We describe experiments on a semiconductor microcavity which provide t
he first demonstration of motional narrowing in semiconductor inter-su
bband optical transitions. Significant narrowing occurs because of the
small mass of the polaritons in a microcavity. The demonstration is m
ade possible by the control provided in a microcavity of the mixing be
tween photon and exciton states, and hence the dispersion of the polar
iton. (C) 1997 Academic Press Limited.