Wireless charging for a moving electronic device such as smartphone is extremely difficult. Owing to energy dissipation during wireless transmission, sophisticated tracking control is typically required for simultaneously efficient and remote energy transfer in mobile scenarios. However, reaching the necessary tracking accuracy and reliability is very hard or even impossible. Here, inspired by the structures of optical resonator and retroreflector, we develop a self-aligned light beam system for mobile energy transfer with simultaneous high efficiency and long distance by exploring radiative resonances inside a double-retroreflector cavity. This system eliminates the requirement for any tracking control. To reduce transmission loss in mobile scenarios, we combine the advantages of energy-concentration using an optical resonant beam and self-alignment using a double-retroreflector cavity. We demonstrate above 5-watt optical power transfer with nearly 100% efficiency to a few-centimeter-size receiver for charging a smartphone, which is moving arbitrarily in the range of 2-meter distance and 6-degree field of view from the transmitter. This charging system empowers a smartphone in mobile operation with unlimited battery life, where cable charging is no longer needed. We validate the simultaneous high efficiency and long distance of the mobile energy transfer system through theoretical analyses and systematic experiments.
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