Comparison and analysis of Mueller-matrix spectra from exoskeletons of blue, green and red Cetonia aurata

• Published in: Thin solid films Vol. 571; pp. 739 - 743
• Main Authors: Arwin, H., Fernández del Río, L., Järrendahl, K.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 28-11-2014; Elsevier
• Subjects: Chiral structures; Circular polarization; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Materials science; Mueller-matrix ellipsometry; Nanoscale materials and structures: fabrication and characterization; Natural photonic structures; Optical instruments, equipment and techniques; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of specific thin films; Other topics in nanoscale materials and structures; Physics; Polarimeters and ellipsometers; Scarab beetles; Chiral structures; Scarab beetles; Circular polarization; Natural photonic structures; Mueller-matrix ellipsometry; Multilayer; Refractive index; Mueller matrix; Regression analysis; Nanostructures; Ellipsometry; Structural models; Reflectivity; Multilayers; Reflection spectrum; Optical properties; Incidence angle; Matrix elements; Nanostructured materials; Structural analysis
• ISSN: 0040-6090; 1879-2731; 1879-2731
• DOI: 10.1016/j.tsf.2014.02.012

The exoskeleton, also called the cuticle, of specimens of the scarab beetle Cetonia aurata is a narrow-band reflector which exhibits metallic shine. Most specimens of C. aurata have a reflectance maximum in the green part of the spectrum but variations from blue–green to red–green are also found. A few specimens are also more distinct blue or red. Furthermore, the reflected light is highly polarized and at near-normal incidence near-circular left-handed polarization is observed. The polarization and color phenomena are caused by a nanostructure in the cuticle. This nanostructure can be modeled as a multilayered twisted biaxial layer from which reflection properties can be calculated. Specifically we calculate the cuticle Mueller matrix which then is fitted to Mueller matrices determined by dual-rotating compensator ellipsometry in the spectral range 400–800nm at multiple angles of incidence. This non-linear regression analysis provides structural parameters like pitch of the chiral structure as well as layer refractive index data for the different layers in the cuticle. The objective here is to compare spectra measured on C. aurata with different colors and develop a generic structural model. Generally the degree of polarization is large in the spectral region corresponding to the color of the cuticle which for the blue specimen is 400–600nm whereas for the red specimen it is 530–730nm. In these spectral ranges, the Mueller-matrix element m41 is non-zero and negative, in particular for small angles of incidence, implicating that the reflected light becomes near-circularly polarized with an ellipticity angle in the range 20°–45°.
•C. aurata exhibits reflection of left-handed circularly polarized light.•Degree of polarization and ellipticity of beetle reflections are quantified.•Analysis of Mueller-matrix ellipsometry data gives details of beetle cuticle structures.•A model with a chiral dielectric layer describes well the Mueller-matrix elements.