Manipulations of atomic below-threshold harmonic generation by electrostatic fields

We have studied the harmonic spectra of hydrogen atoms subjected to different combined laser fields with increasing electrostatic fields, which are based on the numerical solutions of the time-dependent Schrödinger equation. It is found that, as the electrostatic field increases, a hump structure in the below-threshold harmonic spectra will show up. The reasons are that the below-threshold harmonic generation is closely related to the excited states, and the probabilities of the excited states 2s and 2p evidently increase with the increasing electrostatic fields.