Physicists have experimentally reconstructed the quantum probability distribution of an exciton for the first time using super-fast spectroscopy. By simultaneously capturing the spatial and momentum spreads of these short-lived quasiparticles, the work adds to our understanding of how excitons evolve in complex materials. According to members of the research team, this improved understanding could lead to advances in organic photovoltaic devices.
When light shines on materials such as insulators and semiconductors, the energy from these incident photons can sometimes promote electrons in the material to higher energy bands, leaving behind positively charged holes. The electron and hole can then bind together, forming a neutral excited quasiparticle known as an exciton.
Excitons play a key role in light harvesting in optoelectronics devices, but their quantum mechanical structure has proved notoriously difficult to study. Among other reasons, this is because the excited electrons decay back to their original states within just picoseconds (10-12 s), and standard spectroscopic techniques are not fast enough to simultaneously probe the spatial dimensions of the quasiparticles’ orbits and their momentum profiles within that time limit.
Photoemission orbital tomography
In the new work, physicists led by Peter Puschnig of the University of Graz, Austria, together with colleagues at Marburg University and Forschungszentrum Jülich in Germany, got around this problem thanks to a technique called time-resolved photoemission orbital tomography (trPOT). This technique combines pump-probe photoemission spectroscopy with momentum microscopy and offers resolution at the femtosecond (fs, 10-15 s) scale, making it well-suited to capturing the exciton wavefunction.
Puschnig and colleagues began by applying an ultrashort normal-incidence laser pulse with an energy of 2.35 eV to an organic semiconductor called alpha-sexithiophene, which is employed in optoelectronic applications such as solar cells. This pulse generated the excitons. The next step was to eject the electrons from these excitons using a second, linearly polarized laser pulse with a relatively high energy of 21.7 eV. This process is known as photoemission.
“If we then measure the energy and direction of the ejected electrons, theoretical models allow us to infer their quantum-mechanical state,” Puschnig explains. “And by varying the time between the first and second laser pulses, we are able to obtain snapshots of the exciton at different times after its creation.”
The team found that the exciton in the sexithiophene films initially extends over a distance of approximately 1.5 nm, or roughly three molecules. Within 400 fs of its creation, though, its size contracts by about 25%. “What makes this particularly exciting is that we are not just measuring an energy or lifetime but are also gaining access to the quantum-mechanical wave function of the exciton itself, including its spatial structure and phase,” Puschnig says.
Coordinating demanding experiments and theory
Puschnig says the idea for the experiment grew out of the team’s earlier work on photoemission orbital tomography (POT), including a 2021 proof-of-principle study showing that it could be extended into the ultrafast regime. “The main challenge was coordinating demanding experiments and theory: producing well-defined molecular films; transporting them under ultra-high vacuum from Jülich to Marburg in a vacuum suitcase; performing ultrafast photoemission measurements; and carrying out computationally intensive ab-initio calculations in Graz to interpret the data,” he adds.
Direct image of an electron orbit in an exciton is a first, say researchers
The work, which is detailed in Physical Review X, provides a new way to study excitons directly in real space and follow their quantum-mechanical evolution in time. According to the researchers, it could help us understand how excited states evolve into charge-separated states in the donor/acceptor systems that occur in organic photovoltaics, where the electron and hole can begin to separate after photoexcitation.
The researchers say their next goal is to move from the relatively simple exciton they studied to more complex systems. “We want to observe how charge-separation processes are controlled by the molecular and electronic structure of a material,” Puschnig tells Physics World. “Doing this will be particularly interesting because charge separation is the crucial step between absorbing a photon and generating a usable electrical current in an organic solar cell.”
