Phase-Adjusting Layers in the Multilayer Reflector of a Jewel Beetle

In this study, we investigated the iridescence of a jewel beetle Chrysochroa fulgidissima, which is one of the most common examples of structural color produced by multilayer optical interference. We performed detailed measurements and analyses of angle- and polarization-dependent reflection spectra and found that a few layers located near the elytron surface play an important role in the coloration mechanism: they adjust the phases of light waves reflected from the lower part of the multilayer structure such that the waves interfere constructively with the reflection at the top surface. This phase adjustment is crucial to the coloration mechanism since the top surface contributes the most to the overall reflection owing to a large refractive-index difference between air and the cuticle. We also found that the jewel beetle exhibits a green-color-dominant iridescence: the blue color observed at oblique angles looks much weaker than the green color at small angles. We elucidated the physical origins of this peculiar iridescence by performing detailed analyses of the multilayer interference.