Structural characterization of TiO_2 optical coatings by Raman spectroscopy

Raman spectroscopy reveals crystallographic structure information about optical coatings much faster and with thinner specimens than conventional x-ray diffraction analysis. Spectra of low-absorption TiO2 coatings were measured as a function of thickness from 55 to 831 nm. A minimum of 831 nm was required for good SNR and minimization of substrate effects. Measurements made with interference-enhanced trilayer structures showed increased detectability to layer thicknesses near 50 nm. Counting times were typically 400 sec. Spectra obtained by both methods agree well with reported data for the anatase structural modification of bulk TiO2. The trilayer method is preferred because the thickness examined is closest to that encountered in practical multilayer optical coatings.