Semiconductor excitations can hybridize with cavity photons to form exciton-polaritons (EPs) with remarkable properties, including light-like energy flow combined with matter-like interactions. To fully harness these properties, EPs must retain ballistic, coherent transport despite matter-mediated interactions with lattice phonons. Here we develop a nonlinear momentum-resolved optical approach that directly images EPs in real space on femtosecond scales in a range of polaritonic architectures. We focus our analysis on EP propagation in layered halide perovskite microcavities. We reveal that EP–phonon interactions lead to a large renormalization of EP velocities at high excitonic fractions at room temperature. Despite these strong EP–phonon interactions, ballistic transport is maintained for up to half-exciton EPs, in agreement with quantum simulations of dynamic disorder shielding through light-matter hybridization. Above 50% excitonic character, rapid decoherence leads to diffusive transport. Our work provides a general framework to precisely balance EP coherence, velocity, and nonlinear interactions.

The optoelectronic and transport properties of two-dimensional transition metal dichalcogenide semiconductors (2D TMDs) are highly susceptible to external perturbation, enabling precise tailoring of material function through post-synthetic modifications. Here we show that nanoscale inhomogeneities known as nanobubbles can be used for both strain and, less invasively, dielectric tuning of exciton transport in bilayer tungsten disulfide (WSe2). We use ultrasensitive spatiotemporally resolved optical scattering microscopy to directly image exciton transport, revealing that dielectric nanobubbles are surprisingly efficient at funneling and trapping excitons at room temperature, even though the energies of the bright excitons are negligibly affected. Our observations suggest that exciton funneling in dielectric inhomogeneities is driven by momentum-indirect (dark) excitons whose energies are more sensitive to dielectric perturbations than bright excitons. These results reveal a new pathway to control exciton transport in 2D semiconductors with exceptional spatial and energetic precision using dielectric engineering of dark state energetic landscapes. Main text:Two-dimensional transition metal dichalcogenide semiconductors (2D TMDs) are van der Waals materials that hold great promise for nanoscale optoelectronics thanks to their strong light-matter interactions even at the atomically-thin limit. The optoelectronic properties of 2D TMDs are in large part governed by their Coulomb-bound electron-hole pairs (excitons), with relatively large binding energies of up to hundreds of milli-electronvolts (meV) due to weak out-of-plane dielectric screening. [1][2][3][4][5][6] Unlike free charges, excitons are charge neutral and are therefore difficult to manipulate with external electric fields in electronic devices. [7][8][9] Therefore, the transport properties of excitons are largely dictated by random, diffusive motion with no long-range directionality, limiting their use as information and energy carriers. Finding new ways to manipulate exciton transport in 2D TMDs without radically altering other material properties would result in excitonic devices that combine strong light-matter interactions and precise control over energy and information flow in atomically thin materials.An attractive route to controlling the properties of 2D TMDs is to leverage their extreme sensitivity to extrinsic factors such as strain, 10-21 and dielectric screening by the environment (Figure 1a), 5,[22][23][24][25][26] enabling post-synthetic tuning of their optoelectronic and transport properties. For example, tensile strain reduces the optical transition energy of 2D TMDs; 16,18,27,28 localized strain regions thus create energy gradients that can funnel and trap excitons in nanoscale low-energy sites, a process that was leveraged to create long-lived quantum emitters. 14,[29][30][31][32][33] Strain engineering, however, is difficult to control over macroscopic scales and can introduce undesired disorder.A less invasive and in principle more contr...

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