Dynamical decoherence of the light induced interlayer coupling in   YBa$_{2}$Cu$_{3}$O$_{6+\delta}$
  Optical excitation of apical oxygen vibrations in YBa$_{2}$Cu$_{3}$O$_{6+\delta}$ has been shown to enhance its c-axis superconducting-phase rigidity, as evidenced by a transient blue shift of the equilibrium inter-bilayer Josephson plasma resonance. Surprisingly, a transient c-axis plasma mode could also be induced above T$_{c}$ by the same apical oxygen excitation, suggesting light activated superfluid tunneling throughout the pseudogap phase of YBa$_{2}$Cu$_{3}$O$_{6+\delta}$. However, despite the similarities between the above T$_{c}$ transient plasma mode and the equilibrium Josephson plasmon, alternative explanations involving high mobility quasiparticle transport should be considered. Here, we report an extensive study of the relaxation of the light-induced plasmon into the equilibrium incoherent phase. These new experiments allow for a critical assessment of the nature of this mode. We determine that the transient plasma relaxes through a collapse of its coherence length rather than its carrier (or superfluid) density. These observations are not easily reconciled with quasiparticle interlayer transport, and rather support transient superfluid tunneling as the origin of the light-induced interlayer coupling in YBa$_{2}$Cu$_{3}$O$_{6+\delta}$. 
C. R. Hunt, D. Nicoletti, S. Kaiser, D. Pr\"opper, T. Loew, J. Porras,   B. Keimer, A. Cavalleri
cond-mat.supr-con cond-mat.str-el