The optical properties of matter are based on the coupling of the incident electromagnetic radiation to oscillators within the material. The oscillators could be electrons, ions or molecules. Close to a resonance the dielectric function indicates strong dispersion and might be negative.
A long-period fiber grating (LPFG) allows light coupling between the guided mode and the cladding modes. The transmission spectrum of a typical LPFG contains a series of attenuation bands centred at discrete wavelengths, leading to extensive studies of its applications as an