Chirality-induced polarization effects in the cuticle of scarab beetles: 100 years after Michelson

Abstract

One hundred years ago Michelson discovered circular polarization in reflection from beetles. Today a novel Mueller-matrix ellipsometry setup allows unprecedented detailed characterization of the beetles' polarization properties. A formalism based on elliptical polarization for description of reflection from scarab beetles is here proposed and examples are given on four beetles of different character: Coptomia laevis – a simple dielectric mirror; Cetonia aurata – a left-hand narrow-band elliptical polarizer; Anoplognathus aureus – a broad-band elliptical polarizer; and Chrysina argenteola – a left-hand polarizer for visible light at small angles, whereas for larger angles, red reflected light is right-handed polarized. We confirm the conclusion of previous studies which showed that a detailed quantification of ellipticity and degree of polarization of cuticle reflection can be performed instead of only determining whether reflections are circularly polarized or not. We additionally investigate reflection as a function of incidence angle. This provides much richer information for understanding the behaviour of beetles and for structural analysis.

Keywords:

• scarab beetles
• Mueller-matrix ellipsometry
• elliptical polarization
• structural colours

Acknowledgments

Financial support was obtained from the Knut and Alice Wallenberg foundation and the Swedish Research Council. The Museum of Natural History in Stockholm is acknowledged for the loan of beetles. Jens Birch is acknowledged for providing beetle images.

Notes

Note

1. In many reports discussing polarizing phenomena in scarab beetles, the term “circular polarization” is not well defined and often refers to the phenomenon that a beetle appears to have different colour when viewed through a left-hand and a right-hand polarizer. We will here use a stricter definition and use the term “circular polarization” only when the polarization ellipse is circular as further detailed in the theory section.