Higgs Mode in the d-Wave Superconductor Bi2Sr2CaCu2O8+x Driven by an Intense Terahertz Pulse

We investigate the terahertz (THz)-pulse-driven nonlinear response in the d-wave cuprate superconductor Bi2Sr2CaCu2O8+x (Bi2212) using a THz pump near-infrared probe scheme in the time domain. We observe an oscillatory behavior of the optical reflectivity that follows the THz electric field squared and is markedly enhanced below Tc. The corresponding third-order nonlinear effect exhibits both A1g and B1g symmetry components, which are decomposed from polarization-resolved measurements. A comparison with a BCS calculation of the nonlinear susceptibility indicates that the A1g component is associated with the Higgs mode of the d-wave order parameter.