[Paper Review] Routing Valley Excitons in a Monolayer MoS2 with a Metasurface

Liuyang Sun, Chunyuan Wang+9 more|arXiv (Cornell University)|Jan 19, 2018

2D Materials and Applications1 references3 citations

TL;DR

This paper demonstrates room-temperature routing of valley-polarized excitons in monolayer MoS2 using a metasurface with asymmetric grooves. By coupling the 2D semiconductor to the engineered metasurface, the authors achieve spatial separation of K and K' valley excitons and directional emission of circularly polarized photons, enabling a photonic interface for valleytronic devices.

ABSTRACT

Excitons in monolayer transition metal dichalcogenides (TMDs) are formed at K and K' points at the boundary of the Brillouin zone. They acquire a valley degree of freedom, which may be used as a complementary platform for information transport and processing. In a different context, metasurfaces consisting of engineered arrays of polarizable inclusions have enabled the manipulation of light in unprecedented ways, and found applications in imaging, optical information processing, and cloaking. Here, we demonstrate that, by coupling a MoS2 monolayer to a suitably designed metasurface consisting of asymmetric grooves, valley polarized excitons can be sorted and spatially separated even at room temperature. Emission from valley excitons is also separated in K-space, i.e., photons with opposite helicity are emitted to different directions. Our work demonstrates that metasurfaces can facilitate valley transport and establish an interface between valleytronic and photonic devices, thus addressing outstanding challenges in the nascent field of valleytronics.

Motivation & Objective

• To enable control of valley excitons in monolayer MoS2 at room temperature.
• To address the challenge of valley transport in valleytronics by interfacing with photonic systems.
• To design a metasurface that selectively manipulates valley-polarized excitons through engineered optical response.
• To achieve spatial separation of valley excitons and helicity-dependent emission in momentum space (K-space).

Proposed method

• A metasurface composed of asymmetric grooves is fabricated on a substrate to create a spatially modulated optical potential.
• The metasurface is coupled to a monolayer MoS2 flake to enable strong interaction with valley excitons.
• The asymmetric geometry breaks inversion symmetry, enabling selective coupling to specific valley states (K or K') in MoS2.
• The system supports momentum-selective emission, where photons with opposite circular polarization are directed to different angles.
• Theoretical modeling based on effective medium theory and coupled-dipole approximation predicts the valley-dependent response.
• Experimental validation is performed via micro-photoluminescence spectroscopy to measure spatial and momentum-space emission patterns.

Experimental results

Research questions

• RQ1 Can valley-polarized excitons in monolayer MoS2 be spatially routed using a metasurface at room temperature?
• RQ2 How does the metasurface geometry influence the momentum-space (K-space) emission pattern of valley excitons?
• RQ3 To what extent can valley information be preserved and manipulated in a hybrid 2D semiconductor-metasurface system?
• RQ4 Can the metasurface enable directional emission of circularly polarized light from specific valley states?
• RQ5 What is the role of structural asymmetry in breaking valley degeneracy and enabling selective excitation?

Key findings

• Valley excitons in monolayer MoS2 are spatially separated into distinct regions upon coupling to the asymmetric metasurface, demonstrating valley sorting.
• Photons emitted from K and K' valley states exhibit opposite circular polarization and are directed to different angular directions in K-space.
• The spatial separation of valley excitons is maintained at room temperature, indicating robustness against thermal decoherence.
• The metasurface induces a valley-selective coupling, with one valley state preferentially coupled to the radiative modes of the structure.
• Theoretical simulations confirm that the asymmetric groove geometry generates a chiral optical response that selectively manipulates valley-polarized excitons.
• The system enables a functional interface between valleytronic and photonic devices, paving the way for integrated optoelectronic circuits.