Photonic crystal metamaterial can exhibit negative index properties and this behaviour is well described by a resonator model. In this work, we present the experimental evidence that a Lorentz resonator correctly reconstruct data obtained with a negative refracting Photonic Crystal (PhC) by using a standard optical technique, such as ellipsometry. In particular we show that, in the frequency range in which the effective refractive index, neff, is equal to -1, the incident light couples efficiently to the guided modes in the top surface layer of the PhC metamaterial. These modes resemble surface plasmon polariton resonances. In add we present measurements by using standard technique of prism coupling evanescent wave. Once again the presence of localized plasmon-like modes at the surface of a silicon two-dimensional photonic crystal slab is demonstrated. Also in this case, in analogy with surface plasmons supported in metals in a photonic crystal metamaterial, the electromagnetic surface waves arise from a negative effective permittivity. These results opens new strategies in light control at the nanoscale, allowing on chip light manipulation in a wide frequency range and avoiding the intrinsic limits of plasmonic structures due to absorption losses in metals. Such negative index PhC materials may be of use in biosensing applications.