InxAl1−xN chiral nanorods mimicking the polarization features of scarab beetles

The scarab beetle Cetonia aurata is known to re ect light with brilliant colors and a high degree of circular polarization. Both color and polarization e ects originate from the beetles exoskeleton and have been attributed to a Bragg re ection of the incident light due to a twisted laminar structure. Our strategy for mimicking the optical properties of the Cetonia aurata was therefore to design and fabricate transparent, chiral lms. A series of lms with tailored transparent structures of helicoidal InxAl1−xN nanorods were grown on sapphire substrates using UHV magnetron sputtering. The value of x is tailored to gradually decrease from one side to the other in each nanorod normal to its growth direction. This introduces an in-plane anisotropy with di erent refractive indices in the direction of the gradient and perpendicular to it. By rotating the sample during lm growth the in-plane optical axis will be rotated from bottom to top and thereby creating a chiral lm. Based on Muellermatrix ellipsometry, optical modeling has been done suggesting that both the exoskeleton of Cetonia aurata and our arti cial material can be modeled by an anisotropic lm made up of a stack of thin layers, each one with its in-plane optical axis slightly rotated with respect to the previous layer. Simulations based on the optical modeling were used to investigate how pitch and thickness of the lm together with the optical properties of the constitutive materials a ects the width and spectral position of the Bragg re ection band.