Terahertz-induced population transfer between exciton complexes in monolayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>WSe</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math>

Pulsed terahertz radiation is shown here to efficiently convert excitonic many-particle complexes in monolayer semiconductors that are primary carriers of energy and information. The conversion is induced on demand after the optical injection, occurs on picosecond time scales, and persists across a broad temperature range. This offers both access to the formation of excitonic states and can be used for their manipulation using low-energy photons. The universal nature of the approach is expected be useful for a wide range of excitonic materials.