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Inspired by the chiral structure from mode interaction, a chiral terahertz metamaterial and its complementary structure with the coexistence of asymmetric transmission and circular dichroism response are designed. The asymmetric transmission happens in the presence of electric and magnetic modes perpendicular to each other, resulting from intrinsic chirality under normal incidence. Circular dichroism exists in the presence of electric and magnetic modes parallel to each other under oblique incidence because of extrinsic chirality. Both asymmetric transmission and circular dichroism are enhanced by complementary structures, benefitting from the coupling between adjacent units. The maximum of chiral parameter κ can achieve 450, which is one hundred times higher than previously reported. The chiral response can be tuned by the incident angle and is sensitive to the environmental refractive index. The results highlight the potential applications of these metamaterials in chiral sensing and polarization transformation devices. [ABSTRACT FROM AUTHOR]

We demonstrate an electrically triggered tunable terahertz (THz) band-pass filter based on vanadium dioxide (VO2) embedded hybrid metamaterials. The unit cell of the filter consists of two cross resonators and a central bar. The phase transition of VO2 is induced by the ohmic heating of the central bar. The transmission can be modulated by the applied currents. The mechanism of tunability is that the conductivity of the VO2 film changes with the applied currents. The relation between the upper cut-off frequency and the geometries is investigated using finite integration time domain method. The sample is fabricated by a thin film process, and characterized using a THz time domain spectroscopy system. The results show that, when the bias current rises from 0 to 0.27 A, the transmission decreases from 0.85 to 0.01. The maximum modulation depth reaches 96%, and the full width at half maximum is about 0.44 THz. This tunable THz band-pass filter has potential applications in THz communication, imaging, and spectroscopy systems. [ABSTRACT FROM AUTHOR]

The coupling between plasmonic modes and lattice diffraction modes of circular split-ring resonators in terahertz (THz) region has been analyzed by changing the lattice constants. We discovered a blue shift of the individual eigenmode in short lattice constants, which is due to the longitudinal coupling of two adjacent eigenmodes from neighboring unit cells. We found an anti-crossing effect in long lattice constants for both TM and TE incidence as the lattice diffraction modes is fused with the plasmonic modes to form hybrid modes. Moreover, these hybrid modes are more sensitive to the refractive index change than the pure modes, which shows the promise to increase the sensitivity of metamaterials by coupling. [ABSTRACT FROM AUTHOR]

Giant plasmonic mode splitting has been observed in THz metamaterials due to the mediation by the Lorentz phonon dielectric material. This splitting mode is confirmed by the surface current distribution, indicating that plasmonic modes behave like dipole resonances, while the phonon mode behaves like multipole resonance due to coupling. The splitting of the plasmonic modes demonstrates an anti-crossing behavior with the change in Lorentz central frequency, which suggests that there is energy redistribution between plasmon and phonon modes. Similar to the Stark effect, the splitting frequency difference increases with the increasing direct current dielectric function. We also propose an interaction Hamiltonian to understand the physical mechanism of the plasmonic splitting. Furthermore, the splitting is convincible for small Lorentz dielectrics such as sugar and amino acid in the THz region, which could be used for biomolecular sensing applications. [ABSTRACT FROM AUTHOR]

We demonstrated Tai Chi chiral metamaterials of mirror-symmetric and complementary structures in terahertz (THz) region. We investigated the properties of these structures by calculating the transmissions under different circularly polarization waves and analyzing the surface current distributions. The chiral mirror-symmetric structure has circular dichroism and strong optical activity due to electromagnetic field coupling between layers with sub-wavelength thickness. Moreover, the structures with multiple metallic layers or twist angle between layers can realize chirality tunability, which can be used for THz polarized devices. Besides, the complementary structure has potential applications for polarization-insensitive devices in THz region. [ABSTRACT FROM AUTHOR]

Chirality, as the core code of life forms, connects the ultimate secret of life and universe evolution. In article number 1900398, Changji Liu and co-workers propose an intrinsic chiral metamaterial and its complementary structures with coexistence of circular asymmetric transmission and circular dichroism in the THz region. The analysis reveals the chirality in metamaterials mainly comes from mode interaction and can be tuned by the incident angle. [Extracted from the article]

Two-dimensional chirality induced polarization conversion and asymmetric transmission have great potential for polarization applications. We propose graphene-based planar chiral metamaterial and its complementary structure with dual functionality of both circular polarization conversion and asymmetric transmission in terahertz (THz) region. The conversion efficiency of chiral structure can reach up to 37.33/λ, which is comparable with three-dimensional chiral structure made of metal due to the unique optoelectronic properties of graphene. The transmission asymmetry of complementary chiral metamaterial originating from the high order plasmonic resonance can be tuned from 0.049% to 3.504% by changing Fermi energy, and the asymmetry can also be enhanced via geometrical optimization. This work raises the prospect of switchable, compact, and on-chip THz polarized devices with graphene materials. [ABSTRACT FROM AUTHOR]

