A microfluidic liquid sheet apparatus for extreme ultraviolet high-order harmonic generation.

High-order harmonic generation (HHG), as an important ultrafast light source, can be applied in different fields. In contrast to the HHG from gases, the technical extension to liquids remains challenging and recently receives growing attention. Here, we report an apparatus for extreme ultraviolet HHGs from a microfluidic liquid sheet by the excitation of intense femtosecond laser pulses. The liquid sheet is produced with a single-channel borosilicate glass chip, showing a size around 9 mm (length) × 2 mm (width) and a thickness around 1 μm. The HHGs up to twenties eV are successfully produced from the liquid sheets of water, methanol, and ethanol. We scrutinize the HHG dependences on the central photon wavelength and peak intensity of the femtosecond laser pulses and the laser focusing position on the liquid sheet. Furthermore, we propose that the electron radiation-induced chemical reactions in the liquid, as recently explored with our time-delayed mass spectrometry, could be revealed more explicitly with HHG spectroscopy.