Mid-infrared vertical-cavity surface-emitting lasers based on lead salt/BaF2 Bragg mirrors

We demonstrate mid-infrared continuous-wave vertical-cavity surface-emitting lasers based on Bragg mirrors using IV-VI semiconductors and BaF2. This material combination exhibits a high ratio between the refractive indices of up to 3.5, leading to a broad mirror stop band with a relative width of 75%. Thus, mirror reflectivities higher than 99.7% are gained for only three layer pairs. Optical excitation of microcavity laser structures with a PbSe active region results in stimulated emission at various cavity modes between 7.3 mu m and 5.9 mu m at temperatures between 54 K and 135 K. Laser emission is evidenced by a strong line width narrowing with respect to the line width of the cavity mode and a clear laser threshold at a pump power of 130 mW at 95 K. Furthermore, we study a similar microcavity but without an active region. The resonance of such an empty microcavity has a narrow line width of 5.2 mm corresponding to a very high finesse of 750, in good agreement to transfer matrix simulations and to the expected mirror reflectivities.