  Multi-level exciton-polariton systems offer an attractive platform for
studies of non-linear optical phenomena. However, studies of such consequential
non-linear phenomena as polariton condensation and lasing in planar
microcavities have so far been limited to two-level systems, where the
condensation takes place in the lowest attainable state. Here, we report
non-equilibrium Bose-Einstein condensation of exciton-polaritons and low
threshold, dual-wavelength polariton lasing in vertically coupled, double
planar microcavities. Moreover, we find that the presence of the non-resonantly
driven condensate triggers interbranch exciton-polariton transfer in the form
of energy-degenerate parametric scattering. Such an effect has so far been
observed only under excitation that is strictly resonant in terms of the energy
and incidence angle. We describe theoretically our time-integrated and
time-resolved photoluminescence investigations by a set of rate equations
involving an open-dissipative Gross-Pitaevskii equation. Our platform's
inherent tunability is promising for construction of planar lattices, enabling
three-dimensional polariton hopping and realization of photonic devices, such
as two-qubit polariton-based logic gates.
