Light Emission and Conductance Fluctuations in Electrically Driven and Plasmonically Enhanced Molecular Junctions

Electrically connected and plasmonically enhanced molecular junctions combine the optical functionalities of high field confinement and enhancement (cavity function), and of high radiative efficiency (antenna function) with the electrical functionalities of molecular transport and electrically driven light emission. They are supposed to play a leading role in emerging nanoscale optoelectronic devices; yet, this development is hindered by an insufficient control and understanding of atomic-scale phenomena that govern the optical and electrical behavior of plasmonic nanojunctions under ambient operating conditions. For instance, displacement of a single atom may drastically influence the junction's conductance and its optical near-field distribution. Here, we investigate tunneling-induced light emission from a self-assembled metal-molecule-metal junction embedded in a plasmonic cavity at room-temperature. We find that despite the presence of hundreds of molecules in the junction, electrical conductance and light emission are both highly sensitive to atomic-scale fluctuations - a phenomenology reminiscent of picocavities observed in Raman scattering and of luminescence blinking from photo-excited plasmonic junctions. We present a minimal electrical model that is able to capture all main experimental features. Contrasting with these microscopic fluctuations, the overall plasmonic and electronic functionalities of our devices feature an excellent long-term stability and reproducibility at room temperature and under electrical bias of several volts, allowing for measurements over several months. Our work contributes to the understanding of atomic fluctuations in molecular plasmonic junctions and to the development of more robust and scalable platforms for nanoscale optoelectronics

Continuous-wave frequency upconversion with a molecular optomechanical nanocavity

[EN] Coherent upconversion of terahertz and mid-infrared signals into visible light opens new horizons for spectroscopy, imaging, and sensing but represents a challenge for conventional nonlinear optics. Here, we used a plasmonic nanocavity hosting a few hundred molecules to demonstrate optomechanical transduction of submicrowatt continuous-wave signals from the mid-infrared (32 terahertz) onto the visible domain at ambient conditions. The incoming field resonantly drives a collective molecular vibration, which imprints a coherent modulation on a visible pump laser and results in upconverted Raman sidebands with subnatural linewidth. Our dual-band nanocavity offers an estimated 13 orders of magnitude enhancement in upconversion efficiency per molecule. Our results demonstrate that molecular cavity optomechanics is a flexible paradigm for frequency conversion leveraging tailorable molecular and plasmonic properties.This work received funding from the European Union's Horizon 2020 Research and Innovation Program under grant agreement nos. 829067 (FET Open THOR), 820196 (ERC CoG QTONE), and 732894 (HOT). C.G. acknowledges support from the Swiss National Science Foundation (project nos. 170684 and 198898). This work is part of the research program of the Netherlands Organisation for Scientific Research (NWO). A.I.B. acknowledges financial support by the Alexander von Humboldt Foundation.Chen, W.; Roelli, P.; Hu, H.; Verlekar, S.; Amirtharaj, SP.; Barreda, ÁI.; Kippenberg, TJ.... (2021). Continuous-wave frequency upconversion with a molecular optomechanical nanocavity. Science. 374:1264-1267. https://doi.org/10.1126/science.abk31061264126737

Light Emission and Conductance Fluctuations in Electrically Driven and Plasmonically Enhanced Molecular Junctions

International audienceElectrically connected and plasmonically enhanced molecular junctions combine the optical functionalities of high field confinement and enhancement (cavity function), and of high radiative efficiency (antenna function) with the electrical functionalities of molecular transport. Such combined optical and electrical probes have proven useful for the fundamental understanding of metal–molecule contacts and contribute to the development of nanoscale optoelectronic devices including ultrafast electronics and nanosensors. Here, we employ a self-assembled metal–molecule–metal junction with a nanoparticle bridge to investigate correlated fluctuations in conductance and tunneling-induced light emission at room temperature. Despite the presence of hundreds of molecules in the junction, the electrical conductance and light emission are both highly sensitive to atomic-scale fluctuations─a phenomenology reminiscent of picocavities observed in Raman scattering and of luminescence blinking from photoexcited plasmonic junctions. Discrete steps in conductance associated with fluctuating emission intensities through the multiple plasmonic modes of the junction are consistent with a finite number of randomly localized, point-like sources dominating the optoelectronic response. Contrasting with these microscopic fluctuations, the overall plasmonic and electronic functionalities of our devices feature long-term survival at room temperature and under an electrical bias of a few volts, allowing for measurements over several month

Continuous-wave frequency upconversion with a molecular optomechanical nanocavity

Coherent upconversion of terahertz and mid-infrared signals into visible light opens new horizons for spectroscopy, imaging, and sensing but represents a challenge for conventional nonlinear optics. Here, we used a plasmonic nanocavity hosting a few hundred molecules to demonstrate optomechanical transduction of submicrowatt continuous-wave signals from the mid-infrared (32 terahertz) onto the visible domain at ambient conditions. The incoming field resonantly drives a collective molecular vibration, which imprints a coherent modulation on a visible pump laser and results in upconverted Raman sidebands with subnatural linewidth. Our dual-band nanocavity offers an estimated 13 orders of magnitude enhancement in upconversion efficiency per molecule. Our results demonstrate that molecular cavity optomechanics is a flexible paradigm for frequency conversion leveraging tailorable molecular and plasmonic properties.GR-GALPQ