Non-Perturbative: This term suggests that the interaction between the terahertz field and the material is strong enough that perturbation th...

Non-Perturbative: This term suggests that the interaction between the terahertz field and the material is strong enough that perturbation theory (which assumes weak interactions) is not sufficient to explain the process. Instead, the interaction is so intense that it causes a large-scale nonlinear response in the material, which leads to high-order harmonic generation. This is a key feature of the research you mentioned—where the interaction is "non-perturbative