Tunable optical topological transitions of plasmon polaritons in WTe_(2) van der Waals films

Naturally existing in-plane hyperbolic polaritons and the associated optical topological transitions,which avoid the nano-structuring to achieve hyperbolicity,can outperform their counterparts in artificial metasurfaces.Such plasmon polaritons are rare,but experimentally revealed recently in WTe_(2)van der Waals thin films.Different from phonon polaritons,hyperbolic plasmon polaritons originate from the interplay of free carrier Drude response and interband transitions,which promise good intrinsic tunability.However,tunable in-plane hyperbolic plasmon polariton and its optical topological transition of the isofrequency contours to the elliptic topology in a natural material have not been realized.Here we demonstrate the tuning of the optical topological transition through Mo doping and temperature.The optical topological transition energy is tuned over a wide range,with frequencies ranging from 429 cm^(−1)(23.3 microns)for pure WTe_(2)to 270 cm^(−1)(37.0 microns)at the 50%Mo-doping level at 10 K.Moreover,the temperature-induced blueshift of the optical topological transition energy is also revealed,enabling active and reversible tuning.Surprisingly,the localized surface plasmon resonance in skew ribbons shows unusual polarization dependence,accurately manifesting its topology,which renders a reliable means to track the topology with far-field techniques.Our results open an avenue for reconfigurable photonic devices capable of plasmon polariton steering,such as canaling,focusing,and routing,and pave the way for low-symmetry plasmonic nanophotonics based on anisotropic natural materials.

Optical properties and polaritons of low symmetry 2D materials

Low symmetry 2D materials with intrinsic in-plane anisotropic optical properties and high tunability provide a promising platform to explore and manipulate light–matter interactions.To date,dozens of in-plane anisotropic 2D materials with diverse band structures have been discovered.They exhibit rich optical properties,indicating great potential for novel applications in optics,photonics,and optoelectronics.In this paper,we thoroughly review the anisotropic optical properties and polaritons in many kinds of low symmetry 2D materials,aiming to elicit more research interest in this field.First,the optical properties of anisotropic 2D semiconductors,including interband absorption,photoluminescence,excitons,and band structure engineering for tuning optical responses,are introduced.Then fundamentals and advances in experiments of hyperbolic polaritons in anisotropic 2D materials,including phonon,plasmon,and exciton polaritons,are discussed.Finally,a perspective on promising research directions is given.