非共振激发条件下微腔中激子自发辐射的时间分辨光谱

低温下,测量了InGaAs量子阱平面微腔的时间分辨光谱。在非共振激发条件下,观察到上下两支腔极化激元光荧光的衰退时间与失谐无关;下支腔极化激元光荧光的上升时间也与失谐无关;而上支腔极化激元光荧光的上升时间却与失谐有强烈的依赖关系,随着从负失谐到正失谐的增加,上升时间逐渐减小。对实验结果的物理根源进行了讨论。

Time-Resolved Spontaneous Emission of Excitons in a Microcavity under Nonresonant Excitation

Low-temperature time-resolved luminescence experiments have been performed on a semiconductor planar microcavity which contains two In_0.13Ga_0.87As/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.