Measurement of the Entangled Two‐Photon Absorption Cross‐Sections of 4‐Aminoazobenzene

Recently, entangled two‐photon absorption (ETPA) has attracted considerable attention, yet reported cross‐sections vary widely. In this study, we focused on 4‐aminoazobenzene, an azobenzene derivative that exhibits extremely low one‐photon absorption in the wavelength range of time‐entangled photon pairs. By measuring the time correlation between the two photons transmitted through the sample solution and separating correlated from uncorrelated counts, we distinguished between ETPA and linear‐loss contributions. The ETPA cross‐section is only ≈10 7 ‐fold lower than the one‐photon absorption cross‐section and ≈10 18 ‐fold higher than the classical TPA cross‐section (evaluated under classical two‐photon excitation at a photon‐flux density of ≈10 8 photons·s –1 ·cm –2 ). Moreover, ETPA is measurable at a photon‐flux density of ≈10 8 photons·s –1 ·cm –2 .