Terahertz angle-insensitive 90° polarization rotator using chiral metamaterial

We propose a new uniaxial chiral metamaterial which is composed of four crescent shaped split-ring resonators mutually twisted by 90° and patterned on opposite sides of a dielectric substrate. It illustrates a strong optical activity and circular dichroism in the range from 5 GHz to 10 GHz. The presented design can be fabricated more easily due to its efficient metallic structures. Furthermore, chirality value of this proposed chiral metamaterial is large enough the optical applications. This study also shows that the proposed crescent shaped chiral metamaterials have good responses when compared with other studied samples in literature. All validations are performed by computer simulation program CST Microwave Studio in the 5–10 GHz frequency range.

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.

There has been a considerable effort recently in the development of planar chiral metamaterials. Owing to the lack of inversion symmetry, these materials have been shown to display interesting physical properties such as negative index of refraction and giant optical activity. However, the biosensing capabilities of these chiral metamaterials have not been fully explored. Ultrasensitive detection and structural characterization of proteins adsorbed on chiral plasmonic substrates was demonstrated recently using UV-visible circular dichroism (CD) spectroscopy. Second harmonic generation microscopy is an extremely sensitive nonlinear optical probe to investigate the chirality of biomaterials. In this study, we characterize the chiral response of chiral plasmonic metamaterials using second harmonic generation microscopy and CD spectroscopy. These planar chiral metamaterials, fabricated by electron-beam lithography, consist of right-handed and left-handed gold gammadions of length 400 nm and thickness 100nm, deposited on a glass substrate and arranged in a square lattice with a periodicity of 800nm.

A complementary Y-shaped chiral metamaterial (CYCMM) is proposed for the realization of giant optical activity and circular polarization with strong circular dichroism (CD) simultaneously for terahertz (THz) waves. It is demonstrated that the proposed CYCMM can achieve 90° linear polarization rotation around 2.13 THz and a giant CD effect at 2.38 THz through full-wave simulations. The mechanism of the giant CD effect and optical activity is illustrated by simulated surface current distribution. Due to strong optical activity and the CD effect, the proposed CYCMM is useful for the development of integrated terahertz spectroscopic and imaging devices.

Chiral metamaterials represent a special type of artificial structures that cannot be superposed to their mirror images. Due to the lack of mirror symmetry, cross-coupling between electric and magnetic fields exist in chiral mediums and present unique electromagnetic characters of circular dichroism and optical activity, which provide a new opportunity to tune polarization and realize negative refractive index. Chiral metamaterials have attracted great attentions in recent years and have given rise to a series of applications in polarization manipulation, imaging, chemical and biological detection, and nonlinear optics. Here we review the fundamental theory of chiral media and analyze the construction principles of some typical chiral metamaterials. Then, the progress in extrinsic chiral metamaterials, absorbing chiral metamaterials, and reconfigurable chiral metamaterials are summarized. In the last section, future trends in chiral metamaterials and application in nonlinear optics are introduced.

In recent years, considerable research has been carried out relative to the electromagnetic (EM) propagation and refraction characteristics in metamaterials with emphasis on the origins of negative refractive index. Negative refractive index may be introduced in metamaterials via different methods; one such is the con- dition whereby the Poynting vector of the EM wave is in opposition to the group velocity in the material. Alternatively, negative refractive index also occurs when the group and phase velocities in the medium are in opposition. The latter phenomenon has been extensively investigated in the literature, including recent work involving chiral metamaterials with material dispersion up to the first order. This paper examines the pos- sible emergence of negative refractive index in dispersive chiral metamaterials with material dispersion up to the second order. The motivation is to determine if using second- as opposed to first-order dispersion may lead to more practical negative index behavior. A spectral approach combined with a slowly time-varying phasor analy- sis is applied, leading to the analytic derivation of EM phase and group velocities, and the resulting phase and group velocities and the corresponding phase and group indices are evaluated by selecting somewhat arbitrary dispersive parameters. The results indicate the emergence of negative index (via negative phase indices along with positive group indices, as reported in the literature) or negative index material (NIM) behavior over infor- mation bandwidths in the low RF range. The second-order results are not significantly better than those for first-order results based on the theoretical analysis; however, greater parametric flexibility exists for the second-order system leading to the higher likelihood of achieving NIM over practical frequency bands. The velocities and indices computed using the Lorentzian and Condon models yield an NIM bandwidth around 200 − 400 Mrad∕sec, about 2 orders of magnitude higher than that for the parametric approach; more impor- tantly, NIM is found not to occur in the first order when using practical models. © 2015 Society of Photo-Optical

