Coherent spin dynamics in ensembles of randomly oriented singly charged colloidal nanoplatelets and nanocrystals

We present a theoretical study of the pump-probe Faraday rotation and ellipticity signals in ensembles of uniaxially anisotropic CdSe nanoplatelets and nanocrystals. We use the Faraday rotation mechanism based on the excitation of negative heavy-hole trions for a magnetic field applied in the Voigt geometry. Three types of ensembles with typical spatial distributions of the orientation of the anisotropy axis with respect to the direction of light propagation are considered. Faraday rotation and ellipticity signals are modeled for excitation by single and repeated pump pulses, taking into account the anisotropy of the electron $g$ factor. We show that spin dephasing caused by the electron $g$-factor anisotropy and the arbitrary orientation of nanoplatelets or nanocrystals result only in partial damping of oscillation amplitude in contrast to the dephasing caused by the dispersion of the electron $g$ factor in the ensemble. We demonstrate that regardless of the $g$-factor anisotropy degree the oscillation frequency of the Faraday rotation and ellipticity signals for a randomly oriented ensemble is determined by the transverse electron $g$-factor component.