Multidimensional Phononics in YBa₂Cu₃O_6+x

In underdoped YBa₂Cu₃O_6+x superconductors, resonant photo-excitation of the apical oxygen phonon modes at mid-infared frequencies gives rise to transient features in the terahertz reflectivity that indicate nonequilibrium superconductivity well above the critical temperature and up to the pseudogap temperature.

Recent time-resolved second harmonic generation measurements, combined with the same mid-infrared excitation, revealed a three-orders of magnitude growth of a Josephson plasma polariton mode at 2.5 THz, identified via its unique symmetry, momentum and temperature dependence. The amplification of this Josephson plasma polariton, assumed to be well formed below but incoherent throughout the pseudogap phase, potentially explains the superconducting-like features in the transient THz reflectivity.

However, the nonlinear coupling mechanism between the driven phonons and the amplified plasmon could not be directly observed. Here, we make use of multidimensional nonlinear spectroscopy in the mid-infrared spectral range to isolate the nonlinear phonon-plasmon coupling mechanism.