Exciton and Biexciton Coherent Effects in Quantum Dots

Hideki Gotoh and Hidehiko Kamada
Optical Science Laboratory

 Semiconductor quantum dots have discrete density of states and exhibit strong interaction effects with light. Moreover, optically created excitons and biexcitons exist stably in quantum dots because of confinement effects from all spatial directions. These features promise possible future applications of quantum dots for quantum information processes. In quantum dots, there may be coherent interactions between excitons and biexcitons and the effects of these interactions have not yet been clearly observed. These effects provide a source of quantum correlations by exciton-biexciton interactions and a principle of optical functions by quantum interference effects.
 We examined the photo-absorption properties of excitons and biexcitons in a single InGaAs quantum dot. The coherent effects may appear in these absorption properties. We employed a micro-photoluminescence (PL) method. Figure 1 shows measured absorption spectra of exciton (X) and biexciton (XX) for two excitation conditions. At a low excitation, we obtained usual peak shaped absorption spectra. However, these spectra change greatly with a high excitation. The XX spectrum has a broadened peak. In contrast, the X spectrum has an unusual dip-shaped structure. Figure 2 reveals the physical origin of the unusual spectrum. The figure shows the energy level structure for the measurement result. The energy structure is a coherently interacting three-level system with ground, exciton and biexciton states. This structure varies greatly with excitation. With a high excitation, the exciton and biexciton states split due to Rabi oscillation between the exciton and biexciton states. This causes the exciton state to vanish resulting in the dip-shaped exciton absorption. This Rabi oscillation is a coherent effect and our result confirms that there is a strong coherent effect in the exciton and biexciton states.
 Our results are an important step towards achieving a quantum two-bit gate with an exciton and a biexciton. Moreover, this provide a possible way to demonstrate optical device functions with quantum interference effects.
[1] H.Gotoh, et al., Appl. Phys. Lett. 85 (2004) 3480.
[2] H.Gotoh, et al., Phys. Rev. B in press

Fig. 1. Exciton and biexciton absorption.
Fig. 2. Energy level structure

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