a structure generates electromagnetic radiation having at least one wavelength selected for activating a photo - sensitive substance that is applied to a tissue to be treated . the structure is particularly useful for photo - dynamic therapy applications . the structure includes a body of material , such as a polymer filament , band , or substrate , that contains a gain medium . the gain medium in turn contains a substance for generating a stimulated emission that includes the at least one wavelength when excited by a pump wavelength , and a plurality of scattering sites for scattering the stimulated emission to provide a narrow band emission at the at least one selected wavelength . the narrow band emission in turn activates the photo - sensitive therapeutic substance .