Designing two-photon molecular emitters in nanoparticle-on-mirror cavities.

Two-photon spontaneous emission (TPSE) is a second-order quantum process with promising applications in quantum optics that remains largely unexplored in molecular systems, which are usually very inefficient emitters. In this work, we model the first molecular two-photon emitters and establish the design rules, highlighting their differences from those governing two-photon absorbers. Using both time-dependent density functional theory and Pariser-Parr-Pople calculations, we calculate TPSE in three π-conjugated molecules and identify a dominant pathway. To overcome the inherently low TPSE rates in vacuum, we propose plasmonic nanoparticle-on-mirror cavities, engineered for degenerate TPSE. Our simulations reveal over 10 orders of magnitude enhancement and radiative efficiencies exceeding 50%. Notably, for nitro-substituted phenylene vinylene in an optimized nanocone-on-mirror structure, the two-photon emission rate surpasses that of vacuum one-photon emission from a unit dipole. These findings open new avenues for efficient and molecular-based on-demand sources of entangled photon pairs.