Exploring the nonlinear optical responses in a conventional superconductor NbN by terahertz two-dimensional spectroscopy

Terahertz (THz) nonlinear optical responses in superconductors have emerged as a powerful technique to explore the properties of the superconducting order parameter. However, the origins of the terahertz nonlinear responses are still elusive and under intensive discussion. To elucidate the origin of the nonlinear optical process in superconductors, recently developed THz two-dimensional spectroscopy is promising, which allows us to disentangle multiple contributions in the two-dimensional frequency domain. In this study, we performed THz two-dimensional spectroscopy on conventional superconductor NbN films. We identified a nonlinear signal below the superconducting transition temperature, whose peak is located at the superconducting gap energy. In the talk, we will discuss the origin of the THz nonlinear responses in detail. Our work lays the foundation to investigate unconventional superconductors by THz two-dimensional spectroscopy.