Objective Electromagnetically induced transparency (EIT) effect is a quantum interference phenomenon, which can improve the dispersion properties and suppress the absorption of materials. However, it is difficult to realize EIT by using traditional methods because of ultralow temperature and ultrahigh laser power. Recently, researchers have employed metasurfaces to realize EIT effect. Many different structures of metasurfaces, such as split-ring resonators, cut wires, and plasmonic waveguides, have been proposed to generate EIT effect. Most of previous designed metasurface structures were demonstrated as transmission type. While the reflection of EIT, which is as important as the transmission of EIT, is ignored for a long time. In this work, a reflection-type EIT metasurface structure is designed, which has two transparent windows in THz regime. The transparent windows can be manipulated actively.

Methods We conduct full-wave numerical simulations to test the reflection characteristics of the designed metamaterials using the CST (Computer Simulation Technology) Microwave Studio 2010. The simulation conditions are as followed: the x and y directions are set as the periodic boundary conditions, and both the ports are perpendicular to the z direction and placed at the two surfaces of the sample. The electromagnetic waves with y-polarizization are normally incident on the metasurface structure. The geometrical parameters are as followed: P_x=P_y=100μm, L₁=53μm, L₂=29μm, W=6μm, and d =6μm. The thicknesses of Au-pattern, polyimide layer, and Au plane are 1.9μm, 6.5μm, and 0.1μm, respectively (Fig. 1).

Results and Discussions The spectra of the inner split ring alone and the larger outer closed ring alone are simulated (Fig. 2). We can see that both structures can be independently excited. The main resonances of the inner split ring are located at 2.92THz and 3.93THz. The main resonances of the outer closed ring are located at 2.99THz and 4.45THz, which is similar to that of the inner split ring. The Q factors of these resonances are quite different. For the split ring, the Q factors are 233.8 and 58.18; and for outer closed ring, the Q factors are 52.13 and 7.95. The condition of EIT formation is as followed: electromagnetic waves interact with two modes which have similar resonant frequencies and quite different Q values. When the two structures are combined together, the interference of the resonances generates two transparent windows, which locate at 2.89-3THz and 4.03-4.44THz, respectively. Furthermore, the EIT effect can be actively manipulated by implanting photosensitive silicon between the inner split ring and outer closed ring (Fig.3). The conductivity of silicon increases with light intensity, and hence the resonance frequency changes. The modulation depth at transparent windows is modulated.

In order to analyze the mechanism of the EIT effect, we simulated the electric field distribution at the two transparent windows (Fig. 4). The formation of the two transparent windows is different, as shown in Fig. 4. According to the surface electric field distribution at 2.81THz, the enhanced resonance at the gap of the inner ring is "dark mode", and the suppressed resonance of the closed metal ring is "bright mode". This indicates that through the coupling between the bright and dark modes, the "bright mode" excites the "dark mode". At this time, the absorption of the EIT metasurface to the incident electromagnetic field is inhibited, and thus a transparent window forms with high reflection. At 3.82THz, the surface electric field distribution is mainly concentrated between the two metallic rings, which is consistent with the characteristics of propagating surface plasma. The resonance of the propagating plasma excited by the periodic structure couples with the resonance of the closed metal ring, resulting in destructive interference, thus forming a highly reflective transparent window.

When silicon conductivity increases with light intensity, the inner and outer rings become merging together, and thus the resonant frequency gradually becomes the same (Fig. 5). In this situation, the dual harmonic oscillator coupling model is destroyed and the condition of EIT effect is not satisfied. Thus, when light is incident on photosensitive silicon, the original transparent window has no strong coupling, and active manipulation of transparent windows is achieved.

Conclusions An electromagnetically induced transparent metasurface is proposed, and it consists of split ring resonators, a polyimide dielectric layer, and a metal substrate. This structure can realize reflection-type EIT effect at dual frequency bands. Furthermore, photosensitive silicon can be implanted between the inner and outer rings of the designed structure. Active manipulation of the EIT effect can be realized by employing the conductivity variation at different light intensity. This paper provides a reference for the research of improving the performance of EIT metasurface and broadening the application. Follow-up studies can focus on improving the bandwidth of the transparent window, reducing the loss, and adjusting the frequency range of the transparent window, so as to further improve the performance of reflection-type EIT metasurface.