An optical sensor including a first material layer comprising at least a first material; a second material layer comprising at least a second material that is different from the first material, where a material bandgap of the first material is larger than a material bandgap of the second material; and a graded material layer arranged between the first material layer and the second material layer, the graded material layer comprising an alloy of at least the first material and the second material having compositions of the second material that vary along a direction that is from the first material to the second material.