Optical properties of current carrying molecular wires

TL;DR

This paper develops a theoretical framework to analyze optical phenomena in biased molecular junctions, predicting observable molecular emission and light-induced currents, with implications for molecular optoelectronic device design.

Contribution

It introduces a unified non-equilibrium Green function approach to quantify optical absorption and emission in current-carrying molecular wires, highlighting conditions for observable effects.

Findings

Current-driven molecular emission can be observed under certain conditions.

Resonant light can induce electronic currents in unbiased junctions.

The phenomena depend on charge transfer transitions and bias distribution.

Abstract

We consider several fundamental optical phenomena involving single molecules in biased metal-molecule-metal junctions. The molecule is represented by its highest occupied and lowest unoccupied molecular orbitals, and the analysis involves the simultaneous consideration of three coupled fluxes: the electronic current through the molecule, energy flow between the molecule and electron-hole excitations in the leads and the incident and/or emitted photon flux. Using a unified theoretical approach based on the non-equilibrium Green function method we derive expressions for the absorption lineshape (not an observable but a ueful reference for considering yields of other optical processes) and for the current induced molecular emission in such junctions. We also consider conditions under which resonance radiation can induce electronic current in an unbiased junction. We find that current driven molecular emission and resonant light induced electronic currents in single molecule junctions can be of observable magnitude under appropriate realizable conditions. In particular, light induced current should be observed in junctions involving molecular bridges that are characterized by strong charge transfer optical transitions. For observing current induced molecular emission we find that in addition to the familiar need to control the damping of molecular excitations into the metal substrate the phenomenon is also sensitive to the way in which the potential bias si distributed on the junction.