  Wavefront Shaping has emerged over the past decade as a powerful tool to
control wave propagation through complex media, initially in optics and more
recently also in the microwave domain with important applications in
telecommunication, imaging and energy transfer. The crux of implementing
wavefront shaping concepts in real life is often its need for (direct)
feedback, requiring access to the target to focus on. Here, we present the
shaping of a microwave field based on indirect, unsolicited and blind feedback
which may be the pivotal step towards practical implementations. With the
example of a radiofrequency harvester in a metallic cavity, we demonstrate
tenfold enhancement of the harvested power by wavefront shaping based on
non-linear signals detected at an arbitrary position away from the harvesting
device.
