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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Far ultraviolet absorption spectra of N{sub 3} and N{sub 2}{sup +} generated by electrons impacting gaseous N{sub 2}

Electron bombardment of gaseous N{sub 2} produces N{sub 2} {sup +} and N{sub 3}, which are subsequently trapped in the N{sub 2} matrix at 10 K. Both the infrared and ultraviolet absorption spectra of the matrix sample at various stages of electron irradiation were recorded. Apart from a progression observed below 192 nm, with intervals ∼900 cm{sup –1}, corresponding to the transition of D {sup 2} Π {sub g} ← X {sup 2} Σ {sub g} {sup +} of N{sub 2} {sup +}, three new progressions were recorded in the range 225-192 nm, with intervals ∼1000 cm{sup –1}, that correlated well with variations in intensities of the electronic absorption band of N{sub 3} at 272.7 nm; an absorption coefficient of 3.76 × 10{sup –17} cm molecule{sup –1} for the transition A {sup 2}Σ{sub u} {sup +}← X {sup 2}Π{sub g} of N{sub 3} was estimated for the first time. These newly observed progressions were characterized and the vertical excitation energy and oscillator strength were calculated using time-dependent, density-functional theory. This was based on assigning the three progressions to electronic transitions of N{sub 3} from the ground state to 2 {sup 2}Π{sub u}, 1 {sup 2}Σ{sub g} {sup +}, and 1 {sup 2}Σ{sub g} {sup –}, respectively.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Theory of gyroresonance and free-free emissions from non-Maxwellian quasi-steady-state electron distributions

Currently there is a concern about the ability of the classical thermal (Maxwellian) distribution to describe quasi-steady-state plasma in the solar atmosphere, including active regions. In particular, other distributions have been proposed to better fit observations, for example, kappa- and n-distributions. If present, these distributions will generate radio emissions with different observable properties compared with the classical gyroresonance (GR) or free-free emission, which implies a way of remotely detecting these non-Maxwellian distributions in the radio observations. Here we present analytically derived GR and free-free emissivities and absorption coefficients for the kappa- and n-distributions, and discuss their properties, which are in fact remarkably different from each other and from the classical Maxwellian plasma. In particular, the radio brightness temperature from a gyrolayer increases with the optical depth τ for kappa-distribution, but decreases with τ for n-distribution. This property has a remarkable consequence allowing a straightforward observational test: the GR radio emission from the non-Maxwellian distributions is supposed to be noticeably polarized even in the optically thick case, where the emission would have strictly zero polarization in the case of Maxwellian plasma. This offers a way of remote probing the plasma distribution in astrophysical sources, including solar active regions as a vivid example.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Acoustic phonon assisted free-carrier optical absorption in an n-type monolayer MoS{sub 2} and other transition-metal dichalcogenides

The theory of free-carrier absorption (FCA) is given for monolayers of transition-metal dichalcogenides, particularly for molybdenum disulphide (MoS{sub 2}), when carriers are scattered by phonons. Explicit expressions for the absorption coefficient α are obtained and discussed for acoustic phonon scattering via screened deformation potential and piezoelectric coupling taking polarization of the radiation in the plane of the layer. It is found that α monotonously decreases with the increasing photon frequency Ω, increases with the increasing temperature T, and linearly depends on two-dimensional electron concentration n{sub s}. Effect of screening, which is ignored in all the earlier FCA studies, is found to reduce α significantly, attributing to the larger effective mass of the electrons. Results are also obtained in the classical and quantum limit giving the power laws α ∼ Ω{sup −2} and T. Comparison of the results is made with those in bulk semiconductors and semiconductor quantum wells.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spectrally resolved modulated infrared radiometry of photothermal, photocarrier, and photoluminescence response of CdSe crystals: Determination of optical, thermal, and electronic transport parameters

