Two-plasmon excitations with THz light pulses in superconducting cuprates*

The technical improvements in the generation of intense and phase-stable THz pulses considerably enlarged the possibilities to manipulate material properties via optical nonlinearities triggered by ultra-short light excitations. As a prototypical example, large amplitude fluctuations of Raman-active modes can be driven by means of a two-photon sum-frequency excitation process, where the pump frequency is selectively tuned to match half the value of the mode resonance. A related but different possibility is that strong pulses drive simultaneously two collective modes, with an enhancement in the nonlinear response when the pump is directly resonant with the mode frequency. So far, such a scenario has been mainly discussed within the context of non-linear phononics, where the excitations of two infrared (IR)-active phonons serves as an intermediate step to drive additional Raman-active lattice vibrations by anharmonic coupling. More recently, the same mechanism has been shown to describe the second-order excitation of IR-active phonons in THz Kerr measurements, as well as the non-linear driving of the soft out-of-plane Josephson plasma mode in cuprate superconductors. Starting from the more conventional case of IR-active lattice vibrations, in this talk I will introduce the theoretical framework we recently developed to model two-mode excitations driven by intense THz pulses. This protocol will then be generalized to describe plasma excitations in superconducting cuprates with both one and two layers per unit cell, showing how the inclusion of dispersion effects and the subsequent mixing between in-plane and out-of-plane plasmons influence the nonlinear response at THz frequencies.