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Tuesday, November 15, 2011

“Metamaterial tiles” are hot in many applications – including invisibility cloak!

Various forms of metamaterial have generated a lot of scientific attention in the past few decades. Some exciting “potential” applications include the well-publicized invisibility cloak (Thanks to Harry Potter). As you may know already, metamaterial gains its bizarre optical property (such as negative index of refraction) by its internal composition or structure, rather than its original physical property. Most metamaterial has its magic only in specific wavelength region and this wavelength region is correlated to how small you can make the internal structures of the metamaterial. This is exactly why almost all the research on metamaterial focuses on THz region since THz has very long wavelength and we do not need to make the structures awfully small to concoct the magic (I did read some articles about “universal metamaterials”, but it seems a long way to go. Let’s dream of that coming in CLEO 2012).

Digging into more details, you can have 2D or 3D metamaterial depending on your applications. 2D metamaterial – or so called metamaterial tiles (m-tiles) – seems to make a huge leap in guiding the advance in the invisibility cloak and sensing platform. And they are easier to make (through the help of photo-lithography, or micro-machining on the surface). With this powerful combination, a booming in this field seems inevitable. Let us take a peek of its potential application in invisibility cloak first:

It is realized that for a TE plane wave, it is possible to have a perfect invisibility cloak providing that the metamaterial has the right permittivity and permeability. However this cloak has to have circular inner and outer boundaries. As far as we know, it is very difficult to make exact circular cloak even with nowadays technology. To get around it, certain compromise has to be made. Instead of using hollow sphere or cylinder, we can use hollow polyhedral, each facet of which is made by m-tiles. Having this idea in mind, research group in Germany carried out a simulation study, and the result is really promising. Polyhedral made by m-tiles will actually give quite satisfactory results, and it can hide the structure within it very well (figure 1). In some circumstance, you can even rotate the polyhedral without losing its cloaking magic.

Figure 1. An invisibility cloak made by a faceted dodecahedral. This simulation shows that the plane wave can propagate through it without too much distortion and objects can be hidden inside the dodecahedral. Courtesy of Oliver Paul, Yaroslav Urzhumov, Christoffer Elsen, David Smith, and Marco Rahm.
The powerful units of m-tiles have actually simple internal structures. Described in great details in this article, you can easily change (tune) its optical property by changing its size and shape. As shown as an example on figure 2, three hexagonal m-tiles of slightly different structures have different absorption peaks in THz region. This “easy to fabricate and tune ability” makes m-tiles more and more popular in the research world. Considering making these tiles on a flexible film, you can actually fold them into a functional shape with even more interesting applications. And maybe one day we will have some advanced mosaics made of various m-tiles.

Figure 2. Different shapes of Hexagonal m-tiles. Each side of the structure is ranging from a few um to tens of um. By slightly modifying the structures, each of them absorbs different THz frequency. This flexibility makes m-tiles very versatile. Courtesy of Christopher M. Bingham, Hu Tao, Xianliang Liu, Richard D. Averitt, Xin Zhang, and Willie J. Padilla in Optics Express 16 23 18565 (2008).
How about getting a step further -- making these m-tiles on the paper and transforming them to biosensor platforms? In a nutshell, researchers from Tufts University and Boston University use micromachining to fabricate micro-stencils on silicon nitride film. These micro-stencils have many of m-tiles on it. With the help of micro-stencils, they then imprint the pattern of the m-tiles on the paper by spraying on the paper substrates using electron beam evaporation (figure 3). Using this way, you can make as many m-tiles as you want! Now, this sensor is ready to be radiated by THz radiation. Since paper is relatively transparent in THz region, it is a very good substrate (a disposal one). Once the molecules have attached to the m-tiles, they will change the electric capacity of each m-tile. This change of capacity will reflect on the absorption peak of the m-tiles. And this makes it a good sensor for various molecules. In fact, this is quite a new way to sense the molecules. It can achieve the sensitivity of ~ mmole/L concentration. Not bad as a paper-based sensor!

Figure  3. Using micro-stencils to imprint as many m-tiles as you want on the paper! The inset shows how the absorption spectra of m-tiles are modified when different amounts of the molecules (in this case, urea) are attached to them. Courtesy of Hu Tao , Logan R. Chieffo , Mark A. Brenckle , Sean M. Siebert , Mengkun Liu , Andrew C. Strikwerda , Kebin Fan , David L. Kaplan , Xin Zhang , Richard D. Averitt , and Fiorenzo G. Omenetto in Adv. Mater., 23, 3197–3201 (2011).
In the near future, more applications of m-tiles can be seen, indeed.

Trip note:

Luckily, I attended several conferences in China at the end of October. I spent two weeks visiting several cities (Beijing, Wuxi, and Wuhan). In Wuhan – Optics Valley of China, I met student chapter of OSA in POEM 2011. I felt awesome! OSA had a booth, an eye catching poster, and a bunch of energetic students in this conference. It made me feel like home away from home. Way to go, OSA!

Figure 4. The poster behind OSA booth in POEM 2011, Wuhan, China. Energetic student chapter of China promoted OSA nicely. Felt so warm when I saw this.
The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

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