Probing the formation of dark interlayer excitons via ultrafast photocurrent

Optically dark excitons determine a wide range of properties of photoexcited semiconductors yet are hard to access via conventional spectroscopies. Here, we develop a time-resolved ultrafast photocurrent technique (trPC) to probe the formation dynamics of optically dark excitons. The nonlinear nature of the trPC makes it particularly sensitive to the formation of excitons occurring at the femtosecond timescale after the excitation. As proof of principle, we extract the interlayer exciton formation time 0.4~ps at 160 $\mu$J/cm$^2$ fluence in a MoS$_2$/MoSe$_2$ heterostructure and show that this time decreases with fluence. In addition, our approach provides access to the dynamics of carriers and their interlayer transport. Overall, our work establishes trPC as a technique to study dark excitons in various systems that are hard to probe by other approaches.