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

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 <i>via</i> a fast decay (<i>τ</i>₁) and a slow decay (<i>τ</i>₂), which are contributed by the electron-phonon interactions coupling with high frequency and low frequency optical phonons of SnS, respectively. Both the <i>τ</i>₁ and <i>τ</i>₂ decay times show anisotropy. In the <i>ab</i>-plane, both <i>τ</i>₁ and <i>τ</i>₂ decays have a faster relaxion time in the <i>b</i>-axis direction than in the <i>a</i>-axis direction, which is due to the crystal anisotropy of SnS. The crystal anisotropy of SnS gives rise to more phonon vibrations in the <i>b</i>-axis direction than in the <i>a</i>-axis direction, which leads to stronger electron-phonon coupling along the <i>b</i>-axis direction and manifests as a shorter decay time along the <i>b</i>-axis direction. For the <i>τ</i>₂ decay process, the femtosecond laser pump induces different dielectric responses in the <i>ab</i>-plane. In the <i>b</i>-axis direction, the pump laser induces a reduction in the dielectric coefficient (Δ<i>ε</i> < 0), while it induces an increase in the dielectric coefficient (Δ<i>ε</i> > 0) in the <i>a</i>-axis.