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Concept

Laser au dioxyde de carbone

Résumé

Le laser au dioxyde de carbone (laser au ) est un des plus anciens développé par Kumar Patel dans les laboratoires Bell en 1964 et il garde encore de nos jours de très nombreuses applications. vignette|centré|upright=3|Schéma de principe d'un laser au dioxyde de carbone (). Les lasers au dioxyde de carbone en mode continu ont une grande puissance et sont aisément disponibles. Ils sont également très efficaces ; le rapport entre la puissance de pompage (puissance d'excitation) et la puissance de sortie atteint 20 %. Les lasers au émettent dans l'infrarouge, leur bande de longueur d'onde principale étant centrée entre 9,4 et (micromètres). Amplification Le milieu amplificateur est un gaz — refroidi par un circuit d'eau dans le cas des grandes puissances — dans lequel on fait une décharge électrique. Le gaz utilisé dans le tube à décharge se compose de :

dioxyde de carbone (), environ 10 à 20 % ;
diazote (N2), environ 10 à 20 %,
dihydrogène (H2) et/ou xénon (Xe), qu

Source officielle

https://fr.wikipedia.org/wiki/Laser_au_dioxyde_de_carbone

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The function of a peptide is intimately connected to its structure, which in solution is the result of the balance between non-covalent interactions within the molecule and the molecule-solvent interactions. Theory has the power to predict the properties of a wide variety of molecules, but the accuracy of the theoretical predictions must be verified by experiments. A way to test, and hopefully improve, theory is to study proteins or peptides in the gas-phase. The combination of mass spectrometry and laser spectroscopy is used to investigate properties of biological ions in the gas-phase. Closed-shell biomolecular ions produced by an electrospray source are cooled down in an RF ion trap to less than 10 K and interrogated spectroscopically. Electronic and conformer-specific vibrational spectra have been measured for small protonated peptides applying photofragment-based detection schemes. These measurements allow us to investigate the role of the charge and the influence of the nature and the position of the chromophore in the peptide chain on its conformational preferences. Highly-resolved conformer-specific vibrational spectra of such species, as well as those of a double-chromophore molecule, help elucidate the mechanism of the IR-UV double-resonance methods and provide benchmarks for testing the accuracy of theoretical calculations. This thesis shows also the experimental developments that allow us to use photofragment-based detection scheme to measure highly-resolved electronic spectra of peptides of up to 17 amino acids, containing one or two chromophores, using infrared laser-assisted photofragment spectroscopy (IRLAPS). The fragmentation yield of UV pre-excited molecules increases by as much as two orders of magnitude when they further interact with the CO2 laser. This approach can be also applied in a IR-UV double resonance scheme, allowing measurements of conformer-specific infrared spectra of protonated peptides. The fragmentation channel, which is always greatly enhanced by IRMPE of UV pre-excited molecules, is the loss of the neutral, aromatic side chain via cleavage of the Cα-Cβ bond, independent of the chromophore excited in the peptide. The possible mechanisms, involving the formation of a long-lived intermediate species, that lead to the formation of the observed photofragment are discussed in this thesis work.

EPFL2010

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Ultrafast carbothermal reduction of silica to silicon using a CO2 laser beam

Jaeki Jeong

We report the extraction of silicon via a carbothermal reduction process using a CO2 laser beam as a heat source. The surface of a mixture of silica and carbon black powder became brown after laser beam irradiation for a few tens of seconds, and clear peaks of crystalline silicon were observed by Raman shift measurements, confirming the successful carbothermal reduction of silica. The influence of process parameters, including the laser beam intensity, radiation time, nitrogen gas flow in a reaction chamber, and the molar ratios of silica/carbon black of the mixture, on the carbothermal reduction process is explained in detail.

NATURE RESEARCH2020

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Numerical modeling of planar periodic structures in electromagnetics

Katarina Blagovic

Periodic structures, such as frequency-selective surfaces (FSSs) and photonic band-gap (PBG) materials, exhibit total reflection in specific frequency bands while total transmission in other bands. They find numerous applications in a large field of the electromagnetic (EM) spectrum. For example, in the microwave region, they are used to increase the efficiency of reflector antennas. In the far-infrared region they are used in designing polarizers, beam splitters, mirrors for improving the pumping efficiency in molecular lasers, as components of infrared sensors, etc. To set a solid basis for the analysis of periodic structures, we have first studied the most commonly used technique, the integral equation (IE) solved by the method of moments (MoM). IE-MoM is particularly well-suited for the analysis of printed planar structures. In any IE-MoM numerical implementation the efficient evaluation of the corresponding Green's functions (GFs) is of paramount importance. This is especially true for IE analysis of periodic structures whose GFs are slowly converging infinite sums. The systematic study of existing acceleration algorithms of general and specific types, used to accelerate the evaluation of periodic GFs, has been performed. We propose a new and efficient method for acceleration of multilayered periodic GFs that successfully combines the advantages of Shanks' and Ewald's transform. In structures operating at higher frequencies (thin films in millimeter and submillimeter wave bands or with self supporting metallic plates) the thickness of metallic screens must be taken into account. The existing full-wave approaches for simulating these structures double the number of unknowns as compared to that one of the zero-thickness case. Moreover, the thick aperture problem asks for the computation of cavity Green's functions, which is a difficult and time-consuming task for apertures of arbitrary cross-sections. This thesis addresses the problem of scattering by periodic apertures in conducting screens of finite thickness by introducing an approximate and computationally efficient formulation. This formulation consists in treating the thick aperture as an infinitely thin one and in using the correction term in integral equation kernel that accounts for the screen thickness. The number of unknowns remains the same as in the zero-thickness screens and evaluation of complicated cavity Green's functions is obviated, which yields computationally efficient routines.

EPFL2006

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Numerical modeling of planar periodic structures in electromagnetics

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