  Coherent nonlinear optical micro-spectroscopy is a frequently used tool in
modern material science, as it is sensitive to many different local
observables, which comprise, among others, crystal symmetry and vibrational
properties. The richness in information, however, may come with challenges in
data interpretation, as one has to disentangle the many different effects like
multiple reflections, phase jumps at interfaces, or the influence of the
Guoy-phase. In order to facilitate interpretation, the work presented here
proposes an easy-to-use semi-analytical modeling ansatz, that bases upon known
analytical solutions using Gaussian beams. Specifically, we apply this ansatz
to compute nonlinear optical responses of (thin film) optical materials. We try
to conserve the meaning of intuitive parameters like the Gouy-phase and the
nonlinear coherent interaction length. In particular, the concept of coherence
length is extended, which is a must when using focal beams. The model is
subsequently applied to exemplary cases of second-harmonic and third-harmonic
generation. We observe a very good agreement with experimental data and
furthermore, despite the constraints and limits of the analytical ansatz, our
model performs similarly well as when using more rigorous simulations. However,
it outperforms the latter in terms of computational power, requiring more than
three orders less computational time and less performant computer systems.
