Optical Imaging of Ultrafast Phonon-Polariton Propagation through an Excitonic Sensor

• Published in: Nano letters 2023-11, Vol.23 (21), p.9936-9942
• Main Authors: Cheng, Shan-Wen, Xu, Ding, Su, Haowen, Baxter, James M., Holtzman, Luke N., Watanabe, Kenji, Taniguchi, Takashi, Hone, James C., Barmak, Katayun, Delor, Milan
• Format: Article
• Language: English
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/acs.nanolett.3c02897

Hexagonal boron nitride (hBN) hosts phonon polaritons (PhP), hybrid light–matter states that facilitate electromagnetic field confinement and exhibit long-range ballistic transport. Extracting the spatiotemporal dynamics of PhPs usually requires “tour de force” experimental methods such as ultrafast near-field infrared microscopy. Here, we leverage the remarkable environmental sensitivity of excitons in two-dimensional transition metal dichalcogenides to image PhP propagation in adjacent hBN slabs. Using ultrafast optical microscopy on monolayer WSe2/hBN heterostructures, we image propagating PhPs from 3.5 K to room temperature with subpicosecond and few-nanometer precision. Excitons in WSe2 act as transducers between visible light pulses and infrared PhPs, enabling visible-light imaging of PhP transport with far-field microscopy. We also report evidence of excitons in WSe2 copropagating with hBN PhPs over several micrometers. Our results provide new avenues for imaging polar excitations over a large frequency range with extreme spatiotemporal precision and new mechanisms to realize ballistic exciton transport at room temperature.