Interactions of ultrashort strain solitons and terahertz electronic two-level systems in photoexcited ruby

We examine the interaction between coherent, terahertz strain solitons, and the metastable 29-cm^-1 electronic two-level systems in photoexcited ruby. A large difference in soliton-induced R₂ luminescence is observed for soliton propagation along the crystallographic c axis with respect to earlier experiments along the a direction [Phys. Rev. Lett. 92, 035503 (2004)], that can be explained by the strong anisotropy of the crystal field interaction strength. The behavior of the soliton-induced population of terahertz two-level systems is further investigated in several configurations and explained successfully in the context of a model of nonlinear propagation of the supersonic soliton packets and their coherent interaction with the two-level medium.