Moiré excitons and exciton-polaritons: A review

TL;DR

Moiré excitons in van der Waals heterostructures arise from periodic moiré potentials that confine excitons, enabling miniband formation, long lifetimes, and strong interactions. The review covers the foundational exciton physics in 2D semiconductors, moiré pattern formation, and the resulting quantum many-body phases, including excitonic insulators, Bose-Hubbard-type states, and density waves. It also surveys moiré exciton–polaritons, their enhanced nonlinearities and topological features, and discusses experimental progress, theory, and outlook for quantum simulation and devices. Overall, the field provides a versatile solid-state platform bridging quantum optics, nanophotonics, and correlated electron systems, with tunable interactions, geometry, and light–matter coupling for exploring nonequilibrium and many-body photonics.

Abstract

Distinguished by their long lifetimes, strong dipolar interactions, and periodic confinement, moiré excitons provide a fertile territory for realizing interaction-driven excitonic phases beyond conventional semiconductor systems. Formed in twisted or lattice-mismatched van der Waals heterostructures, these excitons are shaped by a periodic potential landscape that enables the engineering of flat bands, strong interactions, and long-lived localised states. This has opened pathways to explore strongly correlated phases, including excitonic insulators, superfluids, and supersolids, potentially stable even at room temperature. When embedded in optical cavities, moiré excitons hybridize with photons to form moiré exciton-polaritons, a new class of quasiparticles exhibiting enhanced optical nonlinearities and novel topological features. In this review, we survey the theoretical foundations and experimental progress in the field of moiré excitons and polaritons. We begin by introducing the formation mechanisms of moiré patterns in two-dimensional semiconductors, and describe their impact on exciton confinement, optical selection rules, and spin-valley physics. We then discuss recent advances in the realization of many-body excitonic phases and exciton-based probes of electronic correlations. Finally, we explore the novel aspects of moiré polaritons, highlighting their unique nonlinear and topological properties. By bridging quantum optics, nanophotonics, and correlated electron systems, moiré excitons offer a powerful solid-state platform for quantum simulation, optoelectronic applications, and many-body photonics.

Figures (12)

• Figure 1: The interparticle electrostatic potential in van der Waals materials is described by the Keldysh (or Rytova-Keldysh) potential $U_K(\rho)$ (plotted as a function of normalized distance $\rho/r_{\mathrm{eff}}$, with $r_\mathrm{eff}$ the effective screening length). The Keldysh potential exhibits a crossover, from logarithmic behavior at short distances, to a $1/\rho$ (Coulomb) decay at long distances.
• Figure 2: Dipolar, hybrid, and quadrupolar excitons in van der Waals heterostructures.(a) In structures such as MoSe$_2$/WS$_2$ heterostructures, the hybridization of electronic states across layers can lead to hybrid excitons, with mixed character from both monolayers. In trilayer moiré structures like WSe$_2$/WS$_2$/WSe$_2$, two interlayer excitons with opposite dipole moments can hybridize into a quadrupolar exciton, which carries no net dipole but a finite quadrupole moment. These quadrupolar excitons exhibit quadratic Stark shifts and reduced exciton-exciton interactions compared to dipolar excitons, image adapted from Lian et al.Lian2023quadrupolar. (b)-(c) In a MoSe$_2$/WSe$_2$ heterobilayer with type-II band alignment, both intralayer and interlayer excitons can form. Intralayer excitons have electron and hole confined within the same monolayer, while interlayer (dipolar) excitons consist of spatially separated electrons and holes residing in adjacent layers, exhibiting a built-in out-of-plane dipole moment. Electron tunneling allows for the hybridisation of intra- and inter-layer excitons. Image adapted from Huang et al.Huang2022excitonsREVIEW.
• Figure 3: Moiré superlattice in a rotationally aligned MoS$_2$/WSe$_2$ hetero-bilayer.(A) Scanning tunneling microscopy (STM) image of a directly grown monolayer (ML) MoS$_2$/WSe$_2$ heterostructure, showing clear atomic resolution and long-range moiré modulation due to the lattice mismatch $\delta \approx 0.036$. (B) Zoom-in STM image highlighting the hexagonal moiré pattern with a measured periodicity of 8.7 nm, corresponding to the interference of the lattice constants of MoS$_2$ (3.16 Å) and WSe$_2$ (3.28 Å) under R-type stacking. The resulting moiré potential gives rise to a lateral modulation of local electronic structure, forming a 2D electronic superlattice with site-dependent band edges and local bandgaps. This leads to spatially modulated interlayer exciton energies and offers a platform for exploring exciton confinement and quantum dot-like behavior in moiré traps. Figures adapted from Zhang et al.Zhang2017.
• Figure 4: Moiré superlattice and excitonic fine structure in WSe$_2$/WS$_2$ heterobilayers.(a) Atomic-resolution scanning transmission electron microscopy (STEM) image of a near-zero twist angle WSe$_2$/WS$_2$ heterostructure, showing a well-defined moiré pattern with a periodicity of $\sim$8 nm. The lattice vectors $\vec{a}_1$ and $\vec{a}_2$ indicate the moiré superlattice orientation in real space. (b) Reflection contrast spectra for WSe$_2$/WS$_2$ heterostructures with near-zero (top) and large (bottom) twist angles. For the near-zero twist configuration, the periodic moiré potential leads to the appearance of multiple excitonic resonances (labelled I–III) around the WSe$_2$. In contrast, the large twist angle sample displays only the primary intralayer exciton peak, confirming the absence of strong moiré coupling. These observations highlight the formation of flat excitonic bands due to moiré confinement in the strong-coupling regime. Figures adapted from Jin et al.Jin2019observation.
• Figure 5: Nanopatterned spin-optical properties of interlayer moiré excitons. (A) Symmetry properties of exciton wavefunctions at three high-symmetry registries (A, B, C), showing distinct $C_3$ rotation eigenvalues. (B) Left: Spatial modulation of oscillator strength. Center: Circular polarization map of exciton emission, with opposite helicities at A and B sites. Right: Ellipticity of intermediate sites. (C) Schematic exciton potential landscape, showing localised exciton energy minima and helicity-dependent transitions. The energy difference between A and B sites is tunable via an external electric field, allowing for programmable quantum emitter arrays. Figures adapted from Yu et al.Yu2017moire.