Threshold reduction of phonon lasing in coupled microresonators with a feedback waveguide

In this Letter, we propose a scheme to reduce the threshold power of generating phonon lasing in coupled microresonators by introducing a feedback waveguide on the basis of the original experimental scheme. A new, to the best of our knowledge, physical mechanism to improve mechanical gain is revealed based on nonreciprocal coupling and constructive interference. We demonstrate that the threshold power of our proposed scheme can be reduced by half compared with the experiments without the feedback. We also show the threshold reduction of the scheme by calculating the second-order coherence degree of the phonon lasing. This scheme is very simple and feasible and represents a beneficial supplement for the experimental method in this field.