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

使用紧凑型EUV光源的单次检测使得可以简化测量设备，因为可以避免实验误差，例如样品和测量设备振动以及光源的指向稳定性。我们展示了一个高通量，高损伤阈值光束分离器的波长短于30 nm。束分离器使用以布鲁斯特角设置的Si衬底上沉积的10 nm厚的铌氮膜。采用射频反应磁控溅射法在Si衬底上制备了薄膜。对于26 fs泵浦脉冲，光束分离器的衰减比为0.01，损伤阈值强度至少为0.8 TW/cm ^2。在13 ~ 18 nm波长范围内，测得的分束器反射率超过70%。用2 × 10^6次激光脉冲辐照后，反射率没有降低。泵浦脉冲的损伤阈值强度保证在0.8 TW/cm^2以上。该宽带光束分离器可用于消除来自纵向泵浦的X射线激光器以及来自高次谐波的高能激光脉冲，对于使用EUV光学器件的许多应用是有用的。利用该光束分离器，我们还在杨氏双缝实验中演示了13 nm高次谐波光束空间相干性的单次测量。对于13 nm谐波，在最佳相位匹配条件下，获得了0.35 mrad的光束发散角和0.96的高条纹可见度。13 nm谐波的空间相干长度约为光束直径的两倍。据我们所知，这是第一次演示的空间相干性测量的13 nm谐波光束与一个单一的拍摄。这一结果表明，13-nm谐波光束是有用的应用在干涉，成像和显微镜。

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.