Shih et al. Nanoscale Research Letters 2011, 6:409 http://www.nanoscalereslett.com/content/6/1/409 Page 4 of 7 Figure 5 PLE spectrum of QD1 recorded at 1.4 K with detection energy fixed. Figure 6 Magnetic field dependent PLE spectra of QD1 recorded at 1.4 K from 0 to 14 T. PLE spectra of QD1 measured with detection energies fixed at five different positions on the ground state peak. All the absorption peaks, other than 35 meV, were shifted according to the change in detection energies. The peak at 35 meV also did not change in position when the PLE spectra were measured at different mag- netic fields, as shown in Figure 6. All of the evidence indicate that the peak at 35 meV indeed corresponds to the relaxation by the emission of one InAs/GaAs inter- face phonon [14]. The PLE spectra of QD2 at 1.4 K are given in Figure 7 with the detection energy fixed at 1.1 eV (the maximum of the QD2 ground-state transition at 1.4 K). The energy of excited luminescence intensity was also displayed with respect to the detection energy (Edet). Due to the limited laser tuning range, the PLE spectra down to 1.236 eV (i.e., 136 meV above Edet) were recorded only. In contrast to the PLE results of smaller QDs, two reso- nance peaks were resolved clearly at 158 and 222 meV above the Edet. However, the PLE resonance at 222 meV is much broader than the one at a lower energy. When PLE spectra recorded at five different detection energies (as shown in Figure 8) were compared, two PLE reso- nances clearly shifted according to the change in detec- tion energy. Therefore, there is reason to believe that these two resonances are due to the bound-to-bound absorption. To verify the resonances in the PLE signals, which are signatures from the QD excited-states transitions, PLE measurements under a magnetic field were performed. Figure 9 shows the magnetic field dependent PLE spec- tra of the larger QD sample with magnetic field scanned from 0 to 14 T. Splitting and a blue shift in energy were observed with increasing magnetic field for the two resonances at 158 and 222 meV, respectively. Under a magnetic field, the energy levels in 0D quantum Figure 7 PLE spectrum of QD2 was plotted as a function of relaxation energy recorded at 1.4 K.