An Ultra-Wideband Absorber Based on Graphene Featuring Oblique Incidence Stability
Objective Frequency-selective absorbers function as spatial filters that suppress both the reflection and transmission of electromagnetic waves.Commonly used in stealth technology,they have drawn considerable interest from scholars across fields including optical wave,microwave,and terahertz research.In recent years,many novel absorbers have been proposed with improved reconfigurability and functional flexibility.However,their angular stability remains limited,as absorption performance in most absorbers deteriorates as the incident angle increases.Researchers are exploring new design methods to address this issue,such as introducing stronger resonant structures and further miniaturization.However,most research work focuses on relatively low-frequency bands and narrowband FSS,and there is little research that successfully improves the oblique incidence performance of broadband absorbers.Enhancing the angular stability of broadband absorbers remains a challenge.Methods We apply an impedance matching tilt design method to develop a polarization-insensitive absorber based on graphene-metal hybrid ink.achieving stable ultra-wideband absorption under large-angle incidence.This design uses a centrally symmetric multi-layer frequency-selective structure to provide broadband absorption and polarization-insensitive characteristics.The top FSS structure includes a connected zigzag ring frame and an enhanced cross dipole,while the middle layer incorporates a grid-type FSS structure with a butterfly patch for multi-frequency resonance and miniaturization.The bottom layer is a copper-clad plate to block electromagnetic wave penetration.Under normal incidence,this design achieves ultra-wideband absorption and polarization-insensitivity.Unlike conventional absorbers,due to the optimal impedance matching,this absorber's performance does not deteriorate but instead improves over a certain range of incident angles under oblique incidence.In addition,we build an equivalent circuit model to analyze the absorber's mechanism.To examine its exceptional oblique incidence stability,we calculate the real and imaginary parts of impedance at different angles.Current and electric field distributions at the absorption peak are also shown to visually illustrate the mechanism.Results and Discussions The proposed metamaterial absorber achieves stable ultra-wideband absorption under large-angle incidence.As the incident angle increases,absorption performance improves instead of deteriorating.As shown in Fig.7,TE and TM polarized wave absorption of the structure covers a frequency band of 3.7-18.3 GHz under normal incidence,with a relative bandwidth(FBW)of 132％.Within an incident angle range of 0°-55°,the absorption effect for TE waves improves,and the-10 dB relative bandwidth increases from 132％to 146.7％.The fractional bandwidth of TM polarization absorption slightly decreases because the impedance imaginary part of TM polarization waves fluctuates more than that of TE polarization within the 0°-55° range,as shown in Figs.5 and 6.At oblique incidence,the electric field component of the TE polarized wave remains parallel to the FSS plane,with only the incident angle changing.For TM polarization,both the incident angle and the angle between the electric field component and the FSS plane vary,resulting in more significant impedance changes.However,the absorption effect for both polarizations remains below-10 dB,providing stability over large angles.Additionally,Fig.8 displays the absorption rate results across a 0°-70° range,demonstrating performance beyond 55°.At a 70° incidence,the structure maintains absorption above 80％.Conclusions This study employs an impedance matching tilt design method to develop a polarization-insensitive absorber with stable ultra-wideband absorption under large-angle incidence.The curved and folded design achieves a degree of miniaturization beneficial for angular stability.The top FSS structure consists of a connected zigzag ring frame and an enhanced cross dipole,with a middle layer of a grid FSS structure with a butterfly patch.An equivalent circuit model analyzes the absorber's working mechanism.Surface current and propagation electric field distributions at the absorption peak are examined to investigate its angular stability.The design achieves ultra-wideband absorption and polarization-insensitivity under normal incidence.Unlike traditional absorbers,due to optimized impedance matching under oblique incidence,this absorber's performance does not deteriorate but improves within certain incident angles.This design principle applies across microwave,optical,and terahertz waves.The proposed large-angle stable absorber holds promising research value in fields such as optical devices and stealth technology.
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