Correspondence should be addressed to SPHR.Femtosecond pulses from soft-x-ray free-electron lasers (FELs) [1] are ideal for directly probing matter at atomic length scales and timescales of atomic motion. An important component of understanding ultrafast phenomena of light-matter interactions is concerned with the onset of atomic motion which is impeded by the atoms' inertia. This delay of structural changes will enable atomic-resolution flash-imaging [2][3] to be performed at upcoming x-ray FELs [4][5] with pulses intense enough to record the x-ray scattering from single molecules [6]. We explored this ultrafast high-intensity regime with the FLASH soft-x-ray FEL [7][8] by measuring the reflectance of nanostructured multilayer mirrors using pulses with fluences far in excess of the mirrors' damage threshold. Even though the nanostructures were ultimately completely destroyed, we found that they maintained their integrity and reflectance characteristics during the 25-fs-long pulse, with no evidence for any structural changes during that time over lengths greater than 3 Å.In the recently built FLASH FEL [7], x-rays are produced from short electron pulses oscillating in a periodic magnet array, called an undulator, by the principle of self-amplification of spontaneous emission [9][10]. The laser quality of the x-ray pulses can be quantified by the peak spectral brilliance of the source, which is 10 28 photons/(s mm 2 mrad 2 0.1% bandwidth) [8]; this is up to seven orders of magnitude higher than modern thirdgeneration synchrotron sources. For our studies, the machine operated with pulses of 25 fs duration at a wavelength of 32.5 nm and energies up to 21 μJ. We focused these pulses to 3x10 14 W/cm 2 onto our nanostructured samples, resulting in an the unprecedented heating rate of 5x10 18 K/s, while probing the irradiated structures at the nanometer length scale.The x-ray reflectivity of periodic nanometer-scale multilayers [11] is very sensitive to changes in the atomic positions and the refractive indices of the constituent materials, making them an ideal choice to study structural changes induced by ultrashort FEL pulses. The large multilayer reflectivity results from the cooperative interaction of the x-ray field with many layers of precisely fabricated thicknesses, each less than the x-ray wavelength. This Bragg or resonant reflection from the periodic structure is easily disrupted by any imperfection of the layers. The characteristics of the structure, such as periodicity or layer intermixing, can be precisely determined from the measurement of the Bragg reflectivity as a function of incidence angle. These parameters can be easily measured to a small fraction of the probe wavelength, as is well known in x-ray crystallography where average atomic positions of minerals or proteins are found to less than 0.01Å. Thus, we can explore ultrafast phenomena at length scales less than the wavelength, and investigate the conditions to overcome the effects of radiation damage by using ultrafast pulses.For our measurement...

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We compare the reflectance and stability of multilayers comprising either Si/Mo, Si/Mo2C, Si/B4C, Si/C, or Si/SiC bilayers, designed for use as extreme-ultraviolet (EUV) reflective coatings. The films were deposited by using magnetron sputtering and characterized by both x-ray and EUV reflectometry. We find that the new Si/SiC multilayer offers the greatest spectral selectivity at the longer wavelengths, as well as the greatest thermal stability. We also describe the optimization of multilayers designed for the Solar-B EIS instrument. Finally, we compare experimental reflectance data with calculations and conclude that currently available optical constants cannot be used to adequately model the performance of many of these multilayers.

Ni/4H-SiC Schottky photodiodes of 5 mm x 5 mm area have been fabricated and characterized. The photodiodes show less than 0.1 pA dark current at -4 V and an ideality factor of 1.06. A quantum efficiency (QE) between 3 and 400 nm has been calibrated and compared with Si photodiodes optimized for extreme ultraviolet (EUV) detection. In the EUV region, the QE of SiC detectors increases from 0.14 electrons/photon at 120 nm to 30 electrons/photon at 3 nm. The mean energy of electron-hole pair generation of 4H-SiC estimated from the spectral QE is found to be 7.9 eV.

We report progress in using nanoimprint lithography to fabricate high fidelity blazed diffraction gratings. Anisotropically etched silicon gratings with 200 nm period and 7.5°blaze angle were successfully replicated onto 100 mm diameter wafers with subnanometer roughness and excellent profile conformity. Out-of-plane distortion induced by residual stress from polymer films was also analyzed and found to be extremely low. The replicated blazed gratings were tested and demonstrated high x-ray diffraction efficiencies. This process was developed for fabricating blazed diffraction gratings for the NASA Constellation-X x-ray telescope.