Ultrafast anisotropic valleytronics in SnSe.

Understanding and controlling valley-specific light-matter interactions and non-equilibrium microscopic coupling mechanisms lie at the heart of valleytronics. Due to their strong in-plane anisotropy, which enables polarization-controlled optical transitions to distinct non-degenerate valleys, group-IV monochalcogenides have been recently proposed as promising candidates for next-generation valleytronic materials. However, ultrafast non-equilibrium dynamics following the optical preparation of valley-polarized states have received limited attention in these systems. Here, combining time- and angle-resolved extreme-ultraviolet photoemission spectroscopy with time-dependent Boltzmann equation simulations, we investigate ultrafast valley polarization dynamics following polarization-controlled photoexcitation in SnSe. We show that selective excitation to valleys at global conduction minima yields nearly unity and time-independent valley polarization. By contrast, photoexcitation to the other valley channel leads to ultrafast decay and reversal of valley polarization on subpicosecond timescales due to intervalley scattering mediated by strong electron-phonon coupling with an optical phonon mode. Our findings reveal strongly anisotropic and radically different non-equilibrium valley physics than in most common two-dimensional valleytronics materials.