Review on Polarization Selective Terahertz Metamaterials: from Chiral Metamaterials to Stereometamaterials
In this article, recent progress and development of terahertz chiral metamaterials including stereometamaterials are thoroughly reviewed. This review mainly focuses on the fundamental principles of design and arrangement of meta-atoms in metamaterials exhibiting chirality with various asymmetry and symmetry and 2D and 3D configuration. Related optical and propagation properties in chiral metamaterials, such as optical activity, circular dichroism, and negative refraction for each different chiral metamaterials, are compared and investigated. Finally, comparison between chiral metamaterials with stereometamaterials in terms of the polarization selective operation along with the similarity and the distinction is addressed as well.
KeywordsTHz metamaterials Polarization selective Chiral metamaterials Stereometamaterials Optical activity
Metamaterials, a new class of engineered materials consisting of arrays of subwavelength scale resonant building blocks, i.e., meta-atoms, have been the focus of growing interest for both fundamental research and practical applications since they were first predicted by Vesalago and realized by Pendry and Smith [1, 2, 3, 4, 5, 6, 7, 8, 9]. Through proper arrangement of these meta-atoms and related structures, metamaterials can be engineered to exhibit negative index of refraction (n < 0) that does not occur in nature. By manipulating both the electric permittivity, ε(ω) = ε(ω) + iε(ω), and the magnetic permeability, μ(ω) = μ(ω) + iμ(ω), the electric and magnetic responses of metamaterials can be independently adjusted, which can provide a convenient method to realize a wide range of devices. This ability of independent adjustment also provides a significant advantage over conventional materials which, generally, do not support this capability. So far, the most conventional metamaterial structure consists of two parts: one is metal wire which provides negative permittivity, and the other consists of split ring resonators (SRRs) to achieve negative permeability, resulting in a combination that exhibits a negative refractive index at certain resonant frequencies. Another important advantage of metamaterials is that they can be geometrically scaled in order to operate in any desired frequency regime since the fundamental properties of subwavelength structures can be applied to any frequency and wavelength spectrum. These electromagnetically active structures are very promising in the development of low cost, ultrasensitive, and easy-to-use room-temperature devices, especially those operating at terahertz (THz) frequencies to fully exploit the wide range applications of this scientifically rich spectrum [10, 11, 12, 13].
THz waves, spanning from 0.1 to 10 THz, have not been fully utilized mainly due to the lack of high-performance sources, detectors, and availability of other passive components, but are considered one of the most intriguing frequency ranges of electromagnetic (EM) waves that can be used from fundamental physical sciences to biomedical imaging [10, 12, 14, 15, 16, 17, 18]. Meta-atoms have been developed in various types, such as spiral , fishnet , labyrinth , and electric field coupled (ELC)  resonators and could lead to various applications such as image compression, superlenses, subwavelength imaging, cloaking, perfect absorption, and black holes [23, 24, 25, 26].
Following the rapid progress in the development and demonstration of THz metamaterials for various applications during the last few decades, achieving polarization selection and control of propagation wave through metamaterial media became extremely important. With more sophisticated structures, the manipulation of polarization using metamaterials in general becomes more effective as well. Furthermore, chiral metamaterials and stereometamaterials have gained great attention for their unique and exotic propagation properties in terms of polarization.
Chiral metamaterials are based on arrangement of meta-atoms that exhibit polarization properties like left handedness (LH) and right handedness (RH). It is common for most optical systems to exhibit such polarization dependence when they are subjected to circularly polarized light. Chirality, a word that originated from Greek, meaning “hand,” can be found in objects with no internal mirror symmetry in their structures. In fact, a hand is the most easily understood example to explain chirality. The left hand can never be superimposed on the right hand no matter how it is rotated. Chirality is most commonly found in many biomolecules which directly impacts important functions in chemistry, biology, and pharmaceuticals. Like sugar (glucose), many biologically active molecules such as amino acids and enzymes are chiral and lack internal mirror symmetry. Such biomolecules’ chirality has been probed with circularly polarized light, since the polarization dependence is the major distinction between chiral and non-chiral molecules. Furthermore, like the left and right hand, which are mirror images of each other, enantiomers are two stereoisomers that are related to each other by reflection, i.e., mirror images of each other that cannot be superimposed. Therefore, chiral molecules naturally fit into the part of stereoisomers.
