A Princeton-led research team develops three-dimensional "metamaterials," which they hope will someday produce lenses that eliminate image distortion
By Larry Greenemeier
|Image: Courtesy of Princeton University, Keith Drake|
|GOING NEGATIVE: Researchers used alternating layers of semiconductors to create a new "metamaterial" that breaks the laws of nature and bends light backward.|
For years, researchers have struggled to find an efficient way to develop lenses that do not lose portions of light as it passes through—an effect that hinders the performance of lasers, medical diagnostic imaging equipment and sensor systems. Now researchers led by a group at Princeton University have developed a new technique using nanosize materials that sets the stage for new lenses that eliminate the errors and image distortion inherent in today(i)s optical technology—and may one day be used to check for toxic chemicals in the air and the body.
The key component of the research was the creation of a solid-state crystal made of "metamaterials" that had the property of negative refraction, which causes light to curve in the opposite direction from where it naturally would while passing through naturally occurring materials, such as air and water. A lens for negative refractive properties would have a flat surface and would not share the same resolution limitations and image distortions of a normal curved lens with positive refractive properties, says lead study author Anthony Hoffman, a Princeton engineering graduate student.
Hoffman and his colleagues crafted their metamaterial semiconductor by placing alternating 80-nanometer-thick (one nanometer equals 3.94 x 10-8 inch) layers of indium gallium arsenide and indium aluminum arsenide atop an indium phosphide substrate 5.1 centimeters (two inches) in diameter. In all, the stack of ultrathin layers rose eight microns (one micron equals 3.94 x 10-5 inch), which is one-tenth the thickness of a strand of human hair. The researchers claim to have created the first three-dimensional metamaterial constructed entirely from semiconductors, the principal ingredient of microchips and optoelectronics.
In 2005 researchers at Purdue University in West Lafayette, Ind., created a metamaterial with a negative refractive index in the near-infrared portion of the spectrum using ultrathin gold nanorods 100 nanometers by 700 nanometers to conduct clouds of electrons. In another two-dimensional experiment to achieve negative refraction, earlier this year researchers Henri Lezec, Jennifer Dionne and Harry Atwater at California Institute of Technology in Pasadena, Calif., sandwiched a 100-nanometer-thick layer of silver between silicon nitride and gold, with openings on either end to allow laser light to enter and exit the silver.
Although these types of "two-dimensional metamaterials" have been around for a few years, the Princeton-led study offers researchers the ability to work with "something optically thick that could achieve a macroscopic effect," says Claire Gmachl, a Princeton electrical engineering professor and director of the Mid-Infrared Technologies for Health and the Environment (MIRTHE), a research center formed last year by the National Science Foundation.
The initial reason for conducting the research was scientific curiosity—a fascination with optical materials that could bend light in a new way, says Gmachl, who worked with Hoffman on the project. "This had been a theory since the 1960s, but ours is a step toward a simpler system that can be reproduced for manufacturing."
Thermal- and night-imaging equipment used by law enforcement and the military make use of the mid-infrared region of the light spectrum, which is where Hoffman, Gmachl and their fellow researchers have focused their work. MIRTHE, headquartered at Princeton University, also includes the City College of New York, Johns Hopkins University, Rice University, Texas A&M University in College Station and the University of Maryland, Baltimore County.
The Princeton team, which also included researchers from Oregon State University in Corvallis and the Murray Hill, N.J.–based telecommunications firm Alcatel-Lucent, is hoping that improved lenses could lead to sensor systems that can measure low concentrations of chemicals in the air. "Chemical trace gases that are vapors under normal conditions have characteristic absorption features," Gmachl says. Sensors that are able to identify the chemical fingerprints could be used to warn people when harmful chemicals have been released into the air, either on purpose or inadvertently. Medical professionals might also be able to use such a sensor to check a patient’s breath for traces of chemicals that indicate liver disease or internal inflammation.
Beyond the development of new sensors, semiconductor metamaterials such as the one Hoffman and his team created will also improve light amplification used in lasers. "Having a new material with improved optical properties just enhances the toolbox of the things we can work with," Gmachl says, adding, however, that most of this technology today is only in the prototype phase "There is still much work to be done. You won’t find these in commercial deployments yet."