Selective ionization of Rydberg atoms to reduce the energy spread of a cold atom focused ion beam
The energy spread of a focused ion beam causes chromatic aberration that limits the focal spot size and resolution for imaging and fabrication. Ion beams based on photoionization of neutral atoms can have a much smaller energy spread and higher brightness than conventional liquid metal ion sources and are thus capable of higher resolution. We present a method for using selective ionization of Rydberg atoms to reduce the beam energy spread in a cold atom focused ion beam. We produce experimental maps of the ionization rate of Rubidium-85 near the classical ionization threshold, predict the energy spread of an ion beam produced from these states and demonstrate energy spread reduction in situ in a focused ion beam. We use a novel method to measure the energy spread, using only components present in many commercial FIB systems. Selecting different states changed the energy spread by up to 50%, with opportunities to further reduce the energy spread by constructing a more favorable electric field gradient and finding atomic states with better ionization characteristics.