  Conventional wireless power transfer systems are linear and time-invariant,
which sets fundamental limitations on their performance, including a tradeoff
between transfer efficiency and the level of transferred power. In this paper,
we introduce and study a possibility of temporal modulation for inductive
wireless power transfer systems and uncover that this tradeoff is avoided as a
consequence of varying the inductive coupling strength in time. Our theoretical
analysis reveals that under the optimal modulation depth and phase, the time
modulation can yield a substantial improvement in the WPT efficiency, while the
received power at the load is also improved compared to the static WPT
reference system. We experimentally demonstrate the concept with a
low-frequency system and observe a threefold improvement in efficiency over the
reference static counterpart. This technical capability reconciles the inherent
tradeoff between the WPT efficiency and transferred power, paving the way for
simultaneous advancements in both efficiency and delivered power.
