Enhanced high-order harmonic generation from excited argon

We experimentally demonstrate an effective scheme to generate the enhanced high-order harmonics from a coherent superposition state of ground state and excited states in Argon. By controlling the time delay between the pump and probe laser pulses, a surprising enhancement plateau of high-order harmonic generation is observed, with a half lifetime of dozens of picoseconds. In the plateau, the harmonic intensity is enhanced by nearly one order of magnitude compared to the case without pre-excitation. Moreover, the gradual enhancement process is also presented when the pump intensity is increasing for improving the population of excited states, which can be attributed to the Rydberg states created by frustrated tunneling ionization.