EUV- and X-ray sources with laser like properties, e.g. free electron lasers, offer possibilities for many new experiments. In
order to successfully plan and perform experiments at these high flux sources, it is necessary to know which kind of optics,
exposed to the full beam, can be used. Due to the high intensities, it is not clear, whether transmissive diffractive optics are
applicable, because these optics are usually fabricated on thin membranes, thus introducing additional absorption in the
desired energy range. Since diffractive optics, especially zone plates, offer the possibility to achieve small spots when used
as a focussing element and can also achieve good image quality in microscopic setups, their usage would facilitate many
experiments, especially for their easy handling. As a proof of concept, we set up a zone plate based scanning transmission
microscope at the unfocussed beamline BL3 at FLASH (DESY/Hamburg). The operating wavelength was 32 nm and
13.8 nm, respectively. While the first attempt, utilizing a zone plate composed of PMMA on silicon substrate failed due to
ablation of the PMMA, a second zone plate (chromium on silicon nitride) was successfully used to focus the beam onto
different samples (e.g. nickel-mesh and a silicon nitride structured sample). The resulting focal spot size was estimated
from the acquired images to be in the range of 1 μm - 3μm in diameter. After several hours of exposure, no damage was
visible to the optics. Beside the optics, different filters (Silicon/Zirconium, Zirconium and Aluminum) have been placed
in the beam to evaluate possibilities to further reduce intensity which may be necessary if sensitive detectors are involved.
All of the filters withstood the irradiation during the whole experiment.
We report on a compact full-field transmission microscope (CTXM) and a scanning transmission microscope (CSTXM) developed for imaging at laboratory scale X-ray sources. The microscopes are based on zone plates for imaging in the EUV and water window region (wavelength 2.3 nm to 4.4 nm).
The radiation for the full-field microscope is generated by focusing short laser pulses with an energy of 100 mJ on a 20 μm cryogenic liquid nitrogen jet. A condenser zone plate in conjunction with an aperture is used to provide monochromatic sample illumination. This allows for easy wavelength selection within the N2-Emission spectrum. Thus, the presented setup offers the possibility of spectral imaging. A micro zone plate generates a magnified image detected by a back illuminated TE-cooled CCD camera (1,340 x 1,300 pixel). The actual configuration provides magnifications up to 1,000x at exposure times in a range of a few ten minutes with sub-100 nm resolution.
Our compact scanning microscope (CSTXM) operates with a zone plate, focusing the radiation onto a sample which is placed on a piezo driven xy-stage with 1 nm lateral resolution. Using high-harmonic radiation at 13 nm wavelength sub-micron resolution is achieved. With light at 17 nm wavelength originating from the O-VI emission line of a laser plasma source based on an ethanol jet, 500 nm structures were imaged in less than 20 minutes resulting in an 100 x 40 pixel image.