Metamaterial Uses Light to Control Its Motion

This is a cross-section of the metamaterial-based device. The top is a bilayer gold/silicon nitride membrane containing an array of cross-shaped nanoantennas etched into the gold layer. The bottom is a metal reflector that is separated from the gold/silicon nitride bilayer by a three-micron-wide air gap. (Source: UCSD / Nanoengineered Photonics Group)

Cross-section of the metamaterial-based device: The top is a bilayer gold/silicon nitride membrane containing an array of cross-shaped nanoantennas etched into the gold layer. The bottom is a metal reflector that is separated from the gold/silicon nitride bilayer by a three-micron-wide air gap. (Source: UCSD)

Researchers have designed a device that uses light to mani­pulate its mechanical properties. The device, which was fabricated using a plasmo­mechanical meta­material, operates through a unique mechanism that couples its optical and mechanical resonances, enabling it to oscillate indefinitely using energy absorbed from light. This work demonstrates a meta­material-based approach to develop an opti­cally-driven mechanical oscil­lator. The device can poten­tially be used as a new frequency reference to accurately keep time in GPS, computers, wrist­watches and other devices, researchers said. Other potential appli­cations that could be derived from this meta­material-based platform include high precision sensors and quantum transducers.

Researchers engineered the meta­material-based device by integrating tiny light absorbing nano­antennas onto nano­mechanical oscil­lators. The study was led by Ertugrul Cubukcu, a professor of nano­engineering and electrical engineering at the Univer­sity of California San Diego. The work, which Cubukcu started as a faculty member at the University of Pennsylvania and is continuing at the Jacobs School of Engineering at UC San Diego, demonstrates how efficient light-matter inter­actions can be utilized for appli­cations in novel nanoscale devices.

Meta­materials are artificial materials that are engineered to exhibit exotic properties not found in nature. For example, meta­materials can be designed to manipulate light, sound and heat waves in ways that can’t typically be done with conven­tional materials. Meta­materials are generally considered lossy because their metal components absorb light very efficiently. “The lossy trait of meta­materials is considered a nuisance in photonics appli­cations and tele­communications systems, where you have to transmit a lot of power. We’re presenting a unique meta­materials approach by taking advantage of this lossy feature,” Cubukcu said.

The device in this study resembles a tiny capa­citor consisting of two square plates measuring 500 microns by 500 microns. The top plate is a bilayer gold/silicon nitride membrane containing an array of cross-shaped slits – the nano­antennas – etched into the gold layer. The bottom plate is a metal reflector that is separated from the gold/silicon nitride bilayer by a three-micron-wide air gap. When light is shined upon the device, the nano­antennas absorb all of the incoming radiation from light and convert that optical energy into heat. In response, the gold/silicon nitride bilayer bends because gold expands more than silicon nitride when heated. The bending of the bilayer alters the width of the air gap separating it from the metal reflector. This change in spacing causes the bilayer to absorb less light and as a result, the bilayer bends back to its original position. The bilayer can once again absorb all of the incoming light and the cycle repeats over and over again.

The device relies on a unique hybrid optical resonance known as the Fano resonance, which emerges as a result of the coupling between two distinct optical resonances of the meta­material. The optical resonance can be tuned “at will” by applying a voltage. The researchers also point out that because the plasmo­mechanical metamaterial can efficiently absorb light, it can function under a broad optical resonance. That means this meta­material can poten­tially respond to a light source like an LED and won’t need a strong laser to provide the energy.

“Using plasmonic meta­materials, we were able to design and fabricate a device that can utilize light to amplify or dampen microscopic mechanical motion more powerfully than other devices that demonstrate these effects. Even a non-laser light source could still work on this device,” said Hai Zhu. “Optical meta­materials enable the chip-level integration of functiona­lities such as light-focusing, spectral selec­tivity and polari­zation control that are usually performed by conventional optical components such as lenses, optical filters and polarizers. Our particular meta­material-based approach can extend these effects across the electro­magnetic spectrum,” said Fei Yi, a postdoctoral researcher who worked in Cubukcu’s lab. (Source: UCSD)

Reference: H. Zhu et al.: Plasmonic metamaterial absorber for broadband manipulation of mechanical resonances, Nat. Phot, online 10 October 2016, DOI: 10.1038/nphoton.2016.183

Links: Nanoengineered Photonics Group, University of California San Diego, USA

 

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