Nature Materials recently published online the latest research findings of Professor Liu Xiaoze from the School of Physics and Technology. The paper is entitled Nonlinear alley phonon scattering under the strong coupling regime.

The first authors of the paper are Professor. Liu Xiaoze and Dr. Yi Jun of the University of California at Berkeley. The corresponding authors are professors of the University of California at Berkeley and Professor Zhang Xiang, president of the University of Hong Kong. The research partners also include Professor Tian Zhongqun and Professor Li Jianfeng from Xiamen University, and Professor Li Yixian from National Tsing Hua University. Wuhan University is the co-signed unit of the paper.

Exciton polaritons, which are the new quasi-particles under the strong coupling regime of semiconductor optical cavity, are one of the main research objects of quantum studies such as Bose-Einstein Condensation (BEC), superfluids and photon blocking. In all these quantum studies, phonons of lattice vibration in solids play a key role. For example, phonons directly determine the boundaries of the BEC phase diagram of exciton polariton. They can also strengthen the coherence of exciton polariton and the nonlinearity of single photon. However, due to the complex non-radiative property of phonons, the study of the interaction between phonons and exciton polaritons is still at the stage of indirect speculation and theoretical prediction.

This work directly detects nonlinear valley phonon scattering through a strong coupling regime between a two-dimensional semiconductor and a plasmon nanocavity (see the figure below), demonstrating the coherence of nonlinear phonons enabled by the strong coupling. The study illustrates the key role of phonons in exciton polariton BEC and photon blocking. According to the circular polarization selectivity of valley degrees of freedom in two-dimensional semiconductors, the nonlinear scattering of valley phonons has coherent characteristics, which enhances valley polarization and protects it from other decoherence processes.

Nonlinear valley phonon scattering in two-dimensional semiconductor nanocavity under strong coupling regime. a.Schematic diagram of exciton polaritons under strong coupling regime; b.On the left is the phonon scattering schematic under strong coupling. The right side is a schematic of different valley phonon scattering.

These findings can be extended to more complex electronic, phonon and optical interactions, help develop valley electronics applications and provides a new perspective for the study of Bose Einstein condensation and quantum coherence of single photon sources in the framework of cavity quantum electrodynamics.

Link to the paper：https://www.nature.com/articles/s41563-021-00972-x

Written by Zhang Yifei

Rewritten by Shen Yutian

Edited by Zhu Tong