Metamaterials have revolutionized the field of photonics because of their exotic optical properties absent in natural materials. Made from the arrangement of subwavelength units, the properties of metamaterials do not derive from their chemical composition but rather from their physical structure. Therefore, properly designing the geometry, size and media involved in a metamaterial, it is possible to engineer the overall electromagnetic response beyond the conventional behaviors.
One of the most fascinating types of metamaterials are those that present a negative refractive index. In these media, refraction of light occurs oppositely to conventional materials, light bending to negative angles. Besides, with these negative index metamaterials, it is possible to focus light without the need of curved surfaces or to achieve super resolution imaging. However, the implementation of these systems into actual devices working at optical frequencies is not an easy task. The complex nanofabrication processes required are typically limited to small areas of a few hundreds of microns.
Now, researchers from the Nanopto group, within the Enlightment ERC project carried out at the Institute of Materials Science of Barcelona (ICMAB), together with colleagues from the Research Center Paul Pascal (CRPP, Bordeaux) have developed a scalable method for the straightforward fabrication of fishnet-like negative index metamaterials.
As reported in their recently published paper in Advanced Optical Materials, combining the low-cost techniques of soft nanoimprinting and electrodeposition, they produce large area metamaterials made of two-dimensional arrays of pillars surrounded by nanolayers of gold and air gaps. This structure induces an artificial magnetism within the system that together with the electrical response of the gold layers leads to the unusual phenomenon of negative refractive index.
Their versatile approach allows tuning different structural parameters such as the periodicity of the array, the pillars diameter or the number of metallic layers. In this way, they easily tailor the optical response, attaining record values of negative index from the visible to the near infrared with an up scalable method. Furthermore, the metamaterials are prepared on transparent conductive substrates hence ready for device implementation. They also report the closely convergence of their metamaterials to the bulk regime, where the intricate optical response is independent of the number of stacked layers or the angle of incidence.
In addition, they prove the change of the operation wavelength of the metamaterials when infiltrating different liquids in between the gold layers. Therefore, the systems can work as optical sensors or tunable flat lenses with the embedded medium.
This publication reports for the first time the application of large area fishnet metamaterials as optical sensors and pave the way for the implementation of metamaterials in optoelectronic devices.
Article written by Mayte Gómez, Enlightment ERC Project, Nanopto Group
Electrodeposited Negative Index Metamaterials with Visible and Near Infrared Response
Mayte Gómez-Castaño, Juan Luis García-Pomar, Luis Alberto Pérez, Sharvina Shanmugathasan, Serge Ravaine and Agustín Mihi
Adv. Optical Mater. 2020
Here, we present a cost effective and straightforward approach for the fabrication of large area NIMs based on soft nanoimprint lithography and electrodeposition.