Photoreactive third-harmonic generation via either one- or two-photon excitation in diarylethene-polymethylmethacrylate polymer thin films: theory and experiment

Abstract

Photoreactive third-harmonic (TH) generation at 355 nm in diarylethene- polymethylmethacrylate (DE-PMMA) polymer thin films is obtained by either one- or two-photon excitation. TH intensity generated from a DE-PMMA polymer thin film decreases, when it is pumped by either 325 nm or 442 nm, which change molecular structure of DE molecules from open-form (A form) to closed-form (B form). TH intensity recovers to its original intensity level, after all B form DE molecules return to A form induced by 532 nm or 1064 nm laser irradiation. The experimental results reveal that the second-order hyperpolarizability (γ) of A form molecules may be larger than that of B form molecules. Moreover, TH output efficiency is independent of the angle between the pump and probe polarization directions. Those experimental results were explained by using a photoinduced isomerization theory based angular hole burning and angular redistribution mechanisms for two-dimensional structure of DE molecules including the two-photon absorption effect of B form. The simulation results are consistent with those of optical pumping TH experiment.