HARMONIC-GENERATION FROM A COHERENT SUPERPOSITION OF STATES

In this Rapid Communication we present a method of extending the harmo nic generation plateau to high energies with very high conversion effi ciencies using moderate intensities. Preparing the initial state as a coherent superposition of the ground state and an excited state we can induce dipole transitions between the continuum and the ground state via the intermediate (excited) state responsible for the ionization. W e solve numerically the Schrodinger equation and show that the spectru m contains two distinct plateaus, with cutoffs at U-e+3U(p) and U-g+3U (p). Using an essential states method we demonstrate that the plateaus are due to recombination into the excited and ground states. We brief ly discuss possible experimental realizations of the scheme we present .