Abstract
Metamaterials are artificially-engineered materials which possess unique properties not found in natural materials. The properties are derived from the structural designs of metamaterials and they allow the structure to manipulate electromagnetic waves and achieve desired responses in a certain frequency range. This chapter reviews past achievements, recent developments, and future trends on electromagnetic metamaterials in microwave regime. The chapter first briefly introduces electromagnetic metamaterials from a general prospect including the definition, historical overview, and classification of metamaterials. Furthermore, three selected applications of metamaterials which are microwave absorbers, sensors, and energy harvesters are discussed based on their operation principles, designs, and characteristics.
TopElectromagnetic metamaterials are artificial materials engineered to have a geometrical structure which possesses unique electromagnetic properties at a certain range of frequency. The novel macroscopic properties are originated from both the properties of the constituent materials and their designed geometry. The effective properties of metamaterials do depend on its properties of their constituents. The structures are usually in periodic arrangement and the unit cell size is small compared to the wavelength. Resonances with the incident electromagnetic waves are normally occurred at the operating frequencies.
The history of electromagnetic metamaterials begins with the development of artificial materials with desired electromagnetic responses in the 19th century. For example, artificial dielectrics were designed using metallic arrays of small spheres, disc or round wires (Kock, 1948). Artificial magnetics were introduced by using an electrically conducting loops loaded with capacitor (split ring resonator) (Schelkunoff & Friis, 1952). Artificial chiral composites were used as microwave absorber (Varadan, Varadan, & Lakhtakia, 1987).
In 2000, metamaterials with negative refractive index became the significant breakthrough of the research on artificial electromagnetic materials (Smith & Kroll, 2000). Negative refractive index was found to be obtained by combining two structures which individually exhibit negative permittivity and negative permeability. This phenomenon is a resonant effect and it causes the media to be dispersive and dissipative (Ramakrishna, 2005). Different basic elements of metamaterials (electric dipoles, magnetic dipoles or chiral particles) can be combined to realize desired response.
The current flourishing state of metamaterials studies have proven the applicability of metamaterials in various applications including absorbers, sensors, energy harvesters, antennas, lenses and filters. Besides microwave frequency range, research of metamaterials in different frequency ranges of the electromagnetic spectra have been carried out. Based on the properties of metamaterials, there are several types of metamaterials such as double negative (DNG) materials, single negative (SNG) materials, artificial magnetic conductors (AMC), and frequency selective surfaces (FSS). Their characteristics and applications are tabulated in Table 1.
Table 1. Classification of metamaterials
| Metamaterials | Characteristics | Applications |
Double negative materials (DNG)
(also known as left-handed metamaterials (LHM), negative-index materials (NIM) or backward-wave media) | • Negative value of permittivity and permeability simultaneously
• Negative refractive index
• Reverse propagation | Absorbers, Antennas, Waveguides |
Single negative (SNG) materials
(or more specifically known as ε-negative (ENG) materials or μ-negative materials (MNG)) | • Negative value of permittivity or permeability | Antennas |
| Artificial magnetic conductors (AMC) (also known as high impedance surface (HIS)) | • Behave as perfect magnetic conductor although composed by non-magnetic materials
• Provide zero-degree reflection phases at resonant frequency | Antennas, Waveguides |
| Frequency selective surfaces (FSS) | Tailor frequency selectiveness | Filters, Antennas, Microwave ovens |
Key Terms in this Chapter
Resonator: A structure that exhibits resonance at a specific frequency.
Metamaterial: A material which exhibit unique characteristics which are originated from its engineered geometrical structure and depend on the properties of its constituent materials.
Tunable: Able to be tuned, adjusted, switched or shifted to a new desired operating frequency.
Energy Harvester: A device which captures energy from external sources for other usages including power up electronics devices or recharge batteries.
Sensor: A device which is used to detect the changes such as temperature, moisture content, concentration, permittivity, position and velocity.
Absorber: A structure which absorbs the incident electromagnetic energy with negligible reflection and transmission.
Absorption Bandwidth: The range of frequencies in which the absorptivity reaches above a particular level.