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International audience; The advanced understanding of the complete degradation phenomena is crucial in order to develop robust optics for FEL. Under very harsh Synchrotron Radiation conditions, results show that multiscale wavelength damages could be observed, inducing local crystalline structure modifications of the high optical index material with a severe increase of the surface roughness.

Characteristic features of defects in dielectric multilayer UV-mirrors are determined by photothermal displacement imaging. A large number of micron sized absorbing defects are detected, however, their size and absorption strength distribution strongly depends on thin film preparation conditions. It is found that all features well correlate with thin film damage thresholds at 248 nm. An enhanced photothermal amplitude in a halo around ablation spots damaged by high fluence UV light indicates thin film delamination resulting from thermoelastic stress during laser irradiation.

We investigate the momentum dependence of the extended Drell-Hearn-Gerasimov sum rule. An economical formalism is developed which allows to express the extended DHG sum rule in terms of a single virtual Compton amplitude in forward direction. Rigorous results for the small momentum evolution are derived from chiral perturbation theory within the one-loop approximation. Furthermore, we evaluate some higher order contributions arising from $\Delta(1232)$ intermediate states and relativistic corrections. (2 figures available upon request)., Comment: 12 pages, TeX, BUTP-92/51 and CRN-92-53

We analyze the structure of QCD Green functions with one external nucleon, treated as a very massive field. We discuss the matching conditions and relations of various nucleon matrix elements of quark currents to the relativistic approach. Particular emphasis is put on the forward Compton amplitudes. Besides the nucleon electromagnetic polarizabilities we consider the spin-flip amplitude ƒ 2 (ω 2 ) to one-loop order. We predict the slope parameter of ƒ 2 (ω 2 ) which can be determined from the absorption cross sections of circularly polarized photons on polarized nucleons.

We consider pion electroproduction at threshold to one loop order in chiral perturbation theory. We derive novel contributions to the low-energy theorems for the S-wave multipoles E 0+ and L 0+ at threshold to order O ( μ 2 , v ) with μ = M π / m the ratio of the pion and nucleon mass and v = k 2 / m 2 related to the four-momentum of the virtual photon. At next-to-leading order, the effect of infrared singularities generated by virtual pions becomes visible. We also discuss the inclusion of photon-nucleon vertex form factors.

We analyze threshold pion photoproduction in the framework of baryon chiral perturbation theory. In case of neutral pion photoproduction, virtual pions generate infrared singularities which modify the familiar form of the low-energy theorem for the electric dipole amplitude E0+. In particular, we predict E0+π0p(sth)=−(1.33±0.09)×−3/Mπ+. We also find a satisfactory description of the recently measured total cross section for γp→π0p in the threshold region. The process γn→π0n is entirely given by the loop and counterterm contributions. It would be interesting to have an experimental determination of the corresponding cross section in the threshold region. In case of the charged channels, the loop corrections are much more tame.

We evaluate the electric and magnetic polarizabilities of the proton and neutron in chiral perturbation theory to one-loop order. We present a detailed discussion of the polarizabilities in the chiral limit. Our calculation explains the experimentally observed patterns concerning the relative magnitudes of the electric and magnetic polarizabilities of the proton and the neutron. We also point out the need for a higher-loop calculation to include important resonance exchanges.

The laser damage resistance of optical coatings is a critical point for a large number of applications. However improving this resistance is often hard to obtain because of the large number of parameters in the deposition processes than can modify the laser damage threshold and the lack of detailed and exploitable studies published on this subject. Then, the aim of this work is to test and analyze the laser damage resistance of a usual material for high power applications (silica) deposited in various conditions. The thin films of different thicknesses were specially deposited using different techniques available at the Institut Fresnel: Dual Ion Beam Sputtering, Electron Beam Deposition, Ion Assisted Deposition and Ion Plating. The laser-induced damage thresholds of these coatings were determined at 1064nm and 355nm using nanosecond pulsed YAG lasers, with a 1-on-1 test procedure. Other diagnostic tools were used to complete the study and make potential correlations with laser damage: photothermal techniques, luminescence spectrocopy, optical profilometry, dark field and Nomarski microscopy. The comparative study of these results highlight different laser damage behaviors of the silica coatings that we correlate to the density and the nature of the defects.