Optically transparent microwave absorbing metasurfaces have shown great potential and are needed in multiple applications environments containing optical windows such as self-adapting electromagnetic stealth and energy harvesting for satellites or unmanned aerial vehicles, owing to their ability to reduce backscattering electromagnetic (EM) signals while keeping continuous optical observation. However, most of the above absorbers operate in the passive mode, and it makes the absorption characteristics fixed and limits their applications such as fitting into the varying surroundings.
As a general approach, distributed circuit components, including positive-intrinsic-negative diodes and varactors, are integrated with passive absorbing metasurfaces to realize active control of microwave absorption. However, these circuit elements generally require bulky electrical wires and complex control circuits to regulate the operating state, resulting in the absorbing structures being optically opaque, which significantly hinders their practicality in increasingly complex electromagnetic environments.
Recently, some advanced materials with transparent properties, including liquid crystal and graphene, to name a few, which provide multiple tunable or switchable functionalities, have been extensively used for being incorporated into metasurfaces to realize optically transparent reconfigurable EM response. Among these materials, graphene, as the 2D crystal, has been extensively used for electrically active meta-devices owing to its unprecedented ability to control light–matter interaction over a very broad spectrum, ranging from visible to microwave frequencies. Although these multitudinous graphene-based meta-devices have proved superior in steering EM waves, they are usually manually controlled by human participation. Self-operating devices have recently shown remarkable potential and have increased the technology's impact. In order to achieve self-adaptability properties of fitting an external stimulus or non-stationary EM environments, the active meta-device based on graphene should be further developed to form a self-operating closed-loop system to provide desired EM responses without any human participation.
To address the problems, a joint research group led by Weibing Lu, Zhenguo Liu and Mingyang Geng from Southeast University has demonstrated and experimentally verified reported an optically transparent cognitive metasurface system made of patterned graphene sandwich structures, a radio frequency sensor array, and a microcontroller unit.
The closed-loop automatic absorber system prototype of the proposed graphene-based metasurface works in the C band. The frequency signal of the incident electromagnetic wave detected by the sensor array is transmitted to the microcontroller unit returning a precise control of the bias voltage applied on graphene, which results in the adaptive adjustment of the metasurface's optimal absorption frequency to match the incident electromagnetic wave. The relevant research results were published in Photonics Research, Volume 11, No. 1, 2023 (Mingyang Geng, Xiaolu Yang, Hao Chen, Xinzhi Bo, Mengzi Li, Zhenguo Liu, Weibing Lu. Optically transparent graphene-based cognitive metasurface for adaptive frequency manipulation. Photonics Research, 2023, 11(1): 129).
Fig.1 provides a schematic of the presented metasurface, which is composed of the sensing module and frequency-tunable absorbing structure. Under the illumination of a plane wave, the sensing array is able to recognize the incident frequency information and transmit the data to the MCU. After collecting the data, the MCU will determine and provide the voltage on the graphene sandwich structure to generate the desired EM absorption. According to the frequency information from the sensing data, the metasurface can realize resonance frequency manipulation based on the pre-designed procedure. The frequency manipulation of the absorbing structure is driven by the graphene capacitor layer under voltage control.
Fig.1 Schematic of the proposed optically transparent graphene-based cognitive metasurface.
The experiments are carried out in a semi-enclosed microwave anechoic chamber, in which two linearly polarized horn antennas, working from 2 to 18 GHz, are employed as the transmitting and receiving antennas. The distance between the sample and transmitting horn antenna is 90 cm. The graphs in Fig.1 show the measured reflection coefficient when the incident frequency is chosen as 5.5 GHz, 5.7 GHz, and 6.0 GHz, respectively, in which markers at 5.56 GHz, 5.76 GHz, and 6.04 GHz represent the measured resonance frequency. Although there are some deviations between incident frequency and those measured, the experimental results demonstrate that the smart frequency manipulation function is realized, thus achieving the self-adaptive absorption at different frequencies according to the incident EM waves without any human intervention.
These results show that this work is a significant step from active metasurfaces to intelligent meta-devices, which will further encourage research on graphene-based cognitive metasurfaces with more complicated and more high-level EM controls.
