Abstract
High–harmonic generation (HHG) in gases has been the main enabling technology of attosecond science since its discovery. Recently, HHG from solids has been demonstrated, opening a lively area of research. In contrast, harmonic generation from liquids has so far remained restricted to low harmonics in the visible regime. Here, we report the observation and detailed characterization of extreme ultraviolet HHG from liquid water and several alcohols extending beyond 20 eV. This advance was enabled by the implementation of the recent liquid flat–microjet technology, which we show to facilitate the spatial separation of HHG from the bulk liquid and the surrounding gas phase. We observe striking differences between the HHG spectra of water and several alcohols. A comparison with a strongly–driven few–band model establishes the sensitivity of HHG to the electronic structure of liquids. Our results suggest liquid–phase high–harmonic spectroscopy as a new method for studying the electronic structure and ultrafast scattering processes in liquids.
Highlights
High–harmonic generation (HHG) in gases has been the main enabling technology of attosecond science since its discovery
We note that HHG from the gas phase located in front of the flat microjet is entirely absorbed by the jet because typical absorption lengths at our extreme ultraviolet (EUV) energies are on the order of ~11 nm at 21 eV26
The EUV HHG from liquids observed in our experiments has a cutoff photon energy of about 20 eV, which is very similar to the maximum photon energy observed in fused silica[15,18]
Summary
High–harmonic generation (HHG) in gases has been the main enabling technology of attosecond science since its discovery. The flat microjet provides an ultrathin (~1.9 μm, see Methods), continuously renewed slab of liquid, which avoids the curvature of the interface inherent to spherical droplets and cylindrical microjets These properties minimize the effect of reabsorption and phase mismatch, sample damage, as well as microfocussing of the infrared driver leading to divergence of the emitted harmonics, turning flat microjets into robust, reproducible and nearly flawless targets for HHG. This new approach enables us to isolate EUV high-harmonic emission from the bulk of liquids from that of the surrounding gas phase. These combined results suggest liquid-phase high-harmonic spectroscopy as a potential avenue for studying the electronic structure and sub-femtosecond electron scattering processes in liquids
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