(a) Dependence of the lasing threshold (black squares) and the quality factor (red triangles) on the gap separation d. An increase of the lasing threshold is identified that is linked to increased cavity losses as confirmed from an opposing trend of the quality factor recorded in transmission measurements. For purposes of presentation we left out the quality factor for non-split-disks (Q_{d=0 nm} = 2.1 × 10⁴). (b) By stacking PL spectra for various gap widths d, a blue-shift of the lasing mode envelope can be recognized that is also related to the decrease of the quality factor.