Spectrally resolved modulated infrared radiometry (SR-MIRR) with super-band gap photoexcitation is introduced as a self-consistent method for semiconductor characterization (CdSe crystals grown under different conditions). Starting from a theoretical model combining the contributions of the photothermal (PT) and photocarrier (PC) signal components, an expression is derived for the thermal-to-plasma wave transition frequency f{sub tc} which is found to be wavelength-independent. The deviation of the PC component from the model at high frequency is quantitatively explained by a quasi-continuous distribution of carrier recombination lifetimes. The integral, broad frequency band (0.1 Hz–1 MHz) MIRR measurements simultaneously yielded the thermal diffusivity a, the effective IR optical absorption coefficient β{sub eff}, and the bulk carrier lifetime τ{sub c}. Spectrally resolved frequency scans were conducted with interchangeable IR bandpass filters (2.2–11.3 μm) in front of the detector. The perfect spectral match of the PT and PC components is the direct experimental evidence of the key assumption in MIRR that de-exciting carriers are equivalent to blackbody (Planck) radiators. The exploitation of the β spectrum measured by MIRR allowed determining the background (equilibrium) free carrier concentration n{sub 0}. At the shortest wavelength (3.3 μm), the photoluminescence (PL) component supersedes the PC one and has distinct features. The average sample temperature influences the PC component but not the PT one.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Optical properties of cubic boron arsenide

The ultrahigh thermal conductivity of cubic boron arsenide (BAs) makes it a promising material for next-generation electronics and optoelectronics. Here, we report measured optical properties of BAs crystals, including the complex dielectric function, refractive index, and absorption coefficient in the ultraviolet, visible, and near-infrared wavelength range. In this work, the data were collected at room temperature using spectroscopic ellipsometry and transmission and reflection spectroscopy. We further calculated the optical response using density functional theory and many-body perturbation theory, considering quasiparticle and excitonic corrections. The computed values for the direct and indirect bandgaps (4.25 eV and 2.07 eV) agree well with the measured results (4.12 eV and 2.02 eV). Our findings pave the way for using BAs in future electronic and optoelectronic applications that take advantage of its demonstrated ultrahigh thermal conductivity and predicted high ambipolar carrier mobility.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Propagation and absorption of high-intensity femtosecond laser radiation in diamond

Femtosecond interferometry has been used to experimentally study the photoexcitation of the electron subsystem of diamond exposed to femtosecond laser pulses of intensity 10{sup 11} to 10{sup 14} W cm{sup -2}. The carrier concentration has been determined as a function of incident intensity for three harmonics of a Ti : sapphire laser (800, 400 and 266 nm). The results demonstrate that, in a wide range of laser fluences (up to those resulting in surface and bulk graphitisation), a well-defined multiphoton absorption prevails. We have estimated nonlinear absorption coefficients for pulsed radiation at λ = 800 nm (four-photon transition) and at 400 and 266 nm (indirect and direct two-photon transitions, respectively). It has also been shown that, at any considerable path length of a femtosecond pulse in diamond (tens of microns or longer), the laser beam experiences a severe nonlinear transformation, determining the amount of energy absorbed by the lattice, which is important for the development of technology for diamond photostructuring by ultrashort pulses. The competition between wave packet self-focusing and the plasma defocusing effect is examined as a major mechanism governing the propagation of intense laser pulses in diamond. (interaction of laser radiation with matter. laser plasma)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Photochemical CO 2 Reduction Using Rhenium(I) Tricarbonyl Complexes with Bipyridyl-Type Ligands with and without Second Coordination Sphere Effects

