In this work, we present the first experimental evidence of negative dielectric susceptibility in a two-dimensional silicon photonic crystal (PhC) with negative refractive index behavior. 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. This finding was confirmed by ellipsometric measurements, demonstrating the isotropy of the PhC resonances. Such negative index PhC materials may be of use in biosensing applications. Researchers have realized a device that exhibits negative effective refractive index on the surface of a silicon chip. The ability to control light on a semiconductor chip is important for applications that involve the interaction of electronics with light. The approach developed by Vito Mocella and colleagues from the Consiglio Nazionale delle Ricerche in Italy is based on the use of photonic crystals—in this case, a periodic array of holes etched into silicon. The holes change the way light propagates along the surface of the chip such that the structure has a negative refractive index at its resonance wavelength, which means that light bends in a direction opposite to what would usually be expected when passing from one medium to another. This effect could be used for highly sensitive biosensing applications.