Valley-Selective Exciton Bistability in a Suspended Monolayer Semiconductor

Area of Science:

• Condensed Matter Physics
• Materials Science
• Optoelectronics

Background:

• Optical bistability, a phenomenon with two stable optical states, is crucial for all-optical signal processing.
• Monolayer transition metal dichalcogenides (TMDs) like WSe2 exhibit strong light-matter interactions near exciton resonances.
• Achieving low-power optical bistability in 2D materials remains a significant challenge.

Purpose of the Study:

• To demonstrate robust optical bistability in suspended WSe2 monolayers.
• To investigate the underlying photothermal mechanism responsible for the observed bistability.
• To explore the influence of magnetic fields on exciton bistability and polarization control.

Main Methods:

• Utilized continuous-wave optical excitation below the exciton resonance in suspended WSe2 monolayers at low temperatures.
• Investigated the photothermal effect by analyzing optical nonlinearity and passive feedback.
• Applied an out-of-plane magnetic field to study the exciton valley Zeeman effect and polarization-dependent switching.

Main Results:

• Achieved robust optical bistability at low excitation intensities (10^3 W/cm^2) due to the photothermal mechanism.
• Demonstrated that the low thermal conductance of suspended WSe2 is key to achieving bistability at low powers.
• Observed helicity-dependent exciton bistability under magnetic field, enabling polarization-controlled reflectance switching.

Conclusions:

• Suspended WSe2 monolayers exhibit efficient photothermally driven optical bistability.
• The exciton valley Zeeman effect provides a pathway for polarization switching of optical states.
• This work opens avenues for light-by-light control of optical properties in monolayer semiconductors.