Motional narrowing, ballistic transport, and trapping of room-temperature exciton polaritons in an atomically-thin semiconductor

Abstract

Monolayer transition metal dichalcogenide crystals (TMDCs) hold great promise for semiconductor optoelectronics because their bound electron-hole pairs (excitons) are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, excitons can hybridise with cavity photons to form exciton polaritons, which inherit useful properties from their constituents. The ability to manipulate and trap polaritons on a microchip is critical for applications. Here, we create a non-trivial potential landscape for polaritons in monolayer WS2, and demonstrate their trapping and ballistic propagation across tens of micrometers. We show that the effects of dielectric disorder, which restrict the diffusion of WS2 excitons and broaden their spectral resonance, are dramatically reduced for polaritons, leading to motional narrowing and preserved partial coherence. Linewidth narrowing and coherence are further enhanced in the trap. Our results demonstrate the possibility of long-range dissipationless transport and efficient trapping of TMDC polaritons in ambient conditions.

Microscopic modelling of exciton-polariton diffusion coefficients in atomically thin semiconductors

“…Due to the rapidly changing polariton dispersion, we find group velocities in the range of 10 µm/ps, thus principally opening the possibility of ballistic polariton propagation for 10s µm. This has recently indeed been observed in a space-and angle-resolved photoluminescence experiments on a WS 2 monolayer in a distributed Bragg reflector cavity [29]. In addition to the remarkable magnitude difference, the group velocity for polaritons has also a qualitatively different momentum dependence.…”

“…This includes non-classical diffusion [33], transient negative diffusion [34], accelerated hot-exciton diffusion [35] or formation of spatial rings (halos) [36][37][38] and unconventional exciton funneling effects [39]. In view of their light component, polaritons show an interesting transport behaviour resulting in a fast propagation in the ballistic regime [27,29]. The polariton diffusion has already been observed e.g.…”

“…The quasi-bosonic polaritons show intriguing effects, such as Bose-Einstein condensation [10,19,20], (super)fluidity [21,22], topological effects [23], and promising applications from lasing [24], to integrated circuits [25] and quantum computing [26]. Exciton-polaritons maintain characteristics of both photons and excitons, such as a small effective mass, which makes them attractive also for transport purposes [27][28][29]. Previous studies have shown that TMD monolayers with their rich exciton landscape, including dark and bright exciton states [30][31][32], exhibit an interesting spatio-temporal exciton dynamics resulting in an intriguing exciton diffusion behaviour.…”

“…Having determined the expression for polariton energies and polariton-phonon scattering rates, we have all necessary ingredients to investigate the polariton diffusion. Polaritons in general show a peculiar transient spatio-temporal dynamics resulting in a large ballistic propagation even at room temperature [29], as well as a non-linear transport behaviour [21,22]. While a full spatiotemporal polariton dynamics is beyond the scope of this work, we focus the investigation on polariton diffusion coefficients in steady-state limit.…”

Coherent dynamics of Floquet-Bloch states in monolayer WS2 reveals fast adiabatic switching

“…To approximate the region of the WS2 flake sampled by the probe pulse the linewidth was determined from a spatially-averaged region across the flake. It is worth noting that these measures are inclusive of inhomogeneous broadening due to random variations in the dielectric environment 38 , and the homogeneous linewidth is likely narrower, as seen in room temperature coherent four-wave mixing measurements 39 . Still, the observation of a room temperature dephasing time of >38 fs speaks to the high-quality of this exfoliated WS2 flake (without encapsulation), but also of the potential for using these materials in applications where room temperature coherence is important [40][41][42] .…”