Tailoring of the properties of orientation-patterned (OP) GaAs and GaP by growing mixed ternary compounds by heteroepitaxy will enable pumping by Er-fiber laser systems at 1.56 µm and idler wavelengths beyond the mid-IR limit of GaP. We will present transmission measurements and bandgap estimations related to potential two-photon absorption with 167-322-µm thick unpattrened layers of different composition with P-content of x = 0%, 33%, 39.8%, 48.3%, and 100%, after separating them from the substrate and chemically polishing to a roughness of 0.8 nm. Except for pure GaP, which exhibits also an indirect bandgap, the estimated bandgaps are well described by the empirical relation 1.424 + 1.172x + 0.186x2 for the direct band-gap, where 1.424 eV stand for GaAs. A strong absorption band is seen around 13.3 µm in GaP (present also in all ternary samples) and at 19.1 µm in GaAs. However, the parasitic absorption band in the 2-4 µm range known for pure GaP, is absent in the ternary compounds.
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