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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Energy and spectral characteristics of a parametric generator based on a nonlinear ZnGeP{sub 2} crystal pumped by a Ho : YAG laser

We have experimentally realised the average generation power of a parametric generator based on a ZnGeP{sub 2} single crystal, which constituted ∼1.5 W at an efficiency of ∼28.6 % in the wavelength range of 3.5 – 4.8 μm with an average pump radiation power of ∼5.5 W and its energy density of ∼ 0.47 J cm{sup −2}. It is shown that there are threshold conditions for the average pump power, providing the realisation of parametric generation. In this case, an increase in the generation efficiency with an increase in the energy density of pump radiation to ∼ 0.4 J cm{sup −2} is observed; with its further increase, the output power increases due to a decrease in the parametric generation threshold. Parametric generators (PGs) based on ZnGeP{sub 2} single crystals with such characteristics are promising for solving many applied problems, including generation of terahertz radiation when nonlinear crystals are pumped by radiation from these PGs at difference frequency. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Generation regimes of a solid-state coupled-cavity ring laser

Based on the experimental studies we have shown that when an inhomogeneous magnetic field is applied to an active YAG : Nd crystal, a solid-state coupled-cavity ring laser is characterised by the following generation regimes: stationary unidirectional regime, self-modulation regimes of the first and second kinds, and dynamic chaos regime. Transitions between these generation regimes were carried out when the magnet was moved over the surface of the YAG : Nd single crystal, which made it possible to control the radiation dynamics. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Parametric stimulated Raman scattering in barium nitrate crystals

The spectra of multifrequency parametric stimulated Raman scattering in a barium nitrate single crystal is investigated. When this single crystal is excited by ultrashort (80 ps) YAG : Nd{sup 3+} laser pulses with a wavelength of 1064 nm, a frequency comb of eight anti-Stokes components is observed in the visible spectral range. The Raman satellites are due to the totally symmetric vibrational modes of barium nitrate single crystal. The frequencies of the corresponding Raman Stokes components in the IR range (from 1193 to 9662 nm) are calculated. The results of the study open the way to form an array of laser frequencies from the IR to green spectral range with a period of 1047 cm{sup –1}. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Acousto-optic modulators for a controlled frequency shift of light beams in optical and microwave cold-atom frequency standards

A series of specialised acousto-optic modulators/frequency shifters based on paratellurite (TeO{sub 2}) single crystals is developed for systems of laser cooling and frequency stabilisation of laser sources in optical and microwave frequency standards based on cold strontium, rubidium and caesium atoms. The methods of calculation and optimisation of the main parameters of acousto-optic modulators are considered; their basic technical characteristics are experimentally investigated in the spectral range of optical radiation 460 – 900 nm and in the frequency shift range of 5 – 400 MHz. The modulators have a high diffraction efficiency, low power consumption and the ability to work with linearly and randomly polarised optical radiation. (frequency standards)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Growth of Yb : Na{sub 2}SO{sub 4} crystals and study of their spectral – luminescent characteristics

Ytterbium-doped single crystals of the high-temperature hexagonal sodium sulphate modification are grown by the Czochralski method. It is found by energy dispersive analysis that the crystallisation occurs congruently. The Yb : Na{sub 2}SO{sub 4} crystals have additional (with respect to Yb{sup 3+} ions) absorption bands in the regions of 650, 850 and 1150 nm, which obviously belong to colour centres. The luminescence spectrum of the Yb : Na{sub 2}SO{sub 4} single crystal noticeably differs from the luminescence spectrum of Yb : YAG crystals by the presence of a broad band without a prominent peak. The luminescence decay curve for the powder is single-exponential with a time constant of 1175 μs, which is somewhat larger than that for the Yb : YAG crystal. The lifetime of the ytterbium excited state does not change during storage of the samples in air for a week. (active media)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Multifrequency stimulated Raman scattering of light in a calcite single crystal

Using pulses of multifrequency stimulated Raman scattering (SRS) in calcite, nonlinear photoluminescence in a stilbene molecular crystal is excited. When multifrequency SRS is excited by the radiation of a Nd{sup 3+} : YAG laser with a wavelength of 1064 nm, eleven anti-Stokes components are observed in the visible spectrum with an average frequency shift of 1086 cm{sup −1} between them. Using the second harmonic of the Nd {sup 3+} : YAG laser radiation allowed three anti-Stokes and four Stokes components to be recorded. The large spectral range of the frequency comb facilitated effective reduction in the duration and increase in the intensity of the radiation pulse of multifrequency SRS. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Relaxation oscillations in a bipolarised Nd : YAG laser with a Fabry – Perot cavity

