Metamaterials: Invisibility Cloaks and Other Wonders

Metamaterials: Invisibility Cloaks and Other Wonders

By Olimpia Onelli

The advances that material science has made in the last few decades are leading towards smarter and smarter materials. All branches of technology are benefiting from these advances, from semiconductors to sensing devices. There is, however, one class of materials showing to be more special than any other: the metamaterials.

Borrowing their name from the Greek μετά- meaning “to go beyond”, metamaterials are engineered to exhibit properties that do not occur in nature. In particular, they interact with electromagnetic fields in surprising, and sometimes counter-intuitive, ways. For example, negative refractive index metamaterials are able to bend light around an obstacle which would be effectively “hidden” to an external observer.

Typically, metamaterials are fabricated by repeating units and patterns. However, the term is recently acquiring a broader meaning, not restricted to periodic arrangements. What has not changed though, is the necessity of patterning the materials at a scale shorter than the wavelength one wants to enhance. This constraint poses a serious experimental challenge. For example, if one desires to work within the visible light range, the material would need to be engineered on a nanoscale with a very high accuracy. So far, the most appreciable advances in the field have been made using longer wavelengths, such as microwaves and terahertz frequencies.

In a recent paper published in Science, Natalia M. Litchinitser and Jingbo Sun from the State University of New York review a specific class of metamaterials: the nonlinear optical meta-atoms. Nonlinearity is a property shown by those materials that have a response to changes in the electrical or magnetic field that is not proportional to the change. Hence, if fed with little energy, a nonlinear material is able to output a much higher power almost instantaneously. So far, nonlinearity has not been fully exploited as the response enhancements have been generated by local phenomena rather than the engineering of non-linear atoms.

The authors propose optical meta-atoms as the perfect candidates for the production of such materials. In fact, they individuate two fabrication technologies that would fit well with standard fabrication procedures: atomic layer deposition of dielectric materials and quantum engineering of potential wells that would trap the electrons in the desired way.

In conclusion, the race for metamaterials is open and experimentalists all over the world are working at putting into practice what the theory predicts. Will optical meta-atoms lead the way towards a new realm of meta-wonders?


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