Provided in one embodiment is an apparatus, comprising: an optical substrate configured to manipulate light received from a light source; and at least one diffractive element in one layer in the optical substrate, each diffractive element comprising: a plurality of sub-substrate structures separated from one another by a plurality of valleys, the sub-substrate structures comprising a material having a first refractive index; and a filler material filling at least partially at least some of the plurality of valleys, the filler material having a second refractive index that is unequal to the first refractive index.