Lattice-induced dual transparency in terahertz hybrid metasurfaces

The phenomenon of electromagnetically induced transparency (EIT)-like in terahertz (THz) metasurfaces facilitates agile manipulation of electromagnetic wave transmission windows and the deceleration of light, rendering it suitable for applications in modulators, absorbers, slow light devices, and more. Traditional design methodologies focus on the coupling between bright-dark modes and bright-bright modes within the unit cell, leveraging interference cancellation effects to regulate electromagnetic wave transmission. Notably, the periodicity of the array structure also plays a pivotal role in modulating the amplitude and resonance intensity of the transparent window, a phenomenon termed lattice-induced transparency (LIT). In this paper, we introduce a gold nanorod structure and an S-shaped gold split-ring resonator supported on a vanadium dioxide (VO₂) thin film to investigate LIT. Unlike conventional structures that solely consider single bright-bright or bright-dark mode coupling, our proposed structure incorporates both bright-bright and bright-dark modes coupling. Furthermore, the dark mode in our structure is not a conventional multipolar mode but rather a surface lattice resonance (SLR) arising from the coupling between lattice modes and the localized surface plasmon resonance (LSPR) of the structure itself. Through the analysis of simulated transmission spectra for the individual gold nanorod and S-shaped split-ring structures, we observed that the gold nanorod exhibits LSPR at 0.985 THz, whereas the S-shaped split-ring structure demonstrates LSPR and SLR at 0.51 THz and 1.025 THz, respectively. When combined, these structures form transparent windows with transmission rates of 66.03% and 59.4% at 0.643 THz and 1.01 THz due to the interplay of bright-bright and bright-dark modes coupling. Upon examining the electric field distribution in the x-y plane, we found that the electric field energy is predominantly concentrated on the S-shaped split-ring. To gain deeper insights into each resonance mode, we employed multipolar decomposition to quantify resonance scattering energy. Our findings revealed that both transparent windows are predominantly governed by electric dipole scattering energy. Further investigations showed that as the array structure’s period varies from 60 μmto 95 μm, the lattice mode progressively couples into the high frequency transmission valley (1.031 THz), giving rise to a high frequency hybrid mode (HFHM). The Q value of this mode initially increases and then decreases, peaking at 27 when the period is 84 μm. Similarly, as the period continues to increase, the lattice mode couples into the low frequency resonance valley (0.76 THz), forming a low frequency hybrid mode (LFHM) with a Q value that reaches a maximum of 51 at 115 μm—approximately an order of magnitude higher than that at a period of 60 μm. Additionally, as the periodicity increases, the near field coupling effect between adjacent units diminishes, leading to the gradual disappearance of the two transparent windows. To achieve active control over these transparent windows, we varied the conductivity of VO₂ from 20 S/m to 30000 S/m, resulting in a decrease in the transmission amplitudes of the two transparent windows to 37.58% and 3.39%, respectively. Finally, we investigated the slow light effect of the two transparent windows, comparing the maximum group delay between them, which was found to be 8.1 ps. The terahertz metasurface proposed in this study opens up avenues for the design of dynamically tunable sensing and slow light devices in the future.