In high-energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e., the scalar (amplitude) excitation of the Higgs field. In superconductors, Cooper pairs bear a close analogy to the Higgs field. Interaction between the Cooper pairs and other degrees of freedom provides dissipation channels for the amplitude mode, which may reveal important information about the microscopic pairing mechanism. To this end, we investigate the Higgs (amplitude) mode of several cuprate thin films using phase-resolved terahertz third harmonic generation (THG). In addition to the heavily damped Higgs mode itself, we observe a universal jump in the phase of the driven Higgs oscillation as well as a non-vanishing THG above T c. These findings indicate coupling of the Higgs mode to other collective modes and potentially a nonzero pairing amplitude above T c .

Superconducting (SC) fluctuations in cuprate superconductors have been extensively studied to gain a deep insight into the preformed Cooper pairs above the SC transition temperature Tc. While the various measurements, such as the terahertz (THz) optical conductivity, Nernst effect and ARPES measurements have provided the signature of the SC fluctuations, the onset temperature of the SC fluctuations depends on the measurement scheme. Here, we shed light on the Higgs mode to investigate the SC fluctuations, as it is the direct fingerprint of SC order parameter and can help elucidate the development of SC phase coherence. We perform the THz pump-optical probe spectroscopy for underdoped and overdoped Bi2Sr2CaCu2O8+x (Bi2212) thin films. We observe oscillatory behavior in the pump-probe signal (THz Kerr signal) that sharply onsets slightly above Tc and is attributed to the emergence of the Higgs mode. We also measure the THz optical conductivity and extracted the superfluid density Ns using the two-fluid model. The onset temperature of the THz Kerr signal shows a good agreement with that of Ns for both thin films, indicating that the Higgs mode emerges when SC phase coherence is established on the THz time scale. We also study five differently doped Bi2212 single crystals where the onset temperature of the Higgs mode is shown to be located 10-30 K above the SC Tc dome.