Optical metasurfaces, consisting of spatially variant meta-atoms, represent a new kind of optical platform for controlling the wavefront of light, with which many interesting applications, such as metalens and optical holography, have been successfully demonstrated. Further extension of the optical functionalities of metasurfaces into the nonlinear optical regime has led to unprecedented control over the local optical nonlinear generation processes. It has been shown that the nonlinear optical metasurface with achiral geometries could exhibit intrinsic optical activity in second- and third- harmonic generations. In this work, we propose an alternative approach for achieving strong nonlinear optical activity in achiral plasmonic metasurfaces by exploiting the lattice surface modes of plasmonic metasurfaces. Specifically, we theoretically and experimentally demonstrate the strong circular dichroism for second harmonic generation (SHG) on plasmonic metasurfaces consisting of split-ring resonator meta-atoms. The strong nonlinear circular dichroism is attributed to the contribution from lattice surface modes at fundamental wavelengths. Our findings may open new routes to design novel nonlinear optical devices with strong optical activity.

Recently, the terahertz (THz) chiral field control opens a new window to THz devices and their applications. In this Letter, the active manipulation for THz chiral states based on the cholesteric liquid crystal (CLC) has been demonstrated by THz time domain cross-polarization spectroscopy. The results show that the CLC has strong THz optical activity and circular dichroism (CD) effect, and the strongest THz CD of 22 dB and a polarization rotation angle of 88.4° occur around the phase transition temperature TS-N=250K. Rising to a room temperature of 300 K, the CLC turns from a chiral state to an isotropic state for THz waves with the phase transition processes of CLC molecules. Therefore, this CLC device can be performed as a thermally active THz circular polarizer, which brings potential applications in THz polarization imaging, broadband communication, and spectroscopy.

Optical activity (polarization rotation of light) is one of the most desired features of chiral media, as it is important for many polarization‐related applications. However, in the THz region, chiral media with strong optical activity are not available in nature. Here, a chiral metamaterial (CMM) structure composed of pairs of vertical U‐shape resonators of “twisted” arms is studied, and it is revealed that it demonstrates large pure optical activity (i.e., optical activity associated with negligible transmitted wave ellipticity) in the low THz regime. The experimental data show polarization rotation up to 25° for an unmatched bandwidth of 1 THz (relative bandwidth 80%), from a 130 µm‐thickness structure, while theoretical optimizations show that the rotation can reach 45°. The enhanced chiral response of the structure is analyzed through an equivalent RLC circuit model, which also provides simple optimization rules for the enhancement of its chiral response. The proposed chiral design allows for easy fabrication via direct laser writing (DLW) and electroless metal plating, making the associated structures suitable candidates for polarization control applications.

Transmission characteristics of a helically twisted photonic crystal fiber (PCF) are theoretically investigated by a recently developed beam propagation method for a helicoidal system. Experimentally reported features of the twisted PCF, such as periodical transmission dips, optical activity discontinuity, and circular dichroism, are successfully reproduced in a simulation. We reveal unique properties related to circular dichroism and optical activity in the twisted PCF. Through Stokes parameter evaluation, the polarization state of the input light in the twisted PCF is investigated in detail and the transition from linearly polarized that to circularly polarized light is demonstrated. Furthermore, the optical activity discontinuity is confirmed near the transmission dip.

