Anisotropic ultrafast hot carrier dynamics of two-dimensional SnS single crystals

Abstract

In this paper, we investigated the temperature-dependent and anisotropic ultrafast hot carrier decay dynamics of two-dimensional SnS single crystals using femtosecond transient optical spectroscopy. The photo-excited hot carriers in SnS are relaxed via a fast decay (τ₁) and a slow decay (τ₂), which are contributed by the electron–phonon interactions coupling with high frequency and low frequency optical phonons of SnS, respectively. Both the τ₁ and τ₂ decay times show anisotropy. In the ab-plane, both τ₁ and τ₂ decays have a faster relaxion time in the b-axis direction than in the a-axis direction, which is due to the crystal anisotropy of SnS. The crystal anisotropy of SnS gives rise to more phonon vibrations in the b-axis direction than in the a-axis direction, which leads to stronger electron–phonon coupling along the b-axis direction and manifests as a shorter decay time along the b-axis direction. For the τ₂ decay process, the femtosecond laser pump induces different dielectric responses in the ab-plane. In the b-axis direction, the pump laser induces a reduction in the dielectric coefficient (Δε < 0), while it induces an increase in the dielectric coefficient (Δε > 0) in the a-axis.