Terahertz excitation of a coherent three-level {\Lambda}-type   exciton-polariton microcavity mode
  Interactions of few-cycle terahertz pulses with the induced optical polarization in a quantum-well microcavity reveal that the lower and higher exciton-polariton modes together with the optically forbidden 2p-exciton state form a unique {\Lambda}-type three-level system. Pronounced nonlinearities are observed via time-resolved strong-terahertz and weak-optical excitation spectroscopy and explained with a fully microscopic theory. The results show that the terahertz pulses strongly couple the exciton-polariton states to the 2p-exciton state while no resonant transition between the two polariton levels is observed. 
Joseph L. Tomaino, Andrew D. Jameson, Yun-Shik Lee, Galina Khitrova,   Hyatt M. Gibbs, Andrea C. Klettke, Mackillo Kira, and...
cond-mat.mes-hall physics.optics