Correlation and dephasing effects on the non-radiative coherence between   bright excitons in an InAs QD ensemble measured with 2D spectroscopy
  Exchange-mediated fine-structure splitting of bright excitons in an ensemble of InAs quantum dots is studied using optical two-dimensional Fourier-transform spectroscopy. By monitoring the non-radiative coherence between the bright states, we find that the fine-structure splitting decreases with increasing exciton emission energy at a rate of 0.1 $\mu$eV/meV. Dephasing rates are compared to population decay rates to reveal that pure dephasing causes the exciton optical coherences to decay faster than the radiative limit at low temperature, independent of excitation density. Fluctuations of the bright state transition energies are nearly perfectly-correlated, protecting the non-radiative coherence from interband dephasing mechanisms. 
Galan Moody, Rohan Singh, Hebin Li, Ilya A. Akimov, Manfred Bayer,   Dirk Reuter, Andreas D. Wieck, Steven T. Cundiff
cond-mat.mes-hall