  Far and mid infrared optical pulses have been shown to induce non-equilibrium
unconventional orders in complex materials, including photo-induced
ferroelectricity in quantum paraelectrics, magnetic polarization in
antiferromagnets and transient superconducting correlations in the normal state
of cuprates and organic conductors. In the case of non-equilibrium
superconductivity, femtosecond drives have generally resulted in electronic
properties that disappear immediately after excitation, evidencing a state that
lacks intrinsic rigidity. Here, we make use of a new optical device to drive
metallic K$_3$C$_{60}$ with mid-infrared pulses of tunable duration, ranging
between one picosecond and one nanosecond. The same superconducting-like
optical properties observed over short time windows for femtosecond excitation
are shown here to become metastable under sustained optical driving, with
lifetimes in excess of ten nanoseconds. Direct electrical probing becomes
possible at these timescales, yielding a vanishingly small resistance. Such a
colossal positive photo-conductivity is highly unusual for a metal and, when
taken together with the transient optical conductivities, it is rather
suggestive of metastable light-induced superconductivity.
