The proposed photonic crystal mode converter is a guided structure that converts the optical mode of a PhC waveguide of any width to the mode of a larger or smaller guide, still operating on the fundamental mode. Within the periodic environment of holes of a photonic crystal matrix, it consists of a PhC waveguide section were the hole size and depth of the inner row of holes vary gradually. This variation synthesizes an artificial material with a gradient effective index.

This modal converter can be implemented in photonic integrated circuits.