The standing wave is tuned to the deep-blue atomic resonance at 425.43 nanometres, and so the spacing between the low intensity troughs is half that, or 212.78nm. Thus each trough acts like a conventional cylindrical lens but only 0.2 microns across. This lens then focusses the atoms to much smaller lines, perhaps as small as 10nm.
Much of the work shown here was done at NIST in Gaithersburg, MD, USA with Jabez McClelland, Robert Celotta, Rajeev Gupta and others in the Electron Physics Group. See what they're doing at: NIST Electron Physics Group, or see Science 262 877 (1993).
Once the lasers are running, and the vacuum system constructed, we will be able to work on experiments.
For the nanofocusing experiments, it is essential to have a very well collimated (parallel) beam of atoms. This can be achieved using laser cooling, and we have found that cooling chromium in this manner does not conform to expectations. Future work will investigate the anomalous behaviour for rubidium.
The simple standing wave is only the beginning. More complex fields can be used, for example that indicated in this figure.
This shows a field produced by two orthogonal standing waves. Such a scheme can be used to focus atoms into spots rather than lines. Each spot will be separated from the next by half the wavelength of light used - for Rb, that's 390nm. Or, if you prefer, about 1 billion spots per square centimetre.
Now imagine that as you deposit the Rb into 1 billion spots, you move the silicon substrate. Each spot will draw a pattern according to how you move the substrate - like an etch-a-sketch. The pattern will be replicated identically 1 billion times. It would be nice if we could draw the pattern for a transistor or memory cell for example. Of course, the pattern can be no larger than one cell size, where the cell size is 390x390nm in this case.
It's not hard to predict where the atoms go - if you're not fussy about details. You can start with a simple semiclassical model based on Newtonian mechanics and a atom/laser interaction derived from quantum mechanics. It does a good job.
| Here's a plot showing atomic trajectories for an idealised
case, where the atomic beam is perfectly parallel, the atoms all have one
velocity along the beam, and the chromium is assumed to be a simple two-level
The horizontal axis is distance along the standing wave, showing about one period, with the minimum intensity at x=0. The laser pushes the atoms towards this minimum. About one third of the atoms make it; the other two thirds form a thin background on the substrate.
We would like to start with calculations like these, and extend them to include better modeling of other effects such as spontaneous emission.
That's a small sample of some of the things we'll be doing. For more information, contact us via:
A/Prof. Robert Scholten Optics Group School of Physics University of Melbourne Parkville Victoria, 3052 AUSTRALIA Phone: +61 3 8344 5457 (Scholten) +61 3 8344 8974 (The Lab) +61 3 8344 8171 (Optics Computer Room) Fax: +61 3 9347 4783 URL: http://www.ph.unimelb.edu.au/~scholten/home.html email: r.scholten at physics.unimelb.edu.au
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