"Wrong Way" Light Comes Into View
The plastic lenses of your glasses bend light to focus it, but a new high-tech material bends visible light in the "wrong" direction. The advance may open the way for a number of new optical applications, including a revolutionary type of lens that could resolve finer details than now possible.
First, a quick lesson in optics: Stick a pencil in a glass of water, and the pencil appears to bend. That's because light rays bend at the interface of the water and the air, a phenomenon known as refraction. For example, a light ray striking the surface of the water at a 45 degree angle from the right bends farther downward into the water and continues to travel down and to the left. But replace the water with a "left-handed material," and the ray would bend so much that it would travel down and to the right instead. So a pencil would appear not only to kink, but to bend back under itself. In 2001, physicists demonstrated such negative refraction for microwaves using a specially designed material that consisted of small c-shaped rings and little rods.
To bend visible light the wrong way, physicists Henri Lezec, Jennifer Dionne, and Harry Atwater of the California Institute of Technology in Pasadena employed a simpler material. They sandwiched a 50-nanometer-thick layer of insulating silicon nitride between silver on top and gold on the bottom. When light waves zip along between the layers, they stir up ripples in the electrons at the metal-insulator interfaces called surface plasmon polaritons. These ripples then ferry the light until it emerges from the other edge of the slab. Moreover, when the light waves have the right frequency, the ripples in the plasmon bizarrely run upstream against the flow of the light. That's key to producing negative refraction, which the researchers demonstrated by cutting the slab into a wedge and using it as a prism to bend light beams of various colors. The prism bent red light the usual way, but deflected shorter-wavelength green light the wrong way, the team reports online 22 March in Science.
"This is an important step," says Gennady Shvets, a physicist at the University of Texas, Austin. The flat device works only for light traveling in the same plane as the slab, but even so, the advance could lead to novel optical systems on a chip, he says. However, first, he says, researchers will have to find a way to reduce the amount of light it absorbs. Meanwhile, Lezec says he has ideas for using the concept to make a 3-D material that bends light coming from any direction.