  Organic semiconductors have attracted extensive attention due to their
excellent optical and electronic properties. Here, we present an experimental
and theoretical study of Brunel harmonic generation in two types of porphyrin
thin films: tetraphenylporphyrin (TPP) and its organometallic complex
derivative Zinc tetraphenylporphyrin (ZnTPP). Our results show that the
$\pi$-$\pi^\ast$ excitation of the porphyrin ringsystem plays a major role in
the harmonic generation process. We uncovered the contribution of an interband
process to Brunel harmonic generation. In particular, the resonant ($S_0
\rightarrow S_2$ transition) enhanced multiphoton excitation is found to lead
to an early onset of non-perturbative behavior for the 5th harmonic. Similar
resonance effects are expected in Brunel harmonic generation with other organic
materials.
