Energy enhancement of a highly coherent VUV source at 125 nm for nanopatterning applications

Highly coherent, tabletop, and reliable sources in the Vacuum UltraViolet (VUV) region (λ ≪ 180 nm) are desirable for both fundamental studies (e.g. laser cooling of (anti-)hydrogen atoms [1]) and nanopatterning applications. While excimer lasers are well suited for mask exposure in lithography or nanohole drilling, they are far from ideal for interference lithography (and holography), due to their poor coherence. An alternative solution consists in using nonlinear conversion of visible lasers in vapors down to the VUV. This makes sources with better coherence and beam quality but with very low energies (typically ≪ µJ). In order to reach energies allowing patterning standard polymers in a reasonable time (the threshold ablation fluence is ∼1 mJ/cm
²
in PMMA at 125 nm [2]), we recently developed an original concept in which the nonlinear medium is a dense, spatially localized, laser ablation plume of mercury atoms [3].