High Harmonic Generation from Ions in a Capillary Discharge Plasma Waveguide

Abstract

I recent years, high harmonic generation (HHG) has proven to be a useful light source for investigating surface dynamics, holographic imaging and, more recently, probing molecular structure and dynamics. One important goal in the further development of HHG as a light source is the production of higher photon flux at shorter soft X-ray wavelengths. Since HHG is the result of ionization of the nonlinear medium followed by rapid acceleration of the electron in the light field, the highest energy photons are generated from target atoms that ionize at high laser intensities—in other words, those with high ionization potentials, such as helium. However, the highest flux has been observed when high-Z atoms are used for HHG. Although these requirements conflict, there is one attractive method for improving conversion to very high photon energy: to use ions rather than neutral atoms. Gibson et al. showed that it is possible to generate higher energy harmonics from argon ions by using a hollow waveguide to mitigate ionization-induced defocusing of the laser beam—which limits the laser intensity and thus the highest photon energy obtained. However, this geometry does not eliminate the laser loss. Alternatively, using a pre-formed plasma waveguide could overcome defocusing, as well as mitigate the ionization-induced energy loss of the driving laser.2,3 A capillary discharge creates an ideal medium for HHG from ions by providing a preformed plasma waveguide with tunable density and ionization. In this work, we present the first observation of HHG from ions in a plasma discharge. monic generation shows great promise for increasing harmonic flux and extending efficient harmonic generation to shorter wavelengths. t