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Physikgebäude (Neubau Otto-Hahn-Str. 4a) an der TU Dortmund © Nikolas Golsch​/​TU Dortmund

Quantum dance of excitons and phonons observed in perovskite nanocrystals

© Ilja Akimov ​/​ TU Dortmund University
Through international collaboration, researchers at TU Dortmund University have made significant progress in understanding quantum dynamics in semiconductor materials. In perovskite nanocrystals, the scientists were able to directly observe for the first time how excitons and phonons interact coherently—a synchronized “quantum dance” between electronic excitation and lattice vibration. The results have been published in Nature Communications.

Excitons are formed in semiconductors when a photon excites an electron to an excited state, leaving behind a positively charged “hole.”

 

Due to their Coulombic attraction, the two particles form a quasi-particle-like state and move together through the crystal. Phonons, on the other hand, are the quanta of a crystal’s lattice vibrations. In perovskite nanocrystals, which are only a few nanometers in size, these excitations are strongly confined spatially. This results in a particularly strong coupling between excitons and phonons. Optical excitation with ultrashort laser pulses leads to a shared quantum mechanical state—a so-called exciton polaron.
The experiments show that these coupled states remain coherent for several picoseconds at very low temperatures of about 2 Kelvin.

 

© Ilja Akimov ​/​ TU Dortmund University
The top left image shows quantum oscillations on a timescale of one trillionth of a second. A special laser measurement technique, known as two-pulse photon echo, was used to make the measurement. The diagram shows the experimental setup along with a nanocrystal. A large number of such nanocrystals are embedded in a glass sample for analysis.

Using femtosecond laser pulses and the two-pulse photon echo technique, the team at TU Dortmund University was able to directly track the temporal evolution of these states (see Figure 1). In the process, pronounced quantum oscillations were observed, arising from the coherent superposition of different states and revealing the energy exchange processes between the exciton and the phonon. This can be imagined as a quantum mechanical “dance” between the exciton and the phonon. The two dance partners do not move independently of one another, but advance perfectly synchronized and coherently within the perovskite nanocrystal (see Figure 2). This provides direct insight into the interaction between light-induced electronic excitations and the vibrations of the crystal lattice.
Noteworthy is the long coherence time as well as the strong amplitude of these oscillations, which have not previously been accessible in this form in other solid-state systems. In collaboration with theoretical groups at TU Dortmund University and Jackson State University, it was also demonstrated that the coupling strength can be specifically adjusted by varying the size of the nanocrystals: smaller crystals lead to stronger coupling, while larger ones result in longer coherence.
The results make perovskite nanocrystals a promising platform for future quantum technologies. These include new concepts for semiconductor-based quantum systems, quantum light sources, and the targeted generation of individual phonons. At the same time, the work shows that lattice vibrations are not only a source of decoherence but can also be specifically utilized as a quantum mechanical resource.

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