Femtosecond time‐resolved two‐photon absorption resonant four‐wave mixing in three‐dimensional spiropyran–PMMA samples

The potential of four‐wave mixing with two‐photon absorption resonances for time‐resolved studies of ultrafast processes triggering photochromic transformations in three‐dimensional spiropyran–poly(methyl methacrylate) samples is demonstrated. Our experimental approach involves using 100 fs pulses of 800 nm fundamental radiation of a Ti:sapphire laser as a two‐photon absorption resonant pump and 130 fs frequency‐tunable pulses of an optical parametric amplifier as a time‐delayed probe. The use of two‐photon resonances allows strong one‐photon absorption, typical of conventional pump–probe experiments, to be avoided, giving access to any point inside a 3D photochromic sample.