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

TL;DR

This paper presents a cavity-enhanced two-step photoionization method for ultracold rubidium beams, achieving higher ionization degrees and currents, with experimental validation and theoretical modeling for focused ion beam applications.

Contribution

It introduces a cavity-enhanced photoionization technique that improves ionization efficiency and current in ultracold rubidium beams for focused ion beam use, validated by experiments and simulations.

Findings

Achieved 170 pA beam current with 52 μm aperture.

Ionization degree and position distribution match theoretical predictions.

Estimated ion beam brightness of 1×10^7 A/(m^2 sr eV).

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 $\mu$m, a current of $\left(170\pm4\right)$ 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 $1\times10^7$ A/(m$^2\,$sr$\,$eV).