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Results for “refractivity”

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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Implementation of compound refractive lenses for large field-of-view x-ray phase-contrast imaging during hypervelocity impact experiments

Synchrotron x-ray phase-contrast imaging (XPCI) offers time-resolved visualization of dynamic compression phenomena, but its intrinsically small field-of-view (FOV) limits the time that key features remain in frame. A novel approach to enlarge the FOV is achieved by positioning a two-dimensional parabolic compound refractive lens (CRL) upstream of the sample to deliberately defocus the white beam. Ray-tracing simulations and XPCI measurements show that this CRL configuration can expand the beam by ∼50% vertically and ∼15% horizontally based on the full width at half-maximum of the beam. Implementing the CRL, however, attenuates the photon flux and lowers signal-to-noise ratio (SNR). Task-based analysis using a calibration grid (30 μm dots) showed that both setups fail to consistently meet the Rose criterion (SNR ≥ 5) for features of this size in single-bunch imaging. Extrapolating the measured SNR Rose values suggests that the minimum consistently detectable feature lies closer to 30–40 μm for the standard XPCI setup and above 40 μm for CRL-XPCI. Despite this limitation, the CRL configuration nearly doubles the illuminated area, enabling simultaneous tracking of front and rear observations of boron carbide targets subjected to rod and sphere impacts at 1.0–2.6 km/s. Image tracking algorithms and photonic Doppler velocimetry were used to measure penetration and rear-surface velocity histories. Together, these measurements capture crack fronts, penetration, and material breakout, offering new benchmark data for validating high-strain-rate constitutive models of ceramic materials.

Ceramic materials↗
DOE OSTI2011

Effect of optical pumping on the refractive index and temperature in the core of active fibre

This paper examines the refractive index change (RIC) induced in the core of Yb{sup 3+}-doped active silica fibres by pulsed pumping. RIC kinetic measurements with a Mach - Zehnder interferometer make it possible to separately assess the contributions of the electronic and thermal mechanisms to the RIC and evaluate temperature nonuniformities in the fibre.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Refractive index gradient measurement across the thickness of a dielectric film by the prism coupling method

A method is proposed for measuring the refractive index gradient n(z) in nonuniformly thick dielectric films. The method is based on the excitation of waveguide modes in a film using the prism coupling technique and on the calculation of n(z) and film thickness H{sub f} with the help of the angular positions of the TE or TM modes. The method can be used for an arbitrary shape of the index modulation over the film thickness in the limit of a small gradient [{Delta} n(z)/n(z) || 1]. (laser applications and other topics in quantum electronics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Nonlinear waves in an array of zigzag waveguides with alternating positive and negative refractive indices

Interaction of coupled waves propagating in a system of waveguides with alternating positive and negative refractive indices is studied theoretically. The zigzag configuration of the waveguides in the array allows communication not only between the nearest neighbours, but also with the waveguides beyond them. It is shown that the spectrum of linear waves in such a waveguide system has a bandgap. Partial solutions are found to the system of coupled waves corresponding to a stationary electromagnetic field pulse that propagates along the array of tunnel-coupled waveguides as a whole. Investigation of the interaction of nonlinear solitary waves has demonstrated numerically the stability of their relatively weak disturbances and collisions with each other. (nanogradient dielectric coatings and metamaterials)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Determination of refractive index, extinction coefficient and thickness of thin films by the method of waveguide mode excitation

We propose a method for measuring simultaneously the refractive index n{sub f}, extinction coefficient m{sub f} and thickness H{sub f} of thin films. The method is based on the resonant excitation of waveguide modes in the film by a TE- or a TM-polarised laser beam in the geometry of frustrated total internal reflection. The values of n{sub f}, m{sub f} and H{sub f} are found by minimising the functional φ = [N{sup -1}Σ{sup N}{sub i=1}(R{sub exp}(θ{sub i}) – R{sub thr}(θ{sub i})){sup 2}]{sup 1/2}, where R{sub exp}(θ{sub i}) and R{sub thr}(θ{sub i}) are the experimental and theoretical coefficients of reflection of the light beam from the interface between the measuring prism and the film at an angle of incidence θ{sub i}. The errors in determining n{sub f}, m{sub f} and H{sub f} by this method are ±2 × 10{sup -4}, ±1 × 10{sup -3} and ±0.5%, respectively. (fiber and integrated optics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Integrated optical devices based on sol – gel waveguides using the temperature dependence of the effective refractive index

