Experimental geometry for Gabor holography. An extreme ultraviolet beam illuminates a sample.

Margaret Murnane and Henry Kapteyn are developing compact coherent (i.e., laserlike) X-ray sources that have pulse durations of femtoseconds to attoseconds. The short wavelength of these pulses is well matched to the primary atomic resonances of most elements. Thus, these new light sources are ideal for use in ultrafast element- and chemical-specific spectroscopies and spectromicroscopies. Applications include monitoring the switching times in molecular electronics, ultrahigh-resolution microscopy, nano lithographic patterning, monitoring the first steps in catalysis, and developing attosecond reaction-dynamics microscopes that are capable of following the motion of both atoms and electrons in a molecule during a chemical reaction. The new probes, which compliment such existing nanoprobes such as atomic-force microscopes, could become indispensable in the quest to develop practical nanoscale "machines." They are also expected to aid our understanding of chemical reactions at the level of individual electrons, atoms and molecules.

Recent research in the Kapteyn/ Murnane group is opening the door to new nanoscience capabilities. In one experiment, the group made dynamic Gabor holographic measurements of surface deformations that had femtosecond time resolution and picometer spatial sensitivity in the vertical dimension. The short wavelength, short pulse duration, and high coherence of the X-ray beams made the experimental measurements possible. In the experiment, an ultrafast laser was first used to rapidly heat a surface. A second delayed X-ray probe beam then created a dynamic hologram that measured the ultrafast deformation and subsequent acoustic oscillations of the surface of the material after the optical excitation. This method will enable the investigation of nanoscale surface acoustic dynamics and the study of interfaces, thin films, and heat transport in nanostructured materials.

In other work, the Kapteyn/Murnane group used ultrafast X-ray sources to monitor the movement of specific atoms as a fast chemical reaction unfolded on a surface. Until recently such observations were not possible because visible laser techniques were not sensitive enough to discern the making and breaking of chemical bonds, intermediate reaction steps, or transient reaction products in surface chemistry.

Finally, in exciting new work the Kapteyn/Murnane group is exploring "lensless" coherent imaging techniques using ultrafast X-rays. Lensless imaging is akin to holography - except that no reference beam is required. It was first demonstrated in 1999 by John Miao and collaborators using synchrotron radiation at wavelengths around 1.7 nm. In this technique, a fully spatially coherent beam illuminates an object, and the scatter pattern (diffracted light) from the object is collected on an X-ray CCD camera. (The unscattered X-ray beam is blocked.) Provided that the region illuminated by the X-ray beam is "over" sampled (i.e., the object is surrounded by a region from which no X-ray light is diffracted), the image can be reconstructed using iterative algorithms that retrieve both the amplitude and phase. This imaging technique has applications in ultrafast nanoscale dynamics as well as in metrology and the nanoinspection of materials.

For additional information please see http://jilawww.colorado.edu/kmgroup

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