Microcavity VCSELs

Microcavity lasers potentially will have ultra-low thresholds in the micro ampere range or even lower. The vertical-cavity surface-emitting laser (VCSEL) structure represents the most straightforwardly produceable device which can scale down to ultra-small cavity dimensions and active material volumes. By a simple scaling, one can calculate that a single-quantum-well (SQW) microcavity VCSEL, given a 100 A/cm2 threshold current density, would have a 200 nA threshold if scaled down to a 0.5 μm diameter. This calculated threshold does not rely on any enhancement of spontaneous emission, which could further reduce the threshold. The spontaneous emission factor in a GaAs/AlAs microcavity VCSEL of this diameter is predicted to be greater than 0.1, even at room temperature, compared to ~10-4 for conventional laser diodes [1], or ~10-2 for cryogenic-temperature planar microcavity VCSELs [2]. It is questionable, however, whether such a device is feasible. A SQW microlaser cavity must have extremely low loss, well under 1 % per pass. At some minimum diameter, almost certainly above 0.2 μm, diffraction and scattering due to sidewall roughness and the mirror-layer interfaces will cause losses to be excessively high. For an ultra-small microlaser to be electrically pumped with practical efficiency formidable fabrication challenges must be overcome.