Creating Dark Lines in Space with Linear Zone Plates


We investigated dark focal lines created by linear zone plates. Zone plates are diffractive optical elements capable of focusing all electromagnetic radiation even x-rays. There were four species of linear zone plates in this project, two normal zone plates and two with a π-phase jump, both of which were tested on binary and sinusoidal zone plates. Transmittance equations were derived and plotted in Mathematica to obtain the designs of the zone plates. These designs were sent to Darkroom Specialties LLC for manufacturing, which printed them onto 35mm transparent film and placed them in plastic frames. The normal binary and sinusoidal zone plates were expected to produce bright foci, and the zone plates with the π-phase jump were expected to produce dark foci. We also expected the sinusoidal zone plates to produce sharper foci than their binary counterparts. An attenuated and collimated laser beam illuminated the zone plates, and the resulting interference pattern was projected directly onto a lens-less CCD camera. We compressed the images with XV image editor, and plotted the intensity distributions with GNUplot. In general it was observed that the normal sinusoidal zone plate produced a finer and sharper bright focal line than the binary zone plate. The binary zone plate with a π-phase jump produced a dark line as expected, however the sinusoidal variation failed to do so. This could perhaps be attributed to a lack of quality in the design or manufacture of the zone plate.