In recent years, seeded free-electronlasers (FEL’s) have demonstrated to be very attractive sources for in- tense light production, revealing improved shot-to-shot stability, tunability, spatial quality and longitudinal co- herence [1–3]. Notably, the high-gain harmonic genera- tion (HGHG) scheme  enables the generation of pow- erful and ultrashort extreme-ultraviolet (XUV) pulses. In such a configuration, an external source (the seed) interacts with a relativistic electron beam wiggling in a first undulator chain (the modulator). This interaction leads to an energy modulation of the electrons, further transposed to a spatial bunching after the electrons ex- perience an energy-dependent path into a magnetic chi- cane, called the dispersive section. The bunched electron beam is then injected into a long undulator chain (the radiator). The bunching has a periodicity determined by the seed frequency but also presents significant compo- nents at the harmonics of the latter, so that the electron beam can emit coherently at one of the seed harmon- ics. In the radiator, the light is amplified at the chosen harmonic until the process reaches saturation, due to bunching deterioration.
FreeElectronLasers (FELs) [ 1 , 2 ] deliver ultrashort, narrow-band and ultrabright pulses down to the hard x-ray range [ 3 – 5 ], enabling breakthrough experiments in chemical, physical, and biological sciences. These light
sources rely on relativistic electron beams wiggling in the periodic magnetic ﬁeld of an undulator as gain medium. Interacting with the spontaneous radiation of the undulator or an external seed, the electrons experience an energy modulation at the resonance wavelength which is further transformed into a density modulation by dispersive elements. After this ‘lethargy’ [ 6 ] phase, the beam density modulation allows the emission of a coherent radiation which can then be exponentially ampliﬁed. A saturation is reached when the electrons energy loss is such that the resonance condition is violated, causing a red spectral shift of the FEL line [ 7 , 8 ].
4.2 Effect of electron velocity mismatch
The energy dispersion of the incident electron bunch is a major concern for X-ray freeelectronlasers. In partic- ular, all the simulations on the FEL effect with optical undulators, in the Compton regime, demonstrate that a remarkable value of mono-energeticity is required, typi- cally of the order of 10 −4  to few 10 −4 for electron energies of few tens of MeV . Indeed, in the Compton regime, amplification occurs throughout the laser undu- lator length only if δγ/γ < 1/2N , N being the number of undulator periods over the whole amplification length . The Doppler frequency shift is therefore limited to the emission linewidth due to the finite emission time. This very stringent condition on the electron energy dis- persion is obviously one of the major reasons why this op- tical undulator scheme has not been demonstrated up to now. How the proposed Raman scheme for a X-ray FEL copes with the electron energy dispersion is therefore a major issue; however, a detailed study of Raman ampli- fication with a spread of electron energies is beyond the scope of the present study, leading us to restrict ourselves to discuss the spectral broadening induced the electron energy spread, and the amplification regime between a monochromatic X-ray field, and an out-of-resonance elec- tron population.
X-ray free-electronlasers (XFELs) enable novel experiments because of their high peak brilliance and femtosecond pulse duration. However, non-superconducting XFELs offer repetition rates of only 10 –120 Hz, placing signiﬁcant demands on beam time and sample consumption. We describe serial femtosecond crystallography experiments performed at the European XFEL, the ﬁrst MHz repetition rate XFEL, delivering 1.128 MHz X-ray pulse trains at 10 Hz. Given the short spacing between pulses, damage caused by shock waves launched by one XFEL pulse on sample probed by subsequent pulses is a concern. To investigate this issue, we collected data from lysozyme microcrystals, exposed to a ~15 μm XFEL beam. Under these conditions, data quality is independent of whether the ﬁrst or subsequent pulses of the train were used for data collection. We also analyzed a mixture of microcrystals of jack bean proteins, from which the structure of native, magnesium-containing concanavalin A was determined.
X-ray free-electronlasers (XFELs) provide unprecedented high brilliance, good coherence and very short pulse duration in the X-ray regime. Their specifications have opened new frontiers for experiments in physics and chemistry (see  and references therein). In this context, the use of focusing optics which enable strong enhancement of X-ray intensity con- tributes to the performance of these sources. In particular, reflective mirrors can achieve the highest focusing efficiency . However, XFEL characteristics impose severe requirements on the optics used to guide and shape the x-ray pulses. Here the onset of damage is difficult to evaluate and can occur well before the material reaches its melting temperature. Further- more, in the hard x-ray regime and at grazing incidences, the damage threshold needs to be evaluated by taking into consideration the energetic photoelectrons that can remove de- posited energy from the interaction region. This particular example illustrates how XFELs involve a new regime of radiation-matter interaction which has to be investigated. The development of a theoretical model describing this interaction is difficult since the sample enters a regime refered as warm dense matter (WDM) regime, not very well understood.
