Small area quantum-dot stripe lasers with focused-ion-beam-etched mirrors

Miniaturization of edge-emitting quantum-dot lasers is hampered by increased mirror losses in ultra-short Fabry–Pérot cavities and limited modal gain of quantum dots (QDs). Here, we demonstrate electrically injected InGaAs/GaAs quantum-dot stripe lasers with an 8-μm stripe width and focused-ion-beam (FIB)-etched mirrors defining cavity lengths from 300 to 77 μm. The laser cavities were fabricated using techniques that allow for group fabrication and integration of lasers with other elements. The stripes were formed by deep reactive ion etching, and the laser mirrors were made using FIB etching technique. Five planes of InGaAs QDs grown in the non-Stranski–Krastanov mode were used as the laser active region. QDs of this type are responsible for high optical gain capable of compensating for losses in short cavities. Lasing is observed down to 77 μm, while the minimum threshold current of 23–26 mA is achieved for cavities 130–250 μm long. Comparison with 50-μm-wide lasers demonstrated previously shows that the threshold current approximately scales with stripe width, indicating a route toward further current reduction in narrower devices. However, the rapid increase in threshold current density in the shortest cavities reveals that further length scaling is limited by carrier accumulation and recombination in the GaAs waveguide. A threshold current model based on dominant waveguide recombination reproduces the nonmonotonic dependence of the threshold current on cavity length. These results identify lateral narrowing as an effective route to low-current short-cavity quantum-dot lasers and waveguide recombination as the key limitation for further miniaturization.