Long-range optical modes supported by a strongly absorbing thin organic film

We report the observation of the optical excitation of a long-range mode supported by coupling to Frenkel-type excitons in a highly absorbing organic thin film of metal-free phthalocyanine. The long-range mode is examined over a wavelength range from 400 to 740 nm, and the results are compared both with Fresnel’s theory for planar systems and with analytic expressions that describe the mode in the thin-film limit. Numerical analysis shows that this long-range mode, which like the long-range coupled surface mode has a quadratic dependence of wave vector on film thickness, actually evolves from a TM0 bulk polariton mode. We call the mode that we have observed a long-range excitonic guided mode.