Abstract
The interactions of the excited states of a single chromophore with static and dynamic electric fields spatially varying at the atomic scale are investigated in a joint experimental and theoretical effort. In this configuration, the spatial extension of the fields confined at the apex of a scanning tunneling microscope tip is smaller than that of the molecular exciton, a property used to generate fluorescence maps of the chromophore with intramolecular resolution. Theoretical simulations of the electrostatic and electrodynamic interactions occurring at the picocavity junction formed by the chromophore, the tip, and the substrate reveal the key role played by subtle variations of Purcell, Lamb, and Stark effects. They also demonstrate that hyper-resolved fluorescence maps of the line shift and linewidth of the excitonic emission can be understood as images of the static charge redistribution upon electronic excitation of the molecule and as the distribution of the dynamical charge oscillation associated with the molecular exciton, respectively.
- Received 30 April 2021
- Revised 29 September 2021
- Accepted 3 November 2021
DOI:https://doi.org/10.1103/PhysRevX.12.011012
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The ability to observe fluorescence from a single molecule has revolutionized the world of biological imaging, enabling detailed mapping of cells and membranes with resolutions smaller than 10 nm. By leveraging the sensitivity of molecular fluorescence to electrostatic and electromagnetic fields, microscopies using sharp metallic tips as probes are now used to break this limit and provide resolutions below the size of the molecule. Reaching this threshold raises many questions about how to interpret submolecular optical signals and the role played by coupling between the molecule and electromagnetic fields appearing in the microscope junction. Here, we disentangle the roles of several key electronic effects and how they contribute to the interpretation of fluorescence images.
To take a deep look into the molecule, we establish two modes of light collection based on a scanning tunneling microscope-induced luminescence (STML). Here, tunneling electrons stimulate molecular excited states that decay and emit photons. In the first mode, where we monitor the width of the fluorescence line, our microscope allows for visualizing the oscillation of charges within the spontaneously emitting molecule, providing a direct view of the radiating dipole. In the second, where the energy position of the fluorescence line is recorded, one can visualize how charges redistribute in the molecule upon emission of a photon.
These STML techniques are particularly suited to study the basic building blocks of next-generation organic light-emitting diodes, organic solar cells, or even sources of nonclassical light for quantum technologies.
