Charged Particle Lithography for the Fabrication of Nanostructured Optical Elements

Abstract

This thesis work focuses on nanostructured optical elements for light and matter waves that have been fabricated using helium ion beam lithography and electron beam lithography. The motivation of this thesis has been to develop new optical elements and to contribute with foundational work to instrumentation and characterization of nanostructures. The work has been carried out at the University of Bergen, Nanostructure Laboratory and at the Massachusetts Institute of Technology, Nanostructure Laboratory. The thesis is based on five papers published in international, peer reviewed, Web of science journals. The thesis defender is sole first author on paper I-IV and shared first author on paper V. Paper I presents the first helium ion beam lithography patterning on a non-horizontal surface. Such patterning is possible because of the large field of depth in a helium ion beam instrument. Comparable writing cannot be performed with standard electron beam lithography. Patterning on curved or tilted surfaces is potentially very useful in a range of devices e.g. optical lenses, and is fundamentally an attractive property. Paper II presents a systematic scanning-electron-microscopy study of the charging effect in metal nanostructures on insulating surfaces. Negative charging is found to induce a measurement error in the measured dimensions of the nanostructures comparable to a de-magnified image. In paper III, the optical response of metal nanoparticles mediated by the localized surface plasmon resonance effect are studied using integrating spheres, and the influence of the fabrication method on the optical properties is discussed. Paper IV and V describe optical elements for matter waves. In Paper IV a high-transmission atom sieve for focusing neutral helium atoms is fabricated, showing that focusing below 10 nm should in principle be possible. Paper V demonstrates fast resolution change in the focusing neutral helium microscope by inserting collimating apertures. Without changing the properties of the neutral helium beam and without breaking the vacuum a resolution change by a factor of 4.4 is demonstrated.