Fig. 1. Evolution of the structure and density of state in semiconductor lasers with different dimensionalities

Fig. 2. Optical feedback dynamics of quantum dot lasers and quantum well lasers. (a) Optical feedback of quantum dot lasers; (b) evolution of RF and optical spectra with varying feedback strength in a single-ground-state quantum dot laser and a quantum well laser^[34] (the quantum dot laser shows no chaos under the strongest feedback, whereas the quantum well laser exhibits chaotic oscillations and mode broadening under strong feedback strength); (c) evolution of RF and optical spectra in a dual-state lasing quantum dot laser near the excited state threshold, with the critical feedback strength at the excited state threshold being lower than that below and above the threshold^[31]; (d) the effect of optical feedback on reducing frequency noise in quantum dot lasers and quantum well lasers^[40]

Fig. 3. Modulation characteristics of quantum dot lasers under optical feedback. (a) External modulation with modulation rate of 10 Gbit/s^[34]; (b) direct modulation with modulation rate of 6 Gbit/s^[41]

Fig. 4. Four-wave mixing in the gain materials of semiconductor lasers^[55]. (a) Schematic of the generation of four-wave mixing signal by injecting drive light and probe light into the gain material; (b) the process of increasing intensity of signal with the increase of probe intensity in a p-doped quantum dot laser; (c) four-wave mixing conversion efficiency of p-doped quantum dot laser, undoped quantum dot laser and quantum well laser; (d) fitted four-wave mixing coefficients (ξ) of different lasers, with the coefficient for the quantum dot lasers being an order of magnitude higher than that for the quantum well laser

Fig. 5. Optical frequency combs generated by mode-locked quantum dot lasers. (a) Optical frequency comb generated by a quantum dot fourth-order colliding pulse mode-locked laser, where the comb produced by the single laser can cover a range of over 36 nm, and their combined optical spectra consists of a 60-channel^[69]; (b) frequency-modulated optical frequency comb generated by a quantum dot mode-locked laser, with uniformly distributed difference of mode phases over 2π within the locking range, and the DC component of the time intensity series is greater than half of the peak power level^[68]