Electromagnetically induced transparency (EIT) can be analogically achieved by terahertz (THz) metamaterial, which has extensive applications in sensing, filtering, and slow light devices. Here, we firstly construct a metastructure that can modulate THz transmission, consisting of an outer symmetrical split ring resonator (SRR) embedded with two inner closed ring resonators. The simulated THz transmission spectrum presents a simple lineshape superposition of two resonances, corresponding to the low frequency dipole mode at 1.184 THz from the external SRR and the high frequency dipole mode at 1.757 THz from the closed ring resonators, respectively. However, the EIT phenomenon can be observed by replacing the inner part with two asymmetric split ring resonators. We have attributed this to that the inner metastructure can induce an extremely weak LC resonance at 1.074 THz due to the breaking of structure asymmetry. This mode will couple with the above dipole resonance of the outer SRR to accomplish the EIT effect through the near-field coupling of the weakly accessible bright-mode and the strongly excited bright-mode in this system. By varying different parameters, we have found that the impact of the rings distance on the EIT effect is more obvious. To further modulate the EIT window, the semiconductor silicon was placed at the opening gaps of the two inner asymmetric split ring resonators. Our simulated results indicate that with the increasing of the silicon conductivity from 0 to 9000 S/m, the EIT peak will gradually weaken and finally vanish, which is consistent with the results of closed ring resonators and shows the switch on/off of EIT phenomenon. Our work provides a design approach to control the electromagnetic transparent peak and manipulate EIT effect, for the potential applications in versatile THz devices.
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.
Optical regulation strategy with the aid of hybrid materials can significantly optimize the performance of terahertz devices so as to be used in the real world. Gold nanobipyramids (AuNBPs) with synthetical tunability to the nearinfrared band show strong local electric field enhancement, which improves the optical coupling at the interface and benefits the modulation performance of all-optical devices. Here we design AuNBPs-integrated terahertz modulators with multiple structured surfaces and indicate that introducing AuNBPs can effectively enhance their modulation depths. In particular, an ultrahigh modulation enhancement with one order of magnitude can be achieved in the AuNBPs hybrid metamaterials accompanied with the multifunctional modulation characteristics. Applying the coupled Lorentz oscillator model, the theoretical calculation suggests that the optical regulation with AuNBPs originates from increased damping rate and higher coupling coefficient under pump excitation. With the help of excellent modulation enhancement in the AuNBPs integrated metastructures, a prototype of novel spatial light modulator is constructed. As a novel terahertz photonic device with the low-power consumption and multifunctionality, this modulator is promising for the potential application in spatial and frequency selective imaging.
Coding metasurface has attracted much attention due to its flexible design of coding sequences and powerful control ability of light beams. However, the traditional coding metasurfaces with pin-diode switches between two metallic patches are usually used in the microwave band. Few studies have been carried out in the terahertz (THz) region with tunable metastructures. In order to realize the dynamic modulation of terahertz metasurface, in this paper we use the phase change material vanadium dioxide (VO2) to activate modulation coding metasurface in the terahertz band. We designed a VO2 embedded hybrid structure with metallic patches as the metasurface unit, which can produce a 180- degree phase change near 0.69 THz during the phase transition of VO2 from an insulating state to the metallic state. Meanwhile, we have constructed a metasurface array with the above designed tunable VO2 components and non-tunable metallic units to realize the dynamic switching of the far-field beam at that frequency. Our simulated results indicate that when the VO2 conductivity increases from 200 to 200000 S/m, the far-field reflected beams of the metasurface array can change from the separation of about 41° apart to close together. Notably, this coding metasurface will remain the reflectivity higher than 0.76 at the working frequency and exhibit polarization insensitive feature to the incident light. The active coding metasurface we designed provides a new idea for flexible beam control and has broad application prospects in terahertz functional devices.
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