Nonlinear Absorption Spectrum in MEH-PPV/Chloroform Solution:  A Competition between Two-Photon and Saturated Absorption Processes

Knowledge about the nonlinear absorption spectra of materials used in photonic devices is of paramount importance in determining their optimum operation wavelengths. It is well-know that resonances enhance the nonlinear response of a material, but in this case, slow excited states processes, propagation losses, and photodegradation can take place, which are known to be deleterious for photonic applications. With this concern, we have studied the nonlinear degenerate spectrum of poly(MEH-PPV) using femtosecond laser pulses from 460 to 1000 nm. Such a wide band spectrum has never been investigated in previous works reported in the literature. We were able to distinguish between a strong two-photon absorption (2PA) mechanism at nonresonant conditions and a saturated absorption (SA) that overcomes the 2PA process as we approach the absorptive region. The results were fitted with rate equations obtained from an energy level model.