Higgs amplitude mode in s-wave superconductors revealed by terahertz pump-terahertz probe spectroscopy

摘要

By using terahertz (THz) pump-THz probe spectroscopy, we investigated ultrafast dynamics of s-wave superconductors NbN. After instantaneous excitation with an intense monocycle THz pulse, transient oscillation of the superconducting order parameter is observed in the transmission of the THz probe pulse, which is interpreted as the collective Higgs amplitude mode associated with spontaneous symmetry breaking. We also found that the Higgs mode can be resonantly excited by a sub-gap multi-cycle THz pump pulse in the nonlinear response regime, resulting in efficient third-order harmonics generation. These results shed new light on the ultrafast optical control of quantum condensates and its application to THz nonlinear optics.