An omnidirectional broadband mirror design inspired by biological multilayer reflectors

Biological multilayer reflectors are common in nature and some are able to reflect light across a broad range of wavelengths with a low degree of polarization over all angles of incidence. This inspired us to examine theoretically possible mechanisms that would allow stacks of biological materials to produce broadband omnidirectional reflections. Through the application of anisotropic multilayer theory, we establish that the degree of polarization of light reflected from the structure can be neutralized by birefringent layers with variations in the orientation of their optics axis and random variations in their optical thickness. The degree of polarization of reflected light decreases with the number of crystal layers and can be made arbitrarily low to produce true omnidirectional reflection. We also show that systematic variation in orientation and layer thickness can produce the same effect. This reflective structure is distinct from existing omnidirectional mirrors and can produce omnidirectional reflection even if the refractive index of the external environment is same as the low index isotropic layers.