Headlines 2009

In photography, the scattered light from an illuminated object is recorded with a detector and one get an image of it. If the image is formed with an objective, the optics imposes many limitations (resolution, aberrations ...). To achieve the ultimate resolution, spatially (function of the wavelength of the radiation used) and temporally (function of the "flash" duration), one possible technique (without any optics) is the coherent diffraction. Using a coherent beam like a laser to illuminate the object, a signal modulation due to interference is present and allows digitally reconstructing the exact image of the object with an unprecedented precision. To achieve nanometer or even atomic resolution, we therefore enlighten with a beam of coherent X-rays (radiation laser wavelength nanometer) and record the image. The usually low average illumination requires long accumulations over several laser shots. Recent advances have yielded images with a single shot femtosecond (10^-15 s) from a laser laboratory, opening the way for time resolved studies.

For regular arrangements of elementary objects, the Bragg diffraction of X-rays is a powerful technique for characterization of matter at the atomic scale. It is the primary tool for crystallography. The information contained in the Bragg diffraction is rich: if a is the characteristic size of the elementary object, the Bragg peaks are spaced by 1/a in reciprocal space. However, some information is lost: indeed, the maximum frequency at which the diffraction pattern can be sampled is less than the Nyquist frequency (2a). In particular, if the elementary object has an amplitude and a phase, the phase is lost in the Bragg diffraction.