[Paper Review] Valley selective optical control of excitons in 2D semiconductors using Chiral metasurface

Sriram Guddala, Rezlind Bushati+3 more|arXiv (Cornell University)|Oct 31, 2018

Metamaterials and Metasurfaces ApplicationsMaterials Science13 references3 citations

TL;DR

This paper demonstrates valley-selective optical control of excitons in 2D transition metal dichalcogenides (TMDs) using a chiral photonic metasurface. By engineering a dielectric metasurface to generate chiral optical fields, the system selectively excites valley-polarized excitons, resulting in preferential emission of circularly polarized light with a measured degree of circular polarization exceeding 0.65, enabling on-chip valleytronic photonic integration.

ABSTRACT

Recent advances in condensed matter physics have shown that the valley degree of freedom of electrons in 2D materials with hexagonal symmetry, such as graphene, h-BN, and TMDs, can be efficiently exploited, leading to the emergent field of valleytronics, which offers unique opportunities for efficient data transfer, computing and storage. The ability to couple the valley degree of freedom of electrons with light can further expand the ways one manipulate this degree of freedom, thus envisioning a new class of solid-state-photonic interfaces and devices. Besides this expansion of control of valley by light-waves, coupling of photons with valley-polarized electrons can dramatically expand the landscape of available optical responses, which may bring new means of controlling light in photonic devices. In this work we design such hybrid solid-state photonic metasurface integrating 2D TMD and photonic all-dielectric metasurface. While TMD is naturally endowed with the property of valley to optical-polarization coupling, the photonic metasurface is designed to produce chiral field which selectively couples to the valley degree of freedom of solid-state TMD component. We experimentally demonstrate that such coupling leads to controlled valley polarization due to the coupling of 2D materials with the chiral photonic metasurface. The measured emission from valley excitons in this hybrid system yields the preferential emission of specific helicity.

Motivation & Objective

• To enable optical control of the valley degree of freedom in 2D transition metal dichalcogenides (TMDs) for valleytronics applications.
• To design a hybrid photonic metasurface that generates chiral optical fields tailored to couple selectively with specific valleys in TMDs.
• To experimentally demonstrate valley polarization via selective excitation of excitons using chiral light from the metasurface.
• To establish a solid-state photonic interface that enables efficient coupling between valley-polarized electrons and tailored optical modes.
• To provide a scalable platform for on-chip valleytronic devices with integrated optical control.

Proposed method

• Design of a photonic all-dielectric metasurface with chiral symmetry to generate tailored chiral optical fields at the target frequency.
• Integration of monolayer TMDs (e.g., WSe2) with the chiral metasurface to enable strong coupling between valley-polarized excitons and chiral light.
• Use of geometric phase (Pancharatnam-Berry phase) in the metasurface to induce circularly polarized optical response with valley-selective coupling.
• Employment of polarization-resolved photoluminescence spectroscopy to measure the helicity of emitted light from excitons.
• Engineering of the metasurface unit cell geometry to achieve selective excitation of specific valleys (K or K') in the TMD.
• Validation of valley selectivity through comparison of emission intensity and circular polarization under illumination with left- and right-handed circularly polarized light.

Experimental results

Research questions

• RQ1 Can a chiral photonic metasurface be engineered to selectively couple to a specific valley in a 2D TMD semiconductor?
• RQ2 To what extent can the degree of valley polarization in excitons be controlled using chiral optical fields from a metasurface?
• RQ3 How does the chiral metasurface enhance the circular polarization of photoluminescence from valley excitons in TMDs?
• RQ4 What is the role of geometric phase in enabling valley-selective optical excitation in the hybrid metasurface-TMD system?
• RQ5 Can this approach enable scalable, on-chip integration of valleytronic and photonic functionalities?

Key findings

• The chiral metasurface successfully generated a chiral optical field that selectively excited valley-polarized excitons in the TMD.
• The measured photoluminescence exhibited a degree of circular polarization exceeding 0.65, indicating strong valley selectivity.
• Polarization-resolved measurements confirmed preferential emission of left- or right-handed circularly polarized light depending on the chiral design of the metasurface.
• The system demonstrated valley-selective excitation without external magnetic fields or complex strain engineering.
• The hybrid metasurface-TMD structure enabled efficient coupling between valley degrees of freedom and tailored photonic modes.
• The results validate the feasibility of using chiral metasurfaces as active components for valleytronic photonic devices.