Click on the picture to download the video of the smallest Strip-the-Willow dance in the world (in Quicktime format (732K); alternatively download it as MPEG 4 movie, either small (336K) or BIG (2.5M)). The movie shows an area about 30 microns wide by 23 microns high (1 micron = 1/1000 mm).

Dancing with lasers. Two transparent spheres (red) are held in the foci of two laser beams (green cones). When the foci move, the spheres move with them. For a Strip the Willow, our computer-controlled hologram moves 8 laser-beam foci.

Click on the diagram to learn more about the dance Strip the Willow.

Credits: Gavin Sinclair (experiment), Jonathan Leach (movie editing), Johannes Courtial (idea, programming, web page)

What have we done?

Just for fun we have programmed our computer-controlled optical tweezers to move 8 microscopic glass spheres along trajectories normally taken by 4 gents and 4 ladies in the traditional Scottish dance Strip the Willow. Each sphere is about 2 microns (micro-meters - 1/1000 of a mm) in diameter, the whole dance set is about 20 microns by 20 microns small. Each glass sphere follows the position of the focus of a laser beam (through electrostatic forces); the 8 laser-beam foci are created from a single laser beam using a computer-controlled phase hologram. We've taken a movie of the result (which can be downloaded on the left): the smallest Strip the Willow in the world.

Our optical tweezers experiment.
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Optical tweezers

A single tightly focussed laser beam can exert a small force on transparent particles in the beam. This force is directed towards the brightest region in the beam, the beam's focus, and can be strong enough to "trap" micron-sized particles there. Such optical tweezers are now routinely used in applications such as the positioning of biological cells.

Our computer-controlled dynamic hologram. A laser beam (green) reflected off the surface of the Hamamatsu PAL-SLM has its phase altered locally in proportion to the intensity pattern projected onto the PAL-SLM from behind (red). The projected intensity pattern is created with a conventional spatial light modulator (SLM).
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