  Waves scattered at a self-oscillating mode can exhibit superradiance, or net
amplification of an external harmonic excitation. This exotic behavior, arising
from the nonlinear coupling between the mode and the incident wave, is
theoretically predicted and experimentally confirmed for the first time in this
work. We propose a generic theory of nonlinear wave-mode coupling, which is
derived in analogy to the temporal coupled-mode theory of [Fan et al., J. Opt.
Soc. Am. A 20, 569 (2003)]. A well-reproducible aeroacoustic realization of a
superradiant scatterer was used to test the theory's predictions. It is shown
that the nonlinear wave-mode coupling can be exploited to quasi-passively tune
the reflection and transmission coefficients of a side cavity in a waveguide.
The theoretical framework used to describe this type of superradiance is
applicable to non-acoustic systems and may be used to design lossless
scattering devices.
