We investigate high harmonics generated from rubidium atoms irradiated simultaneously by an intense $3.5\mu \mathrm{m}$ 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.

• Received 31 August 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.113906