Ultra-low threshold quantum dot microdisk laser

Ultra-low threshold lasers have applications in low-power communications. These lasers are also of fundamental interest, where a full understanding of lasing based on a few discrete emitters is evolving. This is especially true in solid-state systems, for instance those with a quantum dot (QD) gain medium, where a typical spectrum of discrete emission lines observed at lower pump power is often highly modified under higher pump powers. Here we discuss a microcavity laser containing a dilute QD gain medium that has an ultra low, sub-microwatt CW lasing threshold. The structure is based on a high-quality factor microdisk cavity of GaAs with a low density of InAs-based QDs embedded in the microdisk. We estimate 250 QDs in the 1.8 $\mu $m diameter microdisk under investigation. Of these QDs approximately 60 are spatially located within the modal region of the disk, which extends inwards approximately 250 nm from the disk edge. Only a small portion of these QDs couple to the narrow cavity modes, which have a free spectral range of 45 nm and an initial linewidth of 0.06-0.07 nm. Linewidth narrowing and lifetime reduction with increasing pump are both observed. Despite the small number of QDs it is unlikely from our estimates the system lases from a single QD state.