MATERIAL SCIENCE The strange world of metamaterials

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Human eyes can only see an object if it reflects or scatters light. Or if light passes through the same. Imagine a material that could bend light to make objects invisible. Think again about a construction material that could make buildings invisible to earthquakes. The answer is metamaterials. These “strangely behaving” materials are slowly becoming the new talk of the tech town. The article introduces metamaterials and their applications. It emphasizes metamaterial use in wireless power transfer.

The word “meta” is derived from a word called “beyond”— meaning “beyond materials”. Metamaterials have been researched since the late nineteenth and early twentieth centuries. The ultimate goal was to build materials that could manipulate electromagnetic waves. Loads of studies were done and experiments were conducted for years until the first metamaterials were invented between 2001 and 2006.

As of 2024, the metamaterial market is valued at USD0.22 billion. The metamaterial market may hit a new high of USD1.38 billion in 2029 at an exceptional CAGR of 44.8 %. Such a high growth rate is uncommon across a wide range of industries.

What are metamaterials and why are they gaining popularity?

Metamaterials are special artificially engineered materials to exhibit unique properties that are not found in naturally occurring substances. They exhibit “smart” properties through their structure to manipulate acoustic, electromagnetic, and seismic waves. These unique properties are visible through their behavior, size, geometry, orientation, and shape.

Some examples of unique metamaterial properties include:

• Negative refractive index.

• Inverse Doppler’s effect.

• Electromagnetic cloaking.

• Negative thermal conductivity.

• Negative density and stiffness.

• Shape reconfigurability.

• Negative permeability.

• Negative dielectric constant.

• Negative Poisson’s ratio.

• Anomalous wave propagation.

• Perfect wave absorption.

• Chirality.

• Anisotropy.

Generally, all materials exhibit properties similar to their base materials and composition. For example, an integrated circuit manufactured on a silicon substrate exhibits semiconducting behavior like silicon. Such behavior does not apply to metamaterials - their properties are independent of base material and composition.

The nanostructures responsible for building metamaterials are called meta-atoms. Meta atoms are periodically or randomly arranged in groups with size and spacing much smaller than the wavelength of incoming incident electromagnetic light. The main property of metamaterials is their unusual behavior after receiving incident electromagnetic light.

Metamaterials can affect electric permittivity (0) and magnetic permeability (0). Opal and vanadium oxide can also behave like metamaterials due to their unusual behavior against incident light. However, most metamaterials are artificially produced.

The world is constantly running to miniaturize chips, manufacture room-temperature superconductors, and replace silicon and lithium with promising materials. As a result, numerous experiments are conducted worldwide to build metamaterials.

Applications of metamaterials

In addition to their unique properties and behavior, metamaterials are gaining popularity due to their wide range of applications. Metamaterials began commercialization in the early 2000s.

Electronics

Out of 1D, 2D, and 3D metamaterials, compact 1D planar metamaterials are extensively used for electronics, smart home-based IoT, and wireless power transfer applications. 3D metamaterials improve transmission efficiency but have large sizes and offer high losses. In PCBs, metamaterials have improved substrate thickness and efficiency.

Optics

One of the very first experiments of metamaterials enabled building materials with negative refractive index. These metamaterials are called electromagnetic metamaterials. They tend to bend light backward. Negative refractive index metamaterials build superlens— to reach infinite resolution beyond the diffraction limit.

Metamaterials can be used for polarization demultiplexers, on-chip biosensors, optical waveguides, light generation, and antireflection structures. Metamaterials have given rise to a new field of optics known as transformation optics. Metamaterials offer different electric permittivity (0) and magnetic permeability (0) values to manipulate light wave propagation.

Hyperbolic metamaterials

Hyperbolic metamaterials behave like metals for certain polarization angles and dielectric materials for others. Due to their anisotropic nature, they are applicable in sensing, modulation, imaging, optical signal processing, and non-linear optics.

Thermal industry

Metamaterials are used in fibers and carbon nanotubes to manipulate their heat-related behavior and negative thermal conductivity.

Renewables

As mentioned above, “perfect absorption” is a metamaterial property. It facilitates metamaterial use for renewable technologies like solar photovoltaic and photodetection systems. In addition, metamaterials can be used for energy harvesting and management in solar cells.

RF applications

Tunable metamaterials adjust their properties with respect to frequency changes. They can interact with terahertz radiation. Such metamaterials help to build antennas to enhance their radiated power for wireless applications and satellite communication.

