  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
