Effect of the odd-photon destructive interference on laser-induced transparency and multiphoton excitation and ionization in rubidium

We report experimental results on two-color multiphoton ionization and four-wave-mixing production under conditions where one form of laser-induced transparency (LIT) occurs. Under the conditions of LIT, multiphoton-ionization line shapes obtained by tuning one laser through a two-photon resonance exhibit a pronounced Autler-Townes splitting at low concentrations, when a second laser couples the two-photon state to a third level. In this concentration region we observed a four-wave-mixing field that increased quadratically with concentration. As the concentration is increased the Autler-Townes splitting starts to decrease, and disappears completely as a critical concentration is reached. Simultaneously, the four-wave-mixing field intensity starts to level off and finally becomes concentration independent. These observations are explained in terms of odd-photon destructive interference between different excitation pathways. In rubidium, we demonstrate that above concentrations of n≃5×[Formula Presented] [Formula Presented] the four-wave-mixing field is concentration independent and the multiphoton-ionization line shape shows no Autler-Townes splitting, and the width of the line shape is determined by the laser bandwidth. The results presented here demonstrate that destructive interference significantly limits the high-efficiency and high-intensity nonlinear optical generation promised in early studies on LIT, at least for the multimode laser system used in the present experiment.