High-Order Sum and Difference-Frequency Generation in Helium

Abstract

High-order harmonic generation provides a new method for generating coherent, XUV radiation.^1,2 These harmonics are characterized by a rapid, perturbative drop at low orders, followed by a broad plateau extending to photon energies of 150 eV in the lighter, rare gas atoms.^3,4 An experimentally observed limit coincides with the theoretical limit for harmonic generation in neutral atoms given by the expression E_c(eV)=IP(0)+3U_p(I)⁵, where E_c is the energy cutoff of the harmonic plateau, IP(0) is the field-free ionization potential and Up is the electron quiver energy at the maximum intensity, I, seen by the atom. As part of a broad effort to develop this technique into a general purpose, XUV source, extensive work to understand the phase-matching between the harmonic and driving fields, and the resulting effect on the conversion efficiency, angular distribution and spectral brightness has been undertaken at several laboratories.^6–8 Nevertheless, certain aspects of the harmonically generated radiation such as the polarization, relative strength of a given harmonic, and the plateau extent, are defined by the single atom-field interaction. Specifically, the single-atom harmonic spectrum is determined primarily by the interaction of a driven, quasi-free electron with the atomic potential. Using two, independent fields one can affect the electron motion by controlling the relative strength, polarization, and phase of the fields and alter the harmonic spectrum.