Asymmetric electronic states are essential for generating second-order optical nonlinearity, which enables terahertz-wave generation via difference-frequency mixing. While surface electric fields are known to break the symmetry of carrier envelope functions, lattice strain modifies the band structure and can induce carrier localization. In this study, we investigate the role of strain in InxGa1−xAs/GaAs multiple quantum wells. Even with a small indium concentration of x=0.05, corresponding to a lattice mismatch of less than 0.4%, clear terahertz-wave emission via difference-frequency mixing was observed under above-barrier excitation. Numerical calculations reveal the formation of resonant above-barrier states that are weakly localized by strain and contribute to the observed nonlinear response. Photoreflectance measurements confirm the presence of surface electric fields in all samples. These results indicate that surface electric fields primarily induce asymmetric wavefunctions, while even a small strain plays a crucial role in forming resonant above-barrier states that contribute to second-order nonlinear polarization.
Kojima et al. (Mon,) studied this question.