# Two-State Lasing in Microdisk Laser Diodes with Quantum Dot Active Region
## 2 Materials and Methods  
A laser heterostructure for experimental studies was grown by molecular beam epitaxy on an \(n^{+}\)-GaAs substrate with a 500 nm thick \(n^{+}\)-GaAs buffer layer. The laser light-emitting region represents a QD array formed by consistent deposition of 2.5 monolayers of InAs followed by deposition of 5 nm In\({}_{0.15}\)Ga\({}_{0.85}\)As. The structure contains 10 layers of QDs separated from each other by GaAs spacer layers of 35 nm thickness, with the central part doped with acceptors with concentration of 5 \(\times\) 10\({}^{17}\) cm\({}^{-3}\). The waveguiding layer is sandwiched between \(n^{+}\) and \(p^{+}\) Al\({}_{0.25}\)Ga\({}_{0.75}\)As claddings with a thickness of 2.5 and 2.2 \(\upmu\)m,respectively. The structure was terminated with a heavily doped \(p^{+}\)-GaAs contact layer of 200 nm thick.  
Microdisk cavities were formed from the laser heterostructure by photolithography and plasma-chemical etching. The microlasers are about 28, 24, 20 or 9 \(\upmu\)m in diameter and about 6 \(\upmu\)m in height. According to our estimates, the effective volume of the WGM varied from 35 to 8 \(\upmu\)m\({}^{3}\) for resonators with a diameter within the specified range. Round-shaped metal contacts to the \(p^{+}\)-GaAs layer were formed at the tops of the microcavities using AgMn/Ni/Au metallization. A solid AuGe/Ni/Au metal \(n\)-contact was deposited to the back side of the preliminarily thinned substrate. A scanning electron microscopy image of representative microdisks is depicted in Figure 1a. Chips with several microdisk lasers were soldered with indium to a copper heatsink, which was also used for electrical connection to the \(n\)-contact of the microlasers. A BeCu microprobe was used for individual connection to \(p\)-contact of separate microdisks.  
The bias voltage to the microlasers was applied in a pulsed regime with a pulse width of 300 ns and repetition rate of 4 kHz in order to reduce possible influence of self-heating of the lasers. The pulsed current amplitude was measured using an inductive probe. The collection of electroluminescence from microdisks was carried out with a microobjective (Mitutoyo M Plan APO NIR 50x). Further, the electroluminescence emission was directed via a multimode optical fiber to an Andor Shamrock 500i grating monochromator with a diffraction grating containing 1200 lines per millimeter. The emission was detected by an InGaAs CCD array with thermoelectric cooling. When the integrated intensity of laser emission is discussed, it was obtained as the area under the respective spectral curves of lasing modes within a certain optical transition band, e.g. the GS one. The experimental setup is schematically drawn in Figure 1b. The results presented below are stable over time for at least a couple of hours and are reproducible for each specific microdisk.  
For the sake of comparison of the spectral characteristics, broad area lasers were fabricated from the same epitaxial wafer. The stripe width was 100 \(\upmu\)m. The cavity length varied from 0.25 to 2 mm. Cleaved facets were left uncoated. The lasing spectra and light-current curves of the stripe lasers were studied in pulsed regime. The slope efficiency and threshold current density were extracted.  
Figure 1: Scanning electron microscopy image of the microdisks under study; length of the scale bar is 20 \(\upmu\)m (**a**); sketch of the experimental setup (**b**).