Imaging Anisotropic Waveguide Exciton Polaritons in Tin Sulfide

TL;DR

This study uses nano-optical imaging to explore anisotropic waveguide exciton polaritons in tin sulfide, revealing directional propagation and energy-dependent behavior crucial for nanophotonic applications.

Contribution

It provides the first real-space imaging of anisotropic exciton polaritons in SnS, demonstrating their energy and thickness dependence and quasi-one-dimensional propagation.

Findings

Polariton wavelength and propagation length are anisotropic.

EPs exhibit quasi-one-dimensional propagation in a specific spectral range.

Anisotropy originates from different optical bandgaps and exciton binding energies along crystal axes.

Abstract

In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a lot of research interest due to their capability of shaping nanoscale field distributions and controlling nanophotonic energy flows. Here we report a nano-optical imaging study of waveguide exciton polaritons (EPs) in tin sulfide (SnS) in the near-infrared (IR) region using the scattering-type scanning near-field optical microscopy (s-SNOM). With s-SNOM, we mapped in real space the propagative EPs in SnS, which show sensitive dependence on the excitation energy and sample thickness. Moreover, we found that both the polariton wavelength and propagation length are anisotropic in the sample plane. In particular, in a narrow spectral range from 1.32 to 1.44 eV, the EPs demonstrate quasi-one-dimensional propagation, which is rarely seen in natural polaritonic materials. Further analysis indicates that the observed polariton anisotropy is originated from the different optical bandgaps and exciton binding energies along the two principal crystal axes of SnS.