Mapping The Ultrafast Transient Response of Cavity-Embedded WS₂ Monolayers with High Angular Resolution

The behavior of Exciton-Polaritons (EP), arising from strong coupling between the photon mode of a high-quality microcavity and excitonic states in an embedded semiconductor material, have been of intense recent interest¹. EPs of cavity-coupled excitons in Two-Dimensional Transition Metal Dichalcogenides (TMD) are of particular interest due to promising potential applications including low threshold lasing² as well as the opportunity to study many-body physics at ambient temperatures³. Compared to the excitons in bare TMDs, TMD EP states show enhanced oscillator strengths, longer of excitonic and valley coherence times⁴, and may also mediate mixing between otherwise distinct excitonic species⁵. Despite recent progress towards TMD EP-based technologies, there remains significant ambiguity in our understanding of EP dynamics at early times, due in part to the ultrafast timescales (~100s fs) of EP relaxation. Additionally, previous studies of ultrafast EP dynamics have had limited angular resolution due to the use of microscope objectives which average over a large angle range and therefor obscure the EP response.<br/><br/>We study the ultrafast response of a WS₂ monolayer embedded in a metallic cavity measured with a transient reflectance technique with high angular resolution achieved in part through the use of large-area WS₂ monolayer flakes. With this approach, we are able to incisively pump and probe different EP branches as a function of the in plane angular momentum, <i>k</i>, with &lt;1° angular resolution. We generate pseudo-dispersion maps as a function of <i>k</i> and photon energy for a given pump-probe delay time. We use this new approach to provide insight on the source of various nonlinear EP dynamics reported for cavity-coupled TMDs in literature and in particular resolve clear differences in the WS₂-cavity response under different pump energy conditions.<br/><br/>1. Zhang, L. <i>et al.</i> Microcavity exciton polaritons. In <i>Semiconductors and Semimetals</i> (Vol. 105), Elsevier Inc (2020).<br/>2. Paik, E. Y. et al. Interlayer exciton laser of extended spatial coherence in atomically thin heterostructures. Nature 576, 80–84 (2019).<br/>3. Luo, Y. et al. Manipulating nonlinear exciton polaritons in an atomically-thin semiconductor with artificial potential landscapes. <i>Light: Science & Applications</i>, <i>12</i>(1), 220 (2023).<br/>4. Hu, F., & Fei, Z. Recent Progress on Exciton Polaritons in Layered Transition Metal Dichalcogenides. <i>Advanced Optical Materials</i>, <i>8</i>(5), 1901003 (2020).<br/>5. Latini, S. et al. Cavity Control of Excitons in Two-Dimensional Materials. <i>Nano Letters</i>, <i>19</i>(6), 3473–3479 (2019).