Fig. 1

Schematic of biexciton and formation process. a Schematic diagram of exciton and biexciton transitions. Exciton has a bright state which can arrive in the ground state by radiation or non-radiation transitions. The corresponding dark state coupled though a spin–flip process arrives in the ground state mainly by non-radiation transitions. The dark state energy may be higher than that of the exciton due to the large Rashba spin-orbital effect. b The biexciton levels are formed by a four-level system: biexciton level, two bright exciton levels, and ground state. The two bright exciton levels are separated by fine structure splitting. The biexciton decays through a cascade process of either emitting two horizontally or vertically polarized photons. c Schematic diagram of biexciton formation process with nonlinear Fano effect. Nanoplatelets (NPLs) absorb one electron–hole excitation to the exciton state (I) and then reach the biexciton state (II) by absorbing another one, while the electron–hole excitations in metal relax to the ground state. d Schematic view of the structure. The cavity is composed of a Ag nanowire and Ag substrate separated by a SiO₂ gap of 5 nm and perovskite nanoplatelets. e Schematic view of the exciton and biexciton in the cavity. The NPL is covered with 4-Methyl-1-acetoxycalix[6]arene molecular. f Cross-sectional scanning electron microscope (SEM) image of the sample. The scale bar is 20 nm