A type of bilayered chiral structure is introduced. Whereas common bilayered chiral structures are based on rotation, our design is based on the displacement between two U topologies on different layers. This displacement breaks the symmetry of the pattern and generates the chirality. Prototypes are fabricated in the microwave frequency band. Simulated and measured results agree well. Both optical activity and circular dichroism are observed for circularly polarized waves. The amplitude and phase differences for the transmission coefficients of the circularly polarized waves can reach more than 12 dB and 40° at about 13.4 GHz, respectively. Since the chirality of the bilayered structure can be easily controlled by moving the position of one of the two layers, this topology can be used in the designing of tunable chiral metamaterials.

Chiral metamaterial absorbers are instrumental in a wide array of applications, encompassing multifaceted optoelectronic devices, information-encrypting metamaterials, and thermal-controlled imaging and detection systems. In this study, we showcase a Ꞩ-shaped chiral metamaterial integrated with a vanadium dioxide substrate, which enables dynamic tuning of terahertz (THz) dual-band circular dichroism (CD). This absorber boasts spin-selective absorption, stemming from the selective excitation of the plasmon polaron mode on its horizontal microrod. Notably, right-handed circularly polarized light (CPL) experiences near-perfect absorption at resonance, whereas left-handed circular polarization is absorbed to a much lesser degree, yielding pronounced CD. This exceptional chiral selectivity facilitates dual-band CD effects, with peak values reaching 0.814 and 0.784. Furthermore, by manipulating the conductivity of vanadium dioxide, we can dynamically adjust both the CD and absorption peaks of the chiral metamaterial. The maximum modulation depth of the CD response can attain values as high as 0.807 and 0.776. Our work presents a novel design strategy for the active control and modulation of CPL, thereby expanding the horizons of metamaterial absorber applications within the THz spectrum.

Circular dichroism (CD) of chiral metamaterials is of significant interest for bio-sensing and molecular chemistry applications. In this study, using the gammadion shape as an example, the CD effect of gammadion-shaped chiral metamaterials is investigated. We view the gammadion as four Γ-shaped structures. The CD effect of the gammadion is induced by the conductive and magneto-inductive coupling between the weak and strong magnetic fields of the Γ-shaped structures. A large CD effect can be achieved because the magnetic fields excited by the conductive and magneto-inductive coupling show the same and opposing directions for right- and left-handed circular-polarized waves, respectively. This allows analysis of the origin of plasmonic modes in the CD spectrum. We can change the CD effect by changing the conductive and magneto-inductive coupling of the magnetic fields. The CD properties can be tuned by changing either the number of Γ-shaped structures or the layer spacing. These results may guide the design of chiral metamaterials at terahertz frequencies for achieving large CD effects. Furthermore, this approach can be applied to other complex planar structures to gain further insight on the origin of CD modes.

Transmission characteristics of helically twisted photonic crystal fiber is theoretically investigated by using recently developed beam propagation method for helicoidal system. Experimentally reported features of twisted PCF, such as periodical transmission dips, optical activity discontinuity, and circular dichroism are successfully reproduced in the simulation.

Optical activity, a foundational part of chemistry, is not restricted to chiral molecules although generations have been instructed otherwise. A more inclusive view of optical activity is valuable because it clarifies structure-property relationships however, this view only comes into focus in measurements of oriented molecules, commonly found in crystals. Unfortunately, measurements of optical rotatory dispersion or circular dichroism in anisotropic single crystals have challenged scientists for more than two centuries. New polarimetric methods for unpacking the optical activity of crystals in general directions are still needed. Such methods are reviewed as well as some of the 'nourishment' they provide, thereby inviting to new researchers. Methods for fitting intensity measurements in terms of the constitutive tensor that manifests as the differential refraction and absorption of circularly polarized light, are described, and examples are illustrated. Single oriented molecules, as opposed to single oriented crystals, can be treated computationally. Structure-property correlations for such achiral molecules with comparatively simple electronic structures are considered as a heuristic foundation for the response of crystals that may be subject to measurement.