SPAD-Array Characterization of Biexciton Emission from Single Colloidal Quantum Shells

Characterizing multiphoton emission is essential for understanding and developing quantum light sources. Emerging single-photon avalanche diode (SPAD) array detectors offer new opportunities but require tailored analysis strategies. Here we present such approaches for colloidal nanoparticles with long-lived multiexciton states, where multiple bright excitonic and charged states contribute to emission. Using CdS/CdSe/CdS quantum shells as a model system, we combine high-throughput characterization across more than 1000 particles with state-resolved spectroscopy. Double projection of the sample image onto the detector prevents crosstalk artifacts and improves data quality. High-throughput photon-correlation measurements on a frame-based SPAD-array camera quantify biexciton emission efficiency and reveal particle-to-particle variations. Reference time-gated measurements distinguish individual particles from clusters. An event-based linear SPAD array resolves bimodal biexciton spectra but reveals no strong blue shift relative to the exciton, in contrast to previous reports. Our study presents insights into multiphoton emission by colloidal quantum shells and experimental strategies for quantum-emitter characterization.