Bathtub Optics: Bending light also shifts it sideways

The familiar optical illusion that makes a pencil look broken when half-dipped in water just took a new twist. A new experiment shows that when light bends at an interface (such as between water and air), the light’s photons take a sideways shift depending on their polarization—something scientists didn’t even suspect until a few years ago.

DOUBLE VISION. Refraction splits a linearly polarized laser beam (in the animation, arrows forming a sinusoidal curve) into two beams with opposite circular polarizations (rotating arrows). Hosten

Physicists call the broken-pencil effect refraction. It’s how lenses redirect light. When crossing at an oblique angle from air into glass or water, or between any two different transparent materials, light’s path bends.

In 2004, physicists in Japan first suggested that another, subtler effect may occur. Light consists of electromagnetic waves that vibrate sideways with respect to the light’s path. In linearly polarized light, the sideways vibration is all in the same direction, while in circularly polarized light, it spirals around the trajectory in a corkscrew fashion. The Japanese physicists calculated that when a photon crosses an interface, its circular polarization, or spin, will affect its trajectory by a tiny amount depending on whether the spin is clockwise or counterclockwise. “In addition to the bending, it shifts sideways,” says Onur Hosten of the University of Illinois at Urbana-Champaign.

Physicists called this the spin Hall effect of light, in analogy with a just-discovered “spin Hall effect” that affects electrons of different spins moving inside a semiconductor.

Unfortunately, trying this at home in your bathtub won’t work: The shift in a pencil’s image would probably be smaller than the size of the light receptors in your retinas.

To test the Japanese team’s prediction, Hosten and his Illinois colleague Paul Kwiat focused a laser beam onto a glass prism at different angles. As the millimeter-wide beam bent on passing into the glass, they expected photons of opposite spins within the beam to get kicked sideways in opposite directions. But the shift would be measured in nanometers, so the resulting two parallel beams would still mostly overlap. To detect such a tiny separation, Hosten and Kwiat had to devise a new trick.

Their beam’s initial polarization was linear and horizontal, equivalent to equal numbers of photons with opposite circular polarizations. In the vertical plane, the two rotating fields perfectly canceled each other out, while reinforcing each other in the horizontal plane.

But by separating the two components, the spin Hall effect would spoil this symmetry and introduce a small vertical polarization, the team expected. The researchers filtered out almost all the horizontally polarized photons emerging from the prism and combined the remaining photons to effectively amplify the spin Hall effect 10,000 times. That enabled the physicists to detect shifts of up to 60 nm with a precision of 0.1 nm, they report this week in the online edition of Science.

Aephraim Steinberg of the University of Toronto in Canada says that the team devised a “novel approach” that might become useful for other kinds of measurements, though he adds that the spin Hall effect itself is unlikely to find applications anytime soon.

Kwiat, however, says that the spin Hall effect could help physicists test the microscopic components of future computers that use photons instead of electronic circuits.

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