Resonance Effect in Brunel Harmonic Generation in Thin Film Organic Semiconductors

Abstract

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 π–π* 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 (S0 → S2 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.

TURBOMOLE: Today and Tomorrow

“… 89 Recently, the code has been extended to simulate high harmonic generation (HHG) spectra under intense laser pulses and was utilized to complement and analyze the pulse-induced electron dynamics in the organic semiconductor molecules tetraphenylporphyrin (TPP) and zinc-tetraphenylporphyrin (ZnTPP). 326 Figure 19 shows the good agreement of the experimental absorption and HHG spectra with the calculated ones. The difference in the first few harmonics is due to the fact that the experimental spectra contain contributions from the quartz substrate.…”

“…Our simulations combined with experiments showed that π → π* excitation plays a major role in the harmonic generation process in porphyrins. It was also discovered that resonant excitation leads to an early onset of nonperturbative behavior for the fifth harmonic, and similar effects are expected in Brunel harmonic generation with other organic materials …”

“…Previously, the implementation was used to simulate absorption spectra over a wide range of frequencies by perturbing the molecules with weak electric fields . Recently, the code has been extended to simulate high harmonic generation (HHG) spectra under intense laser pulses and was utilized to complement and analyze the pulse-induced electron dynamics in the organic semiconductor molecules tetraphenylporphyrin (TPP) and zinc-tetraphenylporphyrin (ZnTPP) Figure shows the good agreement of the experimental absorption and HHG spectra with the calculated ones.…”

“…(a) Experimental linear absorption spectra and (b) HHG spectra of 100 nm thick TPP and ZnTPP samples. (c) Calculated linear absorption spectra and (d) HHG spectra of TPP and ZnTPP molecules …”

Linking High-Harmonic Generation and Strong-Field Ionization in Bulk Crystals

“…Third, high harmonics have also been explained by intraband currents where the nonlinearity enters through the nonparabolic shape of the conduction (and valence) bands . Fourth, the nonlinearity that is inherent to the process of photoionization was proposed as a possible origin for harmonic generation. , Three possible ionization-related mechanisms have been discussed in the context of HHG. , Brunel harmonics, arising from the acceleration of quasi-free carriers excited by photoionization, have been analyzed in gases, clusters, bulk solids, and thin films. − Moreover, after nonresonant photoionization an excited electron has nonzero velocity, which provides subcycle contributions to the polarization and gives rise to the generation of harmonics. However, the strength of these velocity harmonics is expected to be insignificant under typical experimental conditions due to the modest photon energies that result in small excess velocities.…”