Nonlinear optical response of a polycarbazolyldiacetylene film through femtosecond two-photon spectroscopy

We report the two-photon absorption (TPA) spectrum of a polycarbazolyldiacetylene film in the interval 900–1550 nm. In this range, the TPA coefficient exhibits a broad peak with a maximum of 33 cm/GW around 1125 nm, which indicates a two-photon state positioned at 2.2 eV, slightly below the one-photon state at 2.36 eV. The magnitude of the nonlinear refraction in the infrared is predicted on the base of a Three Level Model, which accounts fairly well for the nonlinear absorption in the explored range