In this paper, a metamaterial (MM) resonator used as a sensitive biosensor is designed and fabricated for monitoring the interaction between bovine serum albumin (BSA) solution and four kinds of drug solutions in real time. The transmission spectra of the resonator are simulated and measured with terahertz time-domain spectroscopy system where the distinct resonance frequency shifts are observed. The experimental results indicate that the interactions between BSA and every kind of solution are violent before the reaction reaches equilibrium, and the reaction solutions manifest varying permittivity. Moreover, different reaction solutions show different frequency shifts and reaction times. The MM resonator worked as an effective biosensor achieves to monitor the interaction between BSA and drug solutions in real time, which is very useful for the development of novel drugs and other biomedical applications. [ABSTRACT FROM AUTHOR]

Plasmonic response in graphene-based metamaterials show great potential for terahertz (THz) wave manipulation. In this work, we study the tunable surface-plasmon-polariton-like modes based on graphene complementary split ring resonators (CSRRs) in THz region. Our study suggests that these modes can be generated by graphene plasmonic metamaterials due to the diffraction coupling of surface plasmon and propagating electromagnetic (EM) wave. Furthermore, the modes can be tuned by stacking graphene plasmonic metamaterials layers or by changing the graphene Fermi level with electric field. Our results suggest graphene plasmonic metamaterials for both physical understanding and novel applications in THz region. [ABSTRACT FROM AUTHOR]

We fabricated a polarization-dependent terahertz (THz) metamaterial absorber consisted of a metal film, a dielectric spacer and a metal pattern layer. The measured absorptivity for x -polarized wave is about 0.9 at 1.42 THz, and that for y -polarized wave is about 0.87 at 2.15 THz. The full width at half maximum (FWHM) of peaks at 1.42 THz and 2.15 THz are about 0.23 THz and 0.38 THz, respectively. The experimental results are in good agreement with the numerical simulation. In addition, the parameters related to the absorption are presented for deeply understanding the absorption mechanism. Most importantly, these two absorption peaks can be tuned respectively by simply changing the length in x and y directions. Our results suggest potential applications based on metamaterial absorber, such as THz polarization imaging, selective spectral detection, THz sensing and polarization multiplexing. [ABSTRACT FROM AUTHOR]

Improving the interaction of graphene with terahertz (THz) waves in experiment - through experimental measurement is a challenge for THz detectors, modulators, and other THz photonic components based on graphene. Hybridization of graphene with metamaterials leads to a strong THz response enhancement. Here, we observed maximum enhancement of 33.0% in non-resonant region and 23.8% in resonant region with the hybridization of graphene and metamaterials in experiment. A coupling model as well as numerical calculation has been carried out to fully investigate the influence of this coupling. The results suggest that there exists an exponential relationship between coupling strength and THz response in both resonant and non-resonant region, while the resonant frequency shift shows a linear growth with coupling strength. The bandwidth of the resonance shows exponential increasing with the damping constant. Correspondingly, the numerical calculation shows the similar dependency with the electrical conductivity of the graphene overlayer. This suggests a higher conductivity for stronger coupling. Substrates could also bring the remote phonon scattering, charge transfer, and dielectric effect, which show the influence such as low dielectric constant for high coupling. [ABSTRACT FROM AUTHOR]

Elliptically polarized second-harmonic generation (SHG) is achieved in the terahertz region with a chiral metamaterial consisting of a gold gammadion and a doped GaAs cut wire under circularly polarized excitation. Remarkable SHG-circular dichroism (SHG-CD) is attributed to the local magnetic field induced by the magnetic resonance in the chiral gammadion and the current generated in the GaAs wire. The SHG-CD follows the parity conservation when the parity of the chiral metamaterial changes. The maximal modulation depth of the ellipticity and the polarization rotation angle of the SHG reach 91% and 72°, respectively, resulting from the change of the second harmonic field current density with different wire lengths. This work deepens the understanding of the nonlinear optical effect in metamaterials and puts forward the chiral metamaterials as efficient and helicity-tunable nonlinear terahertz sources. • The SHG and SHG-CD are achieved from a chiral metamaterial under circularly polarized excitation. • The SHG follows the conservation of parity when the parity of the chiral metamaterial changes. • The maximal modulation depth of the ellipticity and the polarization rotation angle of the SHG reach 91% and 72°. [ABSTRACT FROM AUTHOR]

Abstract: Specific biorecognition is essential for many biological processes, for which highly sensitive and label-free biosensors are strongly demanded. The recently developed metamaterials are a potential choice for biosensing due to their exotic properties. In the current work, a label-free and specific sensor for streptavidin–agarose (SA) was fabricated based on terahertz metamaterial functionlized by octadecanthiols and biotins. Both low and high frequency resonant modes from the metamaterials are found applicable for the detection of SA, and a redshift up to 6.76GHz for the high frequency mode was measured in the undiluted commercial solution. The low frequency mode is attributed to inductor–capacitor (LC) oscillation, while the high frequency mode originates from the plasmonic dipole oscillator, both of which are highly sensitive to the micro-environment change. Adsorption of SA of different concentrations causes different redshifts, and the replacement of high refractive-index substrate with low refractive-index substrate can efficiently promote the sensitivity, well agreeing with the numerical simulation. Moreover, for a particular biomolecule, the sensitivity can be further improved by optimizing the metamaterial design. This method might be very helpful for desirable biorecognition in biology, medicine, and drug industry. [Copyright &y& Elsevier]