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Cold Ion Beams

Ultracold Ion and Electron Sources from Laser Cooled Atoms

We study the effect of Rydberg blockade upon the emission of ions and electrons from an ultracold gas. We hope to demonstrate a quasi-deterministic single ion source. We have two experiments currently running in parallel: one to realise the potential of a compact, transportable grating chip caesium MOT; the other to investigate the effects of Rydberg atoms in cold electron and ion beams. 

If you are interested in this research area and you would like to know more about how to join our team here at Durham University, don't hesitate to get in touch. Please email Dr. Kevin Weatherill (k.j.weatherill@durham.ac.uk) with your queries or for more information.

 

Introduction

Focused ion beams (FIBs) are ubiquitous in nanotechnolgy, where they are used to create the smallest structures that current technology can provide.1 They are employed across a vast range of applications from circuit editing and defect review in the semiconductor industry, to failure analysis, sample preparation, surface analysis and much more. FIBs can be used for imaging, where secondary ions and electrons ejected from the target are detected. FIBs are also used for milling, where the beam sputters material from the target, and deposition, where precursor molecules can be bound to the target using the ion beam. The majority of FIBs use a gallium liquid metal ion source (LMIS) due to its favorable emission characteristics.2 When an electric field is applied to the tip, field evaporation occurs and the LMIS emits ions into a large cone with half angle ~ 20o.  A fine aperture is then required to select the ions with low transverse velocity in order to achieve high beam resolution. Using LMIS, spot sizes of < 10 nm are possible at low currents of ~ 10 pA.

Figure 1: K. J. Weatherill and E. D. J. Vredenbregt (Eindhoven University of Technology), one of the pioneers of cold ion and electron sources, co-authored a review on cold ion beams for Physics World. Click on the image to read the review.

Although FIBs using LMIS have been spectactularly successful, there are some serious limitations. For example, the spot size is fundamentally limited by the chromatic aberrations arising from the emission process (energy spread of ~ 0.5 eV). The source species is limited to gallium because of its unique combination of low melting point, low vapour pressure and relatively unreactive nature. Gallium is destructive when used for imaging but not heavy or reactive enough for many applications. Also gallium implantation can contaminate samples, causing unwanted effects.

 

Ion sources based upon laser cooling techniques3 offer new capabilties for FIBs. Here, we aim to construct and characterise a cold-ion source for FIB applications using recently-developed surface patterned atom chip technology.4 This will provide a compact, relatively inexpensive and easy to use cold-ion source.

 

Grating MOT source

 

Although ion sources based upon laser-cooling have proven to be useful and are a candidate for real technological implementation of cold atom physics, the sheer scale and complexity of the experiments makes embedding them into FIB systems impractical. We aim to use newly-developed surface-patterned (grating) atom chip technology4 to form the MOT for a cold ion source, thereby considerably reducing the size and complexity of the apparatus. 

 

Figure 2: Principle of the grating MOT ion source (GMIS). A single input beam incident on a surface-patterned chip, creates the optical fields necessary for laser-cooling. The cold atoms are then ionised and extracted through the exit aperture to form an ion beam. (Figure created with the assistance of A. Arnold).

 

The chips will be designed and sourced with the assistance of the Strathclyde group, with whom we have a track record of collaboration.5 Recent work in the U.K. from the groups of Arnold (Strathclyde) and Hinds (Imperial) has pioneered novel techniques for the cooling and trapping of atoms. They found that the light field required to create a MOT can be achived using a single laser beam and a planar diffraction element. This technique used a triplet of diffraction gratings to split and steer a single incoming beam into a tripod of reflected beams, and enabling cooling of rudidium atoms to 30 μK.6 Subsequently, this idea was extended to implement microfabricated chips4 and large MOTs of 6 x 10atoms were demonstrated (see Figure 2). Also, the group of Rolston at NIST (Gaithersburg) has used a grating MOT to achieve temperatures as low as 8 μK in rubidium.7 These devices show great promise for a range of new 'quantum' technologies.

 

This grating MOT ion source (GMIS) could then realistically be implemented as the ion source for a FIB system, providing an alternative to the ubiquitous LMIS. This work also provides a vital step toward the deterministic single ion source.8

 

Caesium is chosen as the ion source species because, not only is it easy to laser-cool, but it is also well-suited for sputtering - due to its relatively high mass - and well suited for secondary ion mass spectrometry (SIMS) analysis, where it strongly enhances the secondary ion yield for electronegative elements.

 

References

1N. Yao, Focused Ion Beam Systems, (Cambridge University Press, Cambridge, 2007) 

2J. Orloff, Rev. Sci. Instrum., 64, 1105 (1993)

3J. L. Hanssen et al., Phys. Rev. A., 74, 063416 (2006)

4C. Nshii et al., Nature Nanotech., 8, 321 (2013)

5K. J. Weatherill et al., Rev. Sci. Instrum., 80, 026105 (2009)

6M. Vangeleyn et al., Opt. Lett., 20, 3452 (2010)

7J. Lee et al., J. Opt. Soc. Am. B, 30, 2869 (2013)

8C. Ates et al., Phys. Rev. Lett., 110, 213003 (2013)