Gradient refractive indices enable squid structural color and inspire multispectral materials

The manipulation of light by means of materials with varying refractive index distributions is widespread among natural systems and modern technologies. However, understanding how animals leverage refractive index differences for dynamic color changes and then translating such insight into tunable optical devices remains challenging. We experimentally and computationally demonstrated that iridescent cells (iridophores) containing Bragg reflectors with sinusoidal-wave (rugate) refractive index profiles enable squid dorsal mantle tissues to reversibly transition between nearly transparent and vibrantly colored states. We then drew inspiration from these findings for the design and development of iridophore-inspired multispectral composite materials with tunable visible and infrared functionalities. Our study provides insight into squid dynamic structural coloration mechanisms and furnishes a technology for camouflage, heat management, display, and sensing applications.