by What are Metamaterials ?
Metamaterials are man-made materials engineered to have properties that may not be found in naturally occurring materials. They are designed to manipulate electromagnetic waves and optical properties in unusual and useful ways. Metamaterials derive their properties not from the chemicals they are made of but from their designed internal structure.
Properties of Metamaterials
Negative Refraction
Some metamaterials exhibit negative refraction where the wave propagates in a direction opposite of what is expected based on Snell’s law. Metamaterials is achieved by carefully structuring the material to have negative permittivity and permeability at certain wavelengths. Negative refraction enables the design of superlenses that can image objects much smaller than the wavelength of light used. This overcomes the classical diffraction limit of normal lenses.
Invisibility Cloaking
Inspired by the stories in science fiction, researchers have demonstrated how metamaterials can bend light around an object and make it seemingly disappear. By manipulating the flow of electromagnetic or optical waves around an object, metamaterials allow cloaking devices to be built that can render objects invisible within a specific frequency range. While challenges remain for full three-dimensional cloaking, the concept illustrates the incredible control over light enabled by custom metamaterials.
Flat Lenses
Normally, thinner lenses mean worse optical properties as the curvature needed for focusing cannot be achieved. Metamaterials are changing this by allowing flat lenses that maintain good focusing ability. These superlenses utilize negative refraction near the lens surface and achieve resolutions far below the diffraction limit of normal lenses, even at wavelengths much larger than the structural elements of the metamaterial. Flat lenses have applications in microscopes, endoscopes, cameras and other optical equipment.
Vibration and Sound Control
The mechanical properties of metamaterials allow for unprecedented manipulation of mechanical waves like vibrations and sound. Applications include acoustic concentrators that direct sound to a precise location, sound absorbers that drastically reduce noise pollution from highways or engines, and seismic shields that protect buildings from earthquakes. These developments could significantly alter how vibration and noise are handled in architecture, transportation and manufacturing.
Applications of Metamaterials
Medical Imaging
The super resolving power of metamaterial lenses is enabling new ultrasound and MRI techniques with far higher resolution than before. Researchers have demonstrated the ability to image blood vessels and internal structures much smaller than the diffraction barrier. This could revolutionize early detection of cancers and other diseases. Metamaterials also show promise for developing tunable ultrasound contrast agents and new minimally invasive surgical tools with unprecedented precision.
Optical Communications
New types of metamaterial waveguides concentrated optical signals within subwavelength scales for compact on-chip optical circuits. They enable dramatic miniaturization of photonic chips and improve information throughput. Metacrystals are allowing lasers to be tuned across wide frequency ranges for applications in spectroscopy, optical fiber communications and high performance computing.
Sensors and Detectors
Metastructures are leading to ultra-sensitive detectors and sensors. Metamaterial perfect absorbers concentrate light into nanoscale volumes enabling single molecule detection. Terahertz metamaterials push the frontier of high frequency electronics and enable new security screening and non-destructive evaluation tools. Designer surfaces based on plasmonics exploit metamaterials for Chemical and biological detection with record breaking sensitivity.
Moving Forward with Metamaterials Research
While significant progress has been made in demonstrating advanced functionality in metamaterials across the electromagnetic spectrum, many challenges remain before their full potential is achieved. Fabrication limitations have so far restricted bulk metamaterials to the microwave regime, though nanofabrication is extending their ranges into the terahertz and optical domains. Nonlinearity and active tunability also need further exploration to realize truly dynamic and adaptive metamaterials.
In multi-functional metadevices that integrate several effects simultaneously will be key to practical applications. Significant theoretical developments and large scale computational modeling using machine learning are still needed to guide the rational design of complex metamaterials. Continued research along with new fabrication techniques promises to further expand the exciting possibilities of these designer materials. With concerted global efforts, metamaterials will surely emerge as one of the most transformative technologies of the 21st century.
