Non-equilibrium diffusion of dark excitons in atomically thin semiconductors

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Optics

Background:

• Atomically thin semiconductors are ideal for studying exciton physics.
• Tungsten-based transition metal dichalcogenides feature complex bright and dark exciton states.
• Dark excitons influence relaxation and photoluminescence but their transport role is unclear.

Purpose of the Study:

• Investigate the spatio-temporal dynamics of excitons in WSe2 monolayers.
• Elucidate the role of dark excitons in exciton transport.
• Understand unconventional diffusion mechanisms in 2D materials.

Main Methods:

• Joint theoretical and experimental approach.
• Resonant excitation of hBN-encapsulated WSe2 monolayers.
• Spatio-temporal exciton dynamics resolved on picosecond timescales.

Main Results:

• Observed unconventional, time-dependent exciton diffusion within picoseconds.
• Dark excitons populated by phonon emission from bright states.
• Transient diffusion coefficient increased by over an order of magnitude.

Conclusions:

• Dark excitons play a crucial role in initial exciton spatial propagation.
• Hot exciton expansion leads to transient superdiffusion.
• Findings advance understanding of 2D material exciton dynamics and potential applications.