Low-divergence coherent soft x-ray source at 13 nm by high-order harmonics

We have generated a few tens of nanojoules of coherent soft x-ray pulses at a wavelength around 13 nm with high-order harmonics. Output energy was estimated to be 25 nJ/pulse at the 59th harmonic (13.5 nm), and the beam divergence was measured to be 0.35 mrad full width at half maximum. Since a Mo/Si mirror covers two harmonics in the high reflectivity region, we can use ∼50 nJ harmonic energy per pulse in the 13 nm wavelength. This high-coherent low-emittance x-ray source is useful for the characterization and inspection of an optical system for extreme-ultraviolet lithography