Shih et al. Nanoscale Research Letters 2011, 6:409 http://www.nanoscalereslett.com/content/6/1/409 Broadening of the QD bound states due to the influ- ence of the couplings to the longitudinal optical pho- nons was discussed by Verzelen et al. [10], which revealed an effect of the dot environment to the dot eigenstates. Quantum kinetics of carrier relaxation in self-assembled QDs was investigated by taking into account the influence of the energetically nearby conti- nuum of wetting layer states [11]. The interaction of the discrete QD excited states with a quasicontinuum of states was investigated in Ref. [12], in which a correla- tion between the acoustic phonon broadening efficiency and the background intensity in the PLE spectra of sin- gle InAs QD was found. The continuous background feature experimentally extends downward very deeply in energy, which makes it difficult to associate the fluctua- tions of the WL near the dot. More recently, another source of intrinsic broadening for the dot bound states was demonstrated theoretically. This was stimulated during the interband optical excita- tion when electron-hole pairs are photo-generated in the dots, and is related to the existence of transitions involving one bound state and one delocalized state near the dots [13]. The broadening of the excited QD levels and interband absorption background are attribu- ted to these cross transitions, which are inherent to the joint nature of the valence-to-conduction density of states in QDs. This paper demonstrated that by engineering the QD size and shape, modification of the joint density of the cross transitions and changing the bound excited-state energies with respect to the continuum density of elec- tron-hole states, which extend far below the wetting layer edge, can be made. The PLE spectroscopy of the self- assembled InAs QDs at low temperature and high mag- netic field was reported. The resonances in the PLE spec- tra associated with the QD bound-state transitions were confirmed further by scanning the magnetic field. By evaluating the lineshape change of the PLE resonances, the coupling strength between the bound states and the cross transitions in different QDs was determined. Experimental details The two InAs QD samples studied in this work were grown on GaAs (001) substrates by molecular beam epi- taxy. The substrate was first covered by a 200 nm GaAs buffer layer at 600°C. QDs of two different sizes were formed by depositing 2.4 and 2.6 monolayers of InAs with a growth rate of 0.056 μm/h at growth tempera- tures of 480°C and 520°C under As2 atmosphere, respec- tively. After the QDs were formed, they were then capped with a 150-nm thick GaAs Layer. QDs formed at a lower temperature (this sample is referred to as QD1 in this article) have a lens shape with a smaller average base diameter of about 20 nm and a height of 2 Page 2 of 7 nm. The larger QDs (this sample is referred to as QD2 in this article) self-assembled at a higher temperature have a pyramid shape with an average base diameter of about 40 nm and a height of 14 nm. The areal densities are approximately 1 × 1011 cm-2 and 2 × 1010 cm-2 for QD1 and QD2, respectively. Figure 1a, b shows the AFM images of QD1 and QD2 samples. The conventional PL spectra were obtained directly using an argon ion laser as the excitation source. PLE measurements for the above samples were recorded with a continuous wave (CW) tunable Ti:sapphire laser pumped with a DPSS laser as light source and detected using a 0.18-m double spectrometer equipped with a Figure 1 Atomic force microscopy images of (a) QD1, (b) QD2.