SPONTANEOUS EMISSION FACTOR OF A MICROCAVITY DBR SURFACE-EMITTING LASER

The spontaneous emission factor of a microcavity DBR surface-emitting laser has been theoretically obtained to investigate the possibility of thresholdless lasing operation. First, we obtained the formulas expressing the spontaneous emission in a three-dimensional microcavity. By introducing the distribution of mode density in wavevector space, it was clearly shown that the radiation pattern of spontaneous emission is deeply modified by the microcavity and is different from that in free space. Based on this result, the spontaneous emission factor and the emission lifetime were calculated as a function of emission spectral width and the size of the active region. It was found that the spontaneous emission factor exceeds 0.1, even though the spectral width is as large as 30 nm when the transverse size is smaller than 0.5-mu-m and the DBR reflectivity is larger than 90%. Its upper limit of 0.5 is determined by the polarization probability of a linear polarized mode. In this condition, all of the energy from the spontaneous emission is taken out in the longitudinal direction. In addition, the averaged spontaneous emission lifetime in the microcavity decreases to 0.09 times that in a large bulk if the spectral width is less than 0.5 nm, for example, by employing the excitonic transition. The enhancement of the spontaneous emission factor and the reduction of lifetime may lead to thresholdless lasing operation, high efficiency, high speed, and good spatial coherency of the surface-emitting laser.