We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.
Lu, J. T., Hedegård, P., & Brandbyge, M. (2011). Laserlike Vibrational Instability in Rectifying Molecular Conductors. Physical Review Letters, 107(4), 046801. https://doi.org/10.1103/PhysRevLett.107.046801