In-plane exciton polaritons versus plasmon polaritons: Nonlocal corrections, confinement, and loss

Polaritons are quasiparticles describing the coupling between a photon and a material excitation, which can carry large momentum and confine electromagnetic fields to small dimensions, enabling strong light-matter interactions. In the visible (VIS) to near-infrared (NIR) spectral ranges, the intraband response of metals gives rise to surface plasmon polaritons (SPPs), which have practically governed polaritonic response and its utilization in nanophotonics. Recently, the concept of interband-based VIS/NIR in-plane exciton polaritons has been introduced in two-dimensional materials, such as transition-metal dichalcogenides (TMDs), thus providing an excitonic alternative to plasmonic systems. Here, we compare the properties of such in-plane exciton polaritons supported by monolayer TMDs to the equivalent configuration of SPPs supported by thin metallic films, known as short-range SPPs (SRSPPs). Taking into account both excitonic and plasmonic nonlocal corrections, which play a major role in large momentum modes, we find that in-plane exciton polaritons provide confinement factors that are an order of magnitude larger than those of SRSPPs, and with six times lower propagation losses. In addition, we show that unlike SPPs, in-plane exciton polaritons are coupled to the TMD's valley degree of freedom, leading to directional propagation that depends on the exciton's valley. These properties make in-plane exciton polaritons promising candidates for VIS/NIR nanophotonics and strong light-matter interactions.

Figure 1

The systems under investigation. (a) Monolayer ${\mathrm{WS}}_2$, and (b) 3 nm Au slabs, both encapsulated with semi-infinite layers of hBN.

Figure 2

The Susceptibility, ${\chi }_{\text{TMD}}$, of a hBN encapsulated ${\mathrm{WS}}_2$ monolayer. (a) Real and imaginary parts of the susceptibility in the local model. (b) The real part of the susceptibility in color scale as a function of frequency and momentum in the nonlocal model.

Figure 3

Confinement and loss of the 2DEP and the SRSPP, calculated for both local (dashed lines) and nonlocal models (solid lines). (a) Confinement factor. (b) Propagation losses figure of merit.

Figure 4

Valley coupled, directional 2DEPs on a monolayer ${\mathrm{WS}}_2$. The system schematic depicting the nanoparticle situated above the axis origin, tilted at an angle. Circularly polarized excitonic dipole states in the in-plane direction are presented by blue/yellow curves. Red ribbons illustrate left/right propagating 2DEPs with left/right circular polarization in the out-of-plane direction in orange. (a) Side and (b) perspective views. Cross section in the $y\text{−}z$ of the magnitudes of the electric field for (c) left- and (d) right-handed polarizations. (e) Same cross-sectional view of the magnitude of the electric field as a function of distance in meters $y$ for $z=0$, from the nanoparticle's center position.