“Time-resolved terahertz-pump and optical-probe measurements demonstrate that the 1s heavy-hole and light-hole exciton resonances undergo large-amplitude spectral modulations when the terahertz radiation is tuned near the 1s–2p intraexciton transition” is how the paper begins by researchers from Oregon State, University of Iowa, and others. You can read the full abstract of the paper titled “Ultrafast nonlinear optical effects in semiconductor quantum wells resonantly driven by strong few-cycle terahertz pulses“.