Laser-like vibrational instability in rectifying molecular conductors

TL;DR

This paper explores how electron-hole pair excitations in molecular rectifiers can lead to vibrational amplification, akin to laser action, especially at certain voltages where stimulated emission dominates.

Contribution

It introduces a generic mechanism for vibrational instability in donor-acceptor molecules due to non-equilibrium electron-hole excitations causing negative damping.

Findings

Vibrational amplification occurs at specific voltages.

Electron-hole pair excitations contribute to phonon stimulated emission.

First principles calculations confirm the mechanism in realistic structures.

Abstract

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 non-equilibrium 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 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.