High-Harmonic Generation by Initial Coherent States in a Short Laser Pulse

We study high-order harmonic generation (HHG) in an intense laser pulse when the initial state is prepared as a coherent superposition of the ground and first excited states. By examining the populations of these states for different laser intensity regions, the role of the excited state in the harmonic generation process can be identified. We also find that high conversion efficiency and high cutoff frequency of HHG can be achieved if the intensity of the laser pulse is high enough to cause considerable ionization of the first excited state but not sufficient to ionize the ground state.