In this study, the effect of phenyl, phenol, aniline, amino, and CF 3 substituents of the 2,2'-bipyridine ligand in fac-ReCl(L)(CO) 3 (L=2,2'-bipyridine derivative) catalysts on photochemical CO 2 reduction in dimethylacetamide is examined, in order to understand the structure-function relationships and to compare the catalytic activities with the previously published electrochemical results. All complexes including ReCl(bpy)(CO) 3 have similar excited-state lifetimes and emission spectra, but complex 1 with the Ph-NH 2 moiety exhibits a significantly larger molar absorption coefficient for its metal-to-ligand charge transfer (MLCT) band. All complexes we tested produce CO with only a negligible amount of H 2 and formate in self-sensitized systems in the presence of triethanolamine (TEOA) and in some cases, BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]-imidazole). The presence of the Ph-NH 2 moiety (complex 1) has a beneficial effect on both electrochemical and photochemical activity, allowing a turnover number (TON) of 32 and 120 for photochemical CO production (without and with BIH, respectively). In the case of the Ph-OH group in the second coordination sphere (complexes 4 and 6), these complexes are active for photochemical CO 2 reduction, despite the formation of a stable 6-coordinate Re-OPh intermediate via reductive deprotonation as previously observed under electrochemical conditions. Overall, BIH accelerates the rate of formation of the one-electron reduced species (OERS) of the Re catalysts and allows higher turnover frequency (TOF) and TON for CO formation. The X-ray structures of complexes 1 and 4 were determined to have distorted octahedral Re centres, and show π-π stacking interactions with neighboring molecules as well as intramolecular hydrogen bonds to the internal chloride ligands. The unusually high absorptivity of the MLCT absorption of complex 1 has been explained by TD-DFT calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Noise analysis of optimized Ge/Ge{sub 1−x}Sn{sub x}/Ge p–n–p heterojunction phototransistors for long-wavelength optical receivers

Present literature investigates noise analysis and optimal design for Ge/Ge{sub 1−x}Sn{sub x}/Ge based p–n–p heterojunction phototransistors (HPT) for long-wavelength optical receivers. We calculate gain, noise powers, signal-to-noise ratio, absorption coefficient and optical responsivity of proposed GeSn-based HPT and investigate their dependence on various structural parameters to optimize the design. The proposed HPT incorporates Ge{sub 1−x}Sn{sub x} alloy in the base layer as the active layer, allow to extend the photo detection range from near infrared to mid-infrared to achieve a wide range of detection. The results show that the signal-to-noise ratio (SNR) is strongly dependent on the operating frequency and the introduction of Sn in the base layer can improve the signal-to-noise ratio in the high-frequency region. The calculated results also show that even in the presence of small defects and misfit dislocations at the heterointerfaces, high SNR is still achievable for the proposed structure.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Inkjet-printed vertically emitting solid-state organic lasers

In this paper, we show that Inkjet Printing can be successfully applied to external-cavity vertically emitting thin-film organic lasers and can be used to generate a diffraction-limited output beam with an output energy as high as 33.6 μJ with a slope efficiency S of 34%. Laser emission shows to be continuously tunable from 570 to 670 nm using an intracavity polymer-based Fabry-Perot etalon. High-optical quality films with several μm thicknesses are realized, thanks to ink-jet printing. We introduce a new optical material where EMD6415 commercial ink constitutes the optical host matrix and exhibits a refractive index of 1.5 and an absorption coefficient of 0.66 cm{sup −1} at 550–680 nm. Standard laser dyes like Pyrromethene 597 and Rhodamine 640 are incorporated in solution to the EMD6415 ink. Such large size “printed pixels” of 50 mm{sup 2} present uniform and flat surfaces, with roughness measured as low as 1.5 nm in different locations of a 50 μm × 50 μm AFM scan. Finally, as the gain capsules fabricated by Inkjet printing are simple and do not incorporate any tuning or cavity element, they are simple to make, have a negligible fabrication cost, and can be used as fully disposable items. This work opens the way towards the fabrication of really low-cost tunable visible lasers with an affordable technology that has the potential to be widely disseminated.

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 OSTI2014

Investigation of in-plane- and z-polarized intersubband transitions in pyramid-shaped InAs/GaAs quantum dots coupled to wetting layer: Size and shape matter

In this work, the effects of the shape and size on the intersubband electronic and optical properties of three-dimensional self-assembled pyramid-shaped InAs/GaAs quantum dots (QDs) were investigated in detail. More precisely, in-plane- and z-polarized transitions dipole moment (TDM), oscillator strength (OS), and absorption coefficients of P-to-S, WL-to-P, and WL-to-S transitions were studied as a function of the QD height. The P-to-S TDM showed to be strong and purely in-plane-polarized transition dominating two others. However, the TDMs and OSs of WL-to-P and WL-to-S transitions which are in-plane- and z-polarized transitions, respectively, showed a competition behavior for short and tall QDs. The former dominates for short QDs, and the latter for tall QDs. The physical reasons behind these interesting phenomena were also explained using the probability of finding the carriers in the pyramid region attached to the WL. The theoretical results are in good agreement with experimental data reported for short QDs [Appl. Phys. Lett. 82, 630 (2003)].

