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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 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Investigation of free charge carrier dynamics in single-crystalline CVD diamond by two-photon absorption

By using the methods of transient gratings (TGs) and induced absorption, we have studied the kinetics of plasma of free charge carriers (FCCs) created by the action of a picosecond laser pulse in two high-purity diamond single crystals synthesised from the gas phase. The gratings with different spatial periods have been excited at the wavelengths of 266 or 213 nm (above and below the fundamental absorption edge in diamond) and probed with continuous-wave radiation in the visible region. At the moderate FCC concentrations (∼7 × 10{sup 17} cm{sup -3}), the coefficient of ambipolar diffusion and the carrier recombination time of two crystals are 20.3 and 18.9 cm{sup 2} s{sup -1} and 30 and 190 ns, respectively. The increase in the carrier concentration up to 5 × 10{sup 19} cm{sup -3} reduces the TG lifetime. We have determined the conditions under which the relaxation of the grating of carriers leads to the formation of a thermal grating, with the amplitude sufficient for its experimental observation. (interaction of laser radiation with matter. laser plasma)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Laser radiation frequency doubling in a single-crystal fibre based on a stoichiometric LiNbO{sub 3} crystal

We demonstrate the employment of single-crystal optical fibres based on lithium niobate for doubling the laser radiation frequency. The measured characteristics of the fibre confirm its high quality and spatial homogeneity. Parameters of the frequency doublers for neodymium laser radiation (λ = 1 mm) based on fibre and bulk single crystals are compared. Single crystals are grown by the method of laser-heated pedestal growing with heating by radiation of a CO{sub 2} laser (LHPG-method). (nonlinear optical phenomena)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spectroscopy and tunable continuous-wave operation of Cr{sup 2+}:Zn{sub 1-x}Mg{sub x}Se single crystal around 2.5 μm

Hexagonal single crystals of the Cr{sup 2+}:Zn{sub 1-x}Mg{sub x}Se solid solution are grown by the Bridgman – Stockbarger technique. We study their optical and photoluminescent characteristics and determine the temperature dependences of the photoluminescence kinetics within the range 5 – 440 K. The crystals possess a broad gain band in the range from 2 to 3.5 μm and exhibit a weak quenching of luminescence with a quantum yield exceeding 80% at room temperature. Tunable continuous-wave oscillation is demonstrated in the region of 2.5 μm. (lasers)

36 MATERIALS SCIENCE↗
DOE OSTI2015

Absorption spectra and nonlinear transmission (at λ = 2940 nm) of a diffusion-doped Fe{sup 2+}:ZnSe single crystal

Transmission spectra of a ZnSe sample diffusion-doped with Fe{sup 2+} ions have been measured in the wavelength range 500 – 7000 nm. A broad absorption band in the range 500 – 1500 nm has been observed in both doped and undoped regions of the sample. This band is possibly due to deviations from stoichiometry in the course of diffusion doping. The transmission of the Fe{sup 2+}:ZnSe sample at a wavelength of 2940 nm has been measured at various dopant concentrations and high peak pulse intensities (up to 8 MW cm{sup -2}). The samples have been shown to be incompletely bleached: during a laser pulse, the transmission first increases, reaches a maximum, and then falls off. Our results suggest that the incomplete bleaching cannot be accounted for by excited-state absorption. The incomplete bleaching (as well as the transmission maximum) is due to the heating of the sample, which leads to a reduction in upper level lifetime and, accordingly, to an increase in absorption saturation intensity. (nonlinear optical phenomena)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Carbon photonics

The properties of new carbon materials (single-crystal and polycrystalline CVD diamond films and wafers, single-wall carbon nanotubes and graphene) and the prospects of their use as optical elements and devices are discussed. (optical elements of laser devices)

36 MATERIALS SCIENCE↗
DOE OSTI2016

Room-temperature 1.2-J Fe{sup 2+}:ZnSe laser

The characteristics of a laser based on a Fe{sup 2+}:ZnSe single crystal pumped by an electric-discharge HF laser at room temperature are studied. The HF laser beam diameter on the crystal surface was 17 mm. The achieved laser energy was 1.2 J with an efficiency of ∼ 25% with respect to the pump energy. (letters)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Yb : YAG thin-rod laser amplifier with a high pulse energy for a fibre oscillator

High (more than ten times) small-signal gain is demonstrated in an Yb : YAG single crystal thin rods with pumping by a fibre-coupled diode laser. A four-pass amplifier for a fibre master oscillator with an average output power exceeding 15 W at a pulse repetition rate of 3 MHz is fabricated based on this active element. The small-signal gain in the developed amplifier is 26 dB, which is comparable with the gain in regenerative amplifiers. The possibility of obtaining sub-millijoule pulse energy at a pulse repetition rate of tens of kilohertz is shown. (extreme laser radiation: physics and fundamental applications)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Room-temperature Fe{sup 2+}:ZnS single crystal laser pumped by an electric-discharge HF laser

A room-temperature laser based on a Fe{sup 2+}:ZnS single crystal has been studied. The crystal was pumped by a nonchain electric-discharge HF laser with a light pulse FWHM of ∼140 ns. The pump spot diameter on the crystal surface (d) varied from 3.9 to 11.6 mm. The slope efficiency at these values of d was 37% – 39 %, respectively. The maximum laser energy was 380 mJ (at d=11.6 mm) with the total efficiency with respect to the absorbed pump energy of ∼19%. A further increase in the laser energy was not achieved because the crystal was destroyed at an incident pump energy and power of 3.4 J and ∼24 MW, respectively. The damaged region has a form of a long narrow channel shifted from the entrance surface inside the active element. (lasers)

