Two-dimensional van der Waals semiconductors feature a variety of stable Coulomb-bound electron-hole complexes, which determine the optical response of the materials and serve as primary carriers of energy and spin-valley encoded information. Importantly, transitions between different excitonic states are found in the terahertz spectral range, motivating the use of strong THz radiation for their manipulation on ultrafast timescales. In this work, we apply this technique to efficiently transfer populations within the manifold of excitonic complexes in monolayer ${\mathrm{WSe}}_2$, combining pulsed optical injection with a perturbation induced by a THz free-electron laser source. Monitoring time-resolved photoluminescence, we show conversion between different Coulomb-bound species across biexcitonic and excitonic regimes. Depending on the lattice temperature, these processes involve both short-lived bright and long-lived dark states. Combining experimental findings with theory support, we outline possible dissociation and formation pathways of charged excitons and biexcitons induced by the THz radiation. Finally, we demonstrate access to the formation dynamics of charged biexcitons under controlled conditions of thermalized populations of their constituents, avoiding complications of excess energies that otherwise occur after nonresonant optical excitation.

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• Excitons
• Transition metal dichalcogenides
• Optical microscopy
• Photoluminescence
• Terahertz techniques