December 2015
Spotlight Summary by Jennifer D. T. Kruschwitz
TRACK – A new method for the evaluation of low-level extinction coefficient in optical films
A useful method for determining refractive index and very low absorption values in dielectric films only requires four simple spectrophotometric measurements: reflectance and transmittance at a low incidence angle, and the same at an angle near Brewster’s angle. All four measurements require using P-polarized light. The method is termed TRACK, which stands for Transmission, Reflection, Absorption Combination for K evaluation. The main premise is to determine the refractive index and film thickness from the reflectance (R) and transmittance (T) measurements made at near-normal incidence, and then derive the extinction coefficient, k, from R and T measured at an incidence angle near the film’s Brewster angle. TRACK can be used for bare substrates as well as single layer films. The angles chosen for examples of this method were 10° and 65°. The authors showed that any dispersion equation could be utilized to model the refractive index in the first step of the method. The film thickness is then determined by traditional methods where the film is made optically thick enough to view several maxima/minima over a broad wavelength region. Determination of the extinction coefficient is derived directly from the measured absorption at the higher, near-Brewster, incidence angle. This method can prove to be extremely worthwhile for those invested in dielectric laser coating manufacture without the need for a high-priced calorimeter or spectroscopic ellipsometer.
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Article Information
TRACK – A new method for the evaluation of low-level extinction coefficient in optical films
R. Vernhes and L. Martinu
Opt. Express 23(22) 28501-28521 (2015) View: Abstract | HTML | PDF