Feb 19 2010
Everyone knows holograms from their everyday life, for instance the ones applied to credit cards as security indicators. Unlike a photography of an object which only records the amplitude of the light wave coming from the object, the hologram also includes local information about the light wave's phase. In appropriate lighting, the initial wave front is reconstructed in proper phase and the spectator has a three dimensional sensation of the object.
But it is not this characteristic of holography that is central when it comes to the imaging of small structures, but the fact that for the recording of a hologram no lenses are required at all. In order to conduct research on nanometer scaled objects, light of an equally small wave length is needed (soft X-rays).
The only lenses working in this wave spectrum (so-called Fresnel zone plates) are very sophisticated in design and still yield a quality of imaging one scale inferior then lenses for visible light. The modus operandi of recording holograms without the use of lenses is to superimpose the light wave having radiated the object at the time of recording with a reference wave having a known and stable (coherent) phase.
A spheric wave serves as a reference wave emanating from a few nanometer large puncture located directly near the object. Coherent X-rays are provided at modern synchrotrons or free-electron-lasers like the Hamburg FLASH with a more intense brightness; hence procedures of holographic depiction of nano structures have been tested for several years.A team of the collaborate research center 688 (SFB688) has only recently introduced the first fully functional microscope for the holography of nano structures in cooperation with scientists from the DESY in Hamburg and the ESRF in Grenoble. Thereby, the innovative construction of two slideable silicon nitride membranes assembled directly behind one another allows for approaching and depicting any of the tested object's areas, which has previously not been possible with conventional proceedings.
The trick lies in manufacturing the optically effective components - a one micrometer small hole for the selection of an image detail and the small hole for the generation of the reference wave - from its own impermeable membrane. The object though is prepared on a second, permeable, membrane that can be exchanged separately.The holograms of single image details of a broad object are reconstructed digitally using a simple algorithm (FFT); subsequently the partitions are pieced together. Soft X-rays enable the selective depiction of single chemical elements or their local magnetization, which might be purposefully taken advantage of according to the issue at question.Moreover, the X-ray pulses' timely structure provides information on pico-second scale. Currently it is being worked on the improvement of the spatial resolution to 10nm. The “Applied Physics Letters” journal appreciated the „X-ray holographic microscopy“ (XHM) technology to be so promising, it was awarded the title story.
Source: University of Hamburg