Few alternative strategies have been proposed to sup- ply compact X-ray freeelectronlasers, based on substitu- tion of the LINAC by laser-wakefield acceleration [ 2 , 3 ], use of original compact undulators as an ion channel [ 4 ], or a counter-propagating laser [ 5 – 7 ]. Laser undulators offer indeed a key advantage : laser wavelengths at the µm level allow one to reach X-ray photon energies with moderately relativistic electrons, of kinetic energies of few tens of MeV only. In all cases, the XFEL is expected to operate in the conventional regime of stimulated in- verse Compton scattering, imposing severe limitations on electron emittance, transport, kinetic energy spread, and laser uniformity for laser undulators [ 8 ], thus hindering prospects of experimental demonstrations.
Femtosecond time-resolved experiments employing ad- vanced x-ray probe techniques have been realized in a wide variety of scientific domains since the advent of ultrashort x-ray-pulse sources such as femtoslicing, high harmonic gen- eration, and x-ray free-electronlasers (XFELs). Research ac- tivities range from the investigation of fundamental processes in gas phase experiments [ 1 – 3 ] to the observation of struc- tural rearrangement in biological macromolecules using novel time-resolved crystallography techniques [ 4 , 5 ]. In condensed matter physics, the availability of ultrahort x-ray pulses has allowed us to probe ultrafast charge, spin, and lattice dynamics with chemical selectivity and nanometer spatial resolution [ 6 – 10 ].
2. EDGE-EMITTING LASERS
As a first step towards the fabrication of hybrid GaAs/GaSb IC-VCSELs, we started our investigations by ensuring that laser operation from large-area edge-emitting lasers could be achieved using the chosen 7-stage quantum-well/injection IC active region whose design is fully described in . Two sets of devices were grown and fabricated. As illustrated in Figure 1, the reference structure layout relied on conventional Te-doped AlSb/InAs cladding layers while the second used a metamorphic Te-doped GaAs upper cladding to study the influence of the latter on the device performance.
The work reported in this thesis is devoted to the thermal and dynamics modeling of mid- infrared quantum cascade lasers. The adverse effect of temperature on the performances of quantum cascade lasers arises firstly from the reduction of population in the upper laser level due to the nonradiative relaxation and the escape of electrons by thermionic emission effect, and secondly, by the increase of the broadening term. Our simple model of rate equations relates the degradation of the performances of the quantum cascade laser to the contribution of these sources. The model here developed gives a good agreement with the available experimental results. Then, we show theoretically how the proposed model leads to the analytical expressions permitting the calculation the turn-on and delay times. Concerning the quantum cascade laser dynamics, our results agree with the existing ones found in the published literature and clearly demonstrate the importance of the current injection effect on the temporal variation of the electron and photons populations.
L’émergence de diodes lasers émettant autour de 940 nm a relancé l’intérêt de l’utilisation de l’ion ytterbium comme ion actif. Outre le fait de posséder un diagramme énergétique simplifié, comparé à l’ion néodyme, l’ion ytterbium a un rendement quantique faible (l’énergie dissipée sous forme de chaleur est réduite) et une durée de vie radiative plus importante (permettant ainsi un meilleur stockage d’énergie). L’évaluation des matrices hôtes à l’ion ytterbium a été permise par des figures de mérite : il s’est avéré que les grenats et les sesquioxydes de terre rare sont de très bons candidats en tant que matrices hôtes à l’ion ytterbium pour des applications lasers de forte puissance. Du fait de la température de fusion élevée de ces matériaux, l’obtention de monocristaux s’avère délicate : la solution réside dans la synthétisation de céramique transparente. Ces matériaux lasers sont refroidis jusqu’à la température de l’azote liquide. Lorsque la température du matériau diminue, les propriétés thermiques sont améliorées : la conductivité thermique augmente (jusqu’à atteindre un maximum), les sections efficaces d’émission et d’absorption augmentent et les coefficients de dilatation et thermo-optique dn/dT diminuent.
Fig. 1 (a) optical spectra retrieved with a Fourier transform infrared spectrometer (Bruker Vertex 80V) of the free-running master QCL (in orange) operating at 250 K and 650 mA and of the free-running slave QCL (in cyan) operating at 278K and 450 mA; (b) experimental setup allowing
one-way injection with the optical isolator. MCT : Mercury-Cadmium-Telluride detector, NPBS : non-polarizing beam splitter, Osci : fast oscilloscope, RSA : real time spectrum analyzer, pyro : pyroelectric detector.
et qui correspondaient à la génération d’états quasi classiques du rayonnement.
