Modulation of electromagnetically induced transparency based on bright-bright mode coupling in asymmetric split ring resonators

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.