  We provide experimental and theoretical insight into single-emitter lasing
effects in a quantum dot (QD)-microlaser under controlled variation of
background gain provided by off-resonant discrete gain centers. For that
purpose, we apply an advanced two-color excitation concept where the background
gain contribution of off-resonant QDs can be continuously tuned by precisely
balancing the relative excitation power of two lasers emitting at different
wavelengths. In this way, by selectively exciting a single resonant QD and
off-resonant QDs, we identify distinct single-QD signatures in the lasing
characteristics and distinguish between gain contributions of a single resonant
emitter and a countable number of off-resonant background emitters to the
optical output of the microlaser. We address the important question whether
single-QD lasing is feasible in experimentally accessible systems and show
that, for the investigated microlaser, the single-QD gain needs to be supported
by the background gain contribution of off-resonant QDs to reach the transition
to lasing. Interestingly, while a single QD cannot drive the investigated
micropillar into lasing, its relative contribution to the emission can be as
high as 70% and it dominates the statistics of emitted photons in the
intermediate excitation regime below threshold.
