  Ultrafast responses of BCS superconductor Nb1-xTixN films in a nonadiabatic
excitation regime were investigated by using terahertz (THz) pump-THz probe
spectroscopy. After an instantaneous excitation with the monocycle THz pump
pulse, a transient oscillation emerges in the electromagnetic response in the
BCS gap energy region. The oscillation frequency coincides with the asymptotic
value of the BCS gap energy, indicating the appearance of the
theoretically-anticipated collective amplitude mode of the order parameter,
namely the Higgs amplitude mode. Our result opens a new pathway to the
ultrafast manipulation of the superconducting order parameter by optical means.
