Directly visualizing the momentum-forbidden dark excitons and their dynamics in atomically thin semiconductors

Area of Science:

• Solid-state physics
• Materials science
• Quantum optics

Background:

• Excitons, or electron-hole pairs, are fundamental to semiconductor opto-electronics.
• Understanding exciton momentum is key, but dark excitons are optically inaccessible.
• Atomically thin semiconductors offer unique platforms for studying excitonic properties.

Purpose of the Study:

• To probe the momentum state of excitons in a tungsten diselenide monolayer.
• To directly visualize and characterize momentum-forbidden dark excitons.
• To investigate the formation pathways and energy-momentum landscape of these dark excitons.

Main Methods:

• Photoemission of constituent electrons from excitons.
• Time, momentum, and energy resolved detection of emitted electrons.
• Utilizing atomically thin tungsten diselenide as the material system.

Main Results:

• Direct visualization of momentum-forbidden dark excitons achieved.
• Dark excitons found to be nearly degenerate in energy with bright excitons.
• Dark excitons dominate the excited-state distribution in tungsten diselenide.

Conclusions:

• The study provides direct insight into the properties and behavior of dark excitons.
• Dark excitons play a significant, often dominant, role in atomically thin semiconductors.
• This work opens new avenues for exploring and utilizing dark excitons in opto-electronic devices.