What Are Metamaterials?
Caption: Visualization of an artificial fluid with a negative index of refraction (right) compared to a fluid with positive index of refraction (left). Photo Credit: Karlsruhe Institute of Technology
Metamaterials are artificial materials that can achieve electromagnetic properties that do not occur naturally, such as negative index of refraction or electromagnetic cloaking. The theoretical properties of metamaterials were first described in the 1960s by Victor Veselago, who focused on the purely theoretical (at the time) concept of negative index materials. His concept became a reality in the turn of the century.
A metamaterial typically consists of a multitude of unit cells, i.e. multiple individual elements (sometimes referred to as “meta-atoms”) that each has a size much smaller than the wavelength that it interacts with. These unit cells are microscopically built from conventional materials such as metals and dielectrics like plastics. However, their exact shape, geometry, size, orientation, and arrangement can macroscopically affect light in an unconventional manner such as creating resonances or unusual values for macroscopic permittivity and permeability.
Some examples of available metamaterials are negative index metamaterials, chiral metamaterials, plasmonic metamaterials, photonic metamaterials, etc. Due to their subwavelength nature, metamaterials that operate at microwave frequencies have a typical unit cell size of a few millimetres, while metamaterials operating at the visible part of the spectrum have a typical unit cell size of a few nanometres. Metamaterials are also inherently resonant, i.e. they can strongly absorb light at a certain narrow range of frequencies that can block or absorb a particular colour in the spectrum.
For conventional materials, the electromagnetic parameters such as magnetic permeability and electric permittivity arise from the response of the atoms or molecules that make up the material to an electromagnetic wave being passed through. In the case of metamaterials, these electromagnetic properties are not determined at an atomic or molecular level. Instead, these properties are determined by the configuration of a collection of smaller objects that make up the metamaterial. Although such a collection of objects and their structure do not appear at an atomic level like a conventional material, a metamaterial can nonetheless be designed so that an electromagnetic wave will pass through as if it were passing through a conventional material. Furthermore, because the properties of the metamaterial can be determined from the composition and structure of such small (nanoscale) objects, the electromagnetic properties of the metamaterial such as permittivity and permeability can be accurately tuned on a very small scale.
In the past 10-15 years, significant developments in optical metamaterial design, engineering, and fabrication has created excitement among researchers and consumers alike. Viable products are being developed and commercialized at an astonishing rate. In the coming years, we will marvel at the incredible technological revolution that is made possible by metamaterials and the engineers who are creating them.