Emission linewidths of quantum dot photonic-crystal-slab nanocavities are measured as a function of temperature and fabrication parameters with low-power and high-power, cw and pulsed, nonresonant excitation. The cavity linewidth is dominated by the absorption of the ensemble of quantum dots having a density of Х400/ m 2 ; above the absorption edge, the cavity linewidth broadens considerably compared with the empty cavity linewidth. Gain and lasing are seen for high-power pumping; it is estimated that only a small number of quantum dots contributes to the lasing. DOI: 10.1103/PhysRevB.72.193303 PACS number͑s͒: 42.55.Tv, 42.50.Pq, 42.55.Sa Recently the quality factor Q ͓mode energy divided by full width at half maximum ͑FWHM͒ mode energy linewidth͔ of photonic-crystal-slab cavities has been steadily increased by improved fabrication techniques and designs, while the volume V was kept close to a cubic wavelength in the material. This has made possible not only quantum well 1 and quantum dot 2 lasers but also the observation of strong coupling 3 -vacuum Rabi splitting with a single quantum dot ͑SQD͒. The role of the quantum dots ͑QDs͒ in the lasers is to provide gain, so several layers of high density QDs are often used. In contrast, strong coupling, can best be observed with an isolated SQD, suggesting the use of a single layer of low density QDs. However, to see strong coupling one must search to find two accidental coincidences. The QD must be situated close to an intracavity field maximum. This means it must be within the mode area of 0.15 m 2 , where the intracavity field is strong. It must also have a transition frequency close to a cavity mode; our ensemble QD lowest energy transition has a FWHM of 42.5 meV at 20 K, compared with a maximum dot-nanocavity coupling strength of 0.2 meV. For a reasonable probability for both coincidences, high dot densities ͑300-400/ m 2 ͒ have been used so far. This paper addresses two questions: Is the ensemble QD absorption detrimental to the search for strong coupling? And, if the gain is sufficient for lasing, roughly how many QDs contribute?To fabricate a photonic-crystal-slab nanocavity, a sample is grown by molecular beam epitaxy on a ͑001͒ GaAs substrate starting with a GaAs buffer layer: 800 nm Al 0.94 Ga 0.06 As sacrificial layer, 40 nm GaAs, 20 nm Al 0.1 Ga 0.9 As, single layer of self-assembled InAs QDs ͑den-sity of 300-400 m 2 ͒, and on top of the dots 20 nm Al 0.1 Ga 0.9 As and 40 nm GaAs. 4 Then a two-dimensional triangular photonic-crystal-lattice with three holes missing to form a cavity spacer is fabricated to provide in-plane light confinement. The GaAs-air interfaces on the top and bottom of the 270-nm-thick slab provide vertical confinement by means of total internal reflection, but light with small inplane wave vectors still leaks out of the cavity. As shown by Noda's group using Si, vertical confinement is further enhanced by slightly shifting outward the holes at the ends of the spacer; the Q is increased by confining gently. 5 The quantum dots are e...

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