Measurements of Entangled Two-Photon Absorption in Organic Molecules with CW-Pumped Type‑I Spontaneous Parametric Down-Conversion

Entangled photons exhibit strong nonclassical frequency and time correlations simultaneously, which allow them to excite and extract information about molecules in new ways compared to classical spectroscopy. In this report, we demonstrate the accessibility of entangled two-photon absorption (ETPA) as an analytical technique using CW-pumped type-I degenerate spontaneous parametric down-conversion. We made improvements to lower the noise, error, and limit of detection of the ETPA experiment. We prove and quantify frequency entanglement from the experimentally measured joint frequency spectrum using the Schmidt decomposition. As evidence of the ETPA process, we found a clear linear dependence of the ETPA and ETPA-induced fluorescence rates with the entangled input photon rate for all the studied chromophores. This ETPA experiment can be used to analyze a wide variety of chromophores of chemical and biological significance and shows potential for ETPA-induced fluorescence detection capabilities. As an application of our work, we show that one may control the population of specific excited states in molecules with the use of a spatial light modulator in the setup.