The spontaneous-emission coupling factor β 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 β, 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 β 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.

• Received 8 September 1992

DOI:https://doi.org/10.1103/PhysRevA.47.4451