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

In this paper we will review how the main parameters of the plume (energy density on Hg pool, time delay between the plume creation and the nonlinear interaction, nature of the background gas) can be monitored to maximize the VUV yield. Time-resolved snapshots of the plume, time-resolved spectra of the plume luminescence as well as images of the UV-beam-induced fluorescence, have allowed determining mean velocities and estimates of the atom densities in various cases. The highest energy output at 125 nm (>10 muJ) was obtained when the plume was constrained from expanding in an Argon atmosphere (0.5 bar). With respect to a plume expanding in a vacuum, this configuration allows a higher density and a higher stability since windows are protected; in addition the time delay between the pulse creating the plume and the nonlinear interaction is much longer (up to 20 mus), which opens the way towards quasi-CW operation.