Transient Higgs oscillations and high-order nonlinear light-Higgs coupling in a terahertz wave driven NbN superconductor

We study the nonlinear optical response in a superconducting NbN thin film with strong terahertz (THz)wave. Besides the expected third harmonic generation, we observe a new transient oscillation which softensin frequency with temperature increasing towards superconducting transition temperatureTc. We identify thisnew mode as the Higgs transient oscillation. To verify this proposal, we introduce a time-frequency resolvedtechnique, named spectrogram for visualizing THz spectrum. The dynamic decaying behavior of the mode isobserved, which is consistent with theoretical expectation about intrinsic Higgs oscillation. Moreover, a higherorder nonlinear optics effect,i.e.fifth harmonic generation, has been observed for the first time, which we assignto the higher order coupling between Higgs mode and electromagn