The Technology of MetamaterialsRoger Pink | September 22, 2017
Metamaterials are exotic materials with designable properties that are artificially constructed at the macroscopic level with a two- or three-dimensional periodic structure. They are built to manipulate the electromagnetic or acoustic response of a material at precisely controlled target frequencies. Metamaterials can be used to construct technology that previously only existed in the realm of science fiction, such as invisibility cloaks, superlenses that can overcome the diffraction limit, terahertz imagers, smaller waveguides, acoustic invisibility materials etc.
The number of metamaterials and applications are unlimited and science is just now beginning to scratch the surface. For years the focus of metamaterial research was focused on manipulating electromagnetic waves, to great success. For instance, conventional optical microscopy is limited by the diffraction limit, a fundamental maximum resolution proportional to the wavelength of the light being observed and inversely proportional to the size of its objective. A superlens is a lens that uses metamaterials to go beyond the diffraction limit, allowing microscopes to achieve subwavelength imaging by producing a negative refractive index. The negative refractive index allows near-field rays, which normally decay due to the diffraction limit, to focus outside the lens, allowing subwavelength imaging.
A high-profile application of metamaterials is the invisibility cloak, a device that effectively makes an object disappear by bending light around it. Ordinarily, we see objects by capturing the light reflected off of them with our eyes which then produces an image in our brain. The metamaterials involved for the cloaking are made up of a lattice with the spacing between elements less than the wavelength that can continuously bend the light from behind the object, around the object, so that an observer views the image of what’s behind the cloaked object. In a similar way, an acoustic cloak bends the soundwaves from an acoustic source around an object, making the object acoustically invisible. Despite the amount of attention these applications tend to get, they are far from a working prototype using metamaterials.
A practical application of metamaterials is Terahertz imaging. The scarcity of naturally existing materials that can control terahertz led scientists to construct alternatives with metamaterials. Terahertz radiation holds a tremendous amount of promise in the imaging field because, unlike X-rays, terahertz radiation is not ionizing radiation and its low photon energies generally don’t harm tissues or DNA. Some Terahertz radiation can penetrate several millimeters of tissue and reflect back, making it a useful tool for medical imaging. Terahertz radiation can penetrate fabrics and plastics, so it is also well suited for security applications where many dangerous substances have identifiable spectral signatures in terahertz. Metamaterials designed for manipulating terahertz radiation is making terahertz scanners, imagers and scanners possible.
As technology continues to become more specialized, scientists are finding that they have reached the limits of conventional materials. The periodic structures of metamaterials allow scientists to create materials with specific acoustic and electromagnetic response. These metamaterials can then be used to create devices that were impossible before. As the field of metamaterials matures, we should start to see more applications in our everyday life, from safer medical imaging to faster security scans at airports to improved bandwidth, computer memory, and information processing. All of these applications had approached the physical limits of what was possible with natural materials but are being jumpstarted by emerging metamaterials.