The provision of an ionic self-assembled multilayer (ISAM) film on a fiber-optic element including a long period grating (LPG) by a simple, room temperature process provides tuning of the LPG and/or high sensitivity to the refractive index of the ambient environment of the fiber-optic element. Inclusion of a final layer or a probe, receptor or affinity ligand in the multi-layer film provides a highly sensitive sensor for specific chemical and biological materials. Use of a turn around point (TAP) LPG allows detection to be performed by detection of light intensity rather than wavelength of attenuation. Use of a material having an electro-optical response to an electrical field by change of dimension or refractive index allows the combination of a LPG or TAP LPG and ISAM film to function as a light modulator.