Physics

Laser

Terahertz radiation

Optics

Semiconductor

van der Waals force

Femtosecond

Excited state

Atomic physics

Photoexcitation

Exciton

Ultrashort pulse

Materials science

Waveguide

Condensed matter physics

Molecule

Quantum mechanics

Optoelectronics

Stark effect

Electric field

Nature Communications

Van-der Waals (vdW) atomically layered crystals can act as optical waveguides over a broad range of the electromagnetic spectrum ranging from Terahertz to visible. Unlike common Si-based waveguides, vdW semiconductors host strong excitonic resonances that may be controlled using non-thermal stimuli including electrostatic gating and photoexcitation. Here, we utilize waveguide modes to examine photo-induced changes of excitons in the prototypical vdW semiconductor, WSe 2 , prompted by femtosecond light pulses. Using time-resolved scanning near-field optical microscopy we visualize the electric field profiles of waveguide modes in real space and time and extract the temporal evolution of the optical constants following femtosecond photoexcitation. By monitoring the phase velocity of the waveguide modes, we detect incoherent A-exciton bleaching along with a coherent optical Stark shift in WSe 2 .

Femtosecond exciton dynamics in WSe2 optical waveguides