Revealing and Characterizing Dark Excitons Through Coherent   Multidimensional Spectroscopy
  Dark excitons are of fundamental importance in a broad range of contexts, but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify different types of dark excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms and coupling strengths. The observations of coherent coupling between bright and dark excitons hint at a role for a multi-step process by which excitons in the barrier can relax into the quantum wells. 
Jonathan O. Tollerud, Steven T. Cundiff and Jeffrey A. Davis
physics.optics cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.other