Optical spectroscopy of molecular junctions: Nonequilibrium Green's functions perspective

TL;DR

This paper applies the nonequilibrium Green's function approach to simulate optical spectroscopy in molecular junctions, revealing limitations of traditional perturbation theory and emphasizing the importance of flux conservation.

Contribution

It introduces a flux-conserving NEGF framework for optical spectroscopy in molecular junctions, improving upon traditional perturbation methods.

Findings

NEGF approach ensures flux conservation in optical spectroscopy.

Bare perturbation theory violates conservation laws.

Simulations with realistic parameters demonstrate the differences between methods.

Abstract

We consider optical spectroscopy of molecular junctions from the quantum transport perspective when radiation field is quantized and optical response of the system is simulated as photon flux. Using exact expressions for photon and electronic fluxes derived within the nonequilibrium Green function (NEGF) methodology and utilizing fourth order diagrammatic perturbation theory in molecular coupling to radiation field we perform simulations employing realistic parameters. Results of the simulations are compared to the bare perturbation theory (PT) usually employed in studies on nonlinear optical spectroscopy to classify optical processes. We show that the bare PT violates conservation laws, while flux conserving NEGF formulation mixes optical processes.