S.H.W. Wouters

Performance predictions for a laser-intensified thermal beam for use in high-resolution focused-ion-beam instruments

Photoionization of a laser-cooled and compressed atomic beam from a high-flux thermal source can be used to create a high-brightness ion beam for use in focused-ion-beam instruments. Here we show using calculations and Doppler cooling simulations that an atomic rubidium beam with an equivalent brightness of 2.1 × 107 A/(m2 sr eV) can be created using a compact 5 cm long two-dimensional magneto-optical compressor. If this can be conserved during the photoionization process, this leads to an ion beam brightness an order of magnitude higher than produced by a liquid metal ion source. The source is also capable of producing a flux of 6.2 × 109 s-1 that results in a substantial beam current of 1 nA once fully ionized.

Direct magneto-optical compression of an effusive atomic beam for application in a high-resolution focused ion beam

An atomic rubidium beam formed in a 70-mm-long two-dimensional magneto-optical trap (2D MOT), directly loaded from a collimated Knudsen source, is analyzed using laser-induced fluorescence. The longitudinal velocity distribution, the transverse temperature, and the flux of the atomic beam are reported. The equivalent transverse reduced brightness of an ion beam with properties similar to the atomic beam is calculated because the beam is developed to be photoionized and applied in a focused ion beam. In a single two-dimensional magneto-optical trapping step, an equivalent transverse reduced brightness of (1.0+0.8−0.4)×106  A/(m2 sr eV) is achieved with a beam flux equivalent to (0.6+0.3−0.2)  nA. The temperature of the beam is further reduced with an optical molasses after the 2D MOT. This optical molasses increases the equivalent brightness to (6+5−2)×106  A/(m2 sr eV). For currents below 10 pA, for which disorder-induced heating can be suppressed, this number is also a good estimate of the ion-beam brightness that can be expected. Such an ion-beam brightness would be a 6× improvement over the liquid-metal ion source and could improve the resolution in focused ion-beam nanofabrication.

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

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 85Rb 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 52μm, 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 1×107 A/(m2sreV).

Design and experimental validation of a compact collimated Knudsen source

In this paper, the design and performance of a collimated Knudsen source, which has the benefit of a simple design over recirculating sources, is discussed. Measurements of the flux, transverse velocity distribution, and brightness of the resulting rubidium beam at different source temperatures were conducted to evaluate the performance. The scaling of the flux and brightness with the source temperature follows the theoretical predictions. The transverse velocity distribution in the transparent operation regime also agrees with the simulated data. The source was tested up to a temperature of 433 K and was able to produce a flux in excess of 10(13) s(-1).