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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Sequential Infiltration Synthesis for the Design of Low Refractive Index Surface Coatings with Controllable Thickness

Control over refractive index and thickness of surface coatings is central to the design of low refraction films used in applications ranging from optical computing to antireflective coatings. Here, we introduce gas-phase sequential infiltration synthesis (SIS) as a robust, powerful, and efficient approach to deposit conformal coatings with very low refractive indices. We demonstrate that the refractive indices of inorganic coatings can be efficiently tuned by the number of cycles used in the SIS process, composition, and selective swelling of the of the polymer template. We show that the refractive index of Al2O3 can be lowered from 1.76 down to 1.1 using this method. The thickness of the Al2O3 coating can be efficiently controlled by the swelling of the block copolymer template in ethanol at elevated temperature, thereby enabling deposition of both single-layer and graded-index broadband antireflective coatings. Using this technique, Fresnel reflections of glass can be reduced to as low as 0.1% under normal illumination over a broad spectral range.

Berman, Diana↗
DOE OSTI2015

Compositional dependence of optical band gap and refractive index in lead and bismuth borate glasses

Highlights: • Refractive indices increase with increasing PbO/Bi{sub 2}O{sub 3} content. • Optical band gap arises due to direct forbidden transition. • Optical band gaps decrease with increasing PbO/Bi{sub 2}O{sub 3} content. • New empirical relation between the optical band gap and the refractive index. - Abstract: We prepared a series of lead and bismuth borate glasses by varying PbO/Bi{sub 2}O{sub 3} content and studied refractive index and optical band gap as a function of glass composition. Refractive indices were measured very accurately using a Brewster’s angle set up while the optical band gaps were determined by analyzing the optical absorption edge using the Mott–Davis model. Using the Lorentz–Lorentz method and the effective medium theory, we calculated the refractive indices and then compared them with the measured values. Bismuth borate glasses show better agreement between the calculated values of the refractive index and experimental values. We used a differential method based on Mott–Davis model to obtain the type of transition and optical band gap (E{sub opt}) which in turn was compared with the value of E{sub opt} obtained using the extinction coefficient. Our analysis shows that in both lead and bismuth borate glasses, the optical band gap arises due to direct forbidden transition. With increasing PbO/Bi{sub 2}O{sub 3} content, the absorption edge shifts toward longer wavelengths and the optical band gap decreases. This behavior can be explained in terms of changes to the Pb−O/Bi−O chemical bonds with glass composition. We obtained a new empirical relation between the optical band gap and the refractive index which can be used to accurately determine the electronic oxide polarizability in lead and bismuth oxide glasses.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Refractive indices in the whole transmission range of partially deuterated KDP crystals

Refractive indices of partially deuterated potassium dihydrogen phosphate (DKDP) crystals with 55%, 70% and 80% deuterium contents were measured by auto-collimation method at 293 K between 0.254 to 1.529 μm. Dependence of refractive indices of DKDP on deuterium content show different trend in the infrared region as in uv-visible region. Dependence of n{sup 2} (the square of refractive index) on the mole fraction of deuterium shows a difference between pure KDP and partially deuterated KDP. The Sellmeier equations were obtained by the least square method. The non-critical phase matching angles calculated from the fitted formula were in good agreement with laser experiment results, by which the reliability of these Sellmeier equations was confirmed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Moiré deflectometry using the Talbot-Lau interferometer as refraction diagnostic for High Energy Density plasmas at energies below 10 keV

The highly localized density gradients expected in High Energy Density (HED) plasma experiments can be characterized by x-ray phase-contrast imaging in addition to conventional attenuation radiography. Moiré deflectometry using the Talbot-Lau grating interferometer setup is an attractive HED diagnostic due to its high sensitivity to refraction induced phase shifts. We report on the adaptation of such a system for operation in the sub-10 keV range by using a combination of free standing and ultrathin Talbot gratings. This new x-ray energy explored matches well the current x-ray backlighters used for HED experiments, while also enhancing phase effects at lower electron densities. We studied the performance of the high magnification, low energy Talbot-Lau interferometer, for single image phase retrieval using Moiré fringe deflectometry. Our laboratory and simulation studies indicate that such a device is able to retrieve object electron densities from phase shift measurements. Using laboratory x-ray sources from 7 to 15 μm size we obtained accurate simultaneous measurements of refraction and attenuation for both sharp and mild electron density gradients.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Influence of refraction index strength on the light propagation in dielectrics material with periodic refraction index

