THz driving of superconducting collective modes: new advances and open questions

In the last few years, a number of experiments carried out with very intense THz fields, either in transmission or in pump-probe configuration, have shown the possibility to excite superconducting collective modes, like amplitude (Higgs) and phase (Goldstone) excitation of the order parameter, via light pulses. The signatures of the superconducting excitations manifest either in third-harmonic generation, in the case of narrowband pulses, or in well-defined oscillations of the transmitted field as a function of the pump-probe delay, in the case of broadband pulses. In both cases, one is essentially measuring the properties of the non-linear optical kernel [1,2]. Besides collective electronic excitations, in the superconducting state also the BCS response, connected to the continuum of quasiparticle excitations, can lead to pronounced resonances. How to distinguish and characterise the different possible processes has been the subject of an intense experimental and theoretical research. In this talk I will review the current state of the art, and I will discuss some recent results on the role of disorder[3] and of the layered structure[4], as the one of cuprates, for what concerns the excitations of superconducting phase modes.

[1] F. Giorgianni, T. Cea, C. Vicario, C. P. Hauri, W. K. Withanage, X. Xi, and L. Benfatto, Nat. Phys. 15, 341 (2019).
[2] M. Udina, T. Cea and L. Benfatto, Phys. Rev. B 100, 165131 (2019)
[3] G. Seibold, M. Udina, C. Castellani and L. Benfatto,arXiv:2010.12507
[3] F. Gabriele, M. Udina and L. Benfatto, arXiv:2006.02296.