Long-Range Propagation of Exciton-Polaritons in Large-Area 2D Semiconductor Monolayers

Abstract

Atomically thin transition metal dichalcogenides (TMDs), a subclass of two-dimensional (2D) layered materials, have numerous fascinating properties that make them a promising platform for photonic and optoelectronic devices. In particular, excited state transport by TMDs is important in energy harvesting and photonic switching; however, long-range transport in TMDs is challenging due to the lack of availability of large area films. Whereas most previous studies have focused on small, exfoliated monolayer flakes, in this work we demonstrate metal–organic chemical vapor deposition grown centimeter-scale monolayers of WS2 that support polariton propagation lengths of up to 60 μm. The polaritons form through the strong coupling of excitons with Bloch surface waves (BSWs) supported by all-dielectric photonic structures. We observe that the propagation length increases with the number of dielectric pairs due to the increased quality factor of the supporting distributed Bragg reflector. Furthermore, a longer propagation length is observed as the guided or BSW content of the polariton is increased. Our results provide a practical approach for the systematic engineering of long-range energy transport mediated by exciton-polaritons in TMD layers. Along with the accessibility of large area TMDs, our work enables applications for practical TMD-based polaritonic devices that operate at room temperature.

Guided Bloch Surface Wave Polaritons on Patterned Distributed Bragg Reflectors at Room Temperature

“…Figure c shows the polariton PL intensity along the DBR stripe ( y -direction, data points) fit by applying a polariton self-interference model described previously (solid lines), giving wavelength-averaged polariton propagation lengths of L avg = 32 ± 1 μm and 62 ± 2 μm for GBSWP-4 ( Q = 42) and GBSWP-6 ( Q = 80), respectively. Compared to GBSWP-4, the larger propagation length for GBSWP-6 is due to the increased Q -factor with the number of DBR dielectric pairs . These results are consistent with BSWP systems comprising an unpatterned DBR with a DBP surface film (Figure S6).…”

“…By fitting the propagation profiles for a series of wavelengths from λ = 660 to 720 nm (Figure S7b), the propagation lengths of L 1/ e = 17 ± 1 to 92 ± 4 μm are obtained, respectively. As previously, the lower-energy (longer-wavelength) polariton propagates over a longer distance with the number of dielectric pairs due to the increased BSW fraction. − For polaritons with wavelengths >750 nm, the PL intensity is very low due to the bottleneck effect, and the propagation length cannot be obtained from the fits. Therefore, the wavelength-averaged propagation length depends on the detuning of the polariton mode from the exciton absorption spectrum and is also limited by the relaxation bottleneck in the BSWP emission (Figure d).…”

“…To rule out the weakly coupled exciton emission leakage, a 500 μm × 500 μm square pinhole acts as a k -filter that selects only the high- k (| k // / k 0 | > 1) polariton emission when placed in the reconstructed in-plane k -space detection channel (see the Methods section and Figure S2). , Figure a,b shows the real-space PL images obtained from guided BSWP devices comprising DBRs made of 4 and 6 pairs of Si 3 N 4 /SiO 2 (GBSWP-4 and GBSWP-6, respectively). Figure c shows the polariton PL intensity along the DBR stripe ( y -direction, data points) fit by applying a polariton self-interference model described previously (solid lines), giving wavelength-averaged polariton propagation lengths of L avg = 32 ± 1 μm and 62 ± 2 μm for GBSWP-4 ( Q = 42) and GBSWP-6 ( Q = 80), respectively.…”

“…It has been experimentally demonstrated that BSWs can be guided, refracted, focused, and diffracted using 2D optical components such as ultrathin ridge waveguides, lenses, and gratings on top of a dielectric multilayer structure (e.g., a distributed Bragg reflector or DBR). − Along with the large propagation length due to the low loss of dielectric materials, BSWs are of interest in integrated optics for optical switching and routing . Recently, BSWs have been exploited for strong coupling with excitons in two-dimensional (2D) layered inorganic semiconductors and with a variety of organic materials, giving rise to the formation of hybrid light–matter states called BSW-exciton polaritons (BSWPs). − In contrast to exciton diffusion yielding transport distances of only a few tens to a hundred nanometers, − some organic BSWP systems can support propagation over distances exceeding 100 μm, − providing a promising approach for enhancing and exploiting long-range energy transport.…”

Long-Range Self-Hybridized Exciton-Polaritons in Two-Dimensional Ruddlesden–Popper Perovskites

“…Prominently, transition-metal dichalcogenides (TMDCs) have been used because of their exceptionally high exciton binding energies of a few hundred electronvolts and high oscillator strengths for effective light–matter coupling, particularly in the case of monolayers. , In practice, TMDC monolayers have been merged with high-quality microcavities, photonic and plasmonic crystals, plasmonic nanoantennas, and plasmon polaritons . Recently, layered structures consisting of alternating TMDC monolayers and dielectric insulators, as well as chemical vapor deposition grown TMDC monolayers on truncated all-dielectric photonic structures yielding Bloch surface waves, have demonstrated strong coupling and long polariton propagation. Yet, the handling of TMDC monolayers proves to be quite challenging, and they barely reach lateral dimensions exceeding several tens of micrometers when obtained by mechanical exfoliation.…”