Atom Optics - Nanofocusing

This figure shows how a standing wave (a laser beam reflected back on itself) focuses a beam of atoms. In this example, chromium atoms are directed down onto a surface, typically silicon, where they deposit to form a hard layer. Just before the atoms hit the surface, they are deflected by the standing wave of laser light rather like rain drops falling onto a corrugated iron roof. The atoms collect into lines and so form an array of narrow lines on the surface.

$14k; see Science {\bf 262}, 877 (1993)$
Scanning Atomic Force Microscope (AFM) image of chromium lines deposited on a silicon surface. Just prior to deposition, a standing wave of near-resonant laser light acts as an array of microcylindrical lenses, focusing the atoms into an array of parallel lines.

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.

NIST logo

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).

Future Work

At Melbourne we will be working with rubidium (Rb) because the ground state resonance transition is at 780.1nm, which coincides with the wavelength of low-cost laser diodes - the same ones used in CD players. For more info on the laser diode aspects, click Diode lasers for atomic physics.

Once the lasers are running, and the vacuum system constructed, we will be able to work on experiments.

Molasses - laser cooling of atom beams
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.
2D focussing
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.

Theory

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.

6k - Calculation of atomic trajectories 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 atom.
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

	Atom Optics
	Optics Group
	School of Physics

Created: 10 March, 1995
Last modified:
Authorised by: A/Prof. R. Scholten, School of Physics Maintained by: A/Prof. R. Scholten, School of Physics.