Metamaterial Switches States in Response to Light

A metamaterial has been designed with a switchable metasurface that allows it to either block or transmit light waves in response to light pulses. Developed by researchers at the University of Southampton, the optically switchable metamaterial uses the phase-change medium germanium antimony telluride (GST) to change properties, a capability that may be useful for a range of optical devices.

Switchable metamaterials have previously included a metal component that is structurally engineered to provide the desired optical properties, as well as a phase change component that can be used to tune the properties. Metals tend to absorb light at visible and IR wavelengths, making them unsuitable for many optical device applications. Melting points are suppressed in nanostructured metals, making the metamaterials susceptible to damage from laser beams. Some metals used in metamaterials, such as gold, are not compatible with CMOS technology used for making many integrated devices.

A cross-sectional scanning electron microscopy image of a 750-nm period grating, fabricated by focused ion beam milling in a 300-nm thick amorphous germanium antimony telluride film on silica. Courtesy of Karvounis/Gholipour/MacDonald/Zheludev/Optoelectronics Research Centre, University of Southampton.

The new switchable metamaterial uses GST, but does not use metal.

The researchers created metamaterial grating patterns in an amorphous GST film 300-nm thick, with lines 750 to 950 nm apart. This line spacing has allowed the surfaces to selectively block the transmission of light at NIR wavelengths (between 1300 and 1600 nm). When a green laser converted the surfaces into a crystalline state, they became transparent at these wavelengths. Laser light pulses were used to switch the structure of the GST between nonvolatile random, amorphous or crystalline states.

The team showed that nanostructured, subwavelength-thickness films of GST provide high-quality (Q ≥ 20) NIR resonances that could be spectrally shifted by optically-induced crystallization to deliver reflection and transmission switching contrast ratios up to 5:1 (7 dB) at visible/NIR wavelengths selected by design.

The metasurfaces using chalcogenide demonstrated high-contrast, non-volatile, optically-induced switching of their NIR resonant reflectivity and transmission characteristics. With the novel metamaterial, chalcogenides such as GST may offer a flexible platform for the realization of optically-switchable metamaterials.

"What we've done now is structure the phase-change material itself," said Southampton researcher Kevin MacDonald. "We have created what is known as an all-dielectric metamaterial, with the added benefit of GST's nonvolatile phase-switching behavior."

GST, a material with an established industrial footprint in optical and electronic data storage, can readily be structured for telecommunications applications at 1550 nm, while other members of the chalcogenide family may provide similar active, all-dielectric metasurface functionality in the visible range and at IR wavelengths out to 20 µm.

The ability of a single medium to provide both high and low reflectivity and transmission levels in the same phase state, such that they can be simultaneously inverted via a homogeneous, sample-wide structural transition, may be useful in image processing, as well as metasurface optics applications, according to the researchers. By dynamically controlling the optical properties of materials the various characteristics of light beams such as intensity, phase, color and direction can be modulated, selected or switched.

The research team is now working to make metamaterials that can switch back and forth over many cycles. They are also planning increasingly complex structures to deliver more sophisticated optical functions, including switchable ultra-thin metasurface lenses and other flat optical components.

"Technologies based upon the control and manipulation of light are all around us and of fundamental importance to modern society," MacDonald said. "Metamaterials are part of the process of finding new ways to use light and do new things with it — they are an enabling technology platform for 21st century optics."

The research was published in Applied Physics Letters (doi: org/10.1063/1.4959272).