Stereoisomers have the same molecular formula and same sequence of molecular bonding, but differ in their spatial orientation and/or rotational symmetry. Stereometamaterials stem from the same conceptual structure of stereoisomers, in which their optical and propagation properties differ by spatial constitution rather than the distinction of meta-atoms’ properties. Since some stereoisomers exhibit their chirality, it becomes interesting how polarization sensitivity is revealed in stereometamaterials as well.
In this review, we will focus on recent development of chiral and stereometamaterials in terms of the design of meta-atoms and the optical and propagation properties reported. In the selection of the references, we decided to focus on chiral and stereometamaterials operating at only THz frequencies; therefore, infrared, visible, and microwave chiral and/or stereometamaterials were not included as references. There have been a few review articles on chiral metamaterials so far which included other operating frequencies and wavelength regime [27, 28, 29]. The polarization selective transmission and reflection properties became critical measure of determining the chirality of the reported structures and are compared in some of the reported structures reviewed here. After that, we will compare the polarization properties of stereometamaterials, and finally, the prospective applications will be discussed.
2 Structural Analysis of Chiral THz Metamaterials
2.1 Types of Structures
There are a wide variety of chiral metamaterials that successfully show various chiral optical properties in the THz regime. Although their structures range from simple single-layer metamaterials to complex multilayer ones, we categorize them here based on a few basic principles. First, we look into the overall structure of the device: they can be multilayered or non-planar 3D structures, or more simplified, single-layered 2D structures. Then, we will look specifically into the geometrical patterns used in the devices and categorize them based on their symmetry.
In contrast to 3D and freestanding design, the conventional 2D planar design of chiral metamaterials (CMMs) appears less frequently [42, 43, 44]. Singh et al.  reported 2D chiral pattern based on pairs of split ring resonators adjoined together with orthogonal orientation. The planar chiral metamaterials are made of 200-nm-thick aluminum on a 640-μm-thick silicon substrate as shown in Fig. 1d. The asymmetric transmission was observed from the structure by changing the incident wave direction; it is right-handed when observed from the front and left-handed when observed from the back. Wongkasem et al.  presented a planar metamaterial based on the Y structure as shown in Fig. 1e. In this design, co-polarized and cross-polarized transmission coefficients were successfully measured in the THz regime. This Y-shaped planar CMM demonstrates negative refraction index metamaterial.
2.2 Symmetries in Designing of Chiral Metamaterials
Apart from the overall structure being either 2D or 3D, another interesting feature is the type of symmetry observed in the geometrical designs of these THz chiral metamaterial structures. Ideally, chiral molecules should not have any in-plane mirror symmetry in order for it to display optical activity. However, with metamaterials, we see that this definition does not always hold. Although most geometrical shapes used in chiral metamaterials have no in-plane symmetry, more and more devices with one or more forms of symmetry are now being made that still show optical or extrinsic chirality.
2.2.1 Chiral Structures Without In-Plane Mirror Symmetry
First, we will discuss the most popular case: pure chiral structures. These are geometries without any in-plane mirror symmetry. However, most of them have some form of rotational symmetry. Several of them [30, 31, 35, 36, 37, 40, 45] show fourfold rotational symmetry. Most of these fall within these three common shapes—the rosette, gammadion, and cross-wires.
