Highly efficient epitaxial Bragg mirrors with broad omnidirectional reflectance bands in the midinfrared

Laser-quality low-loss epitaxial Bragg mirrors exhibiting a broad omnidirectional stop band in the midinfrared at wavelengths around $7μm$ are presented. The mirror layers, composed of PbTe and EuTe, exhibit a high refractive index contrast of 80%. Thus, with only three layer pairs, reflectances higher than 99% are achieved for angles of incidence up to 40° and higher than 98% for angles of incidence up to 60°. The angle dependence of the reflectance is accurately determined by analyzing the width of the cavity resonance in a half-wavelength microcavity, formed by two such Bragg mirrors. The experimentally determined omnidirectional stop bandwidth is as high as 50% of the target wavelength. The results are in good agreement to analytical calculations based on the transfer matrix method.