Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams

Abstract

A two-step photoionization strategy of an ultracold rubidium beam for application in a focused ion beam instrument is analyzed and implemented. In this strategy the atomic beam is partly selected with an aperture after which the transmitted atoms are ionized in the overlap of a tightly cylindrically focused excitation laser beam and an ionization laser beam whose power is enhanced in a build-up cavity. The advantage of this strategy, as compared to without the use of a build-up cavity, is that higher ionization degrees can be reached at higher currents. Optical Bloch equations including the photoionization process are used to calculate what ionization degree and ionization position distribution can be reached. Furthermore, the ionization strategy is tested on an ultracold beam of 85 Rb atoms. The beam current is measured as a function of the excitation and ionization laser beam intensity and the selection aperture size. Although details are different, the global trends of the measurements agree well with the calculation. With a selection aperture diameter of 52 µm, a current of (170 ± 4) pA is measured, which according to calculations is 63% of the current equivalent of the transmitted atomic flux. Taking into account the ionization degree the ion beam peak reduced brightness is estimated at 1 × 10 7 A/(m 2 sr eV).

Rubidium focused ion beam induced platinum deposition

“…The development of the FIB system was carried out in our laboratory and is also included in these theses. 13,14 It mainly incorporates two sections, a laser-cooled Rb þ ion source 11 on top of an FEI FIB200TEM column. Neutral Rb atoms are first heated to 160 C in a Knudsen cell and then collimated in a heated thin tube.…”

Brightness measurements of the nano-aperture ion source

“…Making bright, monoenergetic ion beams using laser cooling was proposed by Freinkman et al 7 The ultracold ion source 8 and the magneto-optical trap ion source 9,10 are examples of this idea. Simulations [11][12][13] and some experimental work 14,15 of an improved version of this source, using an atomic beam rather than relying on diffusion, look promising.…”