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Light-matter interaction in quantum materials is a critical aspect that elucidate their intriguing properties. In particular, the terahertz (THz) frequency range is of great interest as it is the natural energy scale of quantum many-body interaction between charge, spin, orbital, and lattice degrees of freedom. Recent advancements in generating an intense THz pulse enabled the investigations of nonlinear light-matter interaction, which can provide information unreachable by linear light-matter coupling. However, the study of nonlinear light-matter interactions is still in its infancy, and experimental investigation is required.

Here, we will present the recent results of THz nonlinear spectroscopy applied to superconductors. Using THz two-dimensional coherent spectroscopy (2DCS), we identified a unique paramagnetic nonlinear response of the amplitude collective mode of the superconducting order parameter, namely the Higgs mode, in conventional superconductors NbN [1]. Our findings demonstrate the ability of THz 2DCS to explore collective excitations inaccessible in other spectroscopies. We will further discuss the results of THz 2DCS in the case of a multi-gap superconductor MgB₂. Finally, given the situation that the paramagnetic light-matter interaction plays an essential role in the THz nonlinearity in superconductors, we reexamine our previous THz pump-probe experiments in high-temperature cuprate superconductors Bi₂Sr₂CaCu₂O_8+x [2].

 

References:

[1] K. Katsumi et al., Phys. Rev. Lett. 132, 256903 (2024)

[2] K. Katsumi et al., Phys. Rev. Lett. 120, 117001 (2018)

 

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