Saltar al contenido

Our Target

The motion of electrons inside atoms and molecules is a central aspect of chemical reactivity. Understanding this motion is also key for the development of novel technologies based on photovoltaic devices, quantum information, or energy storage, just to mention a few. But this motion is very fast: it occurs in the attosecond time scale, which can be interrogated by ultrashort light sources, in combination with electron, ion, or absorption spectroscopy methods, to time resolve and eventually control ultrafast electronic processes. In our group, we focus on a complementary, so far barely explored approach: to image ultrafast electronic motion in individual molecules in real-time but also in real space, with attosecond and picometer resolutions, respectively, by combining ultrashort laser pulses with a scanning tunneling microscope (STM).

Strong coupling in individual molecules

Light interacting with a two-level quantum system can lead to a periodic exchange of population between the two coupled states, known as Rabi oscillations. For a molecule placed inside a nanocavity, coherent strong coupling can occur between the plasmon modes of the nanocavity and the excitonic modes of the molecule. In the spectral domain, this is manifested via Rabi splitting, where the spectrum is no longer plasmonic or excitonic but a convolution between both is observed, giving rise to plexcitonic modes. Using CW lasers in combination with STM we aim to spatially map the Rabi-splitting of an individual molecule. To investigate the ultrafast exchange of population between the coupled states we use pulsed lasers to record the space-time evolution of the electronic population in pump/probe experiments.

a) Rabi oscillations of the population between two coherently coupled energy levels by a laser pulse (red wiggly arrow). b) Rabi splitting: illustration of the plasmonic spectrum of a nanocavity (black), the excitonic spectrum of a molecule (red), and the coherently coupled system (blue)

Charge transfer in donor-acceptor molecular systems

There are multiple factors that affect the efficiency of charge transfer between donor and acceptor elements. Among others, defects induce energy losses by providing different recombination pathways, reducing this efficiency. However, it is still not yet well understood the role of atomic size defects such as vacancies or impurities on the efficiency of the charge transfer at the level of single molecules. Our objective is to improve the current understanding of charge transfer between donor and acceptor molecules and explore the role of atomic size defects by means of tip-enhanced photoluminescence. To investigate the ultrafast electronic motion inside the charge transfer complex we combine ultrashort lasers with STM to map the spatial distribution of the laser-induced tunneling current as a function of the delay between pump-probe pairs of pulses.

Donor-acceptor molecular system. Schematic illustration of the electronic density in a molecular charge transfer complex in a pump-probe experiment. At delay 1 the charge is spatially localized in the donor molecule, whereas at delay 2 it has transferred to the acceptor molecule.