Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “LASER RADIATION”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Terahertz plasmonic laser radiating in an ultra-narrow beam

Plasmonic lasers (spasers) generate coherent surface plasmon polaritons (SPPs) and could be realized at subwavelength dimensions in metallic cavities for applications in nanoscale optics. Plasmonic cavities are also utilized for terahertz quantum-cascade lasers (QCLs), which are the brightest available solid-state sources of terahertz radiation. A long standing challenge for spasers that are utilized as nanoscale sources of radiation, is their poor coupling to the far-field radiation. Unlike conventional lasers that could produce directional beams, spasers have highly divergent radiation patterns due to their subwavelength apertures. Here, we theoretically and experimentally demonstrate a new technique for implementing distributed feedback (DFB) that is distinct from any other previously utilized DFB schemes for semiconductor lasers. The so-termed antenna-feedback scheme leads to single-mode operation in plasmonic lasers, couples the resonant SPP mode to a highly directional far-field radiation pattern, and integrates hybrid SPPs in surrounding medium into the operation of the DFB lasers. Experimentally, the antenna-feedback method, which does not require the phase matching to a well-defined effective index, is implemented for terahertz QCLs, and single-mode terahertz QCLs with a beam divergence as small as 4°×4° are demonstrated, which is the narrowest beam reported for any terahertz QCL to date. Moreover, in contrast to a negligible radiative field in conventional photonic band-edge lasers, in which the periodicity follows the integer multiple of half-wavelengths inside the active medium, antenna-feedback breaks this integer limit for the first time and enhances the radiative field of the lasing mode. Terahertz lasers with narrow-beam emission will find applications for integrated as well as standoff terahertz spectroscopy and sensing. Furthermore, the antenna-feedback scheme is generally applicable to any plasmonic laser with a Fabry–Perot cavity irrespective of its operating wavelength and could bring plasmonic lasers closer to practical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Two-photon absorption of soft X-ray free electron laser radiation by graphite near the carbon K-absorption edge

We have examined the transmission of soft X-ray pulses from the FERMI free electron laser through carbon films of varying thickness, quantifying nonlinear effects of pulses above and below the carbon K-edge. At typical of soft X-ray free electron laser intensities, pulses exhibit linear absorption at photon energies above and below the K-edge, ~308 and ~260 eV, respectively; whereas two-photon absorption becomes significant slightly below the K-edge, ~284.2 eV. The measured two-photon absorption cross section at 284.18 eV (~6 x 10-48 cm4 s) is 7 orders of magnitude above what is expected from a simple theory based on hydrogen-like atoms - a result of resonance effects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Interaction of high-power laser radiation with low-density polymer aerogels

The interaction of high-power subnanosecond laser pulses with low-density targets of cellulose triacetate polymer is considered. An Nd-glass laser setup provides a focal spot intensity of over 10{sup 14} W cm{sup −2}. An investigation is made of absorption of laser radiation, laser-to-X-ray energy conversion, spectra of ions emitted from the plasma, transmission of laser radiation through the target and plasma, as well as volume heating of the target material. It is experimentally determined that the laser energy conversion efficiency to X-rays with photon energies of a few Kiloelectronvolts decreases with increasing target material density. With the use of targets of density 10 mg cm{sup −3} this efficiency is two times lower in comparison to 2 mg cm{sup −3} density targets. The duration and amplitude of laser pulses transmitted through the target decreases with increasing column target density (the product of target material density and its thickness). The spectra of ions emitted from low-density target plasmas are recorded using ion collectors positioned at different angles relative to the direction of laser beam propagation as well as a high-resolution Thomson mass spectrometer. The ion flux and ion energies are found to increase with increasing target material density. The peak of the ion energy spectrum is shifted towards higher energies with increasing laser radiation intensity. (interaction of laser radiation with matter. laser plasma)

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Influence of asymmetric injection of laser radiation into capillary waveguides on wake acceleration of electrons possessing various injection energies

