Terahertz metamaterials with electromagnetically induced transparency (EIT) have attracted extensive attention recently due to the broad application prospects in communication, optical storage, slow light effect, and biosensing. Here, we have studied the EIT effect caused by the interlayer coupling of two asymmetric split ring resonators with four gaps. The upper and the lower layers spaced by the intermediate Si have the same metastructures with the rotated angle of 90° to each other. By varying the length of the metallic arm, we find that the EIT effect becomes increasingly apparent as the asymmetry coefficient decrease. The simulation results indicate that with the increase of the thickness of Si layer, the EIT phenomenon will first emerge, gradually become the strongest with the thickness of 5μm, and finally tend to be weakened after further increasing the Si thickness. Meanwhile, the frequency of the transparency peak exhibits redshift with the Si thickness. It is also found that the EIT effect can be further optimized by adjusting the microstructure width of the split ring resonators. When the asymmetry coefficient and the thickness of the intermediate layer is determined, the EIT effect becomes most obvious with the width of 3 μm, and will gradually weaken with the increase of metallic width. The transparency peak frequency presents blue shift simultaneously. Our designed metastructure could provide the optional approach to modify the EIT behaviors and play an important role in the sensors and modulators.
In recent years, terahertz metamaterials have attracted extensive attention because of their high sensitivity to electromagnetic waves in biosensing application. In order to obtain high-quality factor resonance, we designed and fabricated single split-ring structures with the radius of 25.5 μm. In the experiment, we measured the terahertz transmission spectra when the angle between the open gap direction and the incident wave horizontal polarization is set to be 0, 30, 45, 60 and 90 degrees, respectively, and compared the effects of different angles on the transmission characteristics. With the increase of the rotation angle, the frequency of sharpest resonance dip in the terahertz transmission spectra has a remarkable blue-shift. Furthermore, we further simulated the polarization conversion characteristics of the single splitting rings with different rotation angles. It is found that there is no polarization conversion effect when the sample is placed at 0 and 90 degrees. When the sample is rotated by 30 degrees, 45 degrees and 60 degrees, polarization conversion is observed, with the highest conversion efficiency at 45 degrees. Our obtained results indicate that the terahertz metamaterials have great application prospects in biosensors.
Graphene, made of carbon atoms arranged in a honeycomb lattice, has already attracted intense research and commercial interest in recent years. Early research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The unique properties of graphene have also attracted various researches on carrier dynamics using THz spectroscopy. Here we present an experimental demonstration of monolayer CVD grown graphene via THz time-domain spectroscopy, as well as optical pump terahertz probe system. We observe that the maximum transmittance of the graphene is nearly 96% compared to the ambient signal. However, under the excitation of different optical pump fluences, it is found that unlike the semiconductor material, its transmitted amplitude is enhanced accordingly. We observed a wide-band modulation of the terahertz transmission at the range of 0.3-1.6 THz and a large modulation depth of 16.4% with a certain optical excitation. We attributed it to suppression of the air-adsorbed graphene photoconductivity due to an increase in the carrier scattering rate induced by the increase in the free-carrier concentration by photoexcitation. The obtained results not only highlight the influence of air conditions on how THz characterizations would guide the design and fabrication of graphene-based terahertz modulators and optoelectronic devices, but also show that graphene exhibits the potential for terahertz broadband transmission enhancement with photoexcitation.
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