Metamaterial

Metamaterials get their unusual capabilities from how they are built, not from what they are made of. The word combines Greek *meta* ("beyond") and Latin *materia* ("matter"). Their properties—often impossible to find in nature—come from deliberately arranged internal structures, typically repeating patterns of metals and plastics. These patterns are smaller than the wavelengths of the waves they affect. By controlling shape, geometry, size, orientation, and arrangement, metamaterials can block, absorb, enhance, or bend electromagnetic, acoustic, or seismic waves, outperforming conventional materials. Much research has focused on those that achieve a negative index of refraction for certain wavelengths.

Applications span many areas: sports gear, optical filters, medical devices, aerospace, sensors, infrastructure monitoring, smart solar power, lasers, crowd control, radomes, battlefield communications, high-gain antenna lenses, ultrasonic sensors, and even earthquake shielding. Super-lenses are a promising possibility. A form of invisibility has been demonstrated using gradient-index materials. Acoustic and seismic metamaterials are also active research fields.

The field draws on electrical engineering, electromagnetics, classical optics, solid-state physics, microwave and antenna engineering, optoelectronics, materials science, nanoscience, and semiconductor engineering. Recent work suggests metamaterials could improve optical computing, theoretically performing certain tasks more efficiently than conventional computers.

**History**

Artificial materials for manipulating electromagnetic waves were explored as early as the late 1800s. In 1898, Jagadish Chandra Bose studied substances with chiral properties. Early in the 1900s, Karl Ferdinand Lindman investigated wave interactions with metallic helices as artificial chiral media. In the late 1940s, Winston E. Kock at Bell Labs developed materials with metamaterial-like characteristics. During the 1950s and 1960s, artificial dielectrics were studied for lightweight microwave antennas. Microwave radar absorbers were researched in the 1980s and 1990s as applications for artificial chiral media.

Victor Veselago first theoretically described negative-index materials in 1967, proving they could transmit light and that phase velocity could be anti-parallel to the Poynting vector—opposite to natural wave propagation. In 1995, John M. Guerra fabricated a sub-wavelength transparent grating (later called a photonic metamaterial) with 50 nm lines and spaces, coupling it with a standard oil immersion microscope objective (later called a super-lens) to resolve a 50 nm grating on a silicon wafer using 650 nm illumination.

In 2000, John Pendry identified a practical way to make a left-handed metamaterial, where the right-hand rule does not apply, allowing energy (group velocity) to move against phase velocity. He proposed that metallic wires aligned with a wave could provide negative permittivity (ε < 0). While natural materials like ferroelectrics already show negative permittivity, achieving negative permeability (μ < 0) was the challenge. In 1999, Pendry showed that a split-ring (C-shaped) resonator with its axis along wave propagation could do this, and that a periodic array of wires and rings could yield a negative refractive index. He also proposed the Swiss roll design for negative permeability.

In 2000, David R. Smith and colleagues experimentally demonstrated functioning electromagnetic metamaterials by stacking split-ring resonators and thin wire structures periodically. In 2002, a method using artificial lumped-element loaded transmission lines in microstrip technology realized negative-index metamaterials. By 2003, both the real and imaginary parts of a negative refractive index were demonstrated, along with imaging by a flat lens using left-handed metamaterials. Vladimir Shalaev and others first demonstrated a negative refractive index in the optical range. By 2007, many groups had conducted negative-index experiments. The first imperfect invisibility cloak at microwave frequencies was realized in 2006.

Researchers now classify metamaterials into three branches based on governing equations: electromagnetic/optical wave metamaterials (Maxwell's equations for transverse waves), other wave metamaterials (for longitudinal and transverse waves), and diffusion metamaterials (for diffusion processes). Diffusion metamaterials are designed to control diffusion activities, with diffusion length as the central metric—this parameter changes over time but is unaffected by frequency variations.