Objective Plasmon-induced transparency (PIT) is a quantum interference effect occurring in three-energy-level systems. Under an external laser pump, a substance that is originally opaque becomes transparent within a specific frequency range, and a wide and continuous resonance dip is replaced by a sharp transparency window in the transmission spectrum. Due to its strong dispersion, the PIT effect occurring during electromagnetic wave transmission has great potential for applications such as slow-light devices, optical dynamic storage devices, and high-sensitivity sensors. Subwavelength periodic metasurfaces are one of the most widely used methods to achieve the PIT effect for electromagnetic waves. In previous reports, the external electric field component was usually used as the excitation source, and the magnetic field component,which is an important component of the external field,was rarely used. The aim of current research is to effectively manipulate the PIT effect arising from the interaction between a subwavelength periodic metasurface and the external field. In this study, terahertz (THz) time-domain spectroscopy is adopted to perform a systematic study of PIT metasurfaces placed in a parallel-plate waveguide (PPWG). Under external TE mode excitation, effective modulation of the PIT effect based on a PPWG metasurface system is realized by varying the structural parameters of the metasurfaces both theoretically and experimentally. Our design may provide a new strategy for the design of tunable electromagnetic devices based on the PIT effect.

Methods Based on our previous work, we designed and fabricated a series of aluminum split-ring resonator pairs(SRRPs) with different gap locations on a quartz substrate. There were eight periods of SRRPs along the wave propagation direction, and the gap in split-ring 2 was moved along the x and y axes [Figs. 1(b) and 2]. Then, the sandwich structures (quartz substrate-aluminum structures-quartz substrates) were inserted into a copper PPWG for characterization. The input and output ends of the waveguide were tapered to increase the conversion efficiency from free-space THz radiation to waveguide modes.

To verify the experimental results, SRRPs with the same geometric parameters were simulated using CST Microwave Studio, and the coupled mode theory was used to fit the simulated results. The underlying mechanism of the modulation of the PIT effect was understood through simulations of the surface current and absolute electric field distributions at the frequencies of interest. In simulations, the TE mode of the PPWG was applied to excite the SRRPs; that is, the electric field was parallel to the waveguide plates, and the magnetic field was perpendicular to the waveguide plates. The time-domain solver was adopted, and the background was set as a perfect electric conductor. The electric boundary condition was assigned to all directions, and waveguide ports were set to the input and output ends of the waveguide. In simulations, only one row with eight SRRPs was modeled along the direction of the guided wave, and the aluminum and the quartz substrate were treated as the Drude model and lossless dielectric, respectively. The tapered parts of the waveguide were not considered in the simulations.

Results and Discussions The transmission spectra for the gap in split-ring 2 moving along the x and y axes were measured, simulated, and fitted. The results (Figs. 3 and 5) indicate that the experimental, simulated,and fitted results agreed well. When the gap in split-ring 2 was moved along the x axis, the PIT effect occurred due to the interaction between the subwavelength periodic metasurface and the external field.However, the PIT effect could not be manipulated effectively by changing the gap position. When the gap in split-ring 2 was displaced along the z axis, the transmission spectra demonstrated that the PIT effect gradually disappeared.

To understand the underlying mechanism of the PIT effect, the surface currents in the SRRPs at the resonance dips were simulated. The insets of Figs. 3(b) and 5(b) demonstrate that an antisymmetric mode appeared at the lower resonance frequency, where as a symmetric mode appeared at the higher frequency. When δz=25 μm, the PIT effect vanished, and the single resonance mode was a symmetric mode. The above simulated results are consistent with the plasmon hybridization picture.

The underlying mechanism of the PIT effect was also analyzed by simulating the absolute electric field distributions at the PIT window frequencies. As depicted in Fig.4, the absolute electric field was mainly confined in the gap of split-ring 2, and there was a weak electric field distribution in the gap of split-ring 1 under the same excitation conditions. It is interesting that the intensity of the electric field in the gap of split-ring 1 became stronger when the gap in split-ring 2 was moved (Fig. 6). The absolute electric field distributions in the SRRPs can be explained by the bright-dark mode coupling theory, where split rings 1 and 2 can be treated as the bright and dark modes, respectively. Though the two rings can be excited by the external field, the bright mode can be suppressed by the dark mode through the deep-subwavelength distance between their adjacent arms.

Conclusions Under external TE mode excitation, effective modulation of the PIT effect based on a PPWG metasurface system is realized by varying the structural parameters of the metasurfaces experimentally. The underlying mechanism of the PIT effect is analyzed through simulations of the surface current and absolute electric field distributions at the frequencies of interest. The simulated results demonstrate that the electric field in one ring is suppressed by another ring under the same excitation source at the PIT window frequencies, and the suppression effect can be controlled by varying the structural parameters of the SRRPs. Our work may open new avenues for slow-light devices, optical dynamic storage devices, and high-sensitivity sensors.