  Tunable coherent light sources operating in the vacuum ultraviolet (VUV)
region in 100-200-nm (6-12 eV) wavelength range have important spectroscopic
applications in many research fields, including time-resolved angle-resolved
photoemission spectroscopy (ARPES). Recent advances in laser technology have
enabled the upconversion of visible femtosecond lasers to the vacuum and
extreme ultraviolet regions. However, the complexity of their experimental
setups and the scarcity of bulk nonlinear crystals for VUV generation have
hampered its widespread use. Here, we propose the use of a free-standing
dielectric nanomembranes as a simple and practical method for tunable VUV
generation. We demonstrate that third harmonic VUV light is generated with
sufficient intensity for spectroscopic applications from commercially available
SiO2 nanomemebranes of submicron thicknesses under excitation with visible
femtosecond laser pulses. The submicron thickness of the nanomembranes is
optimal for maximize the VUV generation efficiency and prevents self-phase
modulation and spectral broadening of the fundamental beam. The observed VUV
photons are up to 10^7 photons per pulse at 157 nm with 1-kHz repetition rate,
corresponding to a conversion efficiency of 10^-6. Moreover, the central VUV
wavelength can be tuned in 146-190-nm wavelength range by changing the
fundamental wavelength. We also explore material and thickness dependence with
experiments and calculations. The presented results suggest that dielectric
nanomembranes can be used as a practical nonlinear media for VUV spectroscopic
applications.
