Efficient Upconverted Exciton–Polariton Lasing via Three-Photon Absorption in Colloidal Quantum Wells

Achieving low-threshold lasing under multiphoton excitation is challenging due to weak nonlinear absorption and the requirement for population inversion. We address this by employing colloidal quantum wells (CQWs). They exhibit large three-photon absorption (3PA) cross sections, strong optical gain, and robust excitonic properties, enabling exciton–photon strong coupling and inversion-free polariton lasing. Under 1030 nm femtosecond excitation, CdSe core and CdSe/CdS core/crown CQWs show photoluminescence and amplified spontaneous emission via 3PA. Embedding CdSe/CdS CQWs in Fabry–Pérot cavities yields pronounced polariton dispersions (Rabi splitting of ∼202 meV) and a sharp, blue-shifted emission peak at k = 0 under higher excitation, evidencing exciton–polariton lasing via 3PA. Compared to photonic 3PA lasers, our polaritonic system features a reduced threshold, enabled by bosonic final-state stimulation. This work presents the first upconverted exciton–polariton lasing in colloidal semiconductors, establishing CQWs as a powerful platform for low-threshold, upconverted lasers and nonlinear polaritonic devices.