GOING NEGATIVE: Researchers used alternating layers of semiconductors to create a new "metamaterial" that breaks the laws of nature and bends light backward. Image: Courtesy of Princeton University, Keith Drake
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'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.