Enhanced high harmonic generation from an optically prepared excited medium.

We investigate high harmonics generated from rubidium atoms irradiated simultaneously by an intense 3.5 microm fundamental field and a weak cw diode laser. When 5p, 5d, and 4d excited states are populated through cascade excitation or deexcitation, orders-of-magnitude increases in harmonic yield as compared with the ground state are observed. It appears that, quite unexpectedly, the population accumulated in the 4d state alone is responsible for the observed enhancement.