Scanning electron micrograph of the micromachined switchable metamaterial
Nature Asia Materials – Metamaterials: Chopping and changing Switchable metamaterials allow for the controllable manipulation of light.
The optical properties of a material are predominantly determined by the structure and arrangement of its constituent atoms or molecules. Nature provides a vast array of materials and optical properties to work with, but if the specific properties you are looking for are not available naturally, then synthetic materials may be able to deliver a solution. Such ‘metamaterials’ have been demonstrated to achieve optical effects not found in nature, such as negative refractive index and super lensing. Ai-Qun Liu from Nanyang Technological University in Singapore and colleagues from Singapore, Taiwan, France and Hong Kong have now prepared a metamaterial with optical properties that can be switched dynamically using micromachined actuators.
The researchers fabricated a metamaterial containing microstructures consisting of metal squares broken into two halves separated by a small gap (see image). This type of ‘split-ring’ arrangement is frequently used in metamaterial designs, providing some defined, unnatural optical characteristics. What is remarkably clever about this new structure, however, is that the size of the gap can be controlled using micromachined actuators. “Most metamaterials are limited in action to a narrow band of wavelengths because of the resonance-based nature of the artificial microstructure,” explains Liu. “Our micromachined switchable metamaterials enable us to tune this resonance.”
The split rings were etched through a thin aluminum film and into a silicon substrate by bombarding the surface with reactive ions. Each square was 19 μm across, and the whole block of metamaterial was made up of 160,000 elemental parts. One half of each square was anchored to the substrate, while the other half was attached to a frame connected to an electrostatic actuator. In this way, the researchers were able to close the split-ring gap to form an array of squares, or open it fully so that the half-squares touched back-to-back. Liu and the team showed that increasing the gap size altered the way that their structure reacted to light, switching from positive to negative permittivity at wavelengths between 0.1 and 0.3 mm. Liu soon hopes to extend this idea to visible wavelengths.
A switchable magnetic metamaterial is demonstrated via reshaping micromachined metamaterial molecules. It is experimentally verified that the effective permeability can be switched from negative to positive near the resonant frequency. The micromachining technology offers a powerful platform for switchable metamaterials and provides more degrees of freedom for metamaterial design, easy fabrication, and large tunability.