Signatures of three-dimensional photo-induced superconductivity in YBa$_2$Cu$_3$O$_{6.48}$

Abstract: Optical excitation of large-amplitude apical oxygen phonon oscillations has been shown to renormalize the electronic properties of YBa$2$Cu$_3$O${6+x}$, 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 THz 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 the first 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 photo-induced in-plane optical gap $2\Delta\simeq30$ cm$^{-1}$ and a divergent imaginary conductivity, both consistent with photo-induced superconductivity. A global fit to these data suggests that in- and out-of-plane electronic properties never completely equilibrate during the dynamics.