  Optical excitation in the cuprates has been shown to induce transient
superconducting correlations above the thermodynamic transition temperature,
$T_C$, as evidenced by the terahertz frequency optical properties in the
non-equilibrium state. In YBa$_2$Cu$_3$O$_{6+x}$ this phenomenon has so far
been associated with the nonlinear excitation of certain lattice modes and the
creation of new crystal structures. In other compounds, like
La$_{2-x}$Ba$_x$CuO$_4$, similar effects were reported also for excitation at
near infrared frequencies, and were interpreted as a signature of the melting
of competing orders. However, to date it has not been possible to
systematically tune the pump frequency widely in any one compound, to
comprehensively compare the frequency dependent photo-susceptibility for this
phenomenon. Here, we make use of a newly developed optical parametric
amplifier, which generates widely tunable high intensity femtosecond pulses, to
excite YBa$_2$Cu$_3$O$_{6.5}$ throughout the entire optical spectrum (3 - 750
THz). In the far-infrared region (3 - 25 THz), signatures of non-equilibrium
superconductivity are induced only for excitation of the 16.4 THz and 19.2 THz
vibrational modes that drive $c$-axis apical oxygen atomic positions. For
higher driving frequencies (25 - 750 THz), a second resonance is observed
around the charge transfer band edge at ~350 THz. These observations highlight
the importance of coupling to the electronic structure of the CuO$_2$ planes,
either mediated by a phonon or by charge transfer.
