Study On The Characteristics Of Two Photon Absorption On Excited State Induced By Single Photon Absorption In Compounds NiNc(NCO)_ 4

Nonlinear optical properties, exhibited by the interaction between laser and materials, have important applications. Thus, the research on the nonlinear optical properties of materials and the measurement of optical nonlinear parameters is attracting wide attention. NCO free radical is one of important intermediates, which is produced from the Nitrogen-containing fuel combustion process. Due to its complexity—existing both veronica interaction (Renner-Teller Effect) and Fermi Resonance betweenν₁′and 2ν₂′, NCO free radical has become a focus for researchers.This thesis discussed the optical nonlinear absorption of the compound NiNc(NCO)₄, which included optical nonlinear conversion from saturable absorption to reverse saturable absorption and the measurement of optical nonlinearity absorption parameters. We used rate equation theory and the Z-scan method to study the optical nonlinear absorption properties of four kinds of three-level model, including the model of single photon absorption on ground state and excited state, the model of single-photon and two-photon absorption coexistence on ground state, the model of single-photon absorption on excited state induced by two-photon absorption on ground state and the model of two photon absorption on excited state induced by single photon absorption.The steady-state analysis on the above four models indicated that only the last one, the model of single-photon absorption induced two-photon absorption on excited state, had the conversion phenomena from saturable absorption to reverse saturable absorption. Dynamic analysis was implemented on the model of two-photon absorption on excited state induced by single-photon absorption, from which we found the conversion phenomena from reverse saturable absorption to saturable absorption is also exist. At last, we expanded on the reasons and conditions which resulted in these conversion.Through Z-scan experiment with NiNc(NCO)₄ compounds, compared with the data fit of the theoretic model of two-photon absorption on excited state induced by single-photon absorption, we found that the experimental curves agree well with the theoretical curves, demonstrating that the model of single-photon absorption induced two-photon excited state exist in NiNc(NCO)₄ compounds was effective.