En effet cette nouvelle étape impliquait le recours à une approche quantique du rayon-
nement et de l’émission laser, parce que les niveaux de précision déjà obtenus par les
lasers existants atteignaient les limites fixées par l’inégalité de Heisenberg appliquée aux variables conjuguées du champ électromagnétique. Celle-là impose en effet que le pro- duit des incertitudes affectant la mesure de chacune de celles-ci (par exemple la phase
We study the electron-phonon interaction in n-type PbTe from first-principles calculation and obtain the electron- phonon scattering rates and electron mean free paths at dif- ferent temperatures. The LO phonon in PbTe plays an impor- tant role in determining the lifetime of electrons. The elec- tron mean free path as a function of energy follows almost the same trend as the relaxation time because of the weak energy dependence of group velocity. This makes the electron mean free path decrease monotonically with energy. The screening effect at high carrier concentrations weakens the LO-TO split- ting for phonons and reduces the POP scattering especially for low-energy electron. It also shifts the mean free path dis- tribution towards higher values whilst the integrated transport properties are slightly changed. At elevated temperatures, the scattering rates scale with T and the electron mean free path distribution is shifted towards lower values.
the ·SeCF 3 radical could also be generated from a
nucleophilic source of SeCF 3 via a visible-light-induced SET
(Single Electron Transfer) oxidation. This fluorinated radical would then easily react with aromatic substrates (Scheme 1d). We wish to report herein a new visible-light-promoted aerobic C–H trifluoromethylselenolation of heteroarenes, which proceeds under very mild, metal-free and eco-friendly reaction conditions.
Difficulté de croissance par µPD facile difficile
Tableau I.8.1 Comparaisons des cristaux candidats pour les fibres monocristallins. Comme on peut le voir, le Nd :YVO 4 présente certains avantages, en particulier une section efficace d’émission à 1064 nm très élevée. Néanmoins, sa conductivité thermique est nettement plus faible, et sa croissance est beaucoup plus difficile du fait de son anisotropie. Finalement, il a été jugé préférable d’utiliser le YAG comme matrice cristalline, du fait de ses bonnes propriétés thermomécaniques et de la relative facilité de sa croissance par micro- pulling-down. Utiliser un cristal très bien connu comme le Nd :YAG a de plus l’avantage de faciliter les comparaisons entre notre méthode de croissance et les autres méthodes comme le Czochralski. Le Nd :YAG ne représente pas forcément, à terme, le meilleur choix pour réaliser des lasers à fibres monocristallines de haute puissance moyenne et haute puissance crête. Néanmoins, il apparaît comme une étape incontournable pour valider le concept proposé et identifier les problèmes avec un milieu laser universellement reconnu pour ses qualités, et relativement facile à faire croître.
Mid-infrared lasers are crucial devices to enable optical spectroscopy in this spectral region of strategic importance for molecular sensing. In this presentation, we will review our recent progress in mid-infrared sources based on interband cascade active regions for operation in the L-band (wavelength of ~3.4µm) and in particular towards the demonstration of VCSELs, the most promising laser embodiment for energy-efficient portable tunable sources.
5.1 Structural and electronic properties
In this section I will present a detailed analysis of the theoretical equilibrium lattice pa- rameters and Kohn-Sham band structure of bulk Bi. Special attention will be given to the discussion the effect of the spin-orbit coupling (SOC). The SOC has been included in the ab initio DFT calculations, because it is crucial for the comparison of the Kohn-Sham band structure with the experimental data. Although the electronic band structure of Bi has been extensively studied during the last 70 years, both experimentally and theoreti- cally, until today there has been no comparison of the local density approximation (LDA) and generalized gradient approximation (GGA) in Bi. Therefore, I will present such a study, also by making a comparison with anterior DFT and tight-binding calculations, and various experiments such as Shubnikov-de Haas, de Haas-van Alphen, photoemission. In Sec. 5.1.1 I will present the computational details of the DFT studies. In Sec. 5.1.2 I will make a comparison of the theoretical equilibrium lattice parameters of Bi, within the LDA and GGA, with and without spin-orbit coupling, as obtained in this work and other theoretical and experimental studies. Finally, in Sec. 5.1.3 I will make a detailed analysis of the Kohn-Sham band structure of Bi. Particular attention will be given to the description of the electron and hole pockets, and the effect of the spin-orbit coupling.