  Nonlinear optical excitation of infrared active lattice vibrations has been
shown to melt magnetic or orbital orders and to transform insulators into
metals. In cuprates, this technique has been used to remove charge stripes and
promote superconductivity, acting in a way opposite to static magnetic fields.
Here, we show that excitation of large-amplitude apical oxygen distortions in
the cuprate superconductor YBa2Cu3O6.5 promotes highly unconventional
electronic properties. Below the superconducting transition temperature (Tc =
50 K), interbilayer coherence is transiently enhanced at the expense of
intra-bilayer coupling. Strikingly, even above Tc a qualitatively similar
effect is observed up to room temperature, with transient inter-bilayer
coherence emerging from the incoherent ground state and similar transfer of
spectral weight from high to low frequency. These observations are compatible
with previous reports of an inhomogeneous normal state that retains important
properties of a superconductor, in which light may be melting competing orders
or dynamically synchronizing the interlayer phase. The transient redistribution
of coherence discussed here could lead to new strategies to enhance
superconductivity in steady state.
