Polarization-sensitive Color In Butterfly Scales: Structural Origin And Polarization Mechanism

Diverse biological microstructures that result in advanced optical effects. Polarization-sensitive color is one interesting phenomenon resulting from photonic structures in organisms. Polarization-sensitive color originates from polarization-dependent reflection or transmission, exhibiting abundant light information, including intensity, spectral distribution, and polarization. A wide range of butterflies are physiologically sensitive to polarized light, but the origins of polarized signal have not been fully understood.Here we firstly report a combined architecture comprised of upper deep grating and bottom multilayer in butterfly iridescent scales that leads to the polarization-sensitive color related with scale azimuth. The polarization process is unraveled through detailed investigations on the green scales in butterfly Ornithoptera priamus poseidon and the orange scales in butterfly Ornithoptera croesus lydius. The bright green and orange scales result from the interaction between the transmitted 0th order diffraction of upper grating and the 1st order interference of the bottom multilayer. Combining experiment with calculation, we clarify the structural origin and the mechanism of the polarization conversion. Tapered grating achieves form-birefringence rotating polarized light, and the multilayer selectively reflects the incident light dominating the reflection color. The height of the grating, which determines the phase difference of the two decomposed polarized lights, is essential to the polarization conversion.In order to further reveal the physical origins of polarization-sensitive color, we systematically investigate the colorful scales of six species of butterfly. Microscopic optical images under crossed polarizers exhibit their polarization-sensitive characteristic, and micro-structural characterizations clarify their structural commonality. In the case of the structural scales that have deep ridges, the polarization-sensitive color related with scale azimuth is remarkable. Periodic ridges lead to the anisotropic effective refractive indices in the parallel and perpendicular grating orientations, which achieves form-birefringence, resulting in the phase difference of two different component polarized lights. Simulated results show that ridge structures with reflecting elements reflect and rotate the incident p-polarized light into s-polarized light. The dimensional parameters and shapes of grating greatly affect the polarization conversion process, and the triangular deep grating extends the outstanding polarization conversion effect from the sub-wavelength period to the period comparable to visible light wavelength.The parameters of ridge structures in butterfly scales have been optimized to fulfill the polarization-dependent reflection for secret communication. The structural and physical origin of polarization conversion provides a more comprehensive perspective on the creation of polarization-sensitive color in butterfly wing scales. These findings show great potential in anti-counterfeiting technology and advanced optical material design.