X-ray-like imaging without the harmful radiation and cell phones with more bandwidth are closer to reality now that researchers have developed a novel type of lens that works with terahertz frequencies. The new lens is a metamaterial, an artificial material with a structure made from many tiny parts, and it could drastically expand what lenses can do.
Regular lenses focus and aim visible, infrared, and microwave light, making them useful in a variety of everyday devices such as cameras, cell phones, and eyeglasses. But such lenses have fixed directions and focal points. That's a downside because multiple lenses and complex controls are often needed to guide and focus light with precision. And for some frequencies of light—such as terahertz radiation, a type of radiation that falls between the infrared and microwave bands of the electromagnetic spectrum and passes through many materials that block visual and infrared light—ordinary materials developed so far don't work as lenses at all.
Metamaterial lenses might solve both problems. In theory, they can be designed to alter their own structures in ways that change how they aim and focus light. Also in theory, they can be specifically designed to work with terahertz frequencies.
Physicists from Boston University decided to test the theory by making their own metamaterial lens. They laid out tiny gold rings, just 100 microns across, in a grid on a thin wafer of silicon nitride. Each gold ring had a small cut to make it a tiny circuit called a split-ring resonator. Rotating a split-ring resonator through a light beam will change how it interacts with that light. At some angles the resonator will amplify the light's magnetic field, and at other angles it will amplify the electric field—the same way an atom in the material of a conventional lens interacts with light passing through the lens. The split-ring resonator "atoms," however, can be placed in exactly the right pattern to lens terahertz light. By heating or cooling the material, researchers can make the resonators rotate in ways that change how the lens bends light. They can even force the metamaterial to do things impossible with natural materials, such as switching between a positive and a negative refractive index to flip the direction in which a light beam bends when passing through the material.
The researchers will unveil the new lens tomorrow at the Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference in San Jose, California.
Evan Reed, a physicist at Stanford University in Palo Alto, California, is impressed with the work. "Reconfigurable metamaterials are a quantum leap beyond their static counterparts," he says. "This work could have particularly broad impact if the concepts can be extended" into frequency domains other than terahertz. Reed suggests, for example, that adjustable metamaterials could be adapted to flexibly control and focus light of many different frequencies, which would make a single metamaterial lens capable of replacing entire arrays of conventional lenses.
Adjustable metamaterial lens technology is in its infancy, but the researchers have big plans to refine it. The "ultimate" metamaterial lens, they say, would be able to change all of its properties, including both the spacing and the rotation of the split-ring resonators. That would give users fine control over the frequency and direction of the light beam for applications such as precision scanning, says team member Hu "Tiger" Tao. Tao and colleagues are currently working on quicker methods than temperature changes to rotate and move the resonators.