XUV-assisted high-order-harmonic-generation spectroscopy

Using the analytic time-dependent effective range theory, we study two-color high-order harmonic generation (HHG) involving a weak extreme ultraviolet (XUV) pulse and an intense infrared laser field. Our analysis shows that XUV-assisted HHG spectra contain multiple additional plateau structures originating from absorption of one or more XUV photons at the photorecombination step of HHG. We show also that the HHG rate corresponding to the $n\mathrm{th}$ plateau can be presented in a factorized form involving the XUV-assisted (multiphoton) photorecombination cross section (PRCS) corresponding to absorption of $n$ XUV photons of energy $\mathrm{\ensuremath{\Omega}}$ and emission of a harmonic of energy ${\mathrm{\ensuremath{\Omega}}}_{h}$. This factorization allows one to extract the PRCS from the HHG spectrum and to retrieve the cross section of the inverse process: the photoionization cross section involving absorption of a single photon of energy ${\mathrm{\ensuremath{\Omega}}}_{h}$ and emission of $n$ XUV photons of frequency $\mathrm{\ensuremath{\Omega}}$. The analytic HHG results are in excellent agreement with numerical solutions of the three-dimensional time-dependent Schr\"odinger equation.