Transient Optical Modulation of Two-Dimensional Materials by Excitons at Ultimate Proximity.

Controlling the optical response of two-dimensional (2D) layered materials is critical for their optoelectronic and photonic applications. Current transient optical modulation of 2D semiconductors is mainly based on the band filling effect, which requires internal exciton/charge occupation from photoexcitation or charge injection. However, 2D atomically thin layers exhibit a strong excitonic effect and environmental sensitivity, offering exciting opportunities to engineer their optical properties through an external dielectric or electronic environment. Here, using femtosecond transient absorption spectroscopy as a tool and transition-metal dichalcogenide (TMD) van der Waals heterostructures with type I band alignment, we show the transient absorption modulation of the TMD layer by excitons at ultimate proximity without direct photoexcitation or exciton/charge occupation. Further layer-dependent study indicates the presence of excitons reduces the exciton oscillator strength in adjacent layers through the electric field effect because of environmental sensitivity and proximity of 2D materials. This result demonstrates the transient optical modulation with decoupled light absorption and modulation components and suggests an alternative approach to control the optical response of 2D materials for optoelectronic and photonic applications.