# Two-State Lasing in Microdisk Laser Diodes with Quantum Dot Active Region
## 3 Results and Discussion  
### Spectral Analysis of Microdisk Lasers  
The electroluminescence spectra of spontaneous emission of the microdisk laser structures show several broad bands (full width at half maximum is about 50 nm). They were assigned to radiative recombination of charge carriers occupied several energy levels of the QDs and of the covering InGaAs quantum well. According to the emission spectra of the 20 \(\upmu\)m microdisk presented in Figure 2a, spectral bands of emission related to the GS and three excited-state (ES1, ES2 and ES3) optical transitions of QDs are centered at room temperature near the wavelengths of 1265, 1170, 1100 and 1040 nm, respectively. The quantum well (QW) manifests itself as a weak spectral band located around 970 nm. WGMs involved in lasing are observed in the spectra as narrow lines with a spectral width (limited by the spectral resolution) of less than 0.2 nm. As can be seen from Figure 2a, lasing first starts at the GS optical transitions of QDs; a representative spectrum is shown for a current of 13 mA. As the injection current grows (56 mA), lasing lines appear within the spectral band of the ES1 optical transition, which means that two-state lasing occurs. It should be noted that the additional line arises on the shorter-wavelength side of the spectrum. This distinguishes two-state lasing from injection current-induced switching of the lasing line to a longer wavelength cavity mode. The latter process is related to a temperature increase in the active region and is expected to be much slower compared to switching between the quantum states [16]. In brief, an appearance of two-state lasing in a QD laser [12; 13] can be explained as follows. The charge carriers relax from the excited-state level to the ground-state one. A cascade relaxation of the charge carrier over a ladder of QD electronic states has been confirmed, for example, in experiments on time-resolved luminescence [25]. When lasing on the GS optical transition occurs, the population of the ES1 level does not clamp, and the ES1 gain continues growing with increasing the injection current. Finally, lasing on the ES1 optical transition starts. A further increase in current (180 mA) leads to a gradual decrease in the intensity of lasing lines within the GS optical transition, while the lines within the ES1 optical transition continue to grow in intensity. These trends are clearly visible in Figure 2b, which shows the dependence on the injection current of the integrated intensities of lasing modes within the GS and the ES1 optical transitions for the same 20 \(\upmu\)m microdisk laser. The lasing through the GS optical transition starts with a threshold current \(I_{\mathrm{th}}\)GS of about 6 mA,  
Figure 2: Electroluminescence (EL) spectra of the 20 \(\upmu\)m microdisk measured at different injection currents at room temperature (**a**) and dependence of the integrated intensity of lasing modes for GS (circles) and ES1 (squares) optical transitions (**b**). Several spectra in figure (**a**) are vertically shifted for better clarity with the appropriate coefficients shown in figure. which was estimated from the kink in the GS light-current curve. Similarly, the threshold current of two-state lasing, i.e., the current \(I_{\text{th}}\)\({}^{\text{ESI}}\) when the ES1 laser emission appears, was found to be about 40 mA. Although the integrated intensity of the GS laser emission continues increasing with current, at 60 mA the intensity of the ES1 emission already starts to dominate. After that, the GS intensity begins to decrease with further increasing current. Such a decrease of the GS lasing intensity at high injection currents in the two-state lasing regime was previously observed in edge-emitting QD lasers, and was explained in terms of the competition of GS- and ES1 electrons for \"joint\" holes and the difference in the hole and electron capture rates into QDs [26; 27].