Far out-of-equilibrium spin populations trigger giant spin injection into atomically thin Molybdenum disulfide

Injecting spins from ferromagnetic metals into semiconductors efficiently is a crucial step towards the seamless integration of charge- and spin-information processing in a single device. However, efficient spin injection into semiconductors has remained an elusive challenge even after almost three decades of major scientific effort due to, for example, the extremely low injection efficiencies originating from impedance mismatch, or technological challenges originating from stability and the costs of the approaches. We show here that, by utilizing the strongly out-of-equilibrium nature of sub-picosecond spin-current pulses, we can obtain a massive spin transfer even across a bare ferromagnet/semiconductor interface. We demonstrate this by producing ultrashort spin-polarized current pulses in Co and injecting them into monolayer molybdenum disulfide, a two-dimensional semiconductor. The layer acts both as the receiver of the spin injection and as a selective converter of the spin current into a charge current, whose terahertz emission is then measured. Strikingly, we measure a giant spin current, orders of magnitude larger than typical injected spin-current densities using currently available techniques. Our result demonstrates that technologically relevant spin currents do not require the very strong excitations typically associated with femtosecond lasers. Rather, they can be driven by ultralow-intensity laser pulses, finally enabling ultrashort spin-current pulses to be a technologically viable information carrier for terahertz spintronics.