Laser wakefield acceleration of electron bunches possessing various initial injection energies in capillary waveguides is studied in the conditions of an asymmetric input of laser radiation into a waveguide (the propagation direction of laser radiation deviates from the capillary axis or the laser spot is not symmetric). The factors determining the critical angle of the laser radiation input into the capillary, within which the wake acceleration of electrons is close to optimal, have been found. It is shown that in acceleration stages where electron energies are high, the requirements to angular concentricity of the capillary axis and laser radiation focusing are substantially weaker due to the relativistic 'weighting' of the electron mass. (extreme laser radiation: physics and fundamental applications)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

High-Efficiency Conversion of High-Power-Density Laser Radiation

AlGaAs/GaAs-based semiconductor photovoltaic converters (PVCs) of laser radiation, capable of operating at an illuminance up to 9 kW/cm{sup 2} with retained isothermal state have been studied. This state was confirmed by the logarithmic shape of dependences of the open-circuit voltage on the power density of incident laser radiation. At maximum illuminance, the open-circuit voltage level was close to 1.33 V. It is shown that the proposed PVCs can provide laser radiation conversion at an 840-nm wavelength with efficiency above 51% at an incident radiation power density of 2.5 kW/cm{sup 2}.

14 SOLAR ENERGY↗
DOE OSTI2017

Optical emission of a plasma from low-density targets irradiated with coherence-controllable laser radiation

The results of experiments on the interaction of nanosecond laser radiation with low-density volume-structured targets of different density and thickness are reported. The targets were irradiated by laser radiation with controllable coherence. The primary objective was to investigate the effect of target parameters on the characteristics of radiation scattered by the plasma. The spectral characteristics of the radiation scattered by the plasma in the backward direction and in the direction of laser beam propagation were obtained. Also the radiation scattering patterns were recorded. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Interaction between laser radiation and metallic powder of 316L austenitic steel during selective laser melting

Highlights: • Single and multiple tracks on a loose bulk powder substrate were generated. • Understanding of the microstructure development during SLM was achieved. • Single tracks reveal a nearly symmetrical crystallography with elongated grains. • Multiple tracks show elongated grains perpendicular to the build direction. - Abstract: During selective laser melting (SLM), a complex heat state develops that leads to a characteristic crystallography and microstructure of the processed materials. Depending on the geometry of the processed part, most scan tracks of a new layer, so-called hatches, are located above a dense substrate or already solidified structures whereas others are located above loose powder. This is, inter alia, the case for overhanging structures. Attributable to the lower thermal conductivity of loose powder, temperature gradients and cooling rates of the melt pool differentiate in these areas, resulting in a different microstructural build-up. In this work, the microstructure and the crystallographic orientation of grade 316L austenitic stainless steel processed by SLM was investigated to understand the interaction between the laser radiation and the metallic powder during SLM-processing and to investigate the remelting of a track on a previous SLM-densified track. Single and multiple tracks on a loose bulk powder substrate, as well as single tracks on a dense substrate plate, were generated. A parameter study revealed that high energy densities are necessary to build continuous tracks on a loose bulk powder substrate. In addition, the amount of adhered particles, which are sintered on the fully melted and solidified tracks, is determined in comparison to the melted powder because the sintered particles strongly influence the surface roughness. To understand the microstructure development and, particularly, the influence of adjacent hatches during SLM-processing, investigations on the resulting microstructure and crystallographic orientation of a single track and two connected multiple tracks were carried out. During SLM processing of the tracks, the substrate plate and the solidified structures influence the temperature gradient and cooling rate of the melt pool, thus directionally solidified and elongated grains occur. Furthermore, the solidification is characterized by an epitaxial growth due to a distinct thermal gradient between the melt pool and the surrounding elements.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Modification of a graphene – copper substrate in vacuum by femtosecond laser radiation

The effect of intense femtosecond laser radiation on single-layer graphene fixed on a copper fine-meshed grid is studied using a compact femtosecond electron diffractometer. A modification of the graphene – copper substrate is found. The composition of the resulting compounds is analysed. It is shown that the main new products are copper oxides. The prospects of using such a substrate in the method of ultrafast electron diffraction and microscopy are analysed. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Precise modulation of laser radiation by an acousto-optic modulator for stabilisation of the Nd : YAG laser on optical resonances in molecular iodine

