Semiconductor nanostructures such as quantum dots, confine electron-hole pairs/excitons spatially in one or more dimensions. Consequently, the enhanced exchange interaction between the electron and hole as well as the reduced symmetry give rise to exciton fine structures. Beyond optoelectronics, such fine structure splitting (FSS) states can serve as a prerequisite for quantum information processing. Recently, bright triplet exciton states are found in lead halide perovskite (LHP) nanocrystals (NCs), which underpin their fast exciton recombination and high quantum yields1. Over the past few years, highly efficient and bright light-emitting diodes and lasers have been developed based on these fascinating family of nanocrystals2.
Figure 1: Excitonic quantum coherence in CsPbBr3 NCs. (a) Fine structure splitting of the band edge J = 1 exciton. (b) The Bloch representation of the exciton spin states in CsPbBr3 NCs. (c) Coherent oscillations between x and y FSS states, where Δxy and τ refer to the splitting energy and coherence time, respectively. (d) As measured spin-polarized exciton quantum beating map in CsPbBr3 NCs.
In caesium lead tribromide (CsPbBr3) NCs, the band edge luminescent state splits into mutually orthogonal levels, with dipole orientations along the three crystal axes (Figure 1a). Along the z direction, the x and y exciton FSS states (or horizontal and vertical states) and their coherent superposition can be represented in a Bloch sphere (Figure 1b). Determining the coherence time by directly measuring the emission can be challenging because of fast system relaxation. Typically, direct observation of FSS energy levels is by single-particle photoluminescence measurements that requires a complex high spectral resolution confocal microscope setup with single-particle imaging capabilities. In our recent work3, we utilized transient absorption spectroscopy (TAS) to interrogate the exciton multilevel coherence time in CsPbBr3 NC ensembles (Figure 1c). TAS is one of the most common and elementary third-order susceptibility spectroscopies with sub-picosecond time resolution that is capable of scrutinizing coherent light-matter interactions. After initializing the system longitudinally or transversely by light polarization, exciton quantum beats can be clearly observed at low temperatures (Figure 1d). These beating signals provide a simple approach to evaluate the exciton FSS and coherence times but are unfortunately smeared out at higher temperatures. Nevertheless, in our approach using NC ensembles, only the average splitting can be estimated due to the nanocrystal size distribution and their random dipole orientations.
Figure 2 A summary of the dominant spin relaxation mechanisms in CsPbBr3 NCs with increasing temperature. The effective magnetic field originating from fine-structure splitting and momentum scattering are shown by Ω and k, respectively.
Utilizing optical spin orientation of excitons, we re-examine the controversy over the spin decoherence mechanisms in halide perovskites, especially perovskite NCs. Presently, there is an on-going debate over the spin-relaxation mechanisms in halide perovskites following optical spin injection. A clear understanding these mechanisms is of paramount importance for the development of perovskite optospintronics. One of the main mechanisms is the well-established Elliot-Yafet (EY) process which relaxes spins by momentum scattering. It is believed to be inefficient for conduction band electrons in wide-gap Ⅲ-Ⅴ semiconductors due to the rather weak spin-orbit coupling (SOC) and large band gap which reduces spin-mixing strength. We validated the EY mechanism in bulk CsPbBr3 by correlating the linear relationship between the momentum scattering time and spin lifetime (which decreases with temperature).
In confined NCs, the FSS levels serve as an additional spin relaxation channel even at room temperature, thus depolarizing the exciton spin efficiently. The exciton spin lifetime (i.e., longitudinal coherence time) increases with temperature in CsPbBr3 NC ensembles because of a motional narrowing process. The phenomenon of motional narrowing refers to the linewidth decrease of a resonant frequency due to the motion in an inhomogeneous system. By freezing the exciton motion at very low temperatures, the spin precesses along an effective magnetic field originating from the FSS. This can be understood in terms of the Zeeman effect where the field strength can also be deduced from the splitting energy. An upper limit of the dephasing rate can then be inferred from the quantum beating of spin-polarized excitons. Explicating these competing mechanisms, we present an clear physical picture of the exciton spin relaxation in CsPbBr3 NCs (Figure 2), which can be divided into three regimes: 1) the EY mechanism where the scattering with LO phonons depolarizes the spins at high temperatures; 2) the strongly-scattered motional narrowing mechanism where momentum scattering preserves the spin polarization; and 3) the quasi-scattering-free motional narrowing of spins, which yields oscillatory spin dynamics at low temperatures.
Looking ahead towards practical optospintronics, we aim to realize deterministic control over these excitonic quantum coherence with ultrafast laser pulses. For instance, the horizontal and vertical FSS states together with the biexciton state, could possibly be used in logic devices. With a clear understanding of these decoherence mechanisms, further spin engineering of LHP to suppress their spin relaxation rates can be envisaged.
- Becker, M.A. et al. Bright triplet excitons in caesium lead halide perovskites. Nature 553, 189-193 (2018).
- Dey, A. et al. State of the Art and Prospects for Halide Perovskite Nanocrystals. ACS Nano 15, 10775-10981 (2021).
- Cai, R. et al. Zero-Field Quantum Beats and Spin Decoherence Mechanisms in CsPbBr3 Perovskite Nanocrystals. Nat. Commun. 14, 2474 (2023).