Interpretation of optical absorption loss measurements in synthetic sapphire using positron annihilation lifetime spectroscopy

Sapphire should be highly transparent for photon energies less than the band gap, but residual, weak absorption and scattering losses in the near infrared occur as a result of extrinsic and intrinsic defects. Lattice disorder, impurities, and point defects have all been implicated as being the origin of loss phenomena but very little experimental evidence exists to quantitatively establish the relationships that might exist between these defects and optical loss. In this study, three synthetic, c-axis sapphire samples manufactured under similar conditions were characterized using UV-VIS spectroscopy, photothermal common-path interferometry, and positron annihilation lifetime spectroscopy. Model-based interpretation of optical measurements indicated that vacancy-type defects were partially responsible for absorption loss from the ultraviolet to the near-infrared and that the population densities differed among the samples. Positron annihilation lifetime spectroscopy measurements also indicated a higher concentration of cationic vacancy defects near the sample surface which correlates with a higher surface optical loss. This work establishes the use of positron annihilation techniques as a characterization tool for optical materials that could be useful for investigating the origin of weak surface absorption in the transparent region of sapphire.