Based on the model of a bipolarised laser, which takes into account real orientations of absorbing and emitting dipoles of active centres in the unit cell of an Nd : YAG single crystal, we have shown that the appearance of low-frequency relaxation oscillations requires the participation of both competing pump channels through linearly and circularly polarised dipole transitions. (paper)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

Influence of growth temperature of KTiOAsO{sub 4} single crystals on their physicochemical parameters and formation of domain structures

A potassium titanyl arsenate (KTiOAsO{sub 4}, KTA) crystal 50 × 80 × 60 mm in size has been grown by upgraded Czochralski method from flux (TGGS) with a decrease in temperature from 900 to 770 °C during pulling. It is shown that the spectroscopic properties of the parts of KTA crystals grown at 900 and 770 °C are close, whereas the electrical conductivity of the low-temperature (770 °C) KTA part turned out to be an order of magnitude lower than that of the high-temperature part. Visualisation of the domain structure by second-harmonic generation microscopy revealed a more efficient domain intergrowth (throughout the sample) in the low-temperature KTA, which is important for forming a regular domain structure (RDS) in a KTA-based nonlinear optical element. It is established that the quantum efficiency of parametric generation of light in the RDS formed in low-temperature KTA is several times higher than in the case of high-temperature KTA. The results obtained are important for optimising RDS parameters. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Structure and Properties of Boron-Doped LiNbO{sub 3} Single Crystals

We have studied the macro-and microstructure and properties of LiNbO{sub 3}〈B〉 crystals in comparison with LiNbO{sub 3}〈Zn〉 and stoichiometric and congruent lithium niobate crystals. The optical characteristics of the LiNbO{sub 3}〈B〉 crystals have been shown to approach those of the stoichiometric LiNbO{sub 3} and LiNbO{sub 3}〈Zn〉 crystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Thermal Conductivity of Tetragonal Cadmium Diphosphide Crystals

Tetragonal cadmium diphosphide (β-CdP{sub 2}) single crystals with high structural perfection have been grown and their thermal conductivity has been measured. The results indicate that the higher thermal conductivity across the c axis corresponds to the smaller unit-cell parameter of the crystals (a = 5.08 ± 0.01 Å and c = 18.59 ± 0.05 Å). The observed thermal conductivity anisotropy is tentatively attributed to the fact that structural basis of β-CdP{sub 2} is made up of spectral phosphorus chains running in the [100] and [010] directions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Proton Conductivity and Thermal Properties of Ba(H{sub 2}PO{sub 4}){sub 2}

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H{sub 2}PO{sub 4}){sub 2} (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H{sub 2}PO{sub 4}){sub 2}, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H{sub 2}PO{sub 4}){sub 2} samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H{sub 2}PO{sub 4}){sub 2} up to 130°C. The proton conductivity of the Ba(H{sub 2}PO{sub 4}){sub 2} single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10{sup –9} to 2 × 10{sup –7} S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Photoelectric Fields and Band Gap in Doped Lithium Niobate Crystals

Photoinduced light scattering and optical spectroscopy have been used to study the photorefractive effect and determine the band gap in nominally undoped congruent and stoichiometric lithium niobate crystals, as well as in a series of congruent LiNbO{sub 3} crystals doped with Mg, Zn, B, Gd, Y, and Er cations and LiNbO{sub 3} single crystals codoped with Mg:Gd, Mg:Fe, Mg:Y, and Mg:Ta.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Exchange Spin Waves in Inhomogeneous Spinel Ferrite Films

This paper examines the effect of dynamic spin pinning at an interlayer boundary on spin wave propagation in monocrystalline manganese ferrite films magnetized along the normal to their surface. We calculate wavenumbers of exchange spin waves, decay parameters, inhomogeneous exchange parameters, and magnon effective masses. The thickness of the spin pinning layer is shown to influence the linewidth of spin wave modes. We demonstrate that, in the competition with magnetocrystalline anisotropy, a significant role is played by “surface anisotropy” in films of various chemical compositions grown by chemical vapor transport under different process conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Flux Growth, Thermal Properties, and Luminescence Spectra of (Er,Yb,Lu)Al{sub 3}(BO{sub 3}){sub 4} Solid Solutions

