In the extreme ultra-violet region, multilayer coatings are the only technique to obtain high reflectivity in normal incidence optical configurations. The interference process which regulates periodic multilayers behavior offers narrow-band spectral filtering without the use of additional filters, fact that makes these coatings particularly suitable for lines emission observations. Despite the large amount of possible materials combinations, Mo/Si multilayers are the standard choice for space research on plasma physics in the 13 - 30 nm spectral region. In this work Si/B4C is presented as an alternative material couple for the 30.4 nm selection. Attractive features are the better spectral purity and the second order reflectivity reduction. A possible application to the Sounding CORonagraph Experiment is described as an example. B4C thin films have been used to characterize this material in terms of optical constants in the 40 nm - 150 nm spectral region where, currently, only few data are available.
Recent measurements of the absolute diffraction efficiency of plane gratings in the conical diffraction mounting (in which the light approaches the grating in the plane parallel to the direction of the grooves) are presented. Three gratings have been tested at the beamline BEAR (Elettra Synchrotron, Trieste) in the 10-130 nm region, showing a peak efficiency as high as 70%. The aim of these measurements is the use of two gratings in the conical diffraction mounting for the realization of a high-throughput time-compensated monochromator for the spectral selection of high-order harmonic radiation produced by the interaction between an ultrashort laser pulse (less than 100 fs) and a gas jet. The monochromatic and ultrashort pulse at the monochromator exit can be used for the injection of a Free Electron Laser. The theory of the time-compensation with gratings will be briefly resumed, the design of the monochromator will be presented, and the results of the measurements at BEAR will be discussed.
In this work we report on the optimization of the conversion efficiency of the harmonic generation process, by adaptive control of the wavefront of sub-10-fs light pulses, obtained by using a deformable mirror and a genetic algorithm. Sub-10-fs, 0.2-mJ energy light pulses, generated by the hollow-fiber compression technique, were focused in the gas target (argon or neon) by a 250-mm focal-length mirror. Pulse wavefront correction has been achieved by using a deformable mirror (DM) controlled by 37 actuators distributed on a honeycomb pattern of 15 mm diameter. The harmonic radiation was observed by a soft-X-ray spectrometer, with double output: time-integrated high-resolution bidimensional focal-plane image and real-time (1 kHz) intensity of a suitable spectral region. This latter signal was used as fitness parameter for the genetic algorithm; an initial population of DM configurations was initialized with random values of the actuator signals. A new generation of DM configurations is derived from ordering, selection and transformation of previous generation, up to the convergence to the fittest individual. Strong enhancement of the harmonic conversion efficiency of about one order of magnitude, as well as a significant extension of the harmonic spectrum is evident. The initial and optimal wavefronts of the fundamental beam were measured both in real time with an Hartmann sensor and off-line using a ZYGO interferometer. Using the measured beam wavefront were calculated the spatial characteristics of the fundamental beam.
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