Abstract
Fresnel zone plates are widely used for nanofocusing in x-ray
microscopy. The focusing performance is described in terms of the
resolution, related to the width of the smallest outermost zones, and
the efficiency, governed by the thickness of the zones and therefore
the amount of phase shift imparted onto the x-ray beam. The ratio of
zone thickness to width, or “aspect ratio,” is limited in all methods
of zone plate fabrication, requiring compromises between efficiency
and resolution. We propose a new zone plate design method, which
optimizes focusing efficiency within a set of practical constraints.
This phasor-based method is used to optimize subject to a maximum
aspect ratio the design of binary, multilevel, and kinoform zone
plates. A truncated zone plate profile is proposed, which focuses more
efficiently than binary or kinoform zone plates, with considerably
higher manufacturability. Focused ion beam milling was used to
fabricate the designed lenses in gold, and their relative efficiency
was validated at a synchrotron hard x-ray beamline. Our phasor method
provides rapid design optimization, producing the ultimate lens
designs for a given manufacturing limit, and it is fully generalizable
to incorporate any fabrication tolerances such as roughness, zone
displacements, and zone wall inclination.
Funder
Australian Research Council