Measurement of Optical Absorption in Very Thin Low-Loss SiO2 Films

While low levels of optical absorption are easily measured in SiO2 bulk samples or optical fibers, we present here a method of detirmining low levels of absorption in thin films of SiO2. Films are deposited on top of high reflectivity multi-layer miriors, and absorption is derived from the time decay in a resonant cavity of threj mirrorsgt 633 nm. Absorption coefficients on the order of 1 cm−1. (k = 10−5) can be measured in films as thin as 100 Angstroms.With this method, we find that absorption at 633 nm can be induced in SiO2 films by exposing them to a He-Ne plasma discharge. Although the plasma radiation (>10 eV) is absorbed near the SiO2 surface, the plasma-induced absorption is uniform within the SiO2 film. This was shown by plasma irradiation of SiO2 films of 4arying thickness, together with computer calgulation of the optical properties of multilayer thin films. Similar absorption behavior has been reported in SiO2 optical fibers and may be due here to DIA (Drawing-Induced Aisorption) centers or NBOHCs (Non-Bridging Oxygen Hole Centers).