Scientists successfully trapped excitons in electric fields

Electrons and positively charged holes or missing electrons can carry electric current in semiconductor materials. When light strikes a material, electrons can be stimulated to a higher energy band, leaving a hole in the lower energy band. Electrostatic attraction causes the electron and hole to combine to produce an exciton.

Exciton is a quasiparticle that, in its whole, behaves like a neutral particle. Due to their neutrality, excitons have been difficult to hold in a specific location within a material.

The objective of solid-state physics research has been to trap excitons.

For the first time, scientists used adjustable electric fields to trap excitons in a tiny region. They also exhibited exciton motion quantization.

The studies, lead by Ataç Imamolu, professor of Physics, Puneet Murthy, a postdoc in his lab, and David Norris, professor of Mechanical and Process Engineering, might open the way for optical technologies as well as new insights into fundamental physical phenomena.

Scientists built exciton traps by sandwiching a thin layer of the semiconductor material molybdenum diselenide between two insulators and attaching electrodes on the top and bottom. The top electrode only covers a piece of the material in this configuration. As a result, applying a voltage creates an electric field whose intensity changes depending on the position of the material.

Positively charged holes concentrate directly under the top electrode of the semiconductor, whereas negatively charged electrons accumulate elsewhere. This creates an electric field in the plane of the semiconductor between those two zones.

“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,” said Deepankur Thureja, Ph.D. student and main author of the research who conducted the tests with Murthy.

During the experiment, the scientists illuminated the material with laser light of various wavelengths. They then measured the light reflection in each example.

They discovered a sequence of resonances, which means that the light was reflected more strongly than predicted at various wavelengths. Furthermore, the resonances may be modified by varying the voltage on the electrodes.

“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,” Thuja added.

“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,” Murthy explains.

“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.”

“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,” Imamolu added.