Multi-band Terahertz Chiral Metamaterials Based on Complementary Cross-wire Structure

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.

In this paper, a dualband chiral metamaterial (CMM) based on cross-wire structure is proposed and studied numerically. It exhibits dual-band giant optical activity and negative refractive index in terahertz region. The surface current distributions are calculated to explain original physics. The further numerical results show that the efiective frequency bands of the CMMs can be independent adjusted easily by changing the structure geometrical parameter. The designed dual-band terahertz CMMs ofier ∞exibility in the investigation of novel terahertz device application.

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.

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

Chiral metamaterial induced asymmetric transmission (AT) possesses great potential for terahertz (THz) polarization applications, but its design has mainly relied on the conventional trial-and-error forward strategy. Here, based upon massive training samples, we propose a deep-learning-based approach to accelerate the design of chiral metamaterials. The deep learning framework includes two bidirectional networks that allow the model to self-autonomously decipher the nonintuitive relationship between chiral metamaterial structures and their corresponding electromagnetic responses. Our preliminary results show that our model can accurately predict THz responses for any kind of metamaterial structure and inversely retrieve structure parameters from given THz responses. It turns out that the deep-learning-assisted methodology exhibits more efficient design performance than the conventional physical-based metamaterial design approach. This work might provide another artificially intelligent design strategy for chiral metamaterials and shed light on the prosperous development of unprecedented THz applications.

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 this paper, we propose a new structure which is achieved via the combination of twist conjugated gammadion and four-L resonators pairs. The proposed chiral metamaterial can achieve dispersionless and giant optical activity simultaneously. The polarization ellipticity is lower than 0.46° through all function bands, and the polarization azimuth rotation angle is larger than 90.3° from 2.37 to 2.69 THz. Specifically, the structure can achieve 90° dispersionless polarization rotation at f = 2.57 THz. The optical activity is optimized through changing the parameters of the chiral structure and the physical mechanism is also analyzed based on surface current distribution.

Chiral mechanical metamaterial has become topical in energy absorption for the unusual compression-to-twist deformation determined by the asymmetric chirality geometry of their microstructures. However, the insufficient research on the dynamic crash behavior with high nonlinearity and the lack of efficient design and modeling method have blocked the development of chiral mechanical metamaterials considering crashworthiness. In this work, the structure-property relationship of chiral mechanical metamaterials under medium-strain-rate impact was systematically investigated. We proposed an aperiodic framework considering additive manufacturing which enabled both the ordered and disordered chiral mechanical metamaterials utilizing a screw-theory-based assembly rule. The strut components had variable cross-sections and the joints were fillet-enhanced in order to improve the compressive performance. Meanwhile, the corresponding efficient modeling method for finite element analysis (FEA) was introduced. The constitutive relation, damage criteria, and damage revolution of the 316L stainless steel were established considering strain rate effect. Finally, the deformation mechanisms and energy absorbing capabilities of the proposed chiral mechanical metamaterials were demonstrated and discussed based on the mechanical behaviors and the proposed chirality indexes. The numerical results indicated that the proposed design and modeling method provided an effective paradigm for the crashworthiness design of light-weight materials and structures, facilitating the further applications in fields of automobile, civil engineering, aerospace, etc.

Chiral metamaterials have been a research area for many researchers due to their polarization rotation properties on electromagnetic waves. However, most of the proposed chiral metamaterials are designed depending on experience or time-consuming inefficient simulations. A method is investigated for designing a chiral metamaterial with a strong and natural chirality admittance by optimizing a grid of metallic pixels through both sides of a dielectric sheet placed perpendicular to the incident wave by using a genetic algorithm (GA) technique based on finite element method solver. The effective medium parameters are obtained by using constitutive equations and S parameters. The proposed methodology is very efficient for designing a chiral metamaterial with the desired effective medium parameters. By using GA-based topology, it is proven that a chiral metamaterial can be designed and manufactured more easily and with a low cost.

A compact achromatic quarter-wave retarder (QWR) operating in reflection mode is designed for using in terahertz region. It is a composite device utilizing form birefringence and Fabry–Perot (FP) interference. Under illumination of plane waves with incidence angle of 45°, from 1.8 THz to 2.8 THz, the QWR achieved only ±2° variation around 90° phase retardation, enlarging the working bandwidth of ordinary QWR greatly. An analytical model combining transmission-line (TL) theory with effective medium theory (EMT) is presented and results agree well with the time-consuming numerical calculation. The 38 μm thick construction is simple and easy for fabrication by the existing lithographic technique and a promising application in terahertz or other frequency region is believed.

Terahertz angle-insensitive 90° polarization rotator using chiral metamaterial

This study explores how hydrostatic pressure, temperature, incident light intensity, and structural parameters modulate optical absorption coefficients and refractive index changes in GaAs/Al0.3Ga0.7As spherical quantum dots under the screened cosine Kratzer potential. We use the Nikiforov-Uvarov method to solve the Schrödinger equation to determine the relevant wave functions and energy levels. And using iterative method and density matrix theory, we derived analytical expressions for the optical absorption coefficients and refractive index changes. The results of the numerical analyses demonstrate that both the structural parameters of the quantum dots and the external environmental factors significantly influence the optical absorption coefficients and refractive index changes, with varying degrees of impact. It is particularly noteworthy that these factors are extremely important for modulating the peak amplitude and resonance peak position of the optical absorption coefficients and refractive index changes, providing important theoretical basis for us to deeply understand and modulate the nonlinear optical properties of quantum dots.

An ultracompact double eight-shaped plasmonic structure for the realization of plasmon-induced transparency (PIT) in the terahertz (THz) region has been studied. The device consists of a semiconductor–insulator–semiconductor bus waveguide coupled to the dual-disk resonators. Indium antimonide is employed to excite SPP in the THz region. The transmission characteristics of the proposed device are simulated numerically by the finite-difference time-domain method. In addition, a theoretical analysis based on the coupled-mode theory for transmission features is presented and compared with the numerical results. Results are in good agreement. Also, the dependence of PIT frequency characteristics on the radius of the outer disk is discussed in detail. In addition, by removing one of the outer disk resonators, double-PIT peaks can be observed in the transmission spectrum, and the physical mechanism of the appeared peaks is investigated. Finally, an application of the proposed structure for distinguishing different states of DNA molecules is discussed. Results show that the maximum sensitivity with 654 GHz/RIU−1 could be obtained for a single PIT structure. The frequency shifts equal to 37 and 99 GHz could be observed for the denatured and the hybridized DNA states, respectively.

The effect of terahertz laser field on the optical absorption coefficients and refractive index changes of hyperbolic confinement quantum wells

Optical properties of an off-center hydrogenic impurity in a spherical quantum dot with Gaussian potential