A study published in Advanced Optical Materials describes a wireless power transfer (WPT) system that integrates an electromagnetic wave-focusing metasurface lens with a microstrip antenna and a high-frequency complementary metal-oxide-semiconductor (CMOS) rectifier.
The system is designed to improve power transmission efficiency for compact unmanned aerial vehicles (UAVs) and similar small devices. It addresses challenges related to misalignment, polarization variations, and changing incident angles at millimeter-wave (mmWave) frequencies.
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Importance of Wireless Power Transfer Technology
Small, unmanned devices are increasingly used in commercial, communication, monitoring, and defense settings. However, limited battery capacity restricts their operational duration, typically to 20–40 minutes. Solutions such as high-capacity batteries or ground-based recharging can add weight or complexity.
WPT offers an alternative by enabling contactless energy delivery. While near-field methods exist, far-field WPT at mmWave frequencies allows compact receivers with high directional gain.
Challenges to this approach include wave spreading, polarization mismatch, and rectifier inefficiency. Metasurfaces (engineered materials capable of manipulating electromagnetic waves) can help address these challenges by focusing energy onto small receivers to improve efficiency.
Metasurface Lens-Based Wireless Power Receiver
The WPT system developed by the researchers operates at 22.5 GHz and combines a phase-gradient metasurface lens, a microstrip patch antenna, and a CMOS rectifier. The metasurface lens consists of a 13 × 13 array of unit cells, each formed from a dielectric layer sandwiched between two metallic films with symmetrical patterns. This design enables phase control of incoming waves while maintaining transmission efficiency.
The lens, measuring approximately 65 mm × 65 mm, was shown to focus electromagnetic waves effectively, increasing signal strength by around 11.4 dB at the focal point for both x- and y-polarized waves. The lens functioned consistently under different polarizations and incident angles.
A microstrip patch antenna, positioned at the lens’s focal point, was designed for 22.5 GHz operation and matched to the lens size. It achieved gains of approximately 17.9 dBi in the E-plane and 18 dBi in the H-plane, with a beam spread of ±8°. Compared to a conventional patch antenna, this system exhibited an expanded beamwidth—33.3 % in the H-plane and 60 % in the E-plane, indicating improved tolerance to positional changes.
Tests under varying incident angles and receiver displacements showed that the metasurface lens maintained stable focal positions and consistent field intensity. Even at a 10° incident angle, focal shift and intensity reduction were minimal (~0.6 dB). Combined with the antenna, the system delivered consistent gain over a broader angular range, while conventional patch antennas showed reduced gain at certain angles.
Rectification and Power Conversion
The system converted mmWave energy into usable power using a three-stage Dickson voltage multiplier CMOS rectifier tuned for 22.5 GHz. The rectifier achieved a peak power conversion efficiency of 13.52 % at 16.7 dBm input power, producing a DC output voltage of approximately 6.21 V. Reflection coefficient measurements confirmed good impedance matching.
When paired with the metasurface lens antenna, the rectifier produced a peak output voltage of 8 V at the focal point, higher than the 4.4 V achieved by a conventional patch array antenna under similar conditions. Spatial voltage mapping indicated that the metasurface lens system provided more stable and higher output levels, supporting its effectiveness in energy reception and rectification.
Powering a Miniaturized Polymer Actuator
To evaluate practical use, the WPT system powered a miniaturized actuator made from ionic polymer material. This actuator bends due to ion migration in response to an applied voltage, enabling a simple crawling motion.
The rectified DC voltage was converted into a 1 Hz square wave to drive the actuator. Optical measurements showed linear bending displacements up to 5.0 mm as input power increased to 27 dBm. The actuator achieved a crawling distance of approximately 26 mm in 25 seconds, demonstrating the system’s capacity to power small, untethered devices.
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Potential Applications and Future Work
This system illustrates the use of metasurfaces to improve WPT by focusing energy and maintaining performance despite variations in positioning and polarization. It is particularly relevant for small autonomous devices, such as drones and mobile sensors, where space and weight constraints limit battery size.
Future research should focus on tailoring metasurface designs for specific uses, improving rectifier efficiency, and scaling systems for higher power outputs. Adaptive mmWave-based power beaming with tunable metasurfaces could further enhance flexibility and efficiency. These developments could support extended operation of UAVs and other autonomous systems in real-world environments.
Conclusion
The WPT system described in this study outperformed conventional patch antenna systems in terms of reception efficiency, gain, and tolerance to misalignment. The integration of a metasurface lens with a high-frequency rectifier presents a scalable and adaptable strategy for improving wireless power systems.
Continued development in this area could support a wide range of untethered applications in fields such as monitoring, communication, and robotics.
Journal Reference
Lee, W., et al. (2025). Energy Focusing Metasurface Based Wireless Power Transfer System for Compact Unmanned Mobilities. Advanced Optical Materials. DOI: 10.1002/adom.202500378, https://advanced.onlinelibrary.wiley.com/doi/10.1002/adom.202500378
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