Tunable high-order frequency mixing for XUV transient absorption and photoelectron spectroscopies

We generate tunable extreme ultraviolet emission through high-order frequency mixing between a strong near-infrared field and a weak shortwave-infrared pulse whose wavelength can be adjusted. In this two-color driving scheme, new harmonics appear in between the single-color harmonics at energies which are linear combinations of photons from the two pulses. We demonstrate the utility of tunable two-color harmonics by employing them for XUV transient absorption spectroscopy and time-resolved photoelectron spectroscopy. We show that the two-color harmonics can be used to address the dynamics associated with excited states in Helium and Oxygen which are inaccessible using single-color harmonics. Specifically, we show the ability to switch between excitation of 3p and 4p states in Helium, control of transitions to the light induced states, observe new four-wave-mixing emissions, and selectively address different principal and vibrational quantum numbers associated with Oxygen Rydberg states.