Measurement of the Thickness of Deposited Magnesium Fluoride Films by Evanescent-wave Fluorescence: A Critical Test of the General TIRAF Theory

We have recently developed a quantitative theory describing the stimulation of fluorescence by an evanescent wave, with a view to the precise measurement of thin films by otherwise conventional light microscope photometry. In our set-up, total internal reflection of a focused laser beam occurs at a glass/water interface and the presence of the evanescent wave is reported by a fluorescent dye dissolved in the water. For convenience, we refer to the method as TIRAF (total internal reflection aqueous fluorescence). We have tested our general TIRAF theory critically by using glass/MgF 2 /fluorescein layers such that the evanescent wave established in the glass/MgF 2 interface is, in part, separated from the aqueous fluorescein phase by an accurately and independently measured thickness of deposited MgF 2 film. We show that fluorescence measurements on areas as small as 30 µm 2 allow precise determination of film thicknesses in the range 2-160 nm for two different decay constants of the evanescent wave.