Nano-optical studies of anisotropic waveguide exciton polaritons in Tin Sulfide

We report a nano-optical imaging study of waveguide exciton polaritons (EPs) in Tin Sulfide (SnS) in the near-infrared (IR) region by using the scattering-type scanning near-field optical microscope (s-SNOM). The samples that we studied are SnS microcrystals coated with a thin shell of Tin Disulfide. With s-SNOM, we mapped in real space the propagative EPs in SnS, which are sensitively dependent on the excitation energy and sample thickness. Moreover, we found that both the polariton wavelength and propagation length have shown strong in-plane anisotropy between the zigzag (a axis) and armchair (b axis) crystal directions. In particular, at a narrow spectral range from 1.32 to 1.44 eV, the EPs have shown a relatively long propagation length along the b axis but barely propagate along the a axis. The observed quasi-one-dimensional polaritons, which is originated from the different bandgap and exciton energies along the two axes of SnS, are promising for future applications related to directional control of the flow of nanophotonic energy and information.