Radiative corrections to the excitonic molecule state in GaAs microcavities

The optical properties of excitonic molecules (XX's) in GaAs-based quantum well microcavities (MC's) are studied, both theoretically and experimentally. We show that the radiative corrections to the XX state, the Lamb shift ΔXXMC and radiative width ΓXXMC, are large, about 10%--30% of the molecule binding energy εXX, and definitely cannot be neglected. The optics of excitonic molecules is dominated by the in-plane resonant dissociation of the molecules into outgoing 1λ-mode and 0λ-mode cavity polaritons. The later decay channel, ``excitonic molecule →0λ-modepolariton+0λ-mode polariton,'' deals with the short-wavelength MC polaritons invisible in standard optical experiments---i.e., refers to ``hidden'' optics of microcavities. By using transient four-wave mixing and pump-probe spectroscopies, we infer that the radiative width, associated with excitonic molecules of the binding energy εXX0.9--1.1meV, is ΓXXMC0.2--0.3meV in the microcavities and ΓXXQW0.1meV in a reference GaAs single quantum well (QW). We show that for our high-quality quasi-two-dimensional nanostructures the T₂=2T₁ limit, relevant to the XX states, holds at temperatures below 10 K and that the bipolariton model of excitonic molecules explains quantitatively and self-consistently the measured XX radiative widths. A nearly factor 2 difference between ΓXXMC and ΓXXQW is attributed to a larger number of XX optical decay channels in microcavities in comparison with those in single QW's. We also find and characterize two critical points in the dependence of the radiative corrections against the microcavity detuning and propose using the critical points for high-precision measurements of the molecule binding energy and microcavity Rabi splitting.