Nonlinear optical signatures of spin relaxation in 2D perovskites

Spin-orbit coupling splits the exciton resonances of two-dimensional organic-inorganic hybrid perovskites (2D-OIHPs) into an optically active fine structure. Although circularly polarized light can induce macroscopic spin polarizations in ensembles of quantum wells, the orientations of the angular momentum vectors associated with individual excitons generally randomize on sub-picosecond timescales in 2D-OIHPs with single lead-iodide layers. In the present work, we investigate the nonlinear optical signatures of spin depolarization in 2D-OIHP materials with various organic layer thicknesses and polaron binding energies. Transient absorption experiments conducted using circularly polarized laser pulses establish time constants for spin equilibration ranging from 65 to 110 fs in the targeted systems. In addition, with inspiration from time-resolved Faraday rotation spectroscopies, we introduce a transient grating method in which spin relaxation promotes an elliptical-to-linear transformation of the signal field polarization. Spectroscopic signatures for all experiments are simulated with a common third-order perturbative model that incorporates orientationally averaged transition dipoles and the polarizations of the laser pulses. Spectroscopic line broadening parameters obtained for the 2D-OIHP systems are considered in the context of a rate formula for spin relaxation, wherein the spin-orbit coupling is combined with a cumulant expansion for fluctuations of the energy levels. Our analysis suggests that the insensitivity of the measured spin relaxation rates to the polaron binding energies of 2D-OIHPs reflects the suppression of an activation energy barrier due to motional narrowing. Model calculations conducted with empirical parameters indicate that motional narrowing of the spin relaxation processes originates in correlated thermal fluctuations of the energy levels comprising the exciton fine structure.