A system of precise frequency modulation of laser radiation by an acousto-optic modulator, which makes it possible to stabilise the radiation power and simultaneously suppress the residual amplitude modulation to a level of 10{sup -8} of the total laser power at the third harmonic of modulation frequency (∼500 Hz), is presented. The use of this system for the Nd : YAG/I{sub 2} optical frequency standard and application of digital signal synthesis and processing methods provided a level of frequency standard instability as small as ∼10{sup -15} for ∼6 × 10{sup 4} s. (frequency standards)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Modulation of diode laser radiation for the formation of a distance-independent backscattered signal

We propose a probing regime, in which the backscattered signal energy is independent of the distance from the surface of a scattering object. This regime is implemented using a part of the laser radiation power. With respect to the efficiency of using the laser during one measurement, the proposed regime is close to the pseudorandom noise continuous wave lidar regime. Using diode lasers, the regime of the degenerate energy response function and the lidar regime can be implemented in a single device. The relations between the geometric parameters of the probing scheme and the modulation parameters of laser radiation are obtained. The dependence of the linearity range of the diode laser on the relation between the length of the receiving channel near-field zone and the length of the probed trace is substantiated. (control of laser radiation parameters)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Processes of interaction of laser radiation with porous transparent materials during their ablation

The effect of ytterbium fibre laser radiation with λ = 1.07 μm on the pressed micropowders of transparent oxides and fluorides (CaF{sub 2}, Y{sub 2}O{sub 3}, Al{sub 2}O{sub 3}, YSZ, etc.) having low absorption coefficients is theoretically studied. Using the numerical modelling, it is established that the radiation scattering in the medium of particles 0.5 – 4.6 μm in diameter leads to the concentration of radiation in the local regions of the medium, with the intensity level exceeding the intensity of the incident radiation (0.46 MW cm{sup −2}) by many times. It is shown that with the growth of the refractive index n of the material from 1.38 (MgF{sub 2}) to 2.12 (YSZ) the intensity of radiation in the region of the strongest local maximum increases from 3.8 to 31 MW cm{sup −2}. In our opinion, this factor can be dominant in determining the possibility of achieving the ablation of such a medium by means of the radiation having an intensity of no greater than 1 MW cm{sup −2}. This hypothesis is experimentally checked by the example of preparing a nanopowder of CaF{sub 2} (n = 1.43) and 1 % Nd : Y{sub 2}O{sub 3} (n = 1.91) by evaporating the pressed target with the porosity 50 % by the continuous-wave radiation of the ytterbium fibre laser with the power 600 W and the intensity 0.4 MW cm{sup −2}. Inside the CaF{sub 2} target, the scattered radiation intensity is smaller than the optical breakdown threshold, and the target is not evaporated. Under the same conditions, the target made of higher-melting 1 % Nd : Y{sub 2}O{sub 3} evaporates well, and the efficiency of the nanopowder production amounts to 23 g h{sup −1}. (interaction of laser radiation with matter)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Combined Impact of Gamma and Laser Radiation on Peripheral Blood of Rats in vivo

The impact of γ radiation of {sup 137}Сs (doses of 1 and 3 Gy), low-intensity laser radiation (λ = 670 nm, 5.3 or 10.6 J/cm{sup 2}) as well as the influence of consecutive laser and γ radiation on peripheral blood and blood cells (erythrocytes, leukocytes, lymphocytes, granulocytes) were studied by analyzing the number of blood cells, blood absorption spectra, and activity of antioxidant defense enzymes. Two series of experiments were performed on four groups of rats. The rats of the control group (group 1) were not exposed to γ or laser radiation. In the experimental groups, single irradiation of the whole body of rats with γ radiation (group 2), three- or four-day over-vein irradiation of blood in the tail vein by low-intensity laser radiation (group 3), and successive three- or four-day irradiation of blood by laser and then a single irradiation of the whole body with γ radiation (group 4) were performed. It was shown that changes of the blood cell content in the experimental groups are accompanied by changes in the spectral characteristics of the blood and the activity of antioxidant defense enzymes. The radioprotective effect of low-intensity laser radiation is manifested as an increase in the average number of leukocytes and lymphocytes in the group as compared with the postradiation, as well as an increase in the activity of antioxidant protection enzymes. The possibility of using low-intensity optical radiation for correction of hematological disorders caused by ionizing radiation is discussed.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Periodic colour-centre structure formed under filamentation of mid-IR femtosecond laser radiation in a LiF crystal

