Few-cycle high-harmonic generation in liquids: in-operando thickness measurement of flat microjets

Abstract

Extreme ultraviolet high-harmonic generation (HHG) from bulk liquids has only recently been demonstrated (T.T. Luu, Z. Yin et al, Nat. Comm. 9, 3724, (2018)). This has opened new prospects for the development of bright high-harmonic sources and the development of liquid-phase high-harmonic spectroscopy (HHS). Here, we report on the first observation of HHG in liquids driven by few-cycle (~7 fs) pulses. We observe the emission of a broad quasi-continuum in the extreme ultraviolet, which is strongly modulated on a photon-energy scale much finer than the central photon energy of the driver. We show that these modulations arise from an etalon effect inside the flat microjet, which we use to perform an in-operando measurement of the flat-jet's thickness. The possibility to directly characterize flat microjets during HHG will greatly support their optimization for light-source applications and facilitate the development of liquid-phase HHS.