A simplified introduction to Metamaterials and Metasurfaces
Metamaterials
The word meta means beyond in Greek. Therefore, the term metamaterials means “beyond materials”.
Electromagnetic wave interacts with materials which occur in nature, causing them to respond in many specific ways, depending on their microscopic properties. In dielectric (insulating) materials, such as paraffin, teflon, bakelite, etc., their molecules interact with the electric field, producing electric dipoles (or electric polarization) parallel to the direction of the applied electric field, storing electric energy inside the material. Their ability to interact with the electric field is measured via their dielectric permittivity or dielectric constant εr. The larger its magnitude, the stronger the interaction of the material with the electric field and, therefore, its ability to store energy.
In magnetic materials (cobalt, nickel, iron), an applied magnetic field causes the alignment of their magnetic dipoles at a parallel direction (magnetic polarization). Magnetic interaction is measured by the materials’ magnetic permeability μr.
A few decades ago, around the late 90s, some artificial materials were invented, which - when interacting with the electromagnetic field - exhibit properties that are not available in natural materials. For example, the structure illustrated in Fig. 1, which consists of metallic split-rings printed on a dielectric substrate, is able to produce magnetic polarization in the opposite direction (antiparallel, directed on the negative z-axis) with respect to the direction of the magnetic field (directed on the positive z axis), at a specific frequency zone.
These structures are called Split-Ring Resonators (SRRs) and consist the main class of metamaterials. These kinds of structures may be easily fabricated in the laboratories, by the same procedures typical electronic circuits are manufactured (developing and etching of printed circuit boards). Another interesting property of the SRRs is that although they do not contain any magnetic materials (they are made of dielectric and metallic parts) they produce artificial magnetic response. Metamaterials mostly consist of periodically arranged patterns of resonators but in some cases, non-periodic arrangements may also be utilized.
Thus, by the term metamaterials, we name the artificial materials (i.e. those that do not exist in nature) and have the ability to offer exotic electromagnetic properties that the materials found in nature do not possess at all. When the electromagnetic wave interacts with metamaterials, it results in some very unique phenomena. These may be exploited so as to optimize the performance of antennas, waveguides, transmission lines and other electromagnetic structures.
Fig. 1. The Split-Ring Resonator (SRR) particle (left). Printed repetition of SRRs (right).
Metasurfaces
Metasurfaces are the two-dimensional versions of metamaterials: A periodic repetition of resonators in a two-dimensional plane (i.e. on a surface) forms a periodic metasurface. Typically, metasurfaces’ thickness is much smaller than the wavelength of the interacting electromagnetic wave. A typical metasurface is depicted in Fig. 2.
In general, metasurfaces are more easily fabricated than the corresponding metamaterials and are more light-weight. These are the main advantages they possess when compared to their three-dimensional versions.
MTSs have the ability to control the electromagnetic wave in many ways (absorb, block, guide, concentrate) in a great frequency range, from microwave frequencies (GHz) to visible frequencies (THz).
A very popular metasurface application is their utilization as electromagnetic wave absorbers. The periodic structure shown in Fig. 2 is capable of absorbing the incident electromagnetic field, when it impinges normally to the surface. It is designed in such a way, that for microwave frequencies between 8 and 12 GHz, the incident electromagnetic energy is totally absorbed by the metasurface and no energy is reflected or scattered at any other directions.
Fig. 2. Electromagnetic absorber. A metasurface structure designed to absorb electromagnetic energy at microwave frequencies (8 – 12 GHz).
Another application of metasurfaces is their utilization in electromagnetic structures, where the blocking of the electromagnetic wave is desired. A waveguide is a structure that is capable of guiding the electromagnetic wave in desired directions with minimized losses. In Fig. 3 the Split-ring resonator Substrate-Integrated (SRR-SIW) waveguide is illustrated. Here, the resonators are utilized as a means to block the electromagnetic field radiation to directions other than the desired one (which is the one from the input port on the left to the output port on the right). To this end, they are designed so as to be able to block the electromagnetic energy at certain frequency zones.
Another interesting application of metasurfaces and metamaterials is the field of antennas. In the aformentioned waveguide configuration, if several slots are opened through the upper (and/ or) the lower surface of the waveguide, as illustrated in Fig. 4, then a so-called Leaky-wave antenna is designed and the electromagnetic wave may be radiated to free space at a direction normal to the surface of the antenna.
Fig. 3. The Split-Ring Resonator Substrate-Integrated Waveguide (SRR-SIW). A metasurface-based waveguide structure capable of guiding the electromagnetic wave at microwave frequencies.
Fig. 4. A metasurface-based Leaky-Wave antenna. This structure radiates the electromagnetic wave efficiently at a central frequency of 10 GHz.
References
1. M. Nitas and T. V. Yioultsis, "Characterization of Edge-Coupled Broadside- Coupled and Complementary Split-Ring Resonator Periodic Media Based on Numerical Solutions of Eigenvalue Problems," in IEEE Transactions on Microwave Theory and Techniques, vol. 69, no. 12, pp. 5259-5269, Dec. 2021, doi: 10.1109/TMTT.2021.3116023.
2. Nitas, Michalis, Maria‐Thaleia Passia, and Traianos V. Yioultsis. "Fully planar slow‐wave substrate integrated waveguide based on broadside‐coupled complementary split ring resonators for mmWave and 5G components." IET Microwaves, Antennas & Propagation 14.10 (2020): 1096-1107.
3. Li, Aobo, Shreya Singh, and Dan Sievenpiper. "Metasurfaces and their applications." Nanophotonics 7.6 (2018): 989-1011.
4. Holloway, Christopher L., et al. "An overview of the theory and applications of metasurfaces: The two-dimensional equivalents of metamaterials." IEEE antennas and propagation magazine 54.2 (2012): 10-35.
5. Fernández, Alberto Alvarez. Hybridization of block copolymer thin films with plasmonic nanoresonators for optical metamaterials design. Diss. Université de Bordeaux, 2018.