Butterfly Features Structurally Enhanced Fluorescence

Reporting in the Nov. 18 issue of Science, researchers at the University of Exeter in the UK describe how the scales on the wing of the African swallowtail butterfly Papilio nireus resemble high-efficiency LEDs. The blue and blue-green scales, they found, incorporate natural two-dimensional photonic crystals and distributed Bragg reflectors to boost the output of a fluorescent pigment in the wing.

The scales feature a pigment-infused 2-μm-thick slab of quasiperiodic 240-nm-diameter hollow cylinders. A three-layer distributed Bragg reflector lies below the photonic crystal. The structures respectively inhibit fluorescence in the plane of the crystal and reflect downwardly directed out-of-plane emission back up and out of the scale.

The researchers suggest that P. nireus employs the approach in signaling, converting radiation from the sky centered around 420 nm to 505-nm light near the peak sensitivity of its green photoreceptor.