Cross-Wires and Spirals
Ding et al.  uses cross-wires in their complementary chiral MMs (CCMMs) as shown in Fig. 2e. The various parameters for the structure are tm = 0.2 μm, t = 2 μm, p = 20 μm, w = 2 μm, l = 18 μm, and the twist angle ϕ is 30°. The metal used for the resonators is gold due to its very low loss in the terahertz and higher frequency regions, and benzocyclobutene (BCB) as the dielectric spacer. They numerically studied its optical activity, negative refractive index, and also the influence of the twist angle, dielectric layer thickness, width, and length of the complementary cross-wire pairs on the optical activity. Yogesh et al.  reported bilayer-twisted Fermat’s spiral chiral metamaterials as a circular polarizer and a polarization filter. A quadfilar system has four Fermat’s spiral (FS) arms with 90° rotational arrangements, and each spiral consists of a spiral turn of 2п as shown in Fig. 2f. The chiral metamaterials’ design consists of the spiraling constant of A = 4.8 μm/(rad)1/2. This design is based on mutually conjugated quadfilar system, in which the configuration breaks the mirror symmetry throughout the aspect of propagation of light. It successfully demonstrated higher optical isolation, wide-angle operation, and large bandwidth. Tang et al.  recently suggested a new planar spiral chiral structure having the fourfold rotational symmetry. This design has two spiral metallic silver layers on both sides of a dielectric substrate as shown in Fig. 2g. The unit cell’s parameters are a = 31 μm, d2 = 3.7 μm, l = 12 μm, and w = 2 μm. The rotation of angle between front and back metallic layer θ is 45°. Such structure reveals the cross-polarization conversion (CPC) and strong optical activity in THz frequency. Finally, He et al.  also uses a spiral structure of up to six layers as part of their device as shown in Fig. 2h. A single-layer array of metallic spirals, with the helix diameter R = 36 μm and width of 4 μm, arranged to constitute CMM. A BCB dielectric spacer of thickness d = 18 μm and aluminum metal spiral of 200-nm thickness was used. A mutual twist angle θ is maintained between the multiple metal resonator layers. Parameters such as the number of layers and the twist angle θ are varied to study their influence in producing strong chirality and, in turn, achieve a negative refractive index.
Other Exotic Designs of THz Chiral Metamaterials
2.2.2 Chiral Structures with In-Plane Mirror Symmetry
Some structures have both rotational symmetry and in-plane mirror symmetry, which technically makes it achiral. However, when you look through both the layers comprising its 3D structure, the in-plane symmetry is broken, thereby making it a chiral structure after all. Ozer et al.  proposed one such chiral metamaterial device with resonator metallic parts made of 0.2-μm-thick silver placed to the front and back of a 450-μm-thick quartz dielectric substrate. The structure provides asymmetric transmission for linearly polarized EM waves. Here, silver was chosen for its low resistivity.
3 Polarization Selectivity and Other Optical Properties of Chiral THz Metamaterials
3.1 Optical Activity
Another case of dispersionless and giant optical activity is reported by Wu et al. . Here, in between the two resonant frequencies, they see a polarization rotation of 10° while the ellipticity reaches zero. However, Zhou et al.  at zero ellipticity achieve a rotation angle slightly greater than 12° in the off-resonance region. They also dynamically controlled the chirality of the sample by changing the conductivity of the silicon substrate. The conductivity was experimentally varied by using near-infrared femtosecond laser to excite the photo-carriers in the intrinsic silicon. Zalkovskij et al.  was able to show optical activity reaching up to a polarization plane rotation rate of 500°/λ for their V-shaped Babinet asymmetric dimer as shown in Fig. 4a–c. This result clearly shows that a single layer of planar babinet structures is comparable to that of other more complicated, multilayered structures. Also, the results of the V-shaped babinet are compared to the results of the corresponding parallel-shaped babinet metamaterial. The parallel ones exhibit no optical activity, proving that the asymmetric design of the V-shaped slot dimers greatly contributed in the anisotropic transmission behavior. Kanda et al. [50, 51] explains that relatively thin structured samples with a step of only 100 nm cause strong polarization effects for the THz wave. This will open new techniques to control the polarization of THz wave by classic microprinting techniques. Additionally, it may be possible to achieve a THz polarization modulator in combination with MEMS technology as well. The chiral samples indicate that marked orthogonal components of the electric fields and the sign of the electric fields are opposite for right-twisted and left-twisted gammadion. The amplitude of frequency domain spectra, Ex(ω), is almost the same for right-twisted and left-twisted patterns, and much larger than that of the cross pattern. In addition to the mechanical tuning of the optical activity of the THz CMM, photo-control of optical activity has been studied as well by the same group  and another group . The measured transmission spectra and polarization-rotation spectra of chiral and achiral structures concluded that an increase of the pump power prevented the transmittance. The induced optical activity was not linked to the pump beam’s polarization direction.
