We describe a new method providing the complex index of an absorbing material from interferometric measurements of the transmittance and the corresponding phase shift for a series of unbacked films, of medium to large thickness. In fact, the usual method, giving the optical constants independently from the slope of the line obtained by a least-squares fit to the experimental points, may be used only in the thick-film limit, although it has the advantage of being insensitive to the surface conditions. The new method suggested here retains this advantage and allows the thickness range to be extended to more transparent specimens. Furthermore, it provides information on the surface quality and the thin-film model validity. Application to unbacked gold films is shown in the visible and near ultraviolet.
© 1979 Optical Society of America
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