Physicists and electrical engineers around the world are pushing hard to make materials that twist a fundamental law of optics and bend light the "wrong way." Such "left-handed" materials could be used to make lenses that focus light much more tightly than ordinary lenses can, as well as other novel devices. But a critical aspect of the current approach won't work because it violates the principle that cause must precede effect, one theorist argues.
Imagine sticking a straw into a glass of water. The straw appears to kink because light bends as it passes from water to air in a process called refraction. If the straw descends into the water from right to left it will appear to poke into the water in generally the same down-and-to-the-left direction. Replace the water with a left-handed material, however, and the straw will appear to bend back toward the right. Thanks to such negative refraction, a simple slab of left-handed material could serve as a perfect lens, able to focus on detail that ordinary lenses cannot resolve.
In 2000, researchers produced an assemblage of small c-shaped rings and rods called a metamaterial that bent microwaves the wrong way. Ever since, scientists have been striving to make left-handed materials that work for shorter-wavelength visible light, either by miniaturizing metamaterials or by exploiting strange electric waves called plasmons, which ripple along nanometer-thick channels of metal (ScienceNOW, 23 March). But even as researchers make progress, they face a major hurdle: At optical wave lengths, the devices tend to absorb a large fraction of the light, making them at least partially opaque. To get around that problem, scientists hope to amplify light waves and counteract the losses.
But that won't work, reports Mark Stockman, a theoretical physicist at Georgia State University in Atlanta. The electrons in the materials slosh in response to the electromagnetic fields in the light, and causality says that the precise arrangement and motion of the electrons can depend on light that has already passed through them--not on the light that has yet to arrive. Starting from that point, Stockman has shown mathematically that negative refraction and absorption are intertwined so that you cannot have one without the other, as he reports in a paper to be published in Physical Review Letters. "There is no way to decrease the losses," Stockman says. "The [negative refraction] effect will disappear." In particular, simple amplification won't do the trick, he says.
Others aren't giving up their ambitions just yet, however. Stockman's work isn't a rigorous theorem, says John Pendry, a theorist at Imperial College London. "Everybody agrees that loss is an issue," Pendry says. "This paper doesn't say it cannot be dealt with." Pendry argues that it may be possible to engineer the materials so that they produce negative refraction for some wavelengths and absorb at others. Stockman counters that his work shows the two effects cannot be separated completely. One thing seems certain: The debate will likely absorb researchers' attention for quite some time.