Opto-thermal modulation in biological photonic crystals

Photonic structures of biological origin have been well studied for their optical and morphological properties, but light-induced effects have not yet been explored. In this study, we report sizeable modulation in reflectance on iridescent areas of the wing of a butterfly Morpho didius. Chitin, the primary constituent of the sample, exhibits the large thermo-optic effect typically seen in biopolymers. Measurements yield a thermo-optic coefficient of dn/dT = −4.7 × 10− 4 °C− 1. Relatively low intensity levels (~ 0.05 W/cm2) are therefore required to induce measurable reflectance changes, which take place only in the iridescence spectral range and only in areas where chitin is periodically textured. This confirms that the effect originates from photonic band-shifting. For comparison purposes, Kerr nonlinearity is also investigated in chitin and chitosan films.

Research highlights
► We study the optical properties of a butterfly wing (Morpho didius).
► The wing acts as a photonic crystal with a reflection peak in the blue.
► Chitin, the main constituent of the wing, has a high opto-thermal coefficient.
► Laser light modulates the optical properties of chitin.
► We observe a modulation of the photonic band as a result.