A possibility of designing optical waveguide devices based on sol – gel SiO{sub 2} – TiO{sub 2} films using the temperature dependence of the effective refractive index is shown. The dependences of the device characteristics on the parameters of the film and opticalsystem elements are analysed. The operation of a temperature recorder and a temperature limiter with a resolution of 0.6 K mm{sup -1} is demonstrated. The film and output-prism parameters are optimised. (fibreoptic and nonlinear-optic devices)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of temperature on the shape of spatial quasi-periodic oscillations of the refractive index of alkali atoms in an optically dense medium with a closed excitation contour of Δ type

A theory of a closed excitation contour (Δ system) of a three-level atom in an optically dense medium is constructed with allowance for temperature. The spatial quasi-periodic oscillations of the refractive index in the system under study are shown to damp with increasing temperature. The range of temperatures at which these oscillations are most pronounced is found. (quantum optics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Modulation instability in a zigzag array of nonlinear waveguides with alternating positive and negative refractive indices

The modulation instability is analytically investigated in a zigzag array of tunnel-coupled optical waveguides with alternating refractive indices and Kerr nonlinearity. Particular solutions to a system of coupled nonlinear equations are found. They describe the propagation of electromagnetic waves that are uniform along the waveguide and their instability is studied. It is shown that the coupling coefficient between the waveguides, which are non-nearest neighbours, has a significant effect on the instability of the waves in question. When the coupling coefficient exceeds a certain threshold, the modulation instability disappears regardless of the radiation power. The influence of the ratio of the wave amplitudes in adjacent waveguides to the instability of the particular solutions is studied. Different variants of the nonlinear response in waveguides are considered. The studies performed present a new unusual type of the modulation instability in nonlinear periodic systems. (metamaterials)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Generation of subpicosecond pulses due to the development of modulation instability of whispering-gallery-mode wave packets in an optical waveguide with a travelling refractive-index wave

We consider the transformation of a quasi-continuous (with the duration greater than 1 ns) background tunnelling wave of the whispering gallery mode (WGM) type into ultrashort (pico- and subpicosecond) laser pulses, arising due to the development of modulation instability in a cylindrical optical waveguide with travelling refractive-index wave. It is shown that the peak power of the generated laser pulses can exceed the power of the background WGM by a few orders of magnitude. (paper)

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Uniformity simulation of multiple-beam irradiation of a spherical laser target with the inclusion of radiation absorption and refraction

We consider the physicomathematical model for the simulation of the angular laser flux distribution in a spherical target with the inclusion of radiation refraction in the corona. The irradiation uniformity is simulated using the target irradiation scheme on a megajoule multiple-beam laser facility. Calculations are made of the time-dependent angular distribution of absorbed flux and of the angular distribution of the time-integrated characteristic – the absorbed energy with allowance for the temporal shape of laser pulses – as well as of the time dependence of absorption efficiency. Different versions of target irradiation are considered. Under conventional irradiation it is assumed that the axes of all laser beams of equal energy pass through the target centre and the irradiation nonuniformity is due to its geometry only. Also examined are three unconventional irradiation versions: when there is a small spread of beam energies, when the target is displaced from the common beam pointing centre, and when the beam axes have small random deviations from the target centre. It is shown that the nonuniformity of the angular distribution of absorbed flux is greatest when the target is displaced from the beam pointing centre. (interaction of laser radiation with matter. laser plasma)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Femtosecond writing of refractive index structures in multimode and multicore optical fibres

