Femtosecond exciton dynamics in WSe2 optical waveguides

Aaron Sternbach ,Brian Kim,D N Basov,Sanghoon Chae,Hannes Hübener,Umberto De Giovannini,Lin Xiong,James Hone,Guangxin Ni ,Michal Lipson,Norman Nan Shi ,Peter Kissin,Xiaoyang Zhu ,Simone Latini,R D Averitt ,P James Schuck ,Ángel Rubio ,Zhiyuan Sun,Andrew J Millis,M M Fogler ,Nanfang Yu,Yinming Shao,Daniel Rhodes

doi:10.1038/s41467-020-17335-w

Aaron Sternbach , Brian Kim + Show 21 more

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https://doi.org/10.1038/s41467-020-17335-w

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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, WSe2, 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 WSe2.

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Van-der Waals atomically layered crystals can act as optical waveguides over a broad range of the electromagnetic spectrum ranging from Terahertz to visible
In this work we focus on the ultrafast dynamics of the extraordinary waveguide mode in the prototypical Van-der Waals (vdW) crystal, WSe2 (Fig. 1a, b) produced by photoexcitation
When the waveguide mode reaches the edge of the crystal after traveling the distance x from the atomic force microscope (AFM) probe, the mode scatters to free space and reaches the detector

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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. Previous works have demonstrated that the qr formalism applied to scanning near-field microscopy (SNOM) visualization of the steady state waveguide modes is well suited to evaluate the anisotropic dielectric tensor of vdW semiconductors[4]. This class of materials hosts uncommonly strong excitonic resonances, which are of particular interest given their substantial impact on the ab-plane optical polarizability in the near-infrared and visible range[3,6,7,8]. Our work uncovers the utility of waveguide modes in quantifying non-equilibrium lightdriven effects in vdW crystals

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