Fifth-order nonlinear optical response of excitonic states in an InAs   quantum dot ensemble measured with 2D spectroscopy
  Exciton, trion and biexciton dephasing rates are measured within the inhomogeneous distribution of an InAs quantum dot (QD) ensemble using two-dimensional Fourier-transform spectroscopy. The dephasing rate of each excitonic state is similar for all QDs in the ensemble and the rates are independent of excitation density. An additional spectral feature -- too weak to be observed in the time-integrated four-wave mixing signal -- appears at high excitation density and is attributed to the $\chi^{\textrm{(5)}}$ biexcitonic nonlinear response. 
G. Moody, R. Singh, H. Li, I. A. Akimov, M. Bayer, D. Reuter, A. D.   Wieck, and S. T. Cundiff
cond-mat.mes-hall