Computational Study of Entangled Two-Photon Absorption in Organic Dyes

Entangled two-photon absorption (ETPA) offers promising advantages for quantum spectroscopy due to its linear dependence on photon flux and reduced photodamage. However, experimental limitations in distinguishing ETPA from competing one-photon processes have hindered broader applications. Here, we present a predictive computational framework based on time-dependent density functional theory (TD-DFT) that elucidates the fundamental mechanisms controling ETPA in organic chromophores. Using rhodamine 6G (R6G) and zinc tetraphenylporphyrin (ZnTPP) as model systems, our simulations reveal that nondegenerate photon pairs significantly enhance ETPA intensity when one of the photon energies approaches resonance with one-photon absorption (OPA) transitions-a finding with important implications for experimental design. Despite a systematic blue-shift ( 0.5 eV) typical of TD-DFT calculations, our results accurately predict spectral trends across energy ranges broader than those that can be experimentally accessed by spontaneous parametric down-conversion. Potentially, the presented computational ETPA calculations are able to provide critical guidance for experimental studies by narrowing the exploration space for nonlinear crystal selection and optimizing photon pair properties for ETPA investigations.