December 2004 Archives

Scientists at German and American synchrotron facilities have recently reported the significance of lensless imaging in achieving extremely high-spatial resolution. Although lenses are generally good at obtaining a magnified image of a sample, they also unfortunately introduce aberrations in the image, which ultimately limit the spatial resolution obtainable. In principle, one can form an image without a lens, by means of a coherent scattering experiment. The challenge is to solve the so-called phase problem. The team recently developed a new approach to X-ray holography, realizing a Fourier transform holography geometry by use of a micro- and nanostructured mask. Special contrast mechanisms can be exploited by resonant soft x-ray scattering and, in the experiment at BESSY, they recorded an image revealing the randomly organized "north" and "south" magnetic regions of a cobalt-platinum film to a spatial resolution of 50 nm, which is 10 times better than that achievable with conventional X-ray focusing optics. In the future, the technique will be used as a method for ultra-fast stroboscopic imaging on a femtosecond time scale using an X-ray free electron laser such as the Linac Coherent Light Source (LCLS), for example, which is expected to open at Stanford in 2009. For more information, see the paper, "Lensless imaging of magnetic nanostructures by X-ray spectro-holography", S. Eisebitt et al., Nature, 432, 885-888 (2004).

One of the most exciting recent scientific discoveries is that Mars was possibly once wet and salty, suggesting an environment that could serve as a candidate for early life. The two Mars Rovers, Sprit and Opportunity, have been collecting large amounts of data on the soil, rock and atmosphere by utilizing state-of-the-art analytical instruments including an X-ray spectrometer, which recently determined the major and minor elements of soil and rock samples taken from Meridiani Planum. For more information, see the paper, "Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer", R. Rieder et al., Science, 306, 1746-1749 (2004).

The ultra-fast X-ray diffraction technique has now become widely used. Many experiments using this technique are, in principle, a so-called pump-probe measurement, using a Ti:sapphire laser system (wavelength 800 nm, 1-kHz repetition rate with 5-mJ pulse energy and 45-fs duration) and, for example, a moving, 20-mm-thick Cu band to generate characteristic X-ray pulses. Recently, a German group reported the successful imaging of coherent atomic motions in a GaAs/AlGaAs superlattice. The motions are of great interest and are due to the excitation of electron-hole pairs in the GaAs subband. Both expansion of the GaAs layers and contrast of the AlGaAs layers were observed, mainly because bonding in the GaAs layers was affected by the excitation. For more information, see the paper, "Coherent Atomic Motions in a Nanostructure Studied by Femtosecond X-ray Diffraction", M. Bargheer et al., Science, 306, 1771-1773 (2004).

So far, it has been difficult to observe nonlinear responses to an optical field in the extreme ultraviolet (XUV) and soft X-ray regions. A research group from the University of Tokyo recently succeeded in generating intense isolated XUV pulses (photon energy 27.9 eV) that were shorter than 1 femtosecond through high-harmonic (9^th) generation by using a sub-10-femtosecond blue laser (photon energy 3.1 eV) producing a large dipole moment. For more information, see the paper, "Nonlinear optics in the extreme ultraviolet", T. Sekikawa et al., Nature, 432, 605-608 (2004).