  We describe the formation, characterization and theoretical understanding of
microlenses comprised of alternating polystyrene and polymethylmethacrylate
layers produced by multilayer coextrusion. These lenses are fabricated by
photolithography, using a grayscale mask followed by plasma etching, so that
the refractive index alternation of the bilayer stack appears across the radius
of the microlens. The alternating quarter-wave thick layers form a
one-dimensional photonic crystal whose dispersion augments the material
dispersion, allowing one to sculpt the chromatic dispersion of the lens by
adjusting the layered structure. Using Huygen's principle, we model our
experimental measurements of the focal length of these lenses across the
reflection band of the multilayer polymer film from which the microlens is
fashioned. For a 56 micron diameter multilayered lens of focal length 300
microns, we measured a nearly 25 percent variation in the focal length across a
shallow, 50 nm-wide reflection band.
