Non-monotonic dependence of the radiation linewidth in a phonon laser

TL;DR

This paper reveals that the radiation linewidth in a phonon laser exhibits a non-monotonic dependence on pumping due to the coexistence of two solution types with different spectral noise dependencies, influenced by temperature.

Contribution

It demonstrates the non-monotonic behavior of phonon laser linewidth as a function of pumping, linked to solution stability and noise effects, which was not previously understood.

Findings

Line width of zero solution is independent of noise amplitude.

Line width of nonzero solution increases with noise amplitude.

Transition temperature causes non-monotonic linewidth dependence on pumping.

Abstract

A phonon laser is an important device that can generate coherent sound waves at gigahertz frequency. In this paper, we demonstrate that in a phonon laser, the radiation line width can depend on the pumping non-monotonically. This is because there are two different types of solutions whose spectra depend differently on the noise amplitude. The first solution is a zero solution, which is stable before the generation threshold and is nondegenerate. The second solution is a nonzero solution that is stable above the generation threshold and is degenerate with respect to the phase. The line width of peaks in the spectrum of the zero solution does not depend on the noise amplitude. While, the line width of the peak in the spectrum of the nonzero solution increases with increasing noise amplitude. The noise amplitude increases with temperature, and, as a result, there is a temperature above which the transition from the zero solution to the nonzero solution leads to an increase in the radiation line width. In this case, the radiation line width non-monotonically depends on the pumping.