Photon Correlation Spectroscopy of a Single Quantum Dot

It is by now widely accepted that various quantum dot ~QD! structures exhibit features in transport 1,2 or optical spectroscopy 3‐5 that indicate full three-dimensional confinement of carriers. Identification of QD’s as artificial atoms has been strengthened by the recent observation of strong photon antibunching in single-exciton emission, 6,7 which is the typical signature of an anharmonic quantum system: after a photon is emitted from a single two-level ~anharmonic! emitter, the system is necessarily in the radiatively inactive ground state and a second photon cannot be emitted immediately after the first one. Even though the coherence properties of QD single-exciton emission closely follow those of atoms, the overall spectral features of single QD’s are significantly more complicated. Since the size of QD’s is roughly two orders of magnitude larger than those of atoms, multiparticle excitations give rise to emission peaks with energies comparable to that of a single exciton. Of primary importance in QD spectroscopy is the biexciton state, which corresponds to a doubly-excited QD with completely filled lowest electronand hole-energy levels. When the biexciton state decays by radiative recombination, the final state is a single-exciton state and the generated photon is shifted as compared to the single exciton emission due to Coulomb interaction between the carriers. Biexciton emission in QD spectroscopy has been traditionally identified using the ~quadratic! pump-power dependence of the corresponding peak. In this paper, we demonstrate that photon-correlation measurements provide a powerful tool for characterizing the multiexciton spectral features of QD’s. Our measurements provide a strong support for the identification of a biexciton emission peak, by demonstrating its strong correlations with the subsequent single-exciton emission. We observe that biexciton intensity autocorrelation exhibits bunching together with antibunching or only antibunching under continuous-wave ~cw! excitation depending on the excitation level. In contrast, we find strong antibunching under pulsed excitation. The large difference between the levels of antibunching under continuous wave and pulsed excitations points out to the importance of excitation mechanism and the role of free carriers in QD physics. The lack of polarization correlation between biexciton and single-exciton emissions indicates that spin dephasing is likely to play a key role under nonresonant excitation. We also observe that a third emission peak in QD spectra exhibits strong correlations with both exciton and biexciton fluorescence: we argue that these correlation signatures suggest the identification of this additional line as a charged-exciton emission. Our self-assembled InAs QD’s were grown by molecularbeam epitaxy using the partially covered island technique. 8 Growth resulted in typically lens-shaped QD’s with a base diameter of 40‐50 nm and a height of 3 nm, having their single-excitonic emissions between 925 nm and 975 nm in the spectrum. In our sample, the QD’s were embedded in the center of a 200-nm-thick GaAs microdisk structure located above a 0.5-mm-thick Al0.65Ga0. 35As post. The diameter of the disks was 5-mm and the average number of QD’s within the disks was less than one. Details of the microdisk processing can be found elsewhere. 9 Our experimental setup consisted of a combination of a low-temperature diffractionlimited scanning optical microscope and a Hanbury Brown