Optimized spectral and interferometric techniques for the certification of ETPA

TL;DR

This paper introduces spectral and interferometric methods to reliably certify Entangled Two-Photon Absorption (ETPA) by isolating its spectral signature and analyzing experimental detection limits, addressing longstanding measurement discrepancies.

Contribution

It proposes a novel approach combining spectral modeling and interferometry to distinguish ETPA from background effects, with a feasibility analysis and preliminary experimental validation.

Findings

Spectral distortion in transmitted joint spectral intensity indicates ETPA presence.

Detection limits depend on photon flux and detector noise levels.

Preliminary experiment with RhB dye demonstrates method potential.

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 $\sigma_E$ 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.