Resonance Effect in Brunel Harmonic Generation in Thin Film Organic Semiconductors

Nonlinear optical properties of organic semiconductors (OSCs) have been extensively investigated in the perturbative regime, while strong light induced high-order processes in solid-state OSCs are less studied. Here, below-threshold harmonic generation is examined, both experimentally and theoretically, in two solid-state thin film OSCs, that is, tetraphenylporphyrin and zinc tetraphenylporphyrin. Results show that the pi-pi* excitations of the porphyrin ring system generate the harmonic emission. The contribution of the Brunel harmonic to the 5th harmonic emission is uncovered, where the resonant 5-photon transition (S-0 -> S-2 transition) is found to lead to an early onset of non-perturbative behavior. A similar resonance effect is expected in Brunel harmonic generation in other organic materials.

Automated summary

The below-threshold harmonic generation in solid-state organic semiconductors is explored, showing that the pi-pi* excitations of the porphyrin ring system generate the harmonic emission. The study also reveals the contribution of Brunel harmonic to the 5th harmonic emission and the role of resonant 5-photon transition in triggering non-perturbative behavior, suggesting similar resonance effects in other organic materials.