devices. (acouto-optics)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

QR-code optical encryption in the scheme with spatially incoherent illumination based on two micromirror light modulators

An optical encryption scheme with spatially incoherent illumination based on two micromirror light modulators has been experimentally implemented for the first time. Currently, such modulators are the fastest tools for spatio-temporal light modulation; their high frame rates provide possibilities for developing optical encryption systems with a bandwidth of several gigabites per second. (optical encryption)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

MODULATION OF THE GALACTIC LOW-ENERGY PROTON SPECTRUM IN THE INNER HELIOSPHERE

We study the energy spectra of 0.3-100 MeV protons and find that, at the lower part of the galactic particle spectrum, they are significantly steeper than the J(E) {approx} E spectrum predicted by analytical approximations, such as the force-field model of modulation. We select a series of low-flux periods, and approximate the spectral form by J(E) = AE{sup -{gamma}} + CE{sup {nu}}, where the two terms describe solar/heliospheric and galactic components, respectively. By determining the best fit parameters to energy spectra, correlations are sought with solar activity indices and between the parameters themselves. In the majority of cases, {nu} turns out to be between 1.2 and 1.4, with an average of 1.32 {+-} 0.12, significantly greater than the commonly expected {nu} = 1 predicted by the force-field approximation. In modulation theories {nu} > 1 corresponds to a negative Compton-Getting factor, which poses a challenge. Such an inversion may occur if the radial diffusion coefficient {kappa}{sub rr} < rV (where r is heliocentric distance and V solar wind speed), in which case a large fraction of the 10-100 MeV protons reaching 1 AU would have been cooled down within 1 AU and subsequently convected outward by the solar wind. We also find that the position of the intensity minimum of the proton spectrum dividing the solar and galactic populations shifts toward higher values with increasing solar activity. Correlations obtained with solar activity indicate that the slope of the solar/heliospheric spectrum is practically independent of solar activity. Observations are compared with numerical solutions of the modulation equation adopting simple spherical models. Possible interpretations are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ANALYSIS OF SEEING-INDUCED POLARIZATION CROSS-TALK AND MODULATION SCHEME PERFORMANCE

We analyze the generation of polarization cross-talk in Stokes polarimeters by atmospheric seeing, and its effects on the noise statistics of spectropolarimetric measurements for both single-beam and dual-beam instruments. We investigate the time evolution of seeing-induced correlations between different states of one modulation cycle and compare the response to these correlations of two popular polarization modulation schemes in a dual-beam system. Extension of the formalism to encompass an arbitrary number of modulation cycles enables us to compare our results with earlier work. Even though we discuss examples pertinent to solar physics, the general treatment of the subject and its fundamental results might be useful to a wider community.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

GALACTIC COSMIC-RAY MODULATION IN A REALISTIC GLOBAL MAGNETOHYDRODYNAMIC HELIOSPHERE

To understand the behavior of cosmic-ray modulation seen by the two Voyager spacecraft in the region near the termination shock (TS) and in the heliosheath at a distance of >100 AU, a realistic magnetohydrodynamic global heliosphere model is incorporated into our cosmic-ray transport code, so that the detailed effects of the heliospheric boundaries and their plasma/magnetic geometry can be revealed. A number of simulations of cosmic-ray modulation performed with this code result in the following conclusions. (1) Diffusive shock acceleration by the TS can significantly affect the level of cosmic-ray flux and, in particular, its radial gradient profile in the region near the TS and in the inner heliosheath. (2) The radial profile of cosmic-ray flux strongly depends on longitude. There is a slight north-south asymmetry due to an asymmetric TS, but the larger difference in the radial profiles comes from longitudinal variation. Voyager 1 and 2 are separated by {approx} 40 Degree-Sign in longitude. Simulations in these two directions show a large difference in the radial profile of cosmic-ray flux. Thus, it is not appropriate to determine the cosmic-ray radial gradient by directly using the two-point Voyager measurements. Various other simulations are also performed to show how sensitively the modulation level depends on latitude, cosmic-ray energy, and interstellar spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