The trick is to fabricate subwavelength structures that can resonate both electrically and magnetically at the desired transmission frequency, thereby reversing the natural response of metamaterials. This is relatively easy at microwave wavelengths where millimeter-sized spit-ring resonators can easily be fabricated to respond both electrically and magnetically. But shrinking from the millimeter size of microwaves down to the micrometer and nanometer scale of communications and optical wavelengths, respectively, has proven exceedingly difficult. Now the Princeton architecture sidesteps the issue by employing an equivalent mechanism in highly anisotropic materials whose lattice structure has unequal physical properties along different axes.
"We found that it was relatively easy to achieve the necessary electrical resonance by doping appropriately," said Gmachl. "But it is very difficult to achieve a magnetic resonance at this scale, so instead we use anisotropy to substitute for the magnetic resonance."
The electrical resonance in the highly doped semiconductor layer, plus the layer with very high anisotropy, according to the researchers, confines the light in such a way that it has only one possible way to bendin the opposite direction to normal.
Next, the Princeton researchers and their colleagues at the University of Oregon (Corvalis), the University of Massachusetts (Lowell), Purdue University (West Lafayette, Ind.) and at Alcatel-Lucent (Murray Hill, N.J.) will attempt to craft applications of their new metamaterial for communications lasers and waveguides.
"Our main focus now will be to use this new material to make better semiconductor lasersmaking them smaller and cheaperas well as to fabricate better waveguides," said Gmachl.
Members of the Princeton engineering team included doctoral candidates Anthony Hoffman, Leonid Alekseyev, Scott Howard and Kale Franz, as well as Council of Science and Technology fellow Dan Wasserman (now at the University of Massachusetts), and former Princeton electrical engineering professor Evgenii Narimanov, now at Purdue University.
The research was performed at Princeton's Mid-Infrared Technologies for Health and the Environment (MIRTHE) lab and the Princeton Center for Complex Materials, both sponsored by the National Science Foundation. The crystals were grown for Princeton by Alcatel-Lucent.