# Two-State Lasing in Microdisk Laser Diodes with Quantum Dot Active Region
## 1 Introduction  
For the last few decades, microlasers with disk and ring cavities that support propagation of high-Q whispering gallery modes (WGM) have been intensively studied and discussed. Microdisk (MD) and microring lasers based on III-V semiconductors have a high potential for their implementation in the photonic integrated circuits, because they have a small footprint, demonstrate low threshold currents and narrow linewidths, and emit light in the lateral direction [1, 2]. MD lasers with a quantum dot (QD)-based active region demonstrate much better performance compared to their InGaAs QW-based counterparts. Continuous-wave operation of QD microdisks was already observed at elevated temperatures of 100-110 \({}^{\circ}\)C [3, 4]. It is worth mentioning that small sizes of microlasers, in combination with the low sensitivity of QDs to surface recombination and epitaxial defects, open up opportunities for monolithic [5, 6] and hybrid [7, 8] integration of such emitters with silicon or other non-native substrates. At the same time, good thermal conductivity of silicon lowers thermal resistance of hybrid integrated devices and improves their output characteristics [9]. QD-based MD and microring lasers show direct modulation frequencies of about 6-7 GHz for lasers grown both on GaAs [10] and Si [11]; error-free optical data transmission at 10 Gb/s has been demonstrated. A further improvement in the performance of microlasers can be associated with an increase of the throughput of such systems by, for example, transmitting data at multiple carrier frequencies, similar to the wavelength division multiplexing used in fiber optic communication. QD lasers exhibit a specific feature referred to as two-state lasing, which consists of simultaneous emission through the optical transitions involving ground-state and excited-state energy levels of quantum dots [12, 13]. A reason for such a behavior is related tothe non-instant capture of charge carriers from the excited-state (ES) levels to the ground-state (GS) one that results in a gradual increase in population of the excited states. At a sufficiently high current, which can be considered as a threshold of two-state lasing, optical gain on the first excited-state (ES1) optical transition becomes sufficient to balance optical loss in the laser cavity, which leads to the appearance of stimulated emission for the ES1 optical transition. Two-state lasing is somewhat similar to widely tunable lasing, because both exploit the excited-state optical transition of QDs. However, the former effect manifests itself in a conventional laser cavity, whereas the latter requires a certain spectrally selective element. Two-state lasing can be useful for realization of broad-band laser emission. For example, very broad lasing spectra of about 75 nm in the 1.2-1.28 \(\upmu\)m wavelength range have been achieved by intentional inhomogeneous broadening of the GS optical transition peak and contribution of lasing via the excited-state transitions [14]. A possibility of spectral filtering of individual Fabry-Perot modes and their independent external modulation at 10 Gb/s with a bit error rate less than \(10^{-13}\) has been demonstrated [15]. In [16], an all-optical switching mechanism between the GS and the ES emission of a QD laser has been demonstrated. Very fast times on the order of several hundreds of picoseconds were obtained for switching between the states. A novel encoding scheme that combines advantages of multilevel signaling and wavelength division multiplexing by exploiting the switching from the GS to ES lasing in QD lasers with an increase in injection current was proposed in [17]. Groups of two bits have been proposed to assign to lasing spectra that comprise either one or both GS and/or ES1 lasing or absence of lines depending on injection current, which can take one of four levels. The abovementioned results show the aptitude of two-state lasing in QD lasers for high-speed optical data transmission.