An optical device includes at least two materials forming a structure with
a graded bandgap where photocarriers are generated. A first of the at
least two materials has a larger concentration at opposed ends of the
graded bandgap structure than a concentration of the first of the at
least two materials at an interior region of the graded bandgap
structure. The second of the at least two materials has a larger
concentration at the interior region of the graded bandgap structure than
the concentration of the second of the at least two materials at the
opposed ends of the graded bandgap structure.