  Exciton-polaritons are hybrid light-matter excitations arising from the
non-perturbative coupling of a photonic mode and an excitonic resonance.
Behaving as interacting photons, they show optical third-order nonlinearities
providing effects such as optical parametric oscillation or amplification. It
has been suggested that polariton-polariton interactions can be greatly
enhanced by inducing aligned electric dipoles in their excitonic part. However
direct evidence of a true particle-particle interaction, such as superfluidity
or parametric scattering is still missing. In this work, we demonstrate that
dipolar interactions can be used to enhance parametric effects such as
self-phase modulation in waveguide polaritons. By quantifying these optical
nonlinearities we provide a reliable experimental measurement of the direct
dipolar enhancement of polariton-polariton interactions.
