Retrieval parameters of chiral metamaterials with proposed crescent shaped split ring resonators

Terahertz angle-insensitive 90° polarization rotator using chiral metamaterial

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.

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.

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 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.

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.

A chiral metamaterial based on a double-layer twisted Y shape structure is presented that achieves obvious circular dichroism and giant optical rotation at different frequencies. When a y-polarized wave is incident to this chiral metamaterial, circular dichroism with a great difference between the transmission coefficients for the circularly polarized waves is obtained at 12.28 GHz. Meanwhile, 90° optical rotation is observed at 12.70 GHz, where the incident y-polarized wave is transformed into its cross-polarization with a transmission coefficient of -1.15 dB. Surface current distributions are calculated to demonstrate the coupling between the electric and magnetic fields.

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.

A model-agnostic data enhancement (MADE) algorithm is proposed to comprehensively investigate the circular dichroism (CD) properties in the higher-order diffracted patterns of two-dimensional (2D) chiral metamaterials possessing different parameters. A remarkable feature of MADE algorithm is that it leverages substantially less data from a target problem and some training data from another already solved topic to generate a domain adaptation dataset, which is then used for model training at no expense of abundant computational resources. Specifically, nine differently shaped 2D chiral metamaterials with different unit period and one special sample containing multiple chiral parameters are both studied utilizing the MADE algorithm where three machine learning models (i.e, artificial neural network, random forest regression, support vector regression) are applied. The conventional rigorous coupled wave analysis approach is adopted to capture CD responses of these metamaterials and then assist the training of MADE, while the additional training data are obtained from our previous work. Significant evaluations regarding optical chirality in 2D metamaterials possessing various shape, unit period, width, bridge length, and separation length are performed in a fast, accurate, and data-friendly manner. The MADE framework introduced in this work is extremely important for the large-scale, efficient design of 2D diffractive metamaterials and more advanced photonic devices.

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.

In this paper, a terahertz chiral metamaterial (CMM) with complementary U-shaped structure assembly is proposed. The results exhibit that the strong circular dichroism, giant optical activity, and negative refraction could be obtained by numerical simulation and effective EM scattering parameters retrieval method.

A tunable dual-band asymmetric transmission metasurface with strong circular dichroism in the terahertz communication band

Optical rotatory dispersion and circular dichroism properties of yeast mitochondrial DNA's