Optimized spectral and interferometric techniques for the certification of ETPA

Abstract: The phenomenon of Entangled Two-Photon Absorption (ETPA) presents a persistent controversy in the literature, evidenced by a wide disparity in the reported values for the $σ_E$ cross-sections. Much of this discrepancy is attributed to the difficulty in discriminating ETPA from various background processes that can mimic its signal, such as linear absorption or scattering. Given this need to certify the presence of ETPA unequivocally, this work introduces a key strategy to isolate the ETPA contribution through its spectral signature. This involves modeling the molecule as a two-photon notch filter and inducing a controlled asymmetric overlap with the joint spectral intensity (JSI) of the incident photons. This asymmetry is used to generate a measurable distortion in the transmitted JSI, and, complementarily, as a reduction in the visibility of the Hong-Ou-Mandel (HOM) dip. To ensure the experimental feasibility of this technique, a comprehensive analysis of the experimental conditions required for its detection is presented, establishing the absorption efficiency limits that must be overcome given the constraints of entangled-photon-pair flux and detector noise. A preliminary experiment addressing the ETPA detection limits using the standard RhB dye is presented.