This paper proposes cavity resonators under parity-time- ($PT$) symmetric conditions for robust wireless power transfer (WPT) with minimal exposure to electromagnetic (EM) fields. Transceiver cavities with the same resonant frequencies as that of the fundamental mode are coupled through very small evanescent waves leaking out of the lattices on the top surface of the cavities. This reduces the magnitude of the EM fields near the power-delivery system by $10$ to $10000$ times compared with conventional coil systems. Regardless of the small coupling factors between the cavities, the power-transfer efficiency is maintained at a high level due to the high quality factors of the cavities. According to coupled-mode theory, the resonant frequency of the coupled system changes sensitively with respect to the receiver position, making power delivery unstable. In this work, a feedback loop at the transmit cavity along with the saturated gain of the power amplifier achieves the $PT$-symmetric condition. The power-transfer efficiency is robust because the operating frequency is automatically locked to the resonant frequency, regardless of the operation conditions. The proposed cavity system is promising for supplementing the conventional coil system for WPT applications in which minimal field exposure is crucial. The paper is accompanied by videos demonstrating experiments.

• Received 14 January 2021
• Revised 13 April 2021
• Accepted 16 June 2021

DOI:https://doi.org/10.1103/PhysRevApplied.16.014022