Significance Biexcitons are exciton pairs which interact via Coulomb interactions and recombine to emit two photons. Compared with single excitons, biexcitons are achieved under a high excitation intensity. The physical model of the biexciton can be simplified as two electrons at the bottom of the conduction band and two holes at the top of the valence band. The luminescence process of biexcitons involves the cascade recombination of two excitons, and the intensity of the biexcitons shows a quadratic scale as a function of the excitation intensity.
Biexcitons were first observed in CuCl by Mysyrowicz et al. in 1968 and then extended to other semiconductor materials, including II-VI (ZnSe and CdS) and III-V (GaAs). In the 1980s, the study of biexcitons spread to low-dimensional semiconductors, such as quantum wells or quantum dots (QDs), resulting in the discovery of entangled photon pairs as well as predicted advances as a laser gain medium. In the last 40 years, QDs have been widely studied owing to their size-dependent optical properties and potential applications in photonics and optoelectronics. Owing to their well-known quantum confinement effects, QDs offer strong light absorption and a discrete electronic state structure. Therefore, biexcitons in QDs have unique features, including the large binding energy, correlated polarization of the emitted photons, low threshold,and enhanced temperature stability in lasing. These properties provide the advantages of realizing an on-demand entangled photon source and laser diodes.
Progress The entangled photons can be generated by biexciton-exciton cascade emission or Hong-Ou-Mandel (HOM) interference from indistinguishable photons in QDs (Fig. 5). The degenerate exciton state is a superposition state and emits an exciton photon with opposite circular polarization with respect to the polarization of previously emitted biexciton photons. In 2006, Stevenson et al. demonstrated entangled photon pairs from a single InAs QD. Subsequently, InAs QDs were optimized to achieve high-quality entangled photons with reduced exciton polarization splitting. In 2019, Pan et al. performed a Boson sampling experiment using 20 pure single photons generated by a single InAs/GaAs QD. These results are significant breakthroughs in the field of quantum information and have inspired increased efforts to explore QD-based quantum light sources.
In addition to the progress in self-assembled QDs, colloidal QDs have also received considerable attention because of their solution processability and easier spectral tunability. Because of the atom-like discrete structure of the electronic states in QDs, the population inversion is realized only when a fraction of QDs is excited with biexcitons (Fig. 7). The presence of biexcitons leads to an intrinsic nonradiative Auger recombination (Fig. 6). Owing to the efforts of Klimov et al., several strategies have been successfully developed to suppress Auger recombination by controlling the electronic structure of QDs. Recently, Sargent et al. used the long Auger lifetime of thick-shell CdSe/CdS QDs to realize continuous wave (CW) lasing. The observed thresholds were
Conclusions and Prospects Because biexcitons in QDs exhibit unique advantages in achieving entangled photon sources and quantum-dot lasers, exploring the photophysics of biexcitons in QDs is of great interest. Although significant progress has been made in recent years, there are still many challenges, such as increasing the number of entangled photons to enhance the quantum advantage, maintaining the high fidelity of entangled photons in long-distance transmission, and exploring new methods to suppress Auger recombination more effectively to realize electrically pumped colloid-quantum-dot lasers.