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Plasmon generation in sputtered Ga-doped MgZnO thin films for solar cell applications

The crystalline, electrical, morphological, optical properties and plasmonic behaviour of Ga doped MgZnO (GMZO) thin films grown at different substrate temperatures (200–600 °C) by a dual ion beam sputtering (DIBS) system are investigated. Transmittance value of more than ∼94% in 400–1000 nm region is observed for all GMZO films. The particle plasmon features can be detected in the absorption coefficient spectra of GMZO grown at 500 and 600 °C in the form of a peak at ∼4.37 eV, which corresponds to a plasmon resonance peak of nanoclusters formed in GMZO. The presence of such plasmonic features is confirmed by ultraviolet photoelectron spectroscopy measurements. The values of particle plasmon resonance energy of various nanoclusters are in the range of solar spectrum, and these can easily be tuned and excited at the desirable wavelengths while optimizing the efficiency of solar cells (SCs) by simple alteration of DIBS growth temperature. These nanoclusters are extremely promising to enhance the optical scattering and trapping of the incident light, which increases the optical path length in the absorber layer of cost-effective SCs and eventually increases its efficiency.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Energy dependent response of plastic scintillation detectors to photon radiation of low to medium energy

Purpose: Plastic scintillation detectors are promising candidates for the dosimetry of low- to medium-energy photons but quantitative knowledge of their energy response is a prerequisite for their correct use. The purpose of this study was to characterize the energy dependent response of small scintillation detectors (active volume <1 mm{sup 3}) made from the commonly used plastic scintillator BC400. Methods: Different detectors made from BC400 were calibrated at a number of radiation qualities ranging from 10 to 280 kV and at a {sup 60}Co beam. All calibrations were performed at the Physikalisch-Technische Bundesanstalt, the National Metrology Institute of Germany. The energy response in terms of air kerma, dose to water, and dose to the scintillator was determined. Conversion factors from air kerma to dose to water and to dose to the scintillator were derived from Monte Carlo simulations. In order to quantitatively describe the energy dependence, a semiempirical model known as unimolecular quenching or Birks’ formula was fitted to the data and from this the response to secondary electrons generated within the scintillator material BC400 was derived. Results: The detector energy response in terms of air kerma differs for different scintillator sizes and different detector casings. It is therefore necessary to take attenuation within the scintillator and in the casing into account when deriving the response in terms of dose to water from a calibration in terms of air kerma. The measured energy response in terms of dose to water for BC400 cannot be reproduced by the ratio of mean mass energy-absorption coefficients for polyvinyl toluene to water but shows evidence of quenching. The quenching parameter kB in Birks’ formula was determined to be kB = (12.3 ± 0.9) mg MeV{sup −1} cm{sup −2}. Conclusions: The energy response was quantified relative to the response to {sup 60}Co which is the common radiation quality for the calibration of therapy dosemeters. The observed energy dependence could be well explained with the assumption of ionization quenching as described by Birks’ formula. Plastic scintillation detectors should be calibrated at the same radiation quality that they will be used at and changes of the spectrum within the application need to be considered. The authors results can be used to evaluate the range of validity of a given calibration.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Photothermoelastic contrast in nanoscale infrared spectroscopy

The contrast formation mechanism in nanoscale Infrared (IR) Spectroscopy is analyzed. The temperature distribution and elastic displacement across the illuminated T-shape boundary between two materials with different IR-radiation absorption coefficients and thermo-physical and elastic properties located on a rigid substrate are calculated self-consistently for different frequencies f ~ (1 kHz–1 MHz) of IR-radiation modulation (fully coupled problem). Analytical expressions for the temperature and displacement profiles across the “thermo-elastic step” are derived in the decoupling approximation for f = 0 (“static limit”), and conditions for approximation validity at low frequencies of IR-modulation are established. The step height was found to be thickness-independent for thick layers and proportional to the square of the thickness for very thin films. The theoretical results will be of potential interest for applications in the scanning thermo-ionic and thermal infrared microscopies for relatively long sample thermalization times and possibly for photothermal induced resonance microscopy using optomechanical probes.

