Temporal coherence of a photonic crystal nanocavity laser with high spontaneous emission coupling factor

Abstract

Temporal coherence of a continuous-wave photonic crystal nanocavity laser is investigated in detail using interference experiments at room temperature. The nanocavity laser operates at $1.3\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ with InAs quantum dot gain material and has a very high spontaneous emission coupling factor $\ensuremath{\beta}=0.9$ with a threshold absorbed pump power of $\ensuremath{\sim}375\phantom{\rule{0.3em}{0ex}}\mathrm{nW}$. The coherence around the laser threshold of such a high-$\ensuremath{\beta}$ laser is not obvious because spontaneous emission efficiently couples to the lasing mode. The output power dependence of the coherence length shows linearity at and above threshold. This result indicates that the first-order coherence is not greatly reduced even at threshold, where the cumulative power of spontaneous emission of other wavelengths cannot be negligible when compared with the laser power. This can be attributed to the fact that the lasing mode of a high-$\ensuremath{\beta}$ laser has a relatively large portion of the total power compared to lasers with lower $\ensuremath{\beta}$'s due to efficient coupling of the spontaneous emission to the cavity mode.