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The spontaneous-emission coupling factor for planar microcavities without lateral confinement is measured and calculated for gain media with finite spontaneous-emission linewidth. It is shown that in order to maximize, the spontaneous-emission linewidth of the gain medium must be smaller or equal to the cold-cavity linewidth. It is also shown that the planar-cavity geometry will give rise to spatial modes, and in order to maximize one should pump only an area corresponding to one spatial mode. If the pumped area is larger than the lateral extent of a cavity mode, then the mode will grow laterally when pumped above threshold.
Björk et al. (Sat,) studied this question.