  In this experimental study we show that the coupling between Nb/NbN
superconducting microwave resonators and their feedline can be made amplitude
dependent. We employ this mechanism to tune the resonators into critical
coupling condition, a preferable mode of operation for a wide range of
applications. Moreover we examine the dependence of critical coupling state on
other parameters such as temperature and magnetic field. Possible novel
applications based on such nonlinear coupling are briefly discussed.
