  The development of patterned multi-quantum well heterostructures in
GaAs/AlGaAs waveguides has recently allowed to achieve exciton-polariton
condensation in a topologically protected bound state in the continuum (BIC).
Remarkably, condensation occurred above a saddle point of the polariton
dispersion. A rigorous analysis of the condensation phenomenon in these
systems, as well as the role of the BIC, is still missing. In the present
Letter we theoretically and experimentally fill this gap, by showing that
polariton confinement resulting from the negative effective mass and the
photonic energy gap in the dispersion play a key role in enhancing the
relaxation towards the condensed state. In fact, our results show that
low-threshold polariton condensation is achieved within the effective trap
created by the exciting laser spot regardless of whether the resulting confined
mode is long-lived (polariton BIC) or short-lived (lossy mode). In both cases,
the spatial quantization of the polariton condensate and the threshold
differences associated to the corresponding state lifetime are measured and
characterized. For a given negative mass, a slightly lower condensation
threshold from the polariton BIC mode is found and associated to its suppressed
radiative losses as compared to the lossy one.
