Surface figure measurement of EUV optics by coherent EUV light (high-order harmonics).

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Single-shot inspection using a compact EUV light source makes it possible to simplify measurement apparatus, because experimental errors, such as sample and measurement apparatus vibrations and pointing stability of a light source, could be avoided. We demonstrate a high-throughput, high-damage-threshold beam separator for wavelengths shorter than 30 nm. The beam separator uses a 10-nm-thick niobium nitrogen film deposited on a Si substrate set at the Brewster's angle. The film was deposited by RF reactive magnetron sputtering on the Si substrate. The beam separator has an attenuation ratio of 0.01 and a damage-threshold intensity of at least 0.8 TW/cm^2 for a 26-fs pump pulse. The measured reflectivity of the beam separator exceeded 70% in a wavelength range of 13 to 18 nm. The reflectivity did not decrease after irradiation with 2 × 10^6 shot laser pulse. The damage-threshold intensity for the pump pulse is guaranteed to be above 0.8 TW/cm^2. This broadband beam separator, which may be used to eliminate energetic laser pulses from longitudinally pumped x-ray lasers as well as from high-order harmonics, are useful for many applications using EUV optics. Using this beam separator, we also demonstrated a single-shot measurement of spatial coherence of a 13-nm high-order harmonic beam in a Young's double slit experiment. A beam divergence of 0.35 mrad and a high fringe visibility of 0.96 are obtained with optimal phase-matching condition for the 13-nm harmonics. Spatial coherence length of the 13-nm harmonics is about two times larger than the beam diameter. To our knowledge, this is the first demonstration of spatial coherence measurement of the 13-nm harmonic beam with a single-shot. This result shows that the 13-nm harmonic beam is useful for applications in interferometry, imaging and microscopy.