Recently the THz light-induced excitation of Higgs modes, that is oscillations of the order parameter amplitude in superconductors has been observed. The mechanism of this effect has remained not fully understood. In the absence of relaxation processes the order parameter amplitude is not affected by the irradiation since the systems of quasiparticle excitations and Cooper pairs move together and remain in the equilibrium with each other. In this talk I will demonstrate how the relaxation of quasiparticles by impurity scattering leads to the finite coupling between the Higgs mode and the electromagnetic radiation [1]. Besides that, I will discuss the exotic states which occur due to the hybridization between the Higgs mode and the magnetic degrees of freedom in sperconductor/ferromagnetic insulator systems [2]. This mechanism provides the non-linear coupling between the phonons and magnons. It leads to the interesting possibility of generating correlated photon pairs, that is to the dynamical Casimir effect driven by non-equilibrium magnons.

[1] Nonlinear electromagnetic response and Higgs-mode excitation in BCS superconductors with impurities, M Silaev, Physical Review B 99, 224511 (2019)
[2] Spin and charge currents driven by the Higgs mode in high-field superconductors, Mikhail A. Si-laev, Risto Ojajärvi, and Tero T. Heikkilä, Phys. Rev. Research 2, 033416 (2020)