Signatures of three-dimensional photo-induced superconductivity in YBa₂Cu₃O_6.48

Optical excitation of large-amplitude apical oxygen phonon oscillations has been shown to renormalize the electronic properties of YBa₂Cu₃O_6+x, inducing a superconducting-like optical response above equilibrium T_C. All of the evidence collected so far has been based on the changes of the terahertz frequency c-axis response. In these measurements, the capacitive interlayer coupling was seen to transform into a superconducting-like inductive response. This assignment was strengthened by recent measurements of ultrafast magnetic field expulsion. Here, we report an experimental determination of the transient in-plane optical properties, which has so far been elusive due to the high equilibrium reflectivity and the need to evaluate minute changes in the optical response. We report the appearance of a photoinduced in-plane optical gap 2Δ≃30 cm⁻¹ and a divergent imaginary conductivity, both consistent with photoinduced superconductivity. A global fit to these data suggests that in- and out-of-plane electronic properties never completely equilibrate during the dynamics.