Mid-infrared high finesse microcavities and vertical-cavity lasers based on IV–VI semiconductor/BaF2 broadband Bragg mirrors

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We report on molecular beam epitaxially grown high-reflectivity broadband Bragg mirrors for mid-infrared devices 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%. To verify the high quality of the PbEuTe/BaF2 Bragg mirrors, we study a half-wavelength microcavity formed by mirrors with only three periods. The resonance of the microcavity has a narrow linewidth of 5.2 nm corresponding to a very high finesse of 750. From this, a mirror reflectivity higher than 99.7% is deduced, in good agreement to transfer matrix simulations. Furthermore, we demonstrate mid-infrared continuous-wave vertical-cavity surface-emitting lasers based on these mirrors. Optical excitation of laser structures with a PbSe active region results in stimulated emission at various cavity modes between 7.3 and 5.9 μm at temperatures between 54 and 135 K. Laser emission is evidenced by a strong linewidth narrowing with respect to the linewidth of the cavity mode and a clear laser threshold at a pump power of 130 mW at 95 K.