Metamaterials: An Overview
The prefix ‘meta’ attached to a material indicates that the material in question has characteristics which cannot be clearly defined on the basis of what we observe in nature. Metamaterials are artificially crafted composite materials that derive their properties from internal microstructure, rather than chemical composition as usually found in natural materials. They can thus achieve electromagnetic properties that do not occur naturally, such as negative index of refraction or electromagnetic cloaking.
The 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. But the world has come a long way since the 60s. A few wars, a few depressions, a few pandemics (boy oh boy do i not need to bring this one up…) and more than a few technological advances. Metamaterials have come a long way from first base. And by long, i mean long. The next time you see one of those optically camouflaged vehicles in a sci-fi flick, just take a second to appreciate that you are looking at the wonders a metamaterial can achieve.
The Difference(s)
That metamaterials are different is a given. But, the exact reason why they are different is a bit less of a given. To explain it as simply as possible, 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.
Metamaterials are special one-, two- or three-dimensional artificial structures with electromagnetic properties generally not found in nature. Due to the simultaneous negative values of permittivity (ε) and permeability (μ), the wave vector and the vectors of electric- and magnetic- field intensity form a left-handed triplet with the result of antiparallel phase and group velocity and back-wave propagation. These unique properties of the left-handed materials have allowed novel applications and devices to be developed.
Classification of Metamaterials
On the basis of their properties exhibited, targeted application and internal composition, metamaterials can be broadly divided into 3 broad categories:
Electromagnetic Metamaterials can be further classified into:
- Negative refractive index metamaterial
- Single negative metamaterials (SNGs): SNGs have either negative relative permittivity or negative relative permeability, but not both
- Hyperbolic metamaterials (HMMs): HMMs behave as a metal for certain polarization or direction of light propagation and behave as a dielectric for the other due to the negative and positive permittivity tensor components, giving extreme anisotropy.
Applications of Metamaterials
Metamaterial antennas are a class of antennas which use metamaterials to enhance or increase performance of the system. The metamaterials could enhance the radiated power of an antenna. Materials which can attain negative magnetic permeability could possibly allow for properties such as an electrically small antenna size, high directivity, and tunable operational frequency, including an array system.
Furthermore, metamaterial-based antennas can demonstrate improved efficiency-bandwidth performance Metamaterial, high-impedance ground planes can also be used to improve the radiation efficiency, and axial radio performance of low-profile antennas located close to the ground plane surface.
Metamaterials have also been used to increase the beam scanning range by using both the forward and backward waves in leaky wave antennas. Various metamaterial antenna systems can be employed to support surveillance sensors, communication links, navigation systems, command and control systems.
Next up, a super lens uses metamaterials to achieve resolution beyond the diffraction limit. The diffraction limit is inherent in conventional optical devices or lenses. The road to the super lens is its aptitude to significantly enhance and recover the evanescent waves that carry information at very small scales. No lens is yet able to completely reconstitute all the evanescent waves emitted by an object.
Metamaterials are also a basis for attempting to build a practical cloaking device. The cloak deflects microwave beams so they flow around a “hidden” object inside with little distortion, making it appear almost as if nothing were there at all. Such a device typically involves surrounding the object to be cloaked with a shell which affects the passage of light near it.
Specially tuned Acoustic metamaterials are artificially fabricated materials designed to control, direct, and manipulate sound in the form of sonic, infrasonic, or ultrasonic waves, as these might occur in gases, liquids, and solution.
A metamaterial can also act as an absorber, manipulating the loss components of metamaterials’ permittivity and magnetic permeability, to absorb large amounts of electromagnetic radiation. This is a useful feature for photodetection and solar photovoltaic applications. Loss components are also relevant in applications of negative refractive index (photonic metamaterials, antenna systems) or transformation optics (metamaterial cloaking, celestial mechanics), but often are not utilized in these applications.
if you want to look further, consider checking these out:
2.Fundamentals of Metamaterial Structures | SpringerLink
3. Metamaterials — an overview | ScienceDirect Topics
Adieu then…until next time
