Bending Water with Lasers
Ordinary lasers from household electronics can shape water. In the 6 May Physical Review Letters a team reports using a laser as weak as those found in DVD players to overcome water’s surface tension and form a small dimple at the surface. The team was surprised that their technique formed a valley on the surface, rather than a hill, and their theoretical explanation for the reversal is not fully accepted by other experts. Still, the technique could help make miniature, customizable lenses that could be shaped and reshaped instantly.
Researchers have known how to bend water with high-power lasers since 1973, when two physicists at Bell Labs showed that the photon pressure was strong enough to push the surface of a liquid outward . This was surprising in itself, given light’s small momentum and water’s large surface tension.
Previously, researchers thought that only lasers with a power of 10 watts or greater–the kind of lasers used in micro-machining or surgery–had enough oomph to make water budge. But Olivier and Janine Emile of the University of Rennes in France realized that no one had tried a weak laser in the configuration known as total internal reflection, where the force details come out differently.
When you shine light down onto water at any angle, the combined light force of the three beams–incident, transmitted, and reflected–turns out to be entirely vertical; the horizontal force components cancel out. But when the light beam comes up through the water at an angle greater than about 49 degrees, it is almost all reflected back into the water. In this situation it turns out that a horizontal force survives. The beam experiences a phase shift at the surface and a sideways shift of the incoming beam called the Goos-Hänchen effect, which provides the horizontal force. The net force pushes water from the edges of the beam toward its center, which could create a bump at the surface the way you might make a hill on a piece of paper by laying it on a table and pushing the sides together.
The team shined a green, 20-milliwatt argon laser at an angle into a shallow dish of water with a long mirror along the bottom. The laser beam bounced off the mirror and reflected off the water surface, then bounced again off the bottom mirror and into a detector. The highly elongated shape of the beam image revealed the precise amount of bending of the water surface, just as a funhouse mirror distorts human proportions in a way that depends on its shape.
The Emiles were surprised to find a valley instead of a bump on the water. But they showed that the curvature was still determined by the size of the Goos-Hänchen effect, as expected. To explain the surprise, the team points to the small amount of electric field that leaked out to less than one micron above the water’s surface (the evanescent wave). They suggest that this field gradient was so drastic that it raised the air pressure and pushed down on the water.
”I’m not convinced by that,” says Jean-Pierre Delville of the University of Bordeaux. He thinks the model is too simple. “But for sure the experimental results are true. If we’re unable to understand it, that’s another story.” Whether the Emiles’ model is correct or not, their trick could help make small, easily reconfigurable lenses without introducing nonlinear effects or distortions. Such a lens could be used in adaptive optics in telescopes or retinal imaging, or as lenses in camera phones. “You can really manipulate the interface to make small lenses or mirrors with different radii of curvature,” says Olivier Emile.
Lisa Grossman is a freelance science writer.
- A. Ashkin and J. M. Dziedzic, “Radiation Pressure on a Free Liquid Surface,” Phys. Rev. Lett. 30, 139 (1973)