  We report on a photo-induced transient state of YBa2Cu2O6+x in which
transport perpendicular to the Cu-O planes becomes highly coherent. This effect
is achieved by excitation with mid-infrared optical pulses, tuned to the
resonant frequency of apical oxygen vibrations, which modulate both lattice and
electronic properties. Below the superconducting transition temperature Tc, the
equilibrium signatures of superconducting interlayer coupling are enhanced.
Most strikingly, the optical excitation induces a new reflectivity edge at
higher frequency than the equilibrium Josephson plasma resonance, with a
concomitant enhancement of the low frequency imaginary conductivity. Above Tc,
the incoherent equilibrium conductivity becomes highly coherent, with the
appearance of a reflectivity edge and a positive imaginary conductivity that
increases with decreasing frequency. These features are observed up to room
temperature in YBa2Cu2O6.45 and YBa2Cu2O6.5. The data above Tc can be fitted by
hypothesizing that the light re-establishes a transient superconducting state
over only a fraction of the solid, with a lifetime of a few picoseconds.
Non-superconducting transport could also explain these observations, although
one would have to assume transient carrier mobilities near 10^4 cm^2/(V.sec) at
100 K, with a density of charge carriers similar to the below Tc superfluid
density. Our results are indicative of highly unconventional non-equilibrium
physics and open new prospects for optical control of complex solids.
