Theoretical studies of light emission from molecular junctions

• Applicant: Dr. Safa Golrokh Bahoosh, Ph.D.
• Subject Area: Theoretical Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 338583897

The interaction of electrons with collective electromagnetic modes and the coupling of these modes to photons provide a unique probe for tip-surface interactions involving inelastic electron tunneling excitations of local plasmon modes and has become an important topic bridging nanoelectronics and plasmonics. STM-induced light emission due to the damping of the surface plasmon polaritons revealed a strong correlation between the light emission intensity and the current fluctuations at optical frequencies. However, the role of the molecule and how it effects the STM-induced light emission remains debated.My main objective is to describe the emission of plasmonic light in molecular junctions at lower photon energies compare to the applied bias as well as the emitted light at photonenergies which exceed the applied bias significantly . The former will be investigatedby exploring the characteristics of frequency-dependent non-symmetrized current-current correlation function for different quantum models. While I will formulate my theory for a general multilevel system, I will in conceptual studies concentrate on single and two-level models. The two-level model are of particular interest. They constitute a minimal model for a molecular junction, they can treat both Fano resonance and describe transport through dominant HOMO and LUMO levels.Furthermore, I will develop a microscopic theory for plasmonic light emission in molecular junctions based on the Keldysh nonequilibrium Green's function (NEGF) formalism taking into account an energy-dependent transmission and by considering single level and two-level transport, especially to explain the experimentally observed above-threshold emission. Particularly, the possibility of three- and four-electron scattering processes and the relevant self-energy diagrams due to these higher order corrections will be identified.Finally, using a multiscale quantum mechanics/electromagnetics (QM/EM) method, the shot noise characteristics and the light emission rate of such junctions will be calculated quantitatively. To this aim, the QM region is simulated using the DFT method in combination with NEGF, while the EM region is simulated by implementation of Maxwell's equations. QM and EM calculations communicate with each other through the interface by means of boundary conditions between the two regions and are solved iteratively until self-consistency achieved.

• DFG Programme: Research Grants
• Co-Investigators: Professor Dr. Wolfgang Belzig; Professor Dr. Fabian Pauly