We have studied phase relations in the pseudoquaternary system LuAl{sub 3}(BO{sub 3}){sub 4}–(K{sub 2}Mo{sub 3}O{sub 10}–Al{sub 2}O{sub 3}–B{sub 2}O{sub 3}) in the temperature range 1130 to 900°C. The phase relations were represented in graphical form, as a projection of the formation of solid phases in this range onto the composition triangle at 900°C. Optimal results in terms of high-temperature solution growth have been obtained at 25 wt % LuAl{sub 3}(BO{sub 3}){sub 4} in the growth charge. Undoped and erbium–ytterbium codoped LuAl{sub 3}(BO{sub 3}){sub 4} (LuAB) single crystals have been grown from high-temperature solutions on “pointlike” seeds. The (Er,Yb):LuAB crystals are similar in thermal properties and luminescence spectra to (Er,Yb):YAl{sub 3}(BO{sub 3}){sub 4} and (Er,Yb):GdAl{sub 3}(BO{sub 3}){sub 4} crystals and can be used in diode-pumped lasers operating in the nominally eye-safe spectral region 1.5–1.6 μm.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Optical Properties of Copper-Doped Lithium Niobate Crystals

We have studied the optical properties of copper-doped lithium niobate crystals using semiconductor light-emitting diodes and lasers emitting in the visible and ultraviolet spectral regions as excitation sources. The results demonstrate that, at an excitation frequency approaching the frequency of electronic absorption in copper ions, there is discrete resonance Raman scattering in the form of a frequency comb. The observed Raman satellites of discrete light scattering are due to polar longitudinal A{sub 1} optical modes of the lithium niobate single crystal. Under excitation by a light-emitting diode with a wavelength of 520 nm, we observe a sharp increase in discrete light scattering intensity in comparison with the photoluminescence excited by shorter wavelength excitation sources.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Optical Constants of an Apatite Single Crystal in the IR Range of 6–28 μm

On the basis of polarized IR-reflection spectra in the range of 5000–350 cm{sup –1} measured from the natural face of an optically transparent fluorapatite single crystal, components of a complex refractive index (optical constants) for radiation-vector orientations E ‖ c and E ⊥ c have been calculated by the Kramers–Kronig method. The fluorapatite single crystal has been chosen from several samples: it contains a minimum amount of impurities and has a high degree of crystallinity in accordance with the criteria of IR spectroscopy and Raman spectroscopy. Tabular data on optical constants for ordinary and extraordinary rays are given for the IR range of 6–28 μm. The obtained absorption spectra are compared with the results of quantum-chemical ab initio calculations within the B3LYP simulation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Difference-Frequency Generation of THz Radiation via Parametric Three-Wave Interaction in CdTe and ZnTe Crystals

Difference-frequency generation of terahertz (THz) radiation in a collinear single-crystal scheme with distributed feedback implemented by means of parametric interaction of counterpropagating optical waves in a periodically inhomogeneous medium exhibiting quadratic nonlinearity is investigated. Using ZnTe and CdTe crystals optically pumped at λ = 0.77 and 1.06 μm, respectively, as an example, the possibility of obtaining parametric generation of THz radiation with an optical-to-THz conversion efficiency exceeding 10{sup –3} (with an extremely narrow line of oscillation of less than 4 GHz for ZnTe and less than 9 GHz for CdTe at 1 THz) is demonstrated in the undepleted-pump-intensity approximation. The obtained results can be used for construction of narrow-band lasers in the THz frequency range with an output power (in the quasicw regime of oscillation) of about 10{sup 5} W/cm{sup 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Two-Photon Absorption of Nonchain HF Laser Radiation in Germanium Single Crystals

Some results of experimental and numerical studies on the transmission of a nonchain high-frequency (HF) laser beam through germanium (Ge) single crystals of differing thickness and specific resistance are presented. Based on the experimental data for the HF(DF) lasing spectrum, the coefficient of two-photon absorption in germanium has been estimated as K{sub 2} = 55 ± 10 cm/GW at λ = 2.8 μm. The results are in good agreement with theory. The developed experimental data-processing software has enabled the numerical study of the transmission of a high-power beam with λ = 2.6–3 μm through Ge at any laser-pulse-impact moment. It has been shown that thin germanium coatings can efficiently smoothen the distribution of energy over the beam aperture for high-power laser radiation with λ = 2.6–4 μm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