The ability to control the properties of a quantum state is central to the pursuit of new quantum phenomena that can lead to quantum devices for better future technology.
One of the most powerful strategies to manipulate the quantum state is to control the light-matter interaction by engineering the optical field environment of the material. A well-exploited example is an optical microcavity that tightly confines an optical mode and enhances its interaction with a resonant transition in embedded quantum materials. When the material can absorb and emit the photon back and forth coherently, the system enters the strong coupling regime, where new eigenstates called “polaritons” are formed, analogous to a pair of coupled oscillators1. Such polaritons are hybrid modes of the material excitation and cavity photon, therefore they can be manipulated through cavity engineering of the properties of the photon component.
On the other hand, in material science, controlling the material structure at atomic level allows engineering of energy band structures and thus controlling the quantum state of electrons. In the past 15 years since Andre Geim and Konstantin Novoselov’s work on graphene2, we have witnessed many breakthroughs in quantum state engineering brought by a large family of atomically thin materials, or two dimensional (2D) materials3. Recently, stacking and twisting the 2D sheets like “atomic lego” have ushered a new research area of quantum state engineering——“twistronics”. The twisted bilayers host a new in-plane superlattice, with a nanometer-scale period tunable by the twist angles, which is called “moiré lattice” 4–6.
Combining a microcavity and the moiré lattice, we can establish a new solid-sate platform with controllability over both the photon and the material excitations. In 2019, we successfully integrated a heterobilayer WSe2/MoSe2 with a photonic crystal cavity, and demonstrated an atomically thin semiconductor laser7. (https://devicematerialscommunity.nature.com/posts/56689-lasing-from-atomically-thin-2d-material-heterostructures) Limited by the light-matter interaction strength, the device is operated in the weak coupling region, and no new quantum eigenstates are generated in the system. “Can we reach the strong coupling region?” With this question in mind, we started to explore different types of heterobilayers. A particularly interesting combination is “WS2/MoSe2”8. In an earlier work, we found that, in this bilayer, spatially confined electron hole pairs——“moiré excitons” exhibit large optical dipole and can be tuned by twist angles9.
We integrated the MoSe2-WS2 heterobilayers in a planar Fabry Pérot cavity, and demonstrated strong coupling between moiré-lattice excitons and microcavity photons up to liquid-nitrogen temperature, thereby integrating into one platform with versatile controllability over both matter and light.
Through studies of the nonlinear response of this new type of polaritons, we found the moiré excitons exhibit strong saturation due to exciton blockade, suppressed exciton energy shift, and suppressed excitation-induced dephasing. These characters are consistent with those of fully quantum-confined states. Therefore our findings reveal the quantum-dot like character of each moiré cell and establish in a solid state system collectively coupling of an array of two-level systems with photons.
Importantly, due to the exciton blockade, the moiré polaritons acquire a strong nonlinearity which is tunable by the moiré lattice and scales inversely with the size of the moiré cell instead of the exciton. An abnormally large nonlinearity was also observed at very low polariton densities. Further understanding and control of the strong nonlinearity of moiré polaritons may open a new pathway to polariton blockade and ultralow power nonlinear optical switches. Interplay between moiré lattice potential and photonic potentials may open new opportunities in cavity-control of electronic phases and quantum simulations.
If you are interested in learning more about the scientific detail of this topic, please read our paper in Nature: http://www.nature.com/articles/s41586-021-03228-5
- Deng, H., Haug, H. & Yamamoto, Y. Exciton-polariton Bose-Einstein condensation. Rev. Mod. Phys. 82, 1489–1537 (2010).
- Novoselov, K. S. et al. Electric Field Effect in Atomically Thin Carbon Films. Science 306, 666–669 (2004).
- Das, S., Robinson, J. A., Dubey, M., Terrones, H. & Terrones, M. Beyond Graphene: Progress in Novel Two-Dimensional Materials and van der Waals Solids. Annual Review of Materials Research 45, 1–27 (2015).
- Bistritzer, R. & MacDonald, A. H. Moiré bands in twisted double-layer graphene. Proc Natl Acad Sci USA 108, 12233 (2011).
- Cao, Y. et al. Correlated insulator behaviour at half-filling in magic-angle graphene superlattices. Nature 556, 80–84 (2018).
- Cao, Y. et al. Unconventional superconductivity in magic-angle graphene superlattices. Nature 556, 43–50 (2018).
- Paik, E. Y. et al. Interlayer exciton laser of extended spatial coherence in atomically thin heterostructures. Nature 576, 80–84 (2019).
- Alexeev, E. M. et al. Resonantly hybridized excitons in moiré superlattices in van der Waals heterostructures. Nature 567, 81–86 (2019).
- Zhang, L. et al. Twist-angle dependence of moiré excitons in WS2 /MoSe2 heterobilayers. Nature Communications 11, 5888 (2020).