Polarimetric measurements of the bright triplet emission of single cesium lead halide perovskite quantum dots

Abstract

Cesium lead halide perovskite quantum dots (QDs) have emerged as promising platform for quantum light sources. Indeed, they exhibit exceptional photoluminescence properties thanks to the emission from a bright triplet exciton state [1]. The energetic fine structure splitting (FSS), in the order of a few millielectronvolts, can be revealed by performing single QD spectroscopy at cryogenic temperature. In particular, depending on the QD orientation and on the observation direction, up to three orthogonal emitting dipoles can be spectrally detected, each of them with a high degree of linear polarization. To measure the polarization of the exciton FSS, typically a rotating linear polarizer was used. However, this method can only assess the orientation and the degree of linear polarization, while more sophisticated polarimetric techniques are required to distinguish between unpolarized and circularly polarized light. In this work, we fully characterize the polarization state of the exciton FSS of individual cesium lead halide perovskite QDs at 4 K, introducing an experimental setup capable of measuring the four Stokes parameters (I, S, C, M), through which the degree of circular polarization (S/I), the degree of linear polarization (DOLP) and the total degree of polarization (DOP) can be obtained. For these measurements, it is crucial to measure colloidal QDs with very low spectral diffusion and to perform a careful calibration of the optical path with respect to residual, unwanted birefringent phase shifts that are often occurring from optical coatings. Our results summarized in Figure 1 show that, even without applying an external magnetic field, the individual FSSs can exhibit a non-negligible S/I, which reach up to ~38% over 13 measured QDs, and sometimes an additional unpolarized component is observed. This explains why less than 100% DOLP is observed in basic polarization measurements. In addition, for each measured QD in Figure 1 the sign of S/I changes, varying from right-hand circularly polarized (RHCP) component to left-hand circularly polarized (LHCP) component within the fine structure triplet manifold. To conclude, in my presentation I will report the results of the polarimetry of single perovskite QDs, shedding light on the small non-linearly polarized emission fraction and the nature of their peculiar emission properties.