Polariton–polariton collinear interaction in semiconductor microcavities

• Collinear polariton–polariton scattering is found having no analogs in bulk polariton scattering.
• The k=0 state plays a key role, being involved in a great number of different scattering processes.
• Direct population of the k=0 state excludes the explanation in terms of BEC of microcavity polaritons.

It is shown that the existence of a small polariton mass near k=0k=0 and a large exciton mass leads to a special type of polariton–polariton interaction: a collinear scattering, which has no analogs in bulk polariton scattering. This scattering process governed with energy and momentum conservation laws is very sensitive to the population of the k=0k=0 state, which plays a main role in nonlinear relaxation of polaritons.