Uniformly thick tri-color filters capitalizing on an etalon with a nanostructured cavity

Highly efficient tri-color filters having uniform thickness are proposed based on an etalon tapping into a nanostructured cavity, where a hexagonal lattice of nanopillars (NPs) with a high refractive index is embedded in a base material with a low index. The nanostructured cavity is presumed to behave as a homogeneous medium, which provides a wide range for the effective refractive index in accordance with both the volume fraction of the NPs and the index contrast between the NPs and the base. Hence, for the etalon-based filters, the resonance wavelength can be efficiently tuned by simply altering the effective index rather than the thickness of the cavity, so as to span the entire visible regime including red, green, and blue (RGB) colors. In particular, a hexagonal lattice of NPs was introduced to extend the available range of effective index due to its highly flexible volume fraction. The NP-base index contrast has been pertinently maximized to achieve effective indices leading to RGB colors, to the extent that the nanostructured cavity can be safely modeled as a homogeneous medium. The proposed RGB color filters were finally designed to have an identical thickness of 240 nm by setting the diameters of the NPs at 95, 70, and 40 nm, to achieve a periodicity of 100 nm, considering that TiO2 and Al2O3 can be practically selected as the material candidate for the NPs and the base, respectively. A high transmittance of ∼78% and a suitable 1 dB bandwidth of ∼51  nm were obtained for the tri-color filters, which were further confirmed to exhibit polarization-independent transfer characteristics.