Spintronics has shown immense potential for applications in information encoding, storage, and transmission. The optical spin Hall effect has further expanded this field by paving the way for advancements in spin-photonic devices. Recently, the ultrafast spectroscopy team at the Beijing Academy of Quantum Information Sciences (BAQIS) achieved the room-temperature optical spin Hall effect of exciton-polaritons in a formamidinium lead bromide (FAPbBr₃) perovskite microcavity. This milestone establishes a strong foundation for the development of next-generation spin-photonic devices. The findings were published online on October 22, 2024, in Nature Materials under the title “Coherent Optical Spin Hall Transport for Polaritonics at Room Temperature”.

The spin Hall effect, a key phenomenon in spintronics, exploits spin-orbit coupling to separate electrons with opposite spin directions, generating a spin current and offering exciting opportunities for spin-based electronic devices. However, achieving macroscopic-scale pure spin currents has remained a major challenge due to rapid decoherence caused by electron scattering at room temperature. By designing high-quality FAPbBr₃ perovskite semiconductor microcavity devices, the research team addressed the issue of poor room-temperature stability often seen in traditional semiconductor materials. Additionally, this system avoided the linear splitting typically induced by the intrinsic anisotropy of materials, enabling the first-ever observation of the spin separation phenomenon of exciton-polaritons at room temperature. Professor Alberto Amo from the University of Lille, France, lauded this groundbreaking achievement in the same issue of Nature Materials:

"…researchers demonstrate the engineering of polarization transport of polaritons in microcavities at room temperature. To achieve this, the two groups use perovskite thin films as the active materials. The strong binding energy of bound electron–hole pairs or excitons in perovskites ensures strong light–matter coupling, leading to the existence of exciton–polaritons. Meanwhile their high crystalline quality enables the coherent propagation of polaritons over tens of micrometers … These observations at room temperature are a game changer for the development of polarization-based functionalities in photonics."

As electrically neutral quasiparticles, exciton-polaritons effectively mitigate decoherence caused by charge scattering, offering distinct advantages for spin current generation. Experimental results revealed that exciton-polariton spin currents can propagate over distances of up to 60 micrometers while preserving long-range coherence. Building on this discovery, the research team designed and demonstrated two innovative spin-photonic devices: a logic NOT gate and a spin-polarized beam splitter, both based on exciton-polaritons. These devices enable all-optical control on a picosecond timescale, significantly enhancing response speed and opening new avenues for the development of ultrafast spin-photonic technologies.

Figure 1: Spin separation phenomenon of exciton-polaritons in momentum space and real space in a FAPbBr₃ microcavity at room temperature. a, and c show experimental results, while b, and d present theoretical calculations. The experimental data are in excellent agreement with the theoretical results.

Figure 2: a, Schematic diagram of an exciton-polariton logic NOT gate. b, Experimental and theoretical results: Right-handed circularly polarized light resonantly excites the FAPbBr3 microcavity sample, and after transmitting approximately 80 micrometers, the exciton-polaritons undergo complete spin flip to left-handed circularly polarized light, completing a full spin reversal, thus performing the NOT gate operation. c, Schematic diagram of a spin-polarized beam splitter: When exciton-polaritons encounter a high energy potential barrier, they bypass the obstruction and propagate to both sides, splitting into two beams with opposite spin directions as outputs. d, Experimental and theoretical results of the spin-polarized beam splitter.

The paper's first authors are Ying Shi and Yusong Gan, doctoral students in the Department of Physics at Tsinghua University. The corresponding authors are Sanjib Ghosh, an associate researcher at BAQIS; Alexey Kavokin, a professor at Westlake University; and Qihua Xiong, a joint professor at BAQIS and Tsinghua University. Other key contributors include Yubin Wang, a doctoral student at Tsinghua University, and Yuzhong Chen, a postdoctoral researcher at BAQIS who completed his fellowship in January 2024. This research was supported by the National Natural Science Foundation of China, BAQIS, and other collaborating institutions.

Paper Link: https://doi.org/10.1038/s41563-024-02028-2