Here we investigated the Higgs mode in the d-wave cuprate superconductor Bi2Sr2CaCu2O8+δ (Bi2212) by THz pump-optical probe spectroscopy in reflection geometry. Upon entering the superconducting state, the transient reflectivity change exhibits an instantaneous component that follows the squared THz electric field. This instantaneous signal can be thought as the THz-pulse induced Kerr effect and described by the thirdorder nonlinear susceptibility. The polarization-resolved measurement reveals that the THz Kerr signal can be decomposed into the fully symmetric A1g component and the B1g component. While the ratio of the two components strongly depends on the hole doping concentration, the A1g component is always larger than B1g component. Comparison with a mean-field calculation of the nonlinear susceptibility indicates that the A1g component is associated with the Higgs mode of the d-wave order parameter .
 R. Matsunaga, Y. I. Hamada, K. Makise, Y. Uzawa, H. Terai, Z. Wang, and R. Shimano, Phys. Rev. Lett. 111, 057002 (2013).
 R. Matsunaga, N. Tsuji, H. Fujita, A. Sugioka, K. Makise, Y. Uzawa, H. Terai, Z. Wang, H. Aoki and R. Shimano, Science 345, 1145 (2014).
 K. Katsumi, N. Tsuji, Y. I. Hamada, R. Matsunaga, J. Schneeloch, R. D. Zhong, G. D. Gu, H. Aoki, Y. Gallais, and R. Shimano, Phys. Rev. Lett. 120, 117001 (2018).