# Two-State Lasing in Microdisk Laser Diodes with Quantum Dot Active Region
## 3 Results and Discussion
### Spectral Analysis of Microdisk Lasers
Microdisks with cavity diameters more than 20 \(\upmu\)m show similar behavior but with different threshold currents \(I_{\text{th}}\)\({}^{\text{GS}}\) and \(I_{\text{th}}\)\({}^{\text{ESI}}\) (dependences of the threshold current density on the microdisk diameters will be discussed further). In all these cases, lasing also starts at the wavelengths corresponding to the GS optical transition, and at sufficiently high currents, two-state lasing occurs with simultaneous emission via the GS and ES optical transitions. It is worth mentioning that more than one WGM lasing line is typically observed within both the GS and ES1 emission bands. The spectral spacing between neighboring WGMs (free spectral range, FSR) is about 6.8, 5.8 and 5.0 nm for the 20 \(\upmu\)m, 24 \(\upmu\)m and 28 \(\upmu\)m microdisks, respectively. These FSRs are significantly smaller than the minimum spectral distance between groups of the GS and ESI lasing modes, which is 50 nm at least. This facilitates an individual detection of the spectral components involved into two-state lasing even at low spectral resolution of a detection system. A slightly different situation is observed for the microdisk laser of the smallest diameter of 9 \(\upmu\)m, where lasing first starts at the ES1 optical transition (Figure 3a, \(I_{\text{th}}\)\({}^{\text{ESI}}\)\(\approx\) 6 mA), and the GS laser emission is not observed at all. This indicates that the total optical loss, whatever its cause, in this small cavity exceeds the saturated gain achievable at the GS optical transition. Due to the higher degeneracy of the ES1 quantum level compared to the GS one, the optical gain at the ESI optical transition turns out to be large enough to compensate the loss. It is seen that an increase in the injection current above the lasing threshold leads to an increase in the integrated intensity of the ES1 laser emission (Figure 3b). Once the current exceeds 90 mA (\(I_{\text{th}}\)\({}^{\text{ES2}}\)), the emergence of an additional blue-shifted lasing line is observed in the electroluminescence spectrum. This line (group of lines) is within the ES2 optical transition, so the two-state lasing effect in the 9 \(\upmu\)m microdisk represents a combination of the ES1 and ES2 laser emission. Beyond the two-state lasing threshold, the intensity of the ES2 emission increases, while the intensity of the ES1 laser emission diminishes. It is unusual in QD lasers for two-state lasing to occur through ES1 and ES2 optical transitions rather than through the GS and ES1 ones, i.e., that the quantum numbers of lasing transitions are increased by one. As far as we know, the two-state lasing regime involving two excited-state optical transitions of quantum dots has not been observed so far. This suggests that the degree of degeneracy of the ES2 level is even higher than that of ES1. Although Stranski-Krastanow QDs have a more complex shape than a sphere, it can nevertheless be pointed out that the degree of degeneracy of the GS, ES1 and ES2 quantum levels in a sphere is 1, 3 and 5, respectively (without taking into account spin degeneracy).