Low-temperature time-resolved luminescence experiments have been performed on a semiconductor planar microcavity which contains two In

0.13

Ga

0.87

As/GaAs quantum-wells(QWs)embedded in a two-third-wave cavity.The excition-photon mixing was observed in the structure.The spontaneous emission dynamics of the each of the two components were studied as a function of the excition-cavity detuning under nonresonant optical excitation.In the strong-coupling regime

the decay times of the low and the upper branch cavity polaritons are almost independent of the cavity detuning.Considering the special structure of a planar microcavity

the polariton states with

k

∥

in the strong-coupling region hold only a small fraction of the radiative states(roughly 10%).Most of the rediative states are within the leaky modes of the distributed Bragg reflectors.The photoluminescence decay times of the upper and low branch polaritons are uniquely determined by the radiative states within the leaky modes.Therefore

the decay time is independent of detuning.However

we observed that the rise times of the low and the upper polaritons are obviously different.The rise time of the low branch is independent of the detuning and that of the upper branch is a marked dependence on the detuning.Due to the bottleneck effect and the faster radiative rate of the low branch at

k

∥

=0

the rise time of the PL of the low branch at

k

∥

=0 is only determined by the population buildup time at the bottom of the excitonlike branch.Hence it is detuning independent.With respect to the rise time of the upper branch

the population of the upper branch mainly comes from the excitons of the low branch with the same energy.In particular

the energy of the upper branch polaritons with

k

∥

=0 for larger positive detunings is much higher than that of the bottleneck region and cooling excitions can easily reach these states well before reaching the bottom of the excitonlike branch.Consequently

the PL rise time of the upper branch becomes faster.On the opposite side of the detunings

the upper branch becomes excitonlike

the PL rise time approches the value of the low branch.Thus

the rise time of the upper branch depends significantly on the detuning.