Kenanakis et al.  studied numerically the dynamical tuning capabilities of different THz chiral metamaterial structures with specific metallic parts replaced by photo-conductive silicon. The response of the structure analysis depends on the calculation of the transmission spectrum for linearly polarized incident wave. The structures show regions with giant tunable optical activity and impressive dynamical tuning of the ellipticity of the transmitted wave. It demonstrates the notable switchable polarizer capabilities of reported structures in various frequency regimes: regarding the cross-wire structure around 5.5 THz, it can be easily switched from circular to linear polarizer by changing the flux of the excitation power. Similar switchable response is observed also at ~10 THz for the cross-structure and at ~8.8 THz for the second cross-structure.
3.2 Circular Dichroism
Another very prominent property of chiral metamaterials is circular dichroism (CD). It is measured by the difference in transmission (or reflection) of left and right circular polarized light through the chiral medium. A strong CD can also imply that one of circular polarizations passes through the medium by a larger amount than the other, and as a result enables the medium to be polarization selective. There are several chiral metamaterials that exhibit strong CD [33, 37, 40, 41, 46, 52].
3.3 Negative Refractive Index
Theoretically, chirality was originally proposed by several research groups as an alternative route to obtain negative refractive index [53, 54, 55, 56]. Pendry proposed a 3D spiral (helical) structure that can realize negative refractive index in the given medium, which turned out to be a chiral metamaterial . Tretyakov et al. theoretically calculated a negative refraction in chiral composites consisting of chiral and dipole particles. With the theoretical support of realizing negative refraction, most chiral metamaterials reported here actively measured negative refractive index. Negative refraction is one of the most desirable properties to attain from the chiral metamaterials.
4 Polarization Controlled THz Stereometamaterials
As a broader and less investigated class of metamaterials, THz stereometamaterials designate structures that are realized using the same building blocks or meta-atoms but arranged differently in space and exhibiting distinct responses to THz waves as a direct result [57, 58, 59]. Stereometamaterial is still in its infancy, and tremendous fundamental knowledge can be gained through a correlated theoretical and experimental investigation.
This can be illustrated in one of our recent studies of perfect absorber structure consisting of two non-concentric copper rings, which showed rotationally asymmetric THz absorption and reflection properties at resonant frequencies depending on the polarization of the incident wave . The investigation showed that such a THz device had a rotationally asymmetric absorption and reflection characteristics at resonant frequencies, i.e., dependent on the polarization of the incident wave.
Based on these findings, the absorption spectra at 90° and 0° incident wave polarization can be interpreted as a wave interaction phenomenon between two waves. The first one originated from the reflection from the overall effective dipole on the stereometamaterial FSS. The other wave arose from the mirror image dipole to that of the FSS with respect to the backplane, after undergoing multiple internal reflections through a cavity, formed by the FSS and the metal backplane, as it propagates. At resonance, a perfect absorption occurred because of the destructive interference between these two waves.
5 Remark and Conclusion
In this review, we surveyed and thoroughly reviewed recent development of more sophisticated metamaterial structures called chiral metamaterials and stereometamaterials in the THz frequency region and their ability to perform as polarization waveplates, filters, modulators, etc. More and more new and developed concepts of chiral and stereometamaterials have appeared since the last few review articles [27, 28, 29, 67]. THz frequency is critically important and has become the center of research as it has the potential to find and reveal unknown nature of the fingerprints of many naturally occurring molecules. It is reasonable why THz chiral metamaterials and stereometamaterials have become so exciting. The interest in chirality and stereoisomers can be related to naturally occurring molecules and materials that we would like to explore. Using such artificially engineered meta-atoms which constitute metamaterials, we can mimic some of these exotic properties and hope to unveil some of the natural phenomena associated with it. Such new development can bring THz metamaterials to a new horizon of applications. The exploration of such exquisite metamaterials will continue and contribute to the rapidly developing new technologies.
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