Wednesday, July 6, 2022

Scientists successfully trapped excitons in electric fields

The new technique is important for creating single-photon sources as well as for basic research.

The electric current in semiconductor materials can be conducted by electrons and positively charged holes or missing electrons. When light strikes a material, electrons can be excited to a higher energy band, leaving a hole in the original band. The electron and the hole now merge through electrostatic attraction to form an exciton.

Exciton is a quasiparticle that behaves like a neutral particle in its whole. It has proven challenging to keep excitons at a precise spot inside a material due to their neutrality.

Trapping excitons has been an ambitious goal of solid-state physics research.

For the first time, scientists have successfully trapped excitons in a tiny space using controllable electric fields. They also demonstrated the quantization of exciton’s motion.

The findings- by the team of scientists led by Ataç Imamoğlu, professor at the Physics Department, Puneet Murthy, a postdoc in his group, and David Norris, professor at the Department of Mechanical and Process Engineering- could pave the way for optical technologies as well as to new insights into fundamental physical phenomena.

To trap the excitons, scientists created the exciton traps by sandwiching a thin layer of the semiconductor material molybdenum diselenide between two insulators and adding an electrode on the top and bottom. The top electrode only covers a portion of the material in this design. As a result, applying a voltage generates an electric field whose strength varies depending on the material’s position. 

The positively charged holes gather precisely beneath the top electrode in the semiconductor, while negatively charged electrons accumulate elsewhere. This generates an electric field in the plane of the semiconductor between those two zones.

Deepankur Thureja, Ph.D. student and lead author of the paper who carried out the experiments together with Murthy, said, “This electric field, which changes strongly over a short distance, can very effectively trap the excitons in the material. Although the excitons are electrically neutral, they can be polarized by electric fields, which means that the electron and the hole of the exciton are pulled a bit farther apart. This results in an electric dipole field, which interacts with the external field and thus exerts a force on the exciton.”

Scientists used laser light of different wavelengths to illuminate the material during the experiment. They then measured the light reflection in each case.

In doing so, they observed a series of resonances, meaning that at specific wavelengths, the light was reflected more strongly than expected. Furthermore, the resonances could be tuned by changing the voltage on the electrodes.

Thuja said, “For us, that was a clear sign that the electric fields created a trap for the excitons and that the motion of the excitons inside that trap was quantized. Quantized here means that the excitons can only take on certain well-defined energy states, much like electrons inside an atom. From the positions of the resonances, Imamoğlu and his co-workers could deduce that the exciton trap created by the electric fields was less than ten nanometers wide.”

Murthy said, “Such strongly trapped excitons are extremely important for practical applications and basic questions. Electrically controllable exciton traps were a missing link in the chain up to now.”

“For instance, physicists can now string together many such trapped excitons and adjust them so that they emit photons having the same properties. That would allow one to create identical single-photon sources for quantum information processing.”

Imamoğlu said, “Those traps also open up new perspectives for basic research. Amongst other things, they will enable us to study nonequilibrium states of strongly interacting excitons.”

Journal Reference:

  1. Deepankur Thureja et al, Electrically tunable quantum confinement of neutral excitons, Nature (2022). DOI: 10.1038/s41586-022-04634-z 
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