Fabrication of large free-standing nanostructured zone plates for focusing a Bose-Einstein condensate

Abstract

Matter wave optics is a fascinating area of physics touching many scientific disciplines ranging from microscopy to high precision gravity measurements. One of the major recent breakthroughs was the realization in 1995 of a Bose-Einstein condensate of atoms - a perfectly coherent atom source. Bose-Einstein condensates have been studied intensely over the last couple of decades, but surprisingly little work has been done on manipulation of the condensates using their de Broglie wavelength. The original aim of this thesis was to create a Fresnel zone plate for the first focusing of a Bose-Einstein condensate via the de Broglie wavelength. The zone plate design parameters were chosen to fit the experimental setup of Prof. Zeilinger in Vienna, currently one of the only setups in the world where this experiment can be carried out. The original aim of the thesis has been fulfilled. A free-standing silicon nitride zone plate with a diameter of 5 mm, and a focal length of 60 cm for a wavelength of 100 nm has been successfully created, tested with laser light and is now ready for experiments in Vienna. The zone plate has 44 zones, a 2 mm central stop and the width of the outermost zone is 12 µm. To the best of our knowledge this zone plate has a diameter more than an order of magnitude larger than any zone plates previously created for the focusing of matter waves. The much larger wavelength of the Bose-Einstein condensate compared to previous experiments meant that previously used fabrication methods could not be applied and new methods had to be developed as part of this thesis. Aluminium wet etching, dry-etching of SiO2 and silicon nitride using only a PMMA mask and quantified wet-etching of silicon were all established for the first time at the UiB NanoStructures laboratory as part of this thesis.