Using femtosecond differential transmission spectroscopy, we observed a “nondegenerate” biexciton, consisting of an electron-hole pair in the dot ground state and an electron-hole pair in the excited state, in InGaAs self-organized quantum dots. We resonantly pumped the ground state transition in the quantum dots and observed an induced resonance in the probe differential transmission spectrum near the first excited-state transition, which we attribute to the formation of a nondegenerate biexciton state. The binding energy of 15 meV does not change with excitation power, thus reflecting a genuine feature of few-particle states. Our theoretical model calculations show good agreement with these experimental results. When a prepulse is used to generate a population inversion in the quantum dots, we also observed the effects of nondegenerate biexcitonic correlations in differential transmission.

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