## 3 Optical Properties
### Microdisk Lasers Based on QWDs  
#### 3.5.1 CW Performance  
Recently semiconductor microlasers have attracted considerable interest as ultrasmall light sources for photonic integrated circuits and other applications. To meet the requirements of high efficiency, the major part of the output power should be emitted via lasing whispering gallery modes (WGMs), whereas the spontaneous emission should be weak. Although ideal microdisk (MD) can exhibit very high-quality factors [74], the non-ideal shape of actual devices results in an increase in optical loss [75], and thus, MD can emit practically sufficient optical power. For example, in [76], central electrically pumped 3 \(\upmu\)m in radius microlaser with a single layer of InGaAs QDs as an active medium was employed for optical excitation of five closely spaced micropillar cavities. The out-coupling of the laser emission can be significantly enhanced through the use of optical fiber tapers. In reference [77] a laser differential efficiency as high as 16% and out-coupling efficiency in excess of 28% was measured in GaAs microdisk based on InAs QDs embedded in In\({}_{0.15}\)Ga\({}_{0.85}\)As quantum well after accounting for losses in the optical fiber system. Maximal power in the lasing line using free-space collection was estimated as 2.5 nW, whereas it was as high as 60 nW when using fiber taper collection. In reference [78] output power emitted into free space was directly measured for the first time in MD lasers based on InAs/InGaAs/GaAs QDs. The device was 31 \(\upmu\)m in diameter, and its active region contained ten layers of InAs/InGaAs QDs in GaAs matrix. Total emitted optical power reached approximately 0.1 mW in CW regime, while the WGM emission amounted to about 2.8% of the total power. In this section, we describe static characteristics of microdisk lasers based on QWDs [79]. The typical laser structure consisted of \(p\)- and \(n\)-type doped Al\({}_{0.34}\)Ga\({}_{0.66}\)As cladding layers and a 0.8 \(\upmu\)m-thick undoped GaAs waveguiding layer. An active region represents five sheets of QWDs separated with 40 nm-thick GaAs spacers. The QWDs were formed by the deposition of eight monolayers of In\({}_{0.4}\)Ga\({}_{0.6}\)As. Microdisks with diameters from 15 to 31 \(\upmu\)m were fabricated by standard post-growth processing. A micrograph of a 31-\(\upmu\)m microlaser is shown in the inset of Figure 16.  
Figure 16 compares electroluminescence spectra for a 19 \(\upmu\)m in diameter MD lasers based on InGaAs/GaAs QWDs and 1.3 \(\upmu\)m InAs/InGaAs/GaAs self-organized QDs. Both spectra are measured at injection current four times exceeding the threshold value. In the emission spectrum of QD microlaser, there are many narrow lines corresponding to WGM modes of different orders superimposed to a  
Figure 16: Lasing spectra of microdisk lasers based on (red) QWD and (blue) QD active area. The inset shows a microphotograph of the measured device. 180 nm wide spontaneous emission spectrum with the well-resolved peaks from the ground state, first and second excited states. The integrated intensity of all lasing WGMs is 18% of the entire emitted power. In contrast, in MDs with QWDs, the width of the spontaneous emission spectrum is much lower (\(\sim\)70 nm). The peaks due to the excited-state transitions do not emerge up to very high injection currents. For the QWD MD laser, the share of the lasing WGM to the total emitted power is 95%. The lasing mode peak intensity is 29.9 dB higher as compared to that of the side (next brightest) mode. At 60 mA, the total output power emitted into free space reaches 18.5 mW that corresponds to a wall-plug efficiency of about 15% (Figure 17). The share of all lasing WGMs in the total emitted power is 96%, whereas the fraction of dominating WGM at 1086 nm is about 75%. Figure 18 compares the dependencies of threshold current density on mesa diameter for MD lasers based on InGaAs/GaAs QWDs and 1.3 \(\upmu\)m InAs/InGaAs/GaAs self-organized QDs.