Cross-sectional scanning electron microscopy images of a 750-nm-period grating, fabricated by focused ion beam milling in a 300-nm-thick amorphous germanium antimony telluride film on silica. [Image: Karvounis/Gholipour/MacDonald/Zheludev, Optoelectronics Research Centre, University of Southampton][Enlarge image]
Researchers at a British university have applied the chalcogenide glass used in rewritable optical discs to create an all-dielectric metamaterial that can change its properties as easily as a human flips a switch (Appl. Phys. Lett., doi:10.1063/1.4959272). The researchers believe that the proof-of-concept experiment could lead to various types of flat, easily reconfigurable optical components.
The scientists in OSA Fellow Nikolay Zheludev’s group at the University of Southampton (U.K.) employed germanium antimony telluride (Ge2Sb2Te5 or GST), a compound that can make reversible transitions between amorphous and crystalline states—and that also exhibits markedly different properties in each state. The transitions require different heating regimens for each direction: a brief, intense excitation to go from crystalline to amorphous, and a longer annealing process to go the other way. (“Brief” and “longer” are relative—the times involved are a few nanoseconds and an accumulation of femtosecond laser pulses totaling less than a microsecond, respectively.)
To create the metamaterial, the researchers first sputtered a 300-nm-thick layer of GST onto an optically flat piece of quartz. They then etched the GST with a grating pattern with periods of 750 to 950 nm.
The GST metamaterial switched from amorphous to crystalline states when light from a 532-nm-wavelength laser excited it to a temperature between its glass-transition point and its melting point. The light changed the complex refractive index of the metamaterial and thus its transmission and reflection properties: the metamaterial, which had been opaque to near-infrared light in its amorphous state, became transparent to those wavelengths.
In this set of experiments, the researchers did not try to switch the metamaterial from crystalline to amorphous states because the heating process would have deformed the nanoarray. The authors suggest that creating truly reversible GST metamaterials may require some sort of physical support or encapsulation, in the same way GST in an optical disc is sandwiched between clear protective layers.