Dephasing of coherences between ${\sigma }^{+}$ and ${\sigma }^{-}$ exciton states in a ZnSe single quantum well

We investigate interband coherences as well as coherences between ${\sigma }^{+}$ and ${\sigma }^{-}$ exciton states in a 10 nm thick ZnSe/ZnMgSe single quantum well by spectrally resolved three-pulse degenerate four-wave mixing using circularly polarized 100 fs pulses. Polarization and intensity dependent measurements are performed to identify the coherent processes responsible for the nonlinear signal into the ${\mathbf{k}}_3+{\mathbf{k}}_2-{\mathbf{k}}_1$ direction and to study the influence of Coulomb scattering on the dephasing rates. The experiments reveal nearly equal zero-density dephasing rates for coherences between ${\sigma }^{+}$ and ${\sigma }^{-}$ heavy- and/or light-hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state. The measurements also demonstrate that exciton–exciton scattering destroys the phase coherence between heavy- and light-hole excitons about three times more effectively than the coherence between ${\sigma }^{+}$ and ${\sigma }^{-}$ heavy-hole excitons. The characteristic features of the experimental results are explained and reproduced by numerical calculations of the optical Bloch equations for a ten-level model including excitation induced dephasing.