Plasmon-enhanced molecular electroluminescence: Effects of nonlinear excitation and molecular cooperativity

Abstract

Plasmon enhanced electroluminescence of a single as well as a couple of molecules is discussed theoretically. The considered system is formed by a metal nanoparticle (MNP) placed in the proximity of a junction with molecules sandwiched between a left and a right lead. In continuing recent work on a junction with a single molecule [Phys. Rev. B 88, 155426 (2013)] the case of strong charge injection and resulting further enhanced electroluminescence is investigated. Photoemission and plasmon excitation saturate if the molecule-lead coupling enters a range of about 50 meV. This saturation appears long before the molecule-lead coupling approaches the limit at which the considered sequential type of charge transfer becomes invalid. Turning to junctions with up to five molecules stronger plasmon excitation is achieved and a noticeable population of the second and third excited dipole plasmon level appears. This goes along with a nonlinear increase of the photoemission intensity and a decrease of the emission linewidth displaying the onset of cooperative molecule-MNP coupling.

Theoretical study of plasmonic lasing in junctions with many molecules

“…The geometry factor takes the form κ nI = [e n e I ] − 3 [e n n n ] [n n e I ]. Although the coupling strength with higher plasmon modes may be larger than that with dipole plasmons, their corresponding energy transfer can be very weak if the molecules are off-resonant to them [17,18]. This justifies the dipoledipole interaction used here.…”

“…The master equation solutions for ρ α µ,β ν and σ α µ,β ν (B,t) have been obtained for junctions with up to 5 molecules in [18]. There, it was demonstrated that with increasing number of molecules, higher plasmon excited stat es get populated and the emission becomes more narrowed.…”

“…Since the molecular radiative decay rate is orders of magnitude weaker than the molecular charging and discharge rates [18], we are justified to ignore it in the present work.…”

“…III, we present a general RDM approach to solve the system dynamics, the electric current signal and the optical emission spectrum. This approach was previously used to simulate junctions with up to 5 molecules in [18]. In Sec.…”

“…In Ref. [17], we proposed several approaches to describe the optically pumped plasmonic nano-laser, noting that the analyses can be readily extended to the electrically pumped nano-laser [18].…”