36 MATERIALS SCIENCE↗
DOE OSTI2015

SU-E-I-42: Some New Aspects of the Energy Weighting Technique

Purpose: The development in the last few years of photon-counting pixel detectors creates an important and significant opportunity for x-ray spectroscopy to be applied in diagnostics. The energy weighting technique was originally developed to obtain the maximum benefit from the spectroscopic information. In all previous published papers the concept of an energy weighting function was tested on relatively simple test objects having only two materials with different absorption coefficients. Methods: In this study the shape of the energy weighting function was investigated with a set of ten trabecular bone test objects each with a similar geometry and structure but with different attenuation properties. In previous publications it was determined that the function E-3 was a very good choice for the weighting function (wi). Results: The most important Result from this study was the discovery that a single function of the form E-b was not sufficient to explain the energy dependence of the different types of materials that might be used to describe common bodily tissues such as trabecular bone. It was also determined from the data contained in this paper that the exponent b is often significantly dependent upon the attenuation properties of the materials that were used to make the test objects. Conclusion: Studies of the attenuation properties will be useful in further studies involving energy weighting.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

SU-E-T-214: Comparison of Treatment Techniques for Dupuytren’s Contracture and Risk Assessment of Normal Tissue Complications

Purpose: Duputren’s contracture (DC) is a benign disease characterized by abnormal thickening of the fascial surfaces of the hands or feet causing curling of the surface, functional impairment, weakness, and pain. The purpose of the investigation is to describe the radiation treatment approaches, compare these techniques, and discuss the potential side effects and complications of these techniques. Methods: Early stage DC has been treated with 120 kVp X rays but also with high-energy electrons or photons. High-energy electrons have been the radiation of choice but severe contracture of the hand makes it difficult to produce a plan with acceptable dose uniformity. High-energy photons can overcome this difficulty either by directing a beam onto the palmer or back of the surface of the hand, including bolus to maximize the surface dose. We calculated the dose to the bone for the 120 kVp treatment using published %DD data and mass energy absorption coefficients for bone and muscle. Results: The dose to underlying bone from megavoltage photons and electrons is essentially the same, but dose to the bone for using 120 kVp can be 4–5 times greater due to the photoelectric effect. For the 30 Gy dose deliver using this technique, the dose to the bone could be 84–105 Gy after taking the penetration of the beam into account. After radiotherapy, there is often decreased osteoblastic activity and vascular fibrosis that leads to osteitis, atrophy, and decreased metabolic bone activity. Incidence of fractures occurs routinely above 60 Gy with higher doses potentially leading to higher incidences of bone complications. Conclusion: Radiation therapy for DC using low-energy X rays can deliver a prohibitively high dose to the underlying bone potentially leading to severe bone complications.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Study of the impurity photoconductivity in p-InSb using epitaxial p{sup +} contacts

The optical absorption coefficient α in p{sup +}-InSb layers (with hole concentrations of p ≈ 1 × 10{sup 17}–1.2 × 10{sup 19} cm{sup –3}), grown by liquid-phase epitaxy on p-InSb substrates, is measured in the spectral range of 5-12 µm at 90 K, and the impurity photoconductivity is measured (at 60 and 90 K) in p{sup +}–p structures. It is found that a in the p{sup +} layers reaches a value of 7000 cm{sup –1} (at p ≈ 2 × 10{sup 19} cm{sup –1}). It is shown that the measured substrate value of (α ≈1–3 cm{sup –1}) is overestimated in comparison with estimates (α ≈ 0.1 cm{sup –1}) based on comparing the photoconductivity data. This discrepancy is explained by the fact that the optical transitions of holes responsible for photoconductivity are obscured by the excitation of electrons to the conduction band. The photoionization cross section for these transitions does not exceed 1 × 10{sup –15} cm{sup 2}.

36 MATERIALS SCIENCE↗