  Low-energy polaritons in semiconductor microcavities are important for many
processes such as, e.g., polariton condensation. Organic microcavities
frequently feature both strong exciton-photon coupling and substantial
scattering in the exciton subsystem. Low-energy polaritons possessing small or
vanishing group velocities are especially susceptible to the effects of such
scattering that can render them strongly localized. We compare the time
evolution of low-energy wave packets in perfect microcavities and in a model
1$d$ cavity with diagonal disorder to illustrate this localization of
polaritons and to draw attention to the need to explore its consequences for
the kinetics and collective properties of polaritons.
