Maxwell-Bloch modeling of an x-ray pulse amplification in a 1D photonic crystal

We present an implementation of the Maxwell-Bloch (MB) formalism for the study of x-ray emission dynamics from periodic multilayer materials whether they are artificial or natural. The treatment is based on a direct Finite-Difference-Time-Domain (FDTD) solution of Maxwell equations combined with Bloch equations incorporating a random spontaneous emission noise. Besides periodicity of the material, the treatment distinguishes between two kinds of layers, those being active (or resonant) and those being off-resonance. The numerical model is applied to the problem of $K\alpha$ emission in multilayer materials where the population inversion could be created by fast inner-shell photoionization by an x-ray free-electron-laser (XFEL). Specificities of the resulting amplified fluorescence in conditions of Bragg diffraction is illustrated by numerical simulations. The corresponding pulses could be used for specific investigations of non-linear interaction of x-rays with matter

Investigation of the thermal stability of Mg/Co periodic multilayers for EUV applications

We present the results of the characterization of Mg/Co periodic multilayers and their thermal stability for the EUV range. The annealing study is performed up to a temperature of 400\degree C. Images obtained by scanning transmission electron microscopy and electron energy loss spectroscopy clearly show the good quality of the multilayer structure. The measurements of the EUV reflectivity around 25 nm (~49 eV) indicate that the reflectivity decreases when the annealing temperature increases above 300\degreeC. X-ray emission spectroscopy is performed to determine the chemical state of the Mg atoms within the Mg/Co multilayer. Nuclear magnetic resonance used to determine the chemical state of the Co atoms and scanning electron microscopy images of cross sections of the Mg/Co multilayers reveal changes in the morphology of the stack from an annealing temperature of 305\degreee;C. This explains the observed reflectivity loss.Comment: Published in Applied Physics A: Materials Science \& Processing Published at http://www.springerlink.com.chimie.gate.inist.fr/content/6v396j6m56771r61/ 21 page

Introduction of Zr in nanometric periodic Mg/Co multilayers

We study the introduction of a third material, namely Zr, within a nanometric periodic Mg/Co structure designed to work as optical component in the extreme UV (EUV) spectral range. Mg/Co, Mg/Zr/Co, Mg/Co/Zr and Mg/Zr/Co/Zr multilayers are designed, then characterized in terms of structural quality and optical performances through X-ray and EUV reflectometry measurements respectively. For the Mg/Co/Zr structure, the reflectance value is equal to 50% at 25.1 nm and 45deg of grazing incidence and reaches 51.3% upon annealing at 200deg C. Measured EUV reflectivity values of tri-layered systems are discussed in terms of material order within a period and compared to the predictions of the theoretical model of Larruquert. Possible applications are pointed out.Comment: 19 page

Electronic structure of wurtzite and zinc-blende AlN

The electronic structure of AlN in wurtzite and zinc-blende phases is studied experimentally and theoretically. By using x-ray emission spectroscopy, the Al 3p, Al 3s and N 2p spectral densities are obtained. The corresponding local and partial theoretical densities of states (DOS), as well as the total DOS and the band structure, are calculated by using the full potential linearized augmented plane wave method, within the framework of the density functional theory. There is a relatively good agreement between the experimental spectra and the theoretical DOS, showing a large hybridization of the valence states all along the valence band. The discrepancies between the experimental and theoretical DOS, appearing towards the high binding energies, are ascribed to an underestimation of the valence band width in the calculations. Differences between the wurtzite and zinc-blende phases are small and reflect the slight variations between the atomic arrangements of both phases

Characterization method of the valence states : Application to dielectrics and metal-dielectrics interfaces

Electron-induced x-ray emission spectroscopy (EXES) is an efficient technique to study the physicochemical properties of thin films and of buried interfaces. This method analyzes the distribution of the valence states, i.e. the states sensitive to the environment, in a selective way with the depth. The selectivity comes from the use of ionizing particles (electrons) gradually loosing their energy in the matter. Then the incident electron energy can be chosen in order to probe a given thickness of the material under study. Relation between chemical bond and atomic structure is discussed in the case of bare dielectrics (Al2O3 and MgO). Applications to buried metal-dielectric interfaces (AuPd/Al2O3 and Cu/MgO) are discussed as a function of mechanical properties

X-ray spontaneous emission control by 1-dimensional photonic bandgap structure

The possibility of controlling the X-ray spontaneous emission of atoms embedded in a 1-dimensional photonic bandgap structure by the so-called Purcell effect, is studied. Calculations of the spontaneously emitted power are presented from Fermi's golden rule in the framework of the Wigner-time approach extended to absorbing media. Numerical simulations are compared to experimental results for the case of the K emission from silicon atoms excited by electrons within a Mo/Si multilayer Bragg reflector. The inhibition or enhancement of X-ray emission from such structures appear to be feasible