Theory of optical alignment and orientation of excitons in an ensemble of semiconductor nanoplatelets
We present the theory of exciton optical alignment and optical orientation effects in a single semiconductor nanoplatelet (NPL) and in an ensemble of NPLs. The theoretical model is developed to take into account the emission from both bright and dark exciton states in the case of strong electron-hole exchange interaction and an isotropic distribution of in-plane orientations of the NPLs' edges with respect to the linear polarization axis. The conditions for the linear optical response regime on the polarized resonant excitation of the bright exciton are considered, and the steady-state and time-dependent expressions for polarized photoluminescence are presented for the low temperature case in the Faraday magnetic field at normal incidence. The symmetry of the obtained optical response corresponds to the $C_{2}$ point group for the single NPL and the $C_{\infty}$ point group for the NPL ensemble. The bright-to-dark exciton coupling mechanism allowing one to observe the optical alignment of the dark exciton is considered. The example calculations of the effects are presented for the parameters close to those of CdSe/CdS core/shell NPLs. The effect of the fluctuations in the exciton parameters in the ensemble on the polarization dephasing in the transverse effective magnetic field is demonstrated.