Numerical observation of two sets of low-order harmonics near the ionization threshold

The bound states of atoms play important roles during the interaction with a strong laser pulse. In the present work, we observe a set of harmonics which are not located at the usual odd harmonics. These harmonics can be clearly identified when the driving frequency is resonant with the excited states. The observed set of harmonics become stronger than the usual odd harmonics for a rather long laser pulse at a moderate intensity. From ab initio calculations, we find that an atom can be resonantly pumped to the excited states during the interaction with lasers at some wavelengths. After the excitation, the electrons can be ionized from these excited states and then recombine to the ground state, which leads to another set of harmonic peaks near the atomic ionization potential. We investigate the role of different laser wavelengths as well as atomic potentials. In addition, we find that the phases of these harmonics are far more sensitive than the usual harmonics when we change the parameters of the driving pulse, which can cause a large divergent angle. The parameter dependence of the phases of these harmonics is interpreted through an intuitive model.