An optical device is disclosed. The optical device includes a substrate having an optically transparent grating layer disposed thereon, a recess formed in the grating layer, the recess having a depth that changes from a first end of the recess to a second end of the recess, and a plurality of slanted grating structures formed in the grating layer below the recess and partially defining a portion of the recess. A depth of the plurality of slanted grating structures changes from the first end to the second end, and an angle of each of the plurality of slanted grating structures relative to a plane normal to the substrate is between about 25 degrees and 45 degrees.