Physicists Close In on ‘Perfect’ Optical Lens
From ribosomes assembling proteins to viruses attacking cells, the main dramas in biology happen on a scale that is, tantalizingly, just one order of magnitude below the resolution of the best optical microscopes. Conventional lenses have a hard limit: The light waves propagating through them cannot carry details much smaller than their own crests and troughs. Clever workarounds have emerged, such as structured illumination microscopy, but all have limitations: They are too slow to image dynamic processes, or they poison cells with too much light.
Now, following recent breakthroughs, researchers are laying the groundwork for a “perfect lens” that can resolve sub-wavelength features in real time, as well as a suite of other optical instruments long thought impossible. These devices sidestep old optical limits by bending rays of light the “wrong” way — a phenomenon known as negative refraction.
The effect was first demonstrated in limited cases more than a decade ago, but by achieving it in novel ways, two groups “have made negative refraction a practical reality at optical frequencies,” said Sir John Pendry, a professor of physics at Imperial College London who was not involved in the new work.
In addition to biological imaging, perfect lenses could be used for single molecule biosensing, nanofabrication, light harvesting and (in theory) perfectly efficient solar panels, among other possibilities.
“The only prerequisite for realizing [a perfect lens] is negative refraction, which we have demonstrated,” said Hayk Harutyunyan, a postdoctoral researcher at Argonne National Laboratory in Argonne, Ill., and lead author of one of the new studies. “The rest is just technical problems that one has to solve.”
From air to silver, every medium has a “refractive index” relating the velocity of light in a vacuum to its velocity inside the medium. This number, plugged into a thousand-year-old formula known as Snell’s Law, gives the angle to which a beam of light bends when it enters the medium. When light passes from air into glass, for example, the refractive index increases from about 1 to 1.5, meaning that the light slows down and its angle steepens.
In 1967, the Russian scientist Victor Veselago wondered: What if that number, and therefore that angle, was negative? His minus sign completely transformed the equations of optics, yielding fantastic new solutions in which light pulled instead of pushed when striking a surface, and stretched when it would normally compress into a shockwave. Best of all, while regular curved lenses can only form images of objects located at the “focal point,” negative refraction is achieved with a flat lens that can form images of large regions of space.
But it all seemed like make-believe. To negatively refract light, a material must somehow send its waves rippling backward as its energy flows forward. “The reaction of the scientific community to this result was initially not positive,” said Veselago, now 84. “Many believed that the negative sign … in the formula was some ‘mathematical joke’ and cannot be realized physically.”
Veselago spent several years searching for materials with a negative refractive index. “However, all my attempts failed,” he said. The concept was forgotten.
Then, in 2000, a paper by Pendry in Physical Review Letters reignited interest in the idea. Pendry proved that negative refraction enables not only flat but also “perfect” lensing because negatively refracting materials can pick up and amplify the tiny wavelets that hug the fine-grained edges of objects. Ordinarily, this “near field” radiation decays within nanometers of an object and only the larger crests and troughs propagate outward. But when near field light hits a negatively refracting medium, the minus sign transforms its decay into growth, amplifying the signal. In a perfect lens, no information is lost.
“It’s a very beautiful process if you look at the mathematics of it,” Pendry said.
He also discovered a strategy for bringing the enticing possibilities to life. A material’s refractive index is calculated from its response to electric and magnetic fields. By embedding microscopic structures in a material that resonate with these fields in specially tailored ways, the material’s natural, atomic response to light, which always gives a positive refractive index, could be overridden. The first demonstration of negative refraction followed within months. A team led by David Smith, now a physicist and electrical and computer engineer at Duke University, created an artificial material, or “metamaterial,” consisting of a metal mesh imprinted with millimeter-wide geometric patterns. And as reported in a 2001 paper in the journal Science, by reversing electric and magnetic fields of specific strengths, the device negatively refracted 3-centimeter-long microwaves.
