Excitation of optical modes supported by strong absorption in organic thin films at attenuated total reflection geometry

Reflection dips observed at attenuated total reflection (ATR) geometry for metal thin films are interpreted from excitations of surface plasmon polaritons. We have presented that the absorption induced by enhanced electric fields in a metal film is an origin of this phenomenon (Wakamatsu et al (2007 J. Opt. Soc. Am. B 24 2307)). This viewpoint provides that ATR dips can be generated not only in metals but in other materials. Here, the absorption spectra of copper phthalocyanine (CuPc) thin films were observed at ATR geometry without a metal layer. Only from p-polarized ATR measurements was a strong absorption dip observed in the short wavelength region of the CuPc absorption bands. Kramers-Kronig analyses for the transmittance spectra of a CuPc thin film revealed that the refractive index of the film lowers around the spectral position of the p-polarized ATR dip. Transfer matrix analyses indicated that the lowered refractive index of CuPc induces the enhancement of evanescent fields in the film. These results demonstrate that the ATR dip generation is attributable to the absorption derived from enhanced evanescent fields in the CuPc film.