The phenomenon of electromagnetically induced transparency (EIT) was first discovered in atomic systems with three-level distributions. It is a destructive interference phenomenon between strongly coupled light beams in different transmission paths, which makes initially opaque media transparent. However, the implementation of EIT in atomic systems requires extremely strict external conditions, such as ultralow temperatures and intense pumping light, which limit its practical application and development. In recent years, with vigorous research on metasurfaces, the EIT phenomenon has overcome traditional limitations and can be achieved via the coupling of the bright or dark modes of metasurfaces, thereby expanding its applications in molecular sensing, slow-light devices, and other fields. However, a large portion of the research on EIT metasurfaces selects nonadjustable metal structures in their design, which implies that the designed functionalities are limited to the characteristics of EIT. This considerably restricts the applicable scenarios and hinders further development of this research. This paper introduces adjustable materials into the design of EIT metasurfaces and proposes a multifunctional and polarization-independent metasurface based on EIT. By integrating multiple functions into a single structure, it achieves sensing and measurement of sucrose solvents, controllable slow-light effects, and a temperature-and-light dual-control switch. This significantly enhances the functionality of EIT metasurface devices and demonstrates the design concept of using adjustable materials to change the structural resonance and control the electromagnetic response, thereby providing valuable references for EIT metasurface research.
In this study, the EIT phenomenon was mainly achieved by coupling the bright modes. The bright-mode structure can couple with electromagnetic waves in a dipole resonance state with a low-quality factor. The primary metal square ring and cross structures were used as two modes that indirectly couple to form a transparency window, and the relevant mechanism is further explained by introducing the Lorentz resonance model. To expand the application functionality of EIT metasurfaces, we deposited photosensitive silicon on both ends of the cross-structure, which was excited by an 800 nm near-infrared laser pulse. Vanadium dioxide was embedded inside the square ring structure, which underwent a nonmetal to metal phase transition at 68 ℃. Using these two control methods, temperature and light, controllable slow-light effects, and dual-control dual-channel switches were achieved.
The period of the metasurface structure designed in this study is illustrated in Fig. 1. When the two materials are in an unexcited state, the metasurface response exhibits an EIT phenomenon. We discuss the corresponding sensing characteristics when the ambient refractive index changes with a sensitivity of 306.49 GHz/RIU. To validate the potential application of this design in the field of molecular sensing, we introduced relevant data for sucrose molecules and achieved sensing measurements for sucrose molecules with a sensitivity of 97.6 GHz/(kg/m3). This also implies that the design can be extended to other molecular sensing and detection fields, such as tumor cells and hemoglobin. Regarding the slow-light effect, the group delay at the transparency window was 3.03 ps, and the group refractive index was 174.8. When both materials are in the excited state, the slow-light effect disappears, implying that the slow-light effect can be activated by controlling the excitation of the materials. For the temperature-light dual-control switch, when the photosensitive silicon is excited, the cross structure is connected, causing the original resonance of the cross structure to be destroyed and the resonance peak to disappear. The indirect coupling between the cross and square-ring structures weakens. Because the resonance frequency of the cross structure is located on the left side of the EIT response curve, it increases the amplitude of the front dip of the transmission curve. However, when vanadium dioxide is excited, the square ring structure is supplemented with a square structure, causing a change in the resonance position owing to structural alterations. The indirect coupling between the square-ring structure and cross structure is weakened, thus increasing the amplitude of the rear dip.
This paper proposes a versatile, polarization-independent metasurface structure that enables molecular sensing, controllable slow light, and dual-channel light-controlled switching. The basic structure of the metasurface is composed of a cross structure and four square ring structures. Photosensitive silicon and vanadium dioxide are introduced to achieve diverse and controllable functions. The formation mechanism of the EIT phenomenon is explained based on electric-field analysis and theoretical models, where the basic structures act as bright modes and undergo indirect coupling. Sensing measurements of sucrose solutions of different concentrations validate the potential of this design in the field of molecular detection. The slow-light effect of the metasurface is discussed, and its selectivity of the slow light effect is achieved using controllable materials, addressing the limitations of previous slow-light devices that cannot be turned off. Lastly, a controllable dual-channel switch based on controllable materials is realized, with bandwidths of 61.46 and 70.7 GHz, respectively. This provides a new design approach for EIT metasurfaces that disrupts the resonance of the original structure to obtain new electromagnetic responses, offering a reference for future research.
