Jing Wu

Abstract Twisted two-dimensional bilayer materials exhibit many exotic electronic phenomena. Manipulating the ‘twist angle’ between the two layers enables fine control of the electronic band structure, resulting in magic-angle flat-band superconductivity1, 2, the formation of moiré excitons3–8 and interlayer magnetism9. However, there are limited demonstrations of such concepts for photons. Here we show how analogous principles, combined with extreme anisotropy, enable control and manipulation of the photonic dispersion of phonon polaritons in van der Waals bilayers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptical) dispersion contours in bilayers of α-phase molybdenum trioxide (α-MoO3), arising when the rotation between the layers is at a photonic magic twist angle. These transitions are induced by polariton hybridization and are controlled by a topological quantity. At the transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with a resolution of less than λ0/40, where λ0 is the free-space wavelength. Our findings extend twistronics10 and moiré physics to nanophotonics and polaritonics, with potential applications in nanoimaging, nanoscale light propagation, energy transfer and quantum physics.

Black phosphorus, a fast emerging two-dimensional material, has been configured as field effect transistors, showing a hole-transport-dominated ambipolar characteristic. Here we report an effective modulation on ambipolar characteristics of few-layer black phosphorus transistors through in situ surface functionalization with caesium carbonate (Cs2CO3) and molybdenum trioxide (MoO3), respectively. Cs2CO3 is found to strongly electron dope black phosphorus. The electron mobility of black phosphorus is significantly enhanced to ~27 cm2 V−1 s−1 after 10 nm Cs2CO3 modification, indicating a greatly improved electron-transport behaviour. In contrast, MoO3 decoration demonstrates a giant hole-doping effect. In situ photoelectron spectroscopy characterization reveals significant surface charge transfer occurring at the dopants/black phosphorus interfaces. Moreover, the surface-doped black phosphorus devices exhibit a largely enhanced photodetection behaviour. Our findings coupled with the tunable nature of the surface transfer doping scheme ensure black phosphorus as a promising candidate for further complementary logic electronics. Black phosphorus is a graphene-like material that can be harnessed for two-dimensional electronic devices. Here, Xiang et al. demonstrate that adding caesium carbonate or molybdenum trioxide can significantly enhance the electron or hole conduction, respectively, of this promising material.