A method of making a grating, the method including: providing a substrate, placing a first photoresist layer on the substrate, locating a second photoresist layer on the first photoresist layer, wherein a second exposure dose of the second photoresist layer is greater than a first exposure dose of the first photoresist layer; exposing the first photoresist layer and the second photoresist layer; developing the first photoresist layer and the second photoresist layer and removing an exposed photoresist to form a patterned photoresist layer and obtain an exposed surface of the substrate, wherein the patterned photoresist layer defines a plurality of top surfaces and a plurality of side surfaces, each adjacent top surface and side surface, and the exposed surface form a Z-type surface; depositing a preformed layer on the Z-type surface to form a Z-type structure; removing the patterned photoresist layer.