A structure based on hyperbolic metamaterials allows visible light to pass in only one direction, creating new possibilities for optical circuits and biosensing. Developed at the National Institute of Standards and Technology, the device integrates a silver and silicon dioxide glass nanostructure with sub-wavelength diffractive gratings made of chromium. The 20-layer nanostructure, created using thin-film deposition techniques, is opaque to external light but allows light to propagate inside over a narrow range of angles. A grating on one face of the nanostructure bends incident light, allowing it to enter. Another grating on the opposite face permits the light to exit, but at a different angle than it entered. Schematic of NIST's one-way metamaterial. Forward travelling green light (left) or red light passes through the multilayered block and comes out at an angle due to diffraction off of grates on the surface of the material. Light travelling in the opposite direction (right) is almost completely filtered by the metamaterial and can't pass through. Courtesy of Ting Xu/NIST. The gap spacing of the second grating is slightly different from the first, and so it bends incident light either too much or not enough to enter the silver-glass nanostructure. The researchers said about 30 times more light passed through in the forward direction than in reverse. Devices were tested at 532 nm (green) and 633 nm (red) wavelengths and displayed broadband, efficient asymmetric optical transmission with contrast ratios > 14 dB, the researchers said. Similar devices have been made to manipulate IR light and microwaves, but the NIST project is the first applied to the visible spectrum, the researchers said. Without the silver-glass nanostructure, they said, the grates would have to be fabricated and aligned more precisely than is possible with current techniques. “This three-step process actually relaxes the fabrication constraints,” said NIST researcher Henri Lezec. In the future, the researchers said, the new structure could be integrated into photonic chips and nanoparticle detectors for biosensing applications. Like the chromium grates, nanoscale particles also can deflect light at angles steep enough to travel through the hyperbolic metamaterial and come out the other side. "I think it's a cool device where you would be able to sense the presence of a very small particle on the surface through a dramatic change in light transmission," Lezec said. The research was published in Nature Communications (doi: 10.1038/ncomms5141). For more information, visit www.nist.gov.