Mapping the Time-Dependent Dispersion Curve of Cavity-Embedded WS₂ Monolayers with Ultrafast Reflectance Spectroscopy

Strong cavity coupling in cavity-embedded transition metal dichalcogenides (TMD) gives rise to exciton-polariton (EP) quasi-particles with partial photon and exciton nature¹. Compared to the bare excitons, TMD EP states show enhanced oscillator strengths, longer of excitonic and valley coherence times,² and may also mediate mixing between otherwise distinct excitonic species³. Despite recent progress towards TMD EP-based technologies², there remains significant ambiguity in our understanding of EP dynamics at early times, due in part to use of microscope objectives which average over a large angle range and thereby obscure the EP response. We present the ultrafast transient response of a large-area (~mm) WS₂ ML embedded in a metallic cavity measured with high resolution (<1°) along the in-plane wavevector, k. We demonstrate the ability of this instrument to incisively pump and probe different EP branches at arbitrary values of k to generate pseudo-dispersion maps as a function of delay time and photon energy. We use this new approach to provide insight on the source of various nonlinear EP dynamics reported for cavity-coupled TMDs in literature.

1. Zhang, L. et al. Microcavity exciton polaritons. In Semiconductors and Semimetals (Vol. 105), Elsevier Inc (2020).

2. Hu, F. & Fei, Z. Adv. Opt. Mater. 8, 1901003 (2020).

3. Latini, S. et al. Nano Lett. 19, 3473–3479 (2019).