Photonic metamaterials are artificial structures designed to manipulate the flow of light, often in ways that cannot be achieved using natural materials. One of their most promising applications is in optical computing, where logic operations are performed with light. Metamaterials used for this and other applications must be “reconfigurable”, meaning that it must be possible to change their optical properties on demand.
Three of the researchers. Left to right: Prakash Pitchappa, Professor Ranjan Singh, and Manukumara Manjappa.
A team led by Ranjan Singh at Nanyang Technological University (NTU), Singapore, has developed the first metamaterial device capable of switching easily between multiple configurations. In an October 2018 paper published in Nature Communications, the team showed that their metamaterial can even be used to realize logic gates, such as not-AND (NAND) and exclusive-OR (XOR). The metamaterial is based on a micro-electromechanical system (MEMS), containing micrometer-size mechanical arms that bend when electric voltages are supplied.
Scanning electron microscope image of one cell in the metamaterial. As different voltages are applied to the two pitchforks, one or both of them lift off from the surface. Figure credit: Manjappa et al.
Although reconfigurable metamaterials have been demonstrated before, researchers have struggled with designing metamaterials to switch between more than two configurations. Such switching requires an extremely fine level of control over the complicated electromagnetic interactions between different components in the metamaterial. The new MEMS-based device resolves this difficulty by employing Fano resonances, a phenomenon that allows the amount of energy stored in an electromagnetic oscillator to vary strongly as the properties of the oscillator are adjusted.
The metamaterial contains numerous copies of a pair of pitchfork-shaped aluminum antennae, deposited on a silicon chip. The pitchforks, whose arms are only 25 micrometers long, act as tiny electromagnetic oscillators that resonate at terahertz frequencies (the same frequencies used by milimeter-wave security scanners at airports). When an electrical voltage is supplied to each pitchfork, it bends and lifts off from the silicon surface, altering the delicate Fano resonance between the two pitchforks. Different voltage combinations thus dramatically alter how efficiently the device scatters terahertz-frequency light.
To create a logic gate, the team let the voltages on the two pitchforks serve as the logical input bits (00, 01, 10, or 11), and designed the metamaterial so that the amount of light transmitted through the device corresponds to the desired output of the logic gate. For example, in an XOR gate, the output is 1 (high transmission) when the inputs are 01 or 10, and the output is 0 (low transmission) when the inputs are 00 or 11.
“This is a novel and reliable platform for metamaterial devices,” explains Manukumara Manjappa, a PhD student at NTU who is the first author on the Nature Communications paper. “In the future, we envision using this to develop memory devices based on light at infrared and terahertz frequencies. Metamaterials can serve as randomly-accessible memory, performing multi-channel data processing more quickly than current electronic computers.”
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