Phase signatures in the third-harmonic response of Higgs and coexisting modes in superconductors

Third-harmonic generation (THG) experiments on superconductors can be used to investigate collective excitations like the amplitude mode of the order parameter known as Higgs mode. These modes are visible due to resonances in the THG signal if the driving frequency matches the energy of the mode. In real materials, multiple modes can exist giving rise to additional THG contributions, such that it is difficult to unambiguously interpret the results. In this paper, we additionally analyze the phase of the THG signal for the first time, which contains microscopic details beyond classical resonances as well as signatures of couplings between modes that are difficult to observe in the amplitude alone. We investigate how the Higgs mode, impurities, or Coulomb interaction affects the phase response and consider exemplary two systems with additional modes. We argue that extracting this phase information could be valuable in future experiments.