A colour-centre structure formed in a LiF crystal under filamentation of a femtosecond mid-IR laser pulse with a power slightly exceeding the critical power for self-focusing has been experimentally and theoretically investigated. Strictly periodic oscillations have been detected for the first time for the density of the colour centres induced in an isotropic LiF crystal under filamentation of a laser beam with a wavelength tuned in the range from 2600 to 3350 nm. The structure period is found to be about 30 μm. With an increase in the laser radiation wavelength, the period of the oscillations decreases and their amplitude increases. The maximum colour centre density, observed under filamentation of a 3100-nm beam, is related to the increased contribution of the direct generation of colour centres as a result of the absorption of an integer number of photons by the exciton band. It is numerically shown that the formation of a periodic colour-centre structure in LiF is due to the periodic change in the light field amplitude in the light bullet (1.5 optical periods long) formed under filamentation. (extreme laser radiation: physics and fundamental applications)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Ablation of hexagonal boron nitride by UV laser radiation

We report an experimental study of ablation of hexagonal boron nitride ceramics irradiated by 193- and 248-nm laser pulses. The dependences of the material removal rate and the crater wall slope on the energy density in the irradiation spot are obtained. An ablation regime is found, when the material removal rate reaches 5 mm per pulse, which is not typical of nanosecond laser radiation. It is shown that at relatively low radiation intensities (about 5–10 J cm{sup −2}), the slope angle of the formed crater wall does not exceed 20°. (interaction of laser radiation with matter)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Interaction of near-IR laser radiation with plasma of a continuous optical discharge

The interaction of 1.07-μm laser radiation with plasma of a continuous optical discharge (COD) in xenon and argon at a pressure of p = 3–25 bar and temperature of T = 15 kK has been studied. The threshold power required to sustain COD is found to decrease with increasing gas pressure to P{sub t} < 30 W in xenon at p > 20 bar and to P{sub t} < 350 W in argon at p > 15 bar. This effect is explained by an increase in the coefficient of laser radiation absorption to 20−25 cm{sup –1} in Xe and 1−2 cm{sup –1} in Ar due to electronic transitions between the broadened excited atomic levels. The COD characteristics also depend on the laser beam refraction in plasma. This effect can be partially compensated by a tighter focusing of the laser beam. COD is applied as a broadband light source with a high spectral brightness.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Effect of laser radiation on aqueous solutions of beta-active nuclides

Experimental data are presented on the effect of pulsed laser irradiation of aqueous {sup 234}Th, {sup 231}Th and {sup 137}Cs solutions on the concentrations of the beta-active nuclides in the solutions. Central to the investigation technique used is that the solutions contain nanoparticles resulting from the interaction of laser radiation with a target placed in the cuvette. The concentration of the radionuclides was evaluated before, during and after laser irradiation from the area under gamma-ray lines corresponding to spontaneous gamma decay. The gamma photon energies of {sup 234}Th, {sup 231}Th and {sup 137}Cs are 92, 186 and 661 keV, respectively. It has been shown that laser irradiation reduces the concentration of the three beta-active nuclides without excess gamma-ray emission in the energy range of their spontaneous decay. This is possible if decay follows another mechanism, activated by interaction of laser radiation with the aqueous solutions of the nuclides in the presence of nanoparticles.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2016

Microstructure of the multiple-filamentation zone formed by femtosecond laser radiation in a solid dielectric

The regularities of multiple filamentation of gigawatt femtosecond laser pulses in a solid dielectric (optical glass) have been considered. The fine spatial structure of the plasma region that is formed under glass photoionisation and accompanies the formation of light filaments is analysed experimentally and by means of numerical simulation. The dependence of the number, position, and extension of individual 'generations' of plasma channels on the laser pulse energy has been investigated for the first time. It is found that the distribution of the number of plasma channels over the length of a dielectric sample has a maximum, the position of which correlates well with the position of the nonlinear focus of the light beam as a whole; at the same time, the average channel length decreases with increasing pulse power, whereas the number of successive channel 'generations', on the contrary, increases. (interaction of laser radiation with matter. laser plasma)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