Doppler broadening

About: Doppler broadening is a research topic. Over the lifetime, 5509 publications have been published within this topic receiving 92552 citations.

Kinetic to thermal energy transfer and interpenetration in the collision of laser-produced plasmas

Abstract: An experimental and numerical analysis of the collision of two plasmas produced from laser-exploded Al/Al and Al/Mg pairs of foils is presented. Various imaging and spectroscopic x-ray techniques have been used to diagnose the collision over a broad range of intertarget distances and laser intensities. Ion temperatures in the 10 keV range have been measured from Doppler broadening. Electron temperatures and densities have been deduced from line ratios and interpenetration distances have been determined by the spatial extent of Mg and Al x-ray lines. Eulerian multifluid simulations have been developed and coupled to atomic physics postprocessing. The comparison of the measurements with these simulations shows that interpenetration prevails at large intertarget distances and high laser intensities; kinetic to thermal energy transfer then takes place on a ∼200-μm wide region and during ∼150 ps.

Vacancy-oxygen complexes and their optical properties in AlN epitaxial films studied by positron annihilation

Abstract: Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy (MOVPE) and lateral epitaxial overgrowth (LEO) using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3–6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy (VAl) and oxygen atoms substituting nitrogen sites such as VAl(ON)n (...

Theory of terahertz generation by optical rectification using tilted-pulse-fronts.

Abstract: A model for terahertz (THz) generation by optical rectification using tilted-pulse-fronts is developed. It simultaneously accounts for in two spatial dimensions (2-D) (i) the spatio-temporal variations of the optical pump pulse imparted by the tilted-pulse-front setup, (ii) the nonlinear coupled interaction of THz and optical radiation, (iii) self-phase modulation and (iv) stimulated Raman scattering. The model is validated by quantitative agreement with experiments and analytic calculations. We show that the optical pump beam is significantly broadened in the transverse-momentum (kx) domain as a consequence of its spectral broadening due to THz generation. In the presence of this large frequency and transverse-momentum (or angular) spread, group velocity dispersion causes a spatio-temporal break-up of the optical pump pulse which inhibits further THz generation. The implications of these effects on energy scaling and optimization of optical-to-THz conversion efficiency are discussed. This suggests the use of optical pump pulses with elliptical beam profiles for large optical pump energies. Furthermore, it is seen that optimization of the setup is highly dependent on optical pump conditions. Trade-offs in optimizing the optical-to-THz conversion efficiency on the spatial and spectral properties of THz radiation are discussed to guide the development of such sources.

Doppler Broadening and Magnetic Field Effects on the Balmer Lines Emitted at the Edge of a Tokamak Plasma

Abstract: Profiles of the Balmer lines D α (H α ), D β (H β ) and D γ (H γ ) have been measured in the scrape-off layer and within the edge of the TEXTOR (upgrade) plasma, under Ohmic conditions and with neutral-beam injection. Each line profile shows a strong Zeeman effect in the vicinity of line centre, and a marked central dip when mainly the σ components are observed. The line core evidently originates from cold atoms in the edge plasma, excited in the course of molecular dissociation, while the broad pedestal on which the core rests is radiated by excited atoms produced through charge-exchange recombination of deuterons (protons), transported outwards from the much hotter plasma interior, and by atoms heated directly by collisions with the deuterons (protons). Core temperatures of about 0.5 eV and less are obtained from line profile analysis.

Spectral broadening of a multimode continuous-wave optical field propagating in the normal dispersion regime of a fiber.

Abstract: We investigate experimentally and theoretically the broadening of the optical spectrum of a multimode cw field propagating in the normal dispersion regime of a single-mode fiber. The width of the optical spectrum is not a monotonic function of propagation length. This behavior arising from the interplay between the Kerr effect and group-velocity dispersion contrasts with spectral broadening of mode-locked pulses.