Shih et al. Nanoscale Research Letters 2011, 6:409 http://www.nanoscalereslett.com/content/6/1/409 TE-cooled InGaAs photodetector. The samples were mounted in a closed-cycle helium dewar for low-tem- perature measurement. However, for the optical mea- surements under low temperature and high magnetic field, the sample was placed in a sample holder with N- grease at the bottom of an insert equipped with a fiber probe. The insert was placed in a dilution refrigerator and cooled down to 1.4 K. Laser output from the CW Ti:sapphire laser was delivered into the refrigerator using a fiber. The PLE signals collected through the same fiber were dispersed with a 0.55-m spectrometer and detected with a TE-cooled InGaAs photodetector at different magnetic field intensities. Results and discussion Figures 2 and 3 show the excitation power dependence of the PL spectra at liquid nitrogen temperature for the QD1 and QD2 samples, respectively. The state filling of the excited state transitions can be observed for the QDs by increasing the pumping power. In Figure 2, the ground-state energy as well as the barrier and WL emis- sions of the QD1 are observed clearly at around 1.227, 1.509, and 1.422 eV, respectively. Due to the smaller size of the dots, only one excited state (1Pe ® 1Ph tran- sition) was identified and contributed to the PL spectra at 70 meV above the ground state. For the dots with a larger diameter (QD2), there are three excited states contributed to the PL spectrum (as shown in Figure 3) at 86, 161, and 213 meV above the ground state other than the emission peaks from the barrier and the WL. For the PLE experiments, the excitation power was around 10 W/cm2 to avoid any emission from an excited state and to suppress the Auger scattering. Fig- ure 4 shows the PLE spectrum of the QD1 sample at 1.4 K with the detection energy fixed at 1.198 eV (the Page 3 of 7 Figure 3 Excitation power dependent PL spectra of QD2 at 77 K. maximum of the ground state transition at the same temperature). In this figure, the spectrum with the bot- tom horizontal axis was plotted to represent the differ- ence between the excitation and detection energies (Eexc - Edet). At the high-energy end of the PLE spectrum, absorption occurs in the GaAs barrier layer (approxi- mately 1.52 eV) and in the 2D InAs wetting layer (absorption transitions from both the light holes and the heavy holes). At a lower energy, only one resonance at 35 meV above the ground state was observed. However, there was no peak resolved at the energy where the first excited state absorption happens. Figure 5 shows the Figure 2 Excitation power dependent PL spectra of QD1 at 77 K. Figure 4 PLE spectrum of QD1 was plotted as a function of relaxation energy recorded at 1.4 K.