Entangled two-photon absorption cross-section measurements for biologically important fluorophores

Fluorescence microscopy is a powerful tool to investigate biological processes. However, high excitation power required for nearly simultaneous two photon absorption can perturb or damage light-sensitive biological systems.. Unlike classical two photon excitation that requires high pulse power, excitation by entangled photon pairs holds promise for low light two photon excitation, In particular, photon pairs generated by SPDC are highly correlated in space and time. Detecting the location of one photon of a pair also determines the location of its twin. The simultaneous creation of two photons by SPDC ensures the simultaneous arrival of two photons for two-photon excitation of an atom or molecule. While many labs have explored the potential of entangled photon pairs for fluorescence excitation, most of these have used photons of the same energy/wavelength (degenerate photons). A particular emphasis in this work is exploiting non-degenerate entangled photon pairs for two-photon absorption measurements. Here, we explore frequency non-degenerate SPDC sources (signal photons at 545.3nm, idler photons at 1514.6nm) to produce correlated photons pairs and test their efficacy for two-photon excitation. Previous reports on entangled two-photon cross-sections found upper bounds within 10⁻¹⁷ − 10⁻¹⁸cm²/molecule. Here we detail our progress on measuring the entangled two-photon absorption cross-section for several fluorophores, including organic dyes, fluorescent proteins, and quantum dots with non degenerate entangled photon pairs.