Spatially resolved Ar~* and AO~(+*) imaging as a diagnostic for capillary-based high harmonic generation

摘要

Coherent x-ray—ultraviolet radiation can be generated by the highly nonlinear interactionbetween a gas target and high intensity ultrafast laser pulses using the high harmonic generation(HHG) process. Guiding the fundamental laser field inside a hollow capillary waveguideimproves phase matching and extends the potential interaction length. However, the propagationof an intense pulse within a capillary waveguide filled with ionizable gas is complex, as thepulse creates a plasma, which in turn strongly affects the propagation. Previous work has usedemission from the excited gas to study propagation of nanosecond pulses in capillarywaveguides. In this work we demonstrate spectrally resolved imaging of the plasma created byintense femtosecond pulses within a capillary during an HHG experiment. The spectral andspatial resolution is used to separate contributions from ions and neutral species, and is aneffective diagnostic for the local pressure and modal intensity variations along the waveguide.