We report on micro photoluminescence (PL) experiments on single semiconductor microdisks at low temperature T = 5 K. Two types of microdisks were investigated, containing (a) multiple quantum wells (QWs) of ternary AlGaAs alloys and (b) one layer of self‐assembled InAs quantum dots (QDs). At low excitation strength we observe in both type of samples discrete peaks arising from the recombination of excitons localized, (a), at impurities within the ternary alloy QWs and, (b), in the InAs QDs. With increasing excitation strength, the emitted light is concentrated in several well‐defined whispering gallery modes (WGMs) with typical widths of less than 1 meV distributed over an energy range broader than 200 meV. At even higher excitation strengths, we find a clear threshold behavior for the power emitted into the WGMs, suggesting the onset of lasing.

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