Photonics and spectroscopy in nanojunctions: a theoretical insight

Abstract

The progress of experimental techniques at the nanoscale in the last decade made optical measurements in current-carrying nanojunctions a reality, thus indicating the emergence of a new field of research coined optoelectronics. Optical spectroscopy of open nonequilibrium systems is a natural meeting point for (at least) two research areas: nonlinear optical spectroscopy and quantum transport, each with its own theoretical toolbox. We review recent progress in the field comparing theoretical treatments of optical response in nanojunctions as is accepted in nonlinear spectroscopy and quantum transport communities. A unified theoretical description of spectroscopy in nanojunctions is presented. We argue that theoretical approaches of the quantum transport community (and in particular, the Green function based considerations) yield a convenient tool for optoelectronics when the radiation field is treated classically, and that differences between the toolboxes may become critical when studying the quantum radiation field in junctions.

Light‐Stimulated Charge Transport in Bilayer Molecular Junctions for Photodetection

“…We attributed the asymmetry in single‐component carbon‐based MJs to a difference in electronic coupling at the two electrode contacts, one of which is covalent and the other physisorbed 11b. This effect was predicted theoretically by Galperin et al8b,14 Since the internal electric field generated by the effect is small (10–30 mV), the resulting PCs are also small for single‐component MJs, and would likely be absent if the electronic coupling to both electrodes were identical. We recently reported a different approach in which a molecular bilayer was used to create asymmetry, leading to significantly larger PCs .…”

“…While optical spectroscopy is normally a probe of device structure during fabrication and operation, the field of “molecular optoelectronics” investigates stimulation of transport with light or generation of light in response to an external bias . Photocurrent (PC) generation and changes in MJ conductance by incident light have been reported for single molecules and molecular ensembles and examined theoretically 4a,9.…”

Quantum interference and the time-dependent radiation of nanojunctions

“…However, THz intramolecular transport processes are increasingly relevant for determining the operational frequencies of nanodevices beyond the steady state, which may be related, for instance, to dynamical symmetries in periodically driven structures [19][20][21][22], spin-flip processes [23,24], transport statistics [25][26][27][28], and electron traversal times [29,30]. In addition, the optical properties of irradiated molecular structures have wide-ranging uses resulting from their photoluminescence, photodetection, and frequency conversion potential [31][32][33].…”

Photoinduced anomalous Coulomb blockade and the role of triplet states in electron transport through an irradiated molecular transistor. II. Effects of electron-phonon coupling and vibrational relaxation

“…From the theoretical aspects, the Floquet-based methods have successfully demonstrated several fascinating phenomena, including photon-assisted tunneling [45][46][47][48][49][50][51][52] , coherent destruction of tunneling 53,54 , coherent revival of tunneling 55 , and quantum ratchet effect 56 . For a single-photon process, kinetic approaches, e.g., quantum master equation approaches 41,[57][58][59][60][61] or non-equilibrium green's function (NEGF) approaches [62][63][64][65][66][67] , a) Electronic mail: lyhsu@gate.sinica.edu.tw offer a clear description of electron current induced by electronic excitation and optical excitation. However, in these theoretical studies, molecular triplet states are not considered in the modelling because of the slow rate of an intersystem crossing process.…”