A new type of optical metasurface whose properties can be dynamically reconfigured with a laser pulse has been developed by researchers in the UK. The team believes that its technology, which has lower loss than traditional plasmonic resonators, could be useful for reconfigurable optoelectronic components.
Although metamaterials were originally developed to passively manipulate radiation to achieve perfect lenses or cloaking devices, the field has broadened to encompass materials that can be switched or tuned to modulate their properties. These have often featured plasmonic resonators – subwavelength noble-metal structures that interfere directly with the electromagnetic field of radiation and reshape a wavefront.
To reconfigure these, they can be combined with a so-called phase-change material, which alters its properties in some way in response to an external signal. However, plasmonic resonators often have high losses, especially at optical frequencies. In recent years, therefore, many researchers have produced metasurfaces from silicon or other dielectric materials, because their losses are smaller and they are easier to manufacture.
Laser switch
In the new research, Nikolay Zheludev and colleagues at the University of Southampton have produced metasurfaces purely from the chalcogenide germanium antimony telluride. Many chalcogenides – a class of compounds including sulfides, selenides and tellurides – can exist in both amorphous and crystalline phases. Heating the crystal above its melting point for a few nanoseconds destroys the crystalline structure and turns the material into an amorphous glass.
To trigger the reverse transition, the glass has to be heated to a lower temperature for a longer time (but still less than a microsecond). Chalcogenides have often been used in plasmonic metamaterials to shift the resonant frequencies of the plasmonic resonators by altering their surrounding environment. However, the optical properties of crystalline and amorphous chalcogenides themselves are very different and this phase-changing material is being used in rewritable CDs and DVDs, and is also being developed for new types of computer memory.
The researchers deposited 300 nm-thick films of amorphous germanium antimony telluride onto quartz substrates. They measured the near-infrared absorption of the film across a range of near-infrared wavelengths, finding that it was relatively transparent. Next, they used ion beams to selectively etch away the chalcogenide to produce subwavelength nanogratings. Pronounced absorption resonances appeared, with the resonance frequency dependent on the grating periodicity.
Phase changing
Zheludev’s team scanned a green laser beam over the surface. The light heated the material, causing it to crystallise. Upon measuring the optical properties of the crystalline gratings, the researchers found significant differences: after crystallization, one grating reflected only 20% as much light at 1470 nm as it had when the chalcogenide was amorphous. “We show for the first time that a dielectric metamaterial may be switched through the phase change of the dielectric itself,” says Zheludev. The researchers have not yet demonstrated the reverse transition back to the amorphous form of the grating: this will be more difficult, simply because it requires heating the material above its melting point while maintaining the structure of the grating.
Thomas Taubner of RWTH Aachen University in Germany praises the research, which he says forms part of a move towards all-dielectric, reconfigurable metasurfaces that researchers have worked towards in the past few years. He believes the absence of a reversible phase transition makes this “a first step”, but says that “in the nanophotonics community, the first goal is of course to show the concept and then later to do the engineering.”
The research is published in Applied Physics Letters.
