Imaging Anisotropic Waveguide Exciton Polaritons in Tin Sulfide

Abstract

In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a great deal 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 (near-IR) region using 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. A further analysis indicates that the observed polariton anisotropy originates from the different optical band gaps and exciton binding energies along the two principal crystal axes of SnS.

Nano-optical imaging of exciton–plasmon polaritons in WSe2/Au heterostructures

“…By tuning the laser energy, we found that the fringe pattern demonstrates a systematic variation. As introduced in previous studies, 20–22,28–30 such fringes are generated due to the interference between tip-backscattered photons [labeled as “P1” in Fig. 1a] and beam paths involving propagative in-plane modes.…”

“…Based on the interference mechanism involving beam paths ‘P1’ and ‘P2’ (Fig. 1a), we can establish a direct relationship between the fringe period ( ρ ) and the wavelength of the in-plane mode ( λ p ): 20–22 1/ ρ = 1/ λ p − cos α / λ 0 ,where λ 0 is the free-space photon wavelength and α is the incident angle of the laser beam ( α ≈ 30°) [see Fig. 1a].…”

“…1,[12][13][14][15] In recent years, van der Waals (vdW) semiconductors have been widely explored in polaritonic studies due to their strongly-bound excitons. [16][17][18][19][20][21][22] The exciton binding energy of these vdW materials is 60-100 meV in the bulk and can reach hundreds of meV when thinning to atomic layers. [23][24][25][26] As a result, EPs and EPPs in these layered semiconductors are stable even at room temperature.…”

Dynamical Response of Nonlinear Optical Anisotropy in a Tin Sulfide Crystal under Ultrafast Photoexcitation

“…Recently, Luan et al reported the real-time observation of an anisotropic exciton−polariton in an SnS waveguide. 31 Maragkakis et al demonstrated polarizationresolved second harmonic generation imaging of in-plane anisotropy in a layered SnS crystal. 32 Alberding et al investigated the ultrafast optical response of a nano-to micrometer-sized SnS film with transient terahertz spectroscopy, in which the carrier mobility and carrier dynamics of the thin films exhibit the strong particle size dependence.…”

“…The investigation of the dynamical anisotropic response of SnS following photoexcitation is of great interest for understanding the originality of the anisotropy, expanding its possible application in polarization-sensitive photodetectors, − and offering a new degree of freedom for manipulating the electronic, optoelectronic, and optical properties in a non-equilibrium state. Recently, Luan et al reported the real-time observation of an anisotropic exciton–polariton in an SnS waveguide . Maragkakis et al demonstrated polarization-resolved second harmonic generation imaging of in-plane anisotropy in a layered SnS crystal .…”