Low-cost metamaterials with negative refractive index

edició 2019

Mayte Gómez Castaño

Metamaterials have attracted a great deal of attention over the recent decades because of the exotic electromagnetic phenomena that they can present. Their unconventional properties do not derive from their chemical composition but rather from their artificially engineered physical structure. Therefore, the proper design of their geometry and size can enable the appearance of new optical properties not found in nature such as negative refractive index.

Besides the challenging physics behind them, negative-index metamaterials stand out for their potential application as super lenses, which would present an image resolution beyond the diffraction limit. However, their implementation in nowadays technology is limited by the traditionally used top-down techniques, which involve costly and low-throughput fabrication processes.

As alternative, we have developed an inexpensive and up scalable route to fabricate negative-index metamaterials over centimetre-sized areas that relies on the combination of colloidal lithography and metallic electrodeposition. Our structure consists of two gold layers separated by an air gap and symmetrically perforated by a hexagonal array of polystyrene spheres. This fishnet design is known to excite a magnetic resonance within the dielectric gap, responsible of the negative value of the refractive index.

The good agreement between optical measurements and finite-difference time-domain simulations attests the success of the new fabrication process. The effective properties of the whole structure are retrieved from the calculations and demonstrate that we are experimentally able to create a metamaterial exhibiting a refractive index of -1 in the near-infrared and working over a 100 nm wide spectral band.

These results open the door to low-cost and large-area fabrication of negative-index metamaterials for its future applications.