Nonlinear optical nonreciprocity in a surface plasmon–exciton coupled asymmetric cavity system Shengfa Fan, Fengxue Zhou, Fei Xu, Yihong Qi, Yueping Niu and Shangqing Gong Eur. Phys. J. D, 76 11 (2022) 212 Published online: 07 November 2022 DOI: 10.1140/epjd/s10053-022-00541-0