This study investigated the influence of refraction index strength on the light propagation in refraction index-varied dielectric material. This dielectric material served as photonic lattice. The behavior of light propagation influenced by variation of refraction index in photonic lattice was investigated. Modes of the guiding light were determined numerically using squared-operator iteration method. It was found that the greater the strength of refraction index, the smaller the guiding modes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Determining the refractive index of shocked [100] lithium fluoride to the limit of transmissibility

Lithium fluoride (LiF) is a common window material used in shock- and ramp-compression experiments because it displays a host of positive attributes in these applications. Most commonly, it is used to maintain stress at an interface and velocimetry techniques are used to record the particle velocity at that interface. In this application, LiF remains transparent to stresses up to 200 GPa. In this stress range, LiF has an elastic-plastic response with a very low (<0.5 GPa) elastic precursor and exhibits no known solid-solid phase transformations. However, because the density dependence of the refractive index of LiF does not follow the Gladstone-Dale relation, the measured particle velocity at this interface is not the true particle velocity and must be corrected. For that reason, the measured velocity is often referred to as the apparent velocity in these types of experiments. In this article, we describe a series of shock-compression experiments that have been performed to determine the refractive index of LiF at the two most commonly used wavelengths (532 nm and 1550 nm) between 35 and 200 GPa to high precision. A modified form of the Gladstone-Dale relation was found to work best to fit the determined values of refractive index. In addition, we provide a direct relationship between the apparent and true particle velocity to correct experimentally obtained wave profiles by others using these velocimetry techniques.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Near-edge X-ray refraction fine structure microscopy

We demonstrate a method for obtaining increased spatial resolution and specificity in nanoscale chemical composition maps through the use of full refractive reference spectra in soft x-ray spectro-microscopy. Using soft x-ray ptychography, we measure both the absorption and refraction of x-rays through pristine reference materials as a function of photon energy and use these reference spectra as the basis for decomposing spatially resolved spectra from a heterogeneous sample, thereby quantifying the composition at high resolution. While conventional instruments are limited to absorption contrast, our novel refraction based method takes advantage of the strongly energy dependent scattering cross-section and can see nearly five-fold improved spatial resolution on resonance.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Inserting a Low-Refractive-Index Dielectric Rear Reflector into PERC Cells: Challenges and Opportunities

One of the simplest and most effective ways to reduce the operating temperature of photovoltaic systems in the field is to reflect unusable, 'sub-bandgap' light with energies below the cell absorber's bandgap energy. In this work, low-refractive index SiO2 nanoparticle films inserted between c-Si wafers and metal electrodes significantly increase the reflectance of sub-bandgap light in fabricated test structures. These films are then integrated into the standard PERC fabrication sequence, revealing that the reflection benefits remains after some processing steps but are lost in certain conditions. The SiO2 nanoparticle films are easily ablated during laser contact opening, but are undesirably etched during post-laser cleaning. After Al metallization, the fully fabricated devices show enhanced sub-bandgap reflection when annealed at moderate temperatures, but energy-dispersive X-ray spectroscopy shows that high-temperature firing causes unwanted Al penetration into the SiO2 nanoparticle film, compromising reflection.

14 SOLAR ENERGY↗
DOE OSTI2015

Photo-induced refractive index and topographical surface gratings in functionalized nanocarbon solid film

We demonstrate that a single-walled carbon nanotube network noncovalently coupled with a pyrene-modified azo-benzene chromophore functions as a host matrix for a broad range of photo-orientation and photomechanical effects. The chromophore could be efficiently reoriented through repeated trans-cis-trans isomerization under linearly polarized 480 nm light, with Δn of 0.012 at 650 nm and fast characteristic rise-times of 0.12 s. Erasable phase diffraction gratings could also be written, with permanent surface relief gratings forming at sufficiently long irradiation times. In addition to demonstrating a mechanism for photo-manipulation of single-walled carbon nanotubes, these results show photo-orientation of chromophores in azo-functionalized single-walled carbon nanotube networks as a path towards the photosensitive tuning of the electrostatic environment of the nanotube.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