Metamaterials have spawned numerous practical applications, including “cloaking” devices that reduce electrical interference by bending radio waves around receivers, tunable satellite antennas that can access the Internet from anywhere, and vehicle collision avoidance systems. But the Smith team’s metamaterials could not be used to create lenses that negatively refract broadband visible light. They operate at a single wavelength tied to the dimensions of the material, rather than over a spectrum of colors. And their size could not be reduced enough to resonate within the visible 400- to 700-nanometer wavelength range. A new approach was needed.
“After many years of people staring at this problem of negative refraction, we’re finally getting people mastering the very, very difficult technology of making materials which have this property,” Pendry said.
In work that Pendry calls “a technological tour de force,” researchers at the National Institute of Standards and Technology in Gaithersburg, Md., have exploited the optical properties of objects called plasmonic waveguides to create a negatively refracting flat lens like the one Veselago envisioned 45 years ago. “Our goal was to achieve it in the most classical form, as close as possible to the original presentation back in the ’60s,” said Henri Lezec, principal investigator of the project.
For a range of ultraviolet wavelengths, the lens — made of a stack of silver and titanium dioxide layers — has a refractive index of -1, roughly equal and opposite to that of air. When light in this wavelength range bounces off an object and strikes the lens at any angle, interplay between oscillations of electrons in the two types of layers causes the light to bend back to the mirror-image angle as it moves through the stack, converging to form an image of the object on the far side. Because the lens is flat rather than curved like a conventional lens, there are “infinite axes and a continuum of focal planes,” Lezec explained. That means the device can create an image of everything in its vicinity simultaneously. So far, as detailed in the journal Nature in May, the team has created images of test objects such as rings and crosses, but “it could be a cell incorporating some flourophores,” Lezec said.
The device “allows unprecedented control of light,” he said, with immediate applications in 3-D photolithography (micro- or nano-scale printing with light), optical switching (turning light circuits on and off) and imaging. The researchers are also exploring strange physical effects that Veselago argued would be possible with a negatively refracting flat lens, including negative radiation pressure — pulling objects by shining light on them.
“Lezec actually sees this thing fly in space the wrong way; it flies towards light when you illuminate it,” Smith said. The scientists, who do not believe the object actually violates a fundamental principle of physics known as the conservation of momentum, are working on a new theory to make sense of this behavior.
Lezec’s flat lens currently dissipates too much energy to sufficiently amplify near field light and achieve super-resolution, but an almost loss-free approach to negative refraction proposed by Pendry in 2008 has also been demonstrated at optical frequencies for the first time. Harutyunyan, Ryan Beams of the University of Rochester and Lukas Novotny of ETH Zürich used a pair of high-powered laser beams to create a hologram on a flake of multilayer graphene, an extremely thin carbon crystal. Graphene is a highly “nonlinear” material, meaning it enhances the strange effects exhibited by very intense light. When a beam of light strikes the hologram, the nonlinearity causes a time-reversed replica of the beam to form on the far side of the graphene flake. This is effectively equivalent to the original beam negatively refracting as it crosses the graphene.
The results were reported in the July issue of Nature Physics. “There’s a lot of firsts in this,” Smith said. “It’s a beautiful experiment.”
The researchers say that, further optimized, the device could be used for super-resolution imaging of visible light, with some caveats. Smith thinks the laser beams used to generate the hologram could disturb a biological sample. Another challenge will be magnifying the sub-wavelength image so that ordinary cameras can pick up its fine details. This could be done, for example, by combining the graphene structure with a hyperlens, a newly developed negatively refracting lens that is curved.
Pendry, who recently began collaborating with experimentalists to build a perfect lens, believes the coveted object will be realized in the next five to 10 years. Several other scientists concur. Even negatively refracting, but not quite “perfect,” lenses will yield many practical applications, Pendry said. He compares negatively refracting devices to the laser. When its invention was first reported in 1960 “it was couched as a solution in search of a problem,” he said. “That is not the way you’d describe the laser today.”
This article was reprinted on Wired.com.