Acoustics

Metamaterials can be used as sound filters for medical diagnostics and testing. They can aid in making noise-cancellation devices due to sound wave manipulation capabilities.

Electromagnetic cloaking

Cloaking is hypothetical, often shown in sci-fi movies. Electromagnetic cloaking is a concept where an object becomes invisible to electromagnetic waves for a specific set of frequencies. Metamaterials can offer electromagnetic cloaking. However, cloaking in any form is currently a hypothetical technology.

Seismic cloaking

Some researchers claim that mechanical metamaterial use can minimize earthquake-related damage to artificial structures. Some experiments verify that seismic metamaterials are measurable for frequencies below 100 Hz. Simulations and computations show that mechanical metamaterials exhibit seismic cloaking. Long wavelength seismic waves could be directed in a direction away from the artificial structure— as if it was never there!

Metamaterials for wireless power transfer

WPT (Wireless Power Transmission) is a concept in electronics that enables energy transfer without physical contact using electric and magnetic fields. There are two types of WPT: near field and far field. An example of near-field WPT is wireless phone charging. On the other hand, the transmission of signals through MIMO antennas is far-field WPT.

Obstacles should be absent for effective far-field WPT. On the other hand, distance is comparatively low for near-field WPT. However, there is no contact between the transmitter and receiver in both WPT types. Transmission efficiency decreases with distance. During operation, magnetic flux leakage can lead to hazardous situations.

WPT

Wireless power: Present and future developments

Metamaterial types for WPT

Metamaterials are placed between transmitting and receiving coils in a WPT system. There are three types of metamaterials based on electric permittivity (0) and magnetic permeability (0) values. Each material has its own electric permittivity and magnetic permeability. Altering both of them offers control over a material’s electromagnetic response. It can bend light waves or show negative refraction.

Electric permittivity (0) is a measure of the ability of a material to pass an electric field through it. Here, 0 in 0 marks the permittivity of free space— how much does the free space allow electric fields to spread out or resist them? Magnetic permeability (0) is a measure of the ability of a material to pass a magnetic field through it. Here, 0 in 0 represents the permeability of free space— how much does the free space allow magnetic fields to pass through them?

Double positive: Electric permittivity (0) and magnetic permeability (0) are positive. Such metamaterials exist naturally. However, the mainstream metamaterial applications do not rely on such materials.

Double negative: Electric permittivity and magnetic permeability are negative. These metamaterials are left-handed materials that exhibit properties like negative refraction.

Single negative: One of the two, electric permittivity or magnetic permeability, remains negative. Mu-negative metamaterials (MNG) have negative magnetic permeability and positive electric permittivity. Epsilon-negative metamaterials (ENG) have a negative electric permittivity and positive magnetic permeability. MNZ materials are metamaterials with near-zero magnetic permeability.

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Benefits of using metamaterials in WPT

In WPT, negative metamaterials improve transmission efficiency, misalignment tolerance capabilities, and safety. Adjusting meta-atom placements can tailor their permeability property to shield magnetic flux leakage. Placing metamaterials between two dipoles alters their mutual and self-inductances. Simply put, metamaterials increase coupling to improve overall WPT efficiency.

MNG metamaterials amplify evanescent waves to enhance the efficiency of WPT. Evanescent waves, electric or magnetic, do not propagate like electromagnetic waves due to total internal reflection at the boundary. MNG metamaterials amplify these waves to maximize magnetic field transmission.

Coil misalignment is another issue that reduces WPT efficiency. A hybrid metamaterial plate with a tunable capacitor is an optimal solution to combat coil misalignment. MNZ metamaterials provide magnetic shielding to protect WPT systems from unwanted interference without compromising efficiency.

Metamaterial limitations

Metamaterials are still a subject of research. Less practical implementation is across the electronic and optical industries. Some challenges related to metamaterials are listed below.

• High losses.

• Low practical efficiency.

• Poor low-frequency operation.

• Inability to operate at multiple frequencies for wide bands.

• Low dynamic control.

• Large size and weight make them unsuitable for portable and consumer electronic applications. The miniaturization of metamaterials is a subject of ongoing research.

• Complex and costly manufacturing processes.

References

• https://www.sciencedirect.com/topics/materials-science/metamaterial#:~:text=Metamaterial%20is%20a%20kind%20of,negative%20refractive%20index%20%5B6%5D.

• https://www.britannica.com/topic/metamaterial

• https://pmc.ncbi.nlm.nih.gov/articles/PMC10488467/

• https://www.marketsandmarkets.com/Market-Reports/metamaterials-market-139795737.html

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