Left: Schematic view of a nanoantenna array, one of several newplasmonic metasurfaces that could make optical microscopes 10 X more powerful. Right: A hyperbolic metasurface consists of a tinymetallic grating for enhancing quantum emitters, which could makepossible future quantum information systems.
Flat, ultrathin slices of metamaterials, called “metasurfaces,” can control light better than their thicker counterparts and could open the way to applications ranging from aberration-free lenses to quantum computers, as well as the long-sought “invisibility cloak.”
In a recent review article (Science 339, 1232009), three researchers at Purdue University (U.S.A.) predict that these optical metasurfaces could overcome many of the challenges to building practical nanoscale devices, such as light loss and lack of compatibility with device fabrication processes.
Metasurfaces consist of a patterned metal-dielectric layer thinner than the wavelength of the impinging light. The pattern can be ordered or random.
The article’s co-authors—Alexander Kildishev, Alexandria Boltasseva and OSA Fellow Vladimir Shalaev—were part of a group that last year demonstrated broadband negative-refraction and -reflection effects for planar photonic surfaces in the near-infrared region (Science 335, 427). The type of metasurface they used in their experiments consists of arrays of tiny metallic nanoantennas, which modulate and control the light in ways not possible with natural materials. These thin arrays could be used as ultrafast spatial light modulators, for example.
A second type is known as a hyperbolic metasurface, because it disperses light in a hyperbolic pattern. The effective refractive index of such a metamaterial is extremely high, which would be useful in super-resolution imaging.
The noble metals that have gone into many recent bulk metamaterials work well in the microwave and long-infrared regimes, but their large plasma frequency gets in the way for near-infrared and visible applications. Scientists are still developing new plasmonic materials, such as transparent conducting oxides and transition-metal nitrides, to optimize metasurfaces.