42 ENGINEERING↗
DOE OSTI2016

IR luminescence of Fe{sup 2+} : ZnSe single crystals excited by an electron beam

Spectral-kinetic characteristics of the cathodoluminescence of Fe{sup 2+} : ZnSe single crystals are studied at T='300' and 80 K. These characteristics correspond to the IR luminescence spectrum and kinetics of Fe{sup 2+} ions in ZnSe crystals optically excited into the impurity absorption band. The obtained results open a real possibility of pumping Fe{sup 2+} : ZnSe lasers by hot electrons. (letters)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Laser nanoablation of diamond surface at high pulse repetition rates

The chemical etching of the surface of a natural diamond single crystal irradiated by subpicosecond laser pulses with a high repetition rate (f⩽500 kHz) in air is experimentally investigated. The irradiation has been performed by the second-harmonic (515 nm) radiation of a disk Yb : YAG laser. Dependences of the diamond surface etch rate on the laser energy density and pulse repetition rate are obtained. (interaction of laser radiation with matter)

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Laser structuring of the diamond surface in the nanoablation regime

The possibility of using nanoablation (graphitisation-free photoinduced etching) for precise micro- and nanostructuring of the surface of a diamond single crystal has been experimentally investigated. The processing has been performed by femtosecond third-harmonic pulses of a Ti : sapphire laser (λ=266 nm). The specific features of the formation of a surface nanorelief (regularity of the newly formed structures and reduced nanoablation rate near craters) are discussed. The possibility of fabricating a diamond phase diffraction grating with a relief depth of ∼130 nm is demonstrated and the diffraction pattern recorded at λ=532 nm is compared with the results of theoretical analysis. (paper)

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Study of the optical absorption and photoluminescence in (Pb,Gd){sub 3}(Al,Ga){sub 5}O{sub 12} : Ce epitaxial films grown from Pb-containing melt solutions

The optical absorption and photoluminescence properties of cerium-activated (Pb,Gd){sub 3}(Al,Ga){sub 5}O{sub 12} epitaxial films are studied. The films are grown on single-crystal (111)-oriented Gd{sub 3}Ga{sub 5}O{sub 12} substrates by liquid-phase epitaxy from supercooled PbO – B{sub 2}O{sub 3} melt solutions at different concentrations of gadolinium, cerium, and aluminium oxides in the charge. The photoluminescence band of Ce{sup 3+} ions is shown to peak at 532 nm. The highest cathodoluminescence yield of about 51500 photons MeV{sup -1} at a decay time of the slow component of 61.0 ns (light yield fraction 68%) is found for the Pb{sub 0.01}Ce{sub 0.03}Gd{sub 2.96}Al{sub 3.14}Ga{sub 1.86}O{sub 12} film, grown from melt solution with gadolinium oxide, cerium oxide, and aluminium oxide concentrations of 0.4, 0.2, and 4.5 mol % in the charge, respectively. Epitaxial films with these spectroscopic characteristics are promising for application in scintillation screens. (fluorescence and photogeneration)

36 MATERIALS SCIENCE↗
DOE OSTI2018

Defectoscopy of ZnGeP{sub 2} single crystals using a strontium vapour laser

A modified method of optical defectoscopy of ZnGeP{sub 2} single crystal plates using a strontium vapour laser (λ = 1.03 and 1.09 μm) is proposed based on shadow imaging of internal defects in plates cut parallel to the (100) plane. It is shown that the use of a strontium vapour laser with a wavelength of 6.45 mm makes it possible to study inhomogeneities in large-size ZnGeP{sub 2} samples. The possibility of fabricating a projection defectoscope for monitoring breakdown development in ZnGeP{sub 2} crystals is considered. (laser applications)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Low-coherence interferometry as a tool for monitoring laser micro- and nanoprocessing of diamond surfaces

We have demonstrated the possibility of using low-coherence tandem optical interferometry for monitoring the local laser processing of diamond surfaces. Noncontact measurements of the optical thickness of single-crystal diamond plates were performed directly during the exposure of the surface to repetitive intense laser pulses. We investigated the dynamics of the thinning of a single crystal in two radically different regimes of laser etching: ablation (KrF excimer laser, λ = 248 nm, τ = 20 ns) and nanoablation (Ti : sapphire laser, λ = 266 nm, τ = 100 fs). (laser technologies)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Modelling of heating and photoexcitation of single-crystal silicon under multipulse irradiation by a nanosecond laser at 1.06 μm

We report a theoretical study of heating and photoexcitation of single-crystal silicon by nanosecond laser radiation at a wavelength of 1.06 μm. The proposed physicomathematical model of heating takes into account the complex nonlinear dynamics of the interband absorption coefficient of silicon and the contribution of the radial heat removal to the cooling of silicon between pulses under multipulse irradiation, which allows one to obtain a satisfactory agreement between theoretical predictions of silicon melting thresholds at different nanosecond pulse durations and experimental data (both under single-pulse and multipulse irradiation). It is found that under irradiation by nanosecond pulses at a wavelength of 1.06 μm, the dynamic Burshtein – Moss effect can play an important role in processes of photoexcitation and heating. It is shown that with the regimes typical for laser multipulse microprocessing of silicon (the laser spot diameter is less than 100 μm, and the repetition rate of pulses is about 100 kHz), the radial heat removal cannot be neglected in the analysis of heat accumulation processes. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
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↗