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Physical Review Letters

Phase-Space Manipulation of Ultracold Ion Bunches with Time-Dependent Fields

M.P. Reijnders, Nicola Debernardi, S.B. van der Geer, Peter H. A. Mutsaers, Edgar J. D. Vredenbregt, O.J. Luiten

Published Jul 15, 2010• 17 citations
Publisher

Abstract

All applications of high brightness ion beams depend on the possibility to precisely manipulate the trajectories of the ions or, more generally, to control their phase-space distribution. We show that the combination of a laser-cooled ion source and time-dependent acceleration fields gives new possibilities to perform precise phase-space control. We demonstrate reduction of the longitudinal energy spread and realization of a lens with control over its focal length and sign, as well as the sign of the spherical aberrations. This creates new possibilities to correct for the spherical and chromatic aberrations which are presently limiting the spatial resolution.

DOI
10.1103/physrevlett.105.034802

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The ultra-cold ion (UCI) source is based on creating very cold ion beams by nearthreshold\nphoto-ionization of a laser-cooled and trapped atomic gas. The UCI source has\nthe potential of producing ion beams with a brightness and current comparable to the\nliquid-metal ion source (LMIS) [1], which is the current state-of-art for focused ion beam\n(FIB) technology. We have already shown [2] that the UCI source can provide much\nlower energy spread than LMIS, and may therefore offer a route toward 1-nm ion beam\nmilling [3].

