Origin of the excitonic recombinations in hexagonal boron nitride by spatially resolved cathodoluminescence spectroscopy

The excitonic recombinations in hexagonal boron nitride (hBN) are investigated with spatially resolved cathodoluminescence spectroscopy in the UV range. Cathodoluminescence images of an individual hBN crystallite reveals that the 215 nm free excitonic line is quite homogeneously emitted along the crystallite whereas the 220 nm and 227 nm excitonic emissions are located in specific regions of the crystallite. Transmission electron microscopy images show that these regions contain a high density of crystalline defects. This suggests that both the 220 nm and 227 nm emissions are produced by the recombination of excitons bound to structural defects

Analysis of the photon return of a polychromatic laser guide star

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Atomic oxygen detection in flames using two-photon degenerate four-wave mixing

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Atomic oxygen detection in flames using two-photon degenerate four-wave mixing

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Two-color non-linear spectroscopy: application to NO2

International audienceno abstrac

Two-color non-linear spectroscopy: application to NO2

no abstrac

Atomic oxygen detection in flames using two-photon degenerate four-wave mixing

no abstrac

Atomic oxygen detection in flames using two-photon degenerate four-wave mixing

International audienceno abstrac

Laser Spectroscopy for in Situ Diagnostics.

International audienceLaser spectroscopy based on nonlinear processes such as four wave mixing and laser-induced gratings or based on spontaneous Raman and Thomson scattering is widely used, from near IR to near UV, to probe reactive media. The bulk of this monograph is devoted to the application of these techniques to the specific area of plasma, combustion and hypersonic flow field. For each technique a brief description of the signal amplitude is given from which spectral and/or temporal evolution can be predicted as a function of temperature and pressure. Experimentally, the dependence of the signal on parameters such as temperature, pressure, gas composition, or the energy of pump and probe laser pulses have been investigated. Comparison of the experimental results with theoretical predictions allows us to accurately measure these parameters in situ. Therefore, temperature, velocity or number density of important species (major or minor) can be derived from the spectral or temporal content. The synchronization of pulsed laser with particular physical phenomena provides time evolution of the sample over a μs or ms duration, depending on the probed events, thanks to the time resolution (ns) of the lasers. Several examples are given. CARS is applied to a buoyant H2/air flame, a monodisperse droplet stream in combustion and a pulsed plasma in a methane/air flow. In each case, time and space evolution of temperature is recorded.DFWM has proved particularly useful for probing NO, especially when spectral interference is overwhelming the LIF signal; LIF is used otherwise. Laser induced gratings are also valuable tools in some typical cases. Examples are given to illustrate their potential in measuring NO2 molecules in a flame and in measuring the temperature and velocity of a high speed flow. Finally, electron density can be inferred using Thomson scattering and an application in the pulsed plasma is presented