Vacuum ultraviolet (VUV) light, the wavelength being ≲200 nm, is a powerful tool for modern molecular spectroscopy since the majority of outer electronic excitations of molecules lie in this spectral range. In contrast to most liquids and solids that are opaque in the VUV and feature a low damage threshold, gases are promising platforms for nonlinear frequency conversion of near-infrared and visible light into the VUV. These features, along with the long light-matter interaction lengths provided by hollow-core waveguides, enable unique, compact possibilities to reach this spectral domain that complement other existing approaches such as the use of synchrotrons or other building-scale facilities.

In this work, Ackermann and co-workers report an elegant demonstration of pulsed VUV emission from green light via fifth-harmonic generation in a hollow-core waveguide filled with argon. The harmonic emission is remarkably enhanced through the modification of the gas dispersion that occurs close to a multi-photon atomic resonance. The performance of the system is further improved by exploiting the pressure-adjustable dispersion of the waveguide, which ensures that both pump and VUV light are phase-velocity matched over a reasonably long distance. These results pave the way to the development of novel gas-based ultraviolet sources whose efficiency might be boosted through operation near resonances.

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