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All applications of high brightness ion beams depend on the possibility to precisely manipulate the trajectories of the ions or, more generally, to control their phase-space distribution. We show that the combination of a laser-cooled ion source and time-dependent acceleration fields gives new possibilities to perform precise phase-space control. We demonstrate reduction of the longitudinal energy spread and realization of a lens with control over its focal length and sign, as well as the sign of the spherical aberrations. This creates new possibilities to correct for the spherical and chromatic aberrations which are presently limiting the spatial resolution.
The combination of an ultracold ion source based on photoionization of a laser-cooled gas and time-dependent acceleration fields enables precise manipulation of ion beams. We demonstrate reduction in the longitudinal energy spread and transverse (de)focusing of the beam by applying time-dependent acceleration voltages. In addition, we show how time-dependent acceleration fields can be used to control both the sign and strength of the spherical aberrations. The experimental results are in close agreement with detailed charged particle tracking simulations and can be explained in terms of a simple analytical model.
At present, the smallest spot size which can be achieved with state-of-the-art focused ion beam (FIB) technology is mainly limited by the chromatic aberrations associated with the 4.5 eV energy spread of the liquid-metal ion source. Here we numerically investigate the performance of an ultracold ion source which has the potential for generating ion beams which combine high brightness with small energy spread. The source is based on creating very cold ion beams by near-threshold photoionization of a laser-cooled and trapped atomic gas. We present ab initio numerical calculations of the generation of ultracold beams in a realistic acceleration field and including all Coulomb interactions, i.e., both space charge effects and statistical Coulomb effects. These simulations demonstrate that with existing technology reduced brightness values exceeding 105 A m−2 sr−1 V−1 are feasible at an energy spread as low as 0.1 eV. The estimated spot size of the ultracold ion source in a FIB instrument ranges from 10 nm at a current of 100 pA to 0.8 nm at 1 pA.
Near-threshold photo-ionization of laser-cooled atoms provides a new class of ion sources. Characteristic of such sources is an extended source size combined with low angular spread. Whether ultracold sources are viable alternatives for, e.g., the Liquid-Metal Ion source depends on achieving operation at high brightness, low energy spread and/or with alternate ion species that can enable new applications. A number of these properties have already been investigated and some will be reviewed in the talk. Brightness achieved so far is well below that of the LMI and alternatives using laser-cooled atomic beams rather than traps are therefore being investigated. While these promise better performance on paper, disorder-induced heating and technical realization are challenges to overcome.
High-brightness ion sources are important for several applications such as focussed ion beam (FIB) systems, in which an ion beam is focussed on a sample to a spot size of a few nanometer. These systems have become an important tool in a wide array of material science and technological applications because they offer both high-resolution imaging as well as micromachining capabilities. The resolution in the most common FIB system, the Gallium-Liquid Metal Ion Source (Ga-LMIS) based FIB, is limited by chromatic aberrations in the lens column due to the energy spread of o2 eV. To reach a higher resolution, a high-brightness ion source is needed with a lower energy spread. In this thesis such a new source is presented, the ultra-cold ion source (UCIS) based on the photo-ionization of a laser-cooled atomic gas confined in a magneto-optical trap. The low temperature (_ 150 µK) of the source makes it possible to reach high-brightness with an extended source size. The charge density at the source is thereby reduced compared to sources with a sharp tip, such as the LMIS, and thus also the Coulombic interactions that deteriorate the beam and increase the longitudinal energy spread. Additionally many different atomic species can be laser-cooled, making the UCIS a versatile source for both heavy and light ion species. Light ion species such as Li are very suitable for imaging purposes without directly damaging the sample. Heavy ion species on the other hand, can be used for manipulation of a sample by sputtering away material at a small scale. A analytical model has been developed to estimate the initial source performance. The extractable current from the ionization volume is estimated based on the influx of atoms, which depends on temperature and atomic density. According to the model, the energy spread oU is proportional to the finite length of the ionization volume, limited by the size of the laser focus waist, and the acceleration field E0. To study the influence of the ionic interactions in the beam in more detail, particle tracking simulations have been performed which take all mutual Coulombic interaction into account under realistic experimental conditions. From these simulations we learned that when the current is increased, the brightness is lowered due to increased stochastic heating. Beam currents between 1 and 100 pA are feasible, with an energy spread much lower than that of the LMIS. At 1 pA the source has a peak brightness up to Br = 8 × 104 Am-2sr-1 with an energy spread _U = 0.08 eV. At 10 pA it has a brightness up to Br = 2 × 104 Am-2sr-1 with an energy spread oU = 0.14 eV. An experimental setup has been built, consisting of a rubidium magneto-optical trap inside a cylindrically symmetric accelerator structure. By using a two-step photo-ionization process, a small fraction of the cold atoms in the cross-section of two laser beams is ionized. A pulsed ionization laser was used to produce ion bunches. A short beam line with a multichannel plate detector was used to obtain both the temporal and the transverse spatial profileof the bunches. Time-of-flight measurements have been performed to characterize the longitudinal energy spread of the beam as function of the beam energy U. We have demonstrated, as predicted by the model, that the energy spread oU is indeed due to the length of the ionization volume in the acceleration field. By lowering the extraction field, well defined beams with an average energy of only 1 eV have been produced with an energy spread as low as 20 meV, more than two orders of magnitude lower than the LMIS. A transition from ballistic behavior at high beam energies, where oU / U, to a space-charge dominated regime at low beam energies, where oU / pU, has been observed. In the cross-over region an interesting new effect has been observed in which the space charge forces reduce the energy spread. The experimental results agree well with detailed particle tracking simulations. The combination of well-defined ion beams at low energy and time-dependent acceleration fields opens new possibilities. Precise manipulation of the bunch distribution in both longitudinal and transverse phase space has been demonstrated. Reduction of the longitudinal energy spread has been realized by switching the accelerator field off before the ions have had a chance to leave the accelerating structure. A reduction by a factor 3 is achieved, which is limited by the inhomogeneity of the field. Calculations show that much stronger reduction is achievable in a more homogenous acceleration field. This would make low-energy spread possible without the need to go to small _z0 and E0, enabling even lower energy spread. We also demonstrated that the accelerator structure, which normally acts as a fixed negative lens, can be transformed into a versatile lens by using time-dependent fields. Control over the focal length and sign, as well as the sign and strength of the spherical aberrations can be obtained by tailoring the applied time-dependent acceleration voltage. The possibility to create a negative spherical aberration may be used to correct for the spherical aberrations in the focal columns which are presently limiting the spatial resolution of FIBs. The dependence of the properties of this lens on the parameters of the applied tri-polar pulse has been studied in detail, both experimentally and with particle tracking simulations, which are in good agreement. Finally, based on lessons learned, the design of a next generation UCIS is briefly discussed, based on photo-ionization of a laser cooled and intensified atomic beam. The principle is similar as an UCIS based on a MOT, but with a higher current density. Calculations show that this source should be able to produce orders of magnitude brighter beams in a more compact configuration. With this new design it should be possible to beat the LMIS not only in energy spread but now also in brightness.
The ultra-cold ion (UCI) source is based on creating very cold ion beams by nearthreshold\nphoto-ionization of a laser-cooled and trapped atomic gas. The UCI source has\nthe potential of producing ion beams with a brightness and current comparable to the\nliquid-metal ion source (LMIS) [1], which is the current state-of-art for focused ion beam\n(FIB) technology. We have already shown [2] that the UCI source can provide much\nlower energy spread than LMIS, and may therefore offer a route toward 1-nm ion beam\nmilling [3].