Anisotropic Ultrafast Hot Carrier Dynamics of Two-dimensional SnS Single Crystals

Abstract

In this paper, we investigated the temperature-dependent and anisotropic ultrafast hot carriers 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 time show anisotropy. In the ab-plane, both τ₁ and τ₂ decay have a faster relaxion time in a-axis direction than the b-axis direction, which is a result of the crystal anisotropy of SnS. The crystal anisotropy of SnS gives rise to more phonon vibrations in a-axis than b-axis directions, which leading to stronger electron-phonon coupling along a-axis directions and manifesting as a shorter decay time along a-axis directions. For the τ₂ decay process, the femtosecond laser pump induce different dielectric response in ab-plane. In a-axis direction, the pump laser induce a reduce of dielectric coefficient (Δε < 0), while the pump laser induce a increase of dielectric coefficient (Δε > 0).