The possibility of writing fibre Bragg gratings (FBGs) in optical fibres with a complex spatial structure using femtosecond IR laser pulses is considered. In particular, selective writing of uniform FBGs in individual cores of a seven-core fibre and writing of serial and parallel FBG arrays in the core of a multimode fibre with a gradient refractive index profile are demonstrated. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Control of laser-beam spatial distribution for correcting the shape and refraction of eye cornea

The results of calculation and development of a device aimed at forming a laser beam with an annular intensity distribution and spatial mode homogenisation, as well as approbation of this device for biological phantoms and eyes in ex vivo experiments with laser non-ablative correction of the cornea profile, are presented. The radiation intensities and thermal field distributions on the surfaces of a phantom and isolated rabbit’s eye are investigated. The absence of significant heating in the central region of eye is confirmed. It is shown that this device can be used for laser impact in order to change symmetrically eye’s refraction. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Compound refractive lenses as prefocusing optics for X-ray FEL radiation

The performance of X-ray free-electron laser beamlines may be limited by the angular aperture. Compound refractive lenses (CRLs) can be employed to prefocus the X-ray beam, thereby increasing the beamline transmission. A prefocusing CRL was implemented in the X-ray transport of the Matter under Extreme Conditions Instrument at the Linac Coherent Light Source. A significant improvement in the beamline transmission was calculated over the 3–10 keV photon energy range. At 5 keV, the relative X-ray intensity was measured and a factor of four increase was seen in the beamline transmission. As a result, the X-ray focus was also determined by the ablation imprint method.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

Pink-beam focusing with a one-dimensional compound refractive lens

The performance of a cooled Be compound refractive lens (CRL) has been tested at the Advanced Photon Source (APS) to enable vertical focusing of the pink beam and permit the X-ray beam to spatially overlap with an 80 µm-high low-density plasma that simulates astrophysical environments. Focusing the fundamental harmonics of an insertion device white beam increases the APS power density; here, a power density as high as 500 W mm –2 was calculated. A CRL is chromatic so it does not efficiently focus X-rays whose energies are above the fundamental. Only the fundamental of the undulator focuses at the experiment. A two-chopper system reduces the power density on the imaging system and lens by four orders of magnitude, enabling imaging of the focal plane without any X-ray filter. As a result, a method to measure such high power density as well as the performance of the lens in focusing the pink beam is reported.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Nanofocusing with aberration-corrected rotationally parabolic refractive X-ray lenses

Wavefront errors of rotationally parabolic refractive X-ray lenses made of beryllium (Be CRLs) have been recovered for various lens sets and X-ray beam configurations. Due to manufacturing via an embossing process, aberrations of individual lenses within the investigated ensemble are very similar. By deriving a mean single-lens deformation for the ensemble, aberrations of any arbitrary lens stack can be predicted from the ensemble with σ¯ = 0.034λ. Using these findings the expected focusing performance of current Be CRLs are modeled for relevant X-ray energies and bandwidths and it is shown that a correction of aberrations can be realised without prior lens characterization but simply based on the derived lens deformation. As a result, the performance of aberration-corrected Be CRLs is discussed and the applicability of aberration-correction demonstrated over wide X-ray energy ranges.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Using refractive lenses to provide a variable spot size for Kirkpatrick–Baez mirrors

For many X-ray microprobe experiments it is desirable to be able to vary the beam size: using large beams for survey scans and a small beam for the final measurements. Beryllium refractive lenses were found to be a simple and controllable method for enlarging the focus in a Kirkpatrick–Baez-based microprobe. They can provide variable spot size, can be quickly inserted or removed and do not move the beam center on the sample.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Focusing with saw-tooth refractive lenses at a high-energy X-ray beamline

The Advanced Photon Source 1-ID beamline, operating in the 40–140 keV X-ray energy range, has successfully employed continuously tunable saw-tooth refractive lenses to routinely deliver beams focused in both one and two dimensions to experiments for over 15 years. The practical experience of implementing such lenses, made of silicon and aluminium, is presented, including their properties, control, alignment, and diagnostic methods, achieving ∼1 µm focusing (vertically). Ongoing development and prospects towards submicrometre focusing at these high energies are also mentioned.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