A broad-band high-reflectivity mirror using (Al,Ga)As/(Ca,Sr)F (2) multilayer structures grown by molecular beam epitaxy.

By molecular beam epitaxy (MBE) we have grown a quarter-wave stack of lattice-match Al(0.1)Ga(0.9)As/Ca(0.45)Sr(0.55) F (2) multilayer heterostructure which has high reflectivity (89%) over a broad spectral range. Unlike similar structures of (Al,Ga)As/GaAs which require many periods to achieve high reflectivity, the III-V/fluoride structures require only 4 periods due to the large difference in the refractive indices of the III-V compound and the fluoride (3.6 and 1.4 at 900 nm, respectivity). Furthermore, for the (Al,Ga)As/GaAs structures the high-reflectivity plateau is narrower, and because of adsorption in the GaAs layers the reflectivity decreases rapidly at wavelengths shorter than the bandgap wavelength of GaAs. For the III-V/fluoride structures grown the adsorption is not a problem, and the high- reflectivity region extends from 900 nm down to 700 nm.. The ability to grow epitaxial fluoride and (Al,Ga)As with arbitrary AlAs mole fraction and therefore different refractive indices, should stimulate interesting applications in optics.