Design of fully spatially coherent extreme-ultraviolet light sources

We demonstrate experimentally that, in order to generate fully spatially coherent extreme-ultraviolet (EUV) beams using high-harmonic generation, it is necessary to guide the driving laser beam over long interaction lengths in gas-filled hollow waveguides. Numerical simulations show that, in propagating the laser through a long plasma-filled guide, the laser beam forms a stable eigenmode with uniform spatial phase, even at very high levels of ionization. This results in a compact, highly spatially coherent, EUV source useful for applications in EUV metrology, microscopy, interferometry, and holography.