Function photonic crystals

► We give a new kind of function photonic crystals, whose refractive index is a function of space position. ► Based on Fermat principle, we give the motion equations of light in function photonic crystals. ► Then we give the dispersion relation, band gap structure and transmissivity. ► For the vertical and non-vertical incidence light, there are band gap structures. ► By choosing different refractive indexes, we can obtain different width band gap structures.