The team of Prof. Liang-you Peng and Prof. Qihuang Gong of the State Key Laboratory for Mesoscopic Physics and Department of Physics have made important progress in the mechanisms of below threshold harmonic generation. Their results have been published by Physics Review Letters [Phys. Rev. Lett. 112, 233001 (2014)].

The harmonic generation when atoms or molecules interact with strong laser field has been paid lots of attention during the last decades due to its application as coherent XUV and attosecond light sources. When the photon energy emitted is large enough, theories without consideration of the Coulomb potential such as strong field approximation can describe this process. But for the low energy photons, Coulomb potential have critical impact on the harmonic yield and analytical theory does not exist in this range. They numerically solve the time-dependent Schrödinger equation of an atom to investigate the mechanisms in the low energy range by varying the laser wavelength and peak intensity.

The harmonic spectrum with different laser wavelengths is shown in Fig.1. The harmonic yield is modulated by the laser wavelength. Their analysis through a semi-classical model and the generalized quantum path analysis explained these structures. On the one hand, they confirmed the quantum path interference in this energy range. On the other hand, they identified the influence of Coulomb potential when bound states act as intermediate state during the harmonic generation process. Their theory can help to understand the experimental result of the frequency comb and coherent light sources in EUV range.

This work is supported by the 973 Program under Grant No. 2013CB922402, by the National Natural Science Foundation of China under Grants No. 11322437, No. 11174016, and No. 11121091, and by the Program for New Century Excellent Talents in University (NCET). The computational results were obtained by using the computer cluster “MESO” in the State Key Laboratory for Mesoscopic Physics at Peking University.