Depletion effect in high-order harmonic generation with coherent superposition state

Enhancement of the conversion efficiency of high-order harmonics (HHG) generated from an atom in the superposition of two quantum states instead of the ground state is essential because of the recent application in the generation of ultrashort isolated attosecond pulses. In this study, we first confirm the enhancement for a more real system with full three-dimensional geometry and then investigate an interesting effect-the influence of the depletion on the HHG cutoff. For this purpose, we prepare a hydrogen atom initially in the coherent superposition of the ground and the first excited states and calculate the HHG by numerically solving the three-dimensional time-dependent Schrodinger equation (TDSE)-the TDSE method. We find that the plateau of HHG spectra with the coherent superposition state ends earlier than that of the ground state, which can be attributed to the depletion of the excited state. We justify this link by adopting the classical simulation and time-dependent ionization rate. Besides, we show that the conversion efficiency is sensitive to the existence of the excited state. Particularly, for some values of laser field intensity, the excited state population of about 1% increases HHG intensity by more than 5 orders. However, after a critical excited state population, the HHG intensity is stable with continuously increasing initial population of the excited state that is meaningful for experimental setups. (C) 2020 Optical Society of America