Resonance Effect in Brunel Harmonic Generation in Thin Film Organic Semiconductors

摘要

Nonlinear optical properties of organic semiconductors (OSCs) have beenextensively investigated in the perturbative regime, while strong light inducedhigh-order processes in solid-state OSCs are less studied. Here,below-threshold harmonic generation is examined, both experimentally andtheoretically, in two solid-state thin film OSCs, that is, tetraphenylporphyrinand zinc tetraphenylporphyrin. Results show that the – * excitations of theporphyrin ring system generate the harmonic emission. The contribution ofthe Brunel harmonic to the 5th harmonic emission is uncovered, where theresonant 5-photon transition (S_0 → S_2 transition) is found to lead to an earlyonset of non-perturbative behavior. A similar resonance effect is expected inBrunel harmonic generation in other organic materials.