The wings of tropical Morpho butterflies display vivid iridescent coloration, which is not due to pigmentation but is the result of complex interference of light within their surface microstructure. In spite of extensive studies on this subject, a satisfactory model has not yet been developed. We suspect that a full three-dimensional modelling approach will show similarities with 3D photonic crystals.
In previous studies, Morpho-type diffractive structures were modelled as one-dimensional dielectric stacks. Although agreement was found with the experimental data from the actual butterfly wing for normal incidence and reflection, the model proved inaccurate for predicting non-normal reflection.
When considering distributed dielectric stacks using scalar grating there is little improvement. Reflected light shows highly directional diffraction maxima in line with dimensions of the natural grating. This is in contrast to the Morpho-wing, for which the angular dependence of the reflected intensity is different.
In this study we show that a simple diffractive model is inadequate due to the fact that lateral interference between neighbouring diffractive elements is not taken into account. We suggest that a two-dimensional super-cell plane wave approach would be more appropriate, and show how it might be implemented.