The two-photon excitation and recording peculiarities of a femtosecond photon echo in zinc oxide films of nanoscale thickness

This work reports on the experiments on the two-photon excitation of a primary photon echo femtosecond signal in a zinc oxide semiconductor film of nanoscale (100, 600, and 800 nm) thickness at the quantum transition (corresponding to 400 nm) at room temperature. The photon echo was observed on free excitons. The echo signal partially weakened due to the scattering on the optical phonons was recorded at a wavelength of 800–840 nm. The nano-optical effect of the reduction of the environment irreversible transverse relaxation time ($T_2$) with the decrease of the film thickness was detected. The established fact is connected, firstly, with the structural characteristics of the films under study and, secondly, with the decrease in the intensity of the longitudinal interactions between the excited particles along with the reduction of the film thickness.