Moving a step closer to optical cloaking, researchers at the University of Stuttgart (Stuttgart, Germany) recently created a stacked split-ring metamaterial for the optical wavelength range. This breakthrough may eventually lead to superior lenses that can beat the diffraction limit, as well as optical cloaking devices capable of providing invisibility for macroscopic objects.
The race to develop a cloaking device began to heat up in 2006, when Duke University (Durham, North Carolina) scientists became the first to demonstrate an invisibility cloak that deflects microwave beams to flow around a “hidden” object inside with little distortion—nearly making it appear nothing was there. They managed to achieve this by crafting a cloak out of 2-D metamaterials (ordered composites that can be made to interact with electromagnetic waves in ways that natural materials cannot, see www.laserfocusworld.com/articles/259931) with a negative refractive index (RI) for electromagnetic radiation. The metamaterials they used were split-ring resonators (SRR) with a structure size much smaller than a wavelength, which only required stacking ten layers to achieve the desired invisibility effect.
To take cloaking to optical frequencies, and reach beyond Duke University’s work, Harald Giessen’s research group at the University of Stuttgart designed and manufactured a 3-D optical (infrared) metamaterial with a fabrication process that involves planarization, lateral alignment, and a layer-by-layer stacking approach (see Fig. 1).1 One of the group’s primary goals was to build a stacked optical metamaterial using a fabrication method that would avoid delicate lift-off procedures, which have plagued other researchers by causing problems such as nonrectangular side walls and a limited number of layers—neither of which are desirable for stacking.SRR structure
Giessen’s group began by selecting a SRR structure as the basic unit cell in its demonstration, because the SRR exhibits negative permeability in certain frequencies and has already been widely used for metamaterials. The group fabricated three gold alignment marks (4 × 100 µm) with a gold thickness of 250 nm by lift-off on a quartz substrate, which was then covered with a 20 nm layer of gold film using electron-beam evaporation. Next, the SRR structures were defined in a negative resist using electron-beam lithography. This was followed by ion-beam etching of the gold layer to create the gold SRR structures. Then, a 70-nm-thick spacer layer was spin-coated onto the first layer. A baking process hardened the structure, the researchers say, and then a 20 nm gold film and a spin-coated resist layer were deposited onto the sample. Finally, stacking alignment using gold alignment marks was applied to ensure accurate stacking of subsequent structure layers.
A key part of creating their 3-D metamaterial, the researchers note, was combining a planarization method for the rough nanolithography surface with robust alignment marks that survived the dry etching processes during nanofabrication. The resulting 3-D structures, created using the layer-by-layer method (which can be repeated as often as desired), consist of horseshoe-shaped gold nanowires arranged in a square pattern and perfectly stacked above each other (see Fig. 2). This method can produce arbitrary shapes in each layer as well, the group points out. And more complex structures such as twisted or chiral structures are also possible.
Investigating the interaction between adjacent SRRs, Giessen’s group discovered it could control the electromagnetic coupling strength (which is related to spectral splitting of plasmon modes) by adjusting the vertical distance. The researchers believe the vertical interaction between metamaterial slabs can change the optical properties of metamaterials and lead to new characteristic spectral features with an increasing number of stacked layers. The resulting vertical coupling may prove beneficial in the design of broadband materials, and the group expects stronger coupling will lead to increased bandwidth.
Three-dimensional metamaterials possess highly desirable electromagnetic properties such as negative magnetic permeability and negative RIs, which will play a critical role in future applications like negative refraction, superlensing, and invisibility cloaking. Giessen’s group believes the key to real-world applications will be balancing the number of stacked layers vs. intrinsic losses.
REFERENCE
1. Na Liu et al., Nature Materials, DOI: 10.1038/nmat2072.
