ABSTRACT
Electromagnetic (EM) metamaterial is an array of subwavelength structures that can be engineered to achieve specific EM properties. The properties of the metamaterials strongly depend on the geometrical shape and size of the subwavelength unit cell, also termed as “meta-atoms.” This has led to the demonstration of numerous interesting EM properties, such as artificial magnetism [1], negative refractive index [2], subwavelength focusing [3], perfect absorption [4], chirality [5], narrow band emission [6], electromagnetically induced transparency analogue [7–11], and many more, over a wide range of the EM spectrum. The disruptive feature of metamaterials lies in the scalable design and ultrathin size relative to the interacting wavelengths of EM waves. This scalability of the metamaterial design has been instrumental in enabling significant research progress in the field of terahertz (THz) wave interaction and manipulation. The THz part of the EM spectrum (f ∼ 0.1 − 10 THz; λ ∼ 3000 − 30 μm) lies in between the low energy microwaves and high energy infrared regions, and has minimal interaction with naturally occurring materials. Furthermore, the THz spectrum lies in between the electronics and photonics realms used for controlling the EM waves. The electronics components used for interacting with microwaves cannot be scaled up to operate at THz frequencies and the low energy of THz waves compared to IR waves limits the usage of photonics devices for THz wave interaction. This technological vacancy between the electronics and photonics realms of the electromagnetic spectrum is popularly termed the “THz gap” [12]. However, the versatility of the metamaterial resonator designs and the ease of fabrication of metamaterials have led to the realization of various THz devices. Metamaterial resonators are mostly designed to interact with either the magnetic field and/or the electric field of EM waves. The metamaterial acts as an effective medium, and its response to the incident magnetic and/or electric field will provide effective permeability (μr ) and/or effective permittivity (εr ) values, respectively. These two fundamental optical parameters can then be engineered to achieve other optical parameters, such as refractive index, surface impedance, and so on. This allows for the advanced interaction, control, and manipulation of EM waves using metamaterials. The geometrical shape and size of the meta-atom primarily determines the coupling mechanism of the incident field to the metamaterial and the operating spectral range. The most popular meta-atom design for enabling the magnetic response is the split ring resonator (SRR), which is a metal ring with a split gap in one side as shown in Figure 13.1a. The magnetic response of the SRR can be achieved either by having the incident magnetic field be perpendicular to the plane containing the SRR [1] or with the incident electric field parallel to the SRR gap [13]. The magnetic response is characterized by the circulating current induced in the metallic ring with a strong field confinement across the capacitive gap. The magnetic moment generated in response to the circulating currents in the planar SRR will always be in the plane perpendicular to the SRR. The SRR can be modelled as an electrical inductive-capacitive resonator (LC), as shown in Figure 13.1b. Thus, the resonant frequency can be given as fr = 1/√(LC), where “L” is the effective inductance of the metal ring and “C” is the effective capacitance across the split gap. The operational frequencies of the SRR can be scaled by varying the geometrical parameters of the SRR accordingly. Alternatively, a simple cut wire resonator (CWR), which is primarily a dipole resonator, is used as the meta-atom to achieve electrical response of the metamaterial by interacting directly with the electric field of the incident THz waves. The electric field response of the CWR is characterized by the induced surface current that is parallel to the incident electric field. Another popular meta-atom design for interacting with the incident electric field is the electrical split ring resonator (ESRR), which is formed by placing two SRRs facing each other [14]. When the incident electrical field is along the gap-bearing side of the ESRR, antiparallel currents are induced in the two SRRs with strong field confinement in the gap region. Due to causality, the response is electrical, and hence provides an effective permittivity value for the ESRR metamaterials. The resonance frequency of the ESRR, fr = 2/√(LC), and can be engineered based on the geometrical parameters of the ESRR. The metamaterial unit cell can also be made of multiple resonators that are coupled in either in-plane or out-of-plane directions. This enables more advanced THz manipulation. Hence, the versatility of the resonator design by itself offers a wide range of THz properties.
