In this book, a systematic study of laser irradiation effects in polymers and crystals is presented. Several microstructures, craters, diffraction gratings, waveguides and Y couplers were fabricated in polymers and Lithium Niobate crystal using fs laser direct writing technique. Four different polymers PMMA, PDMS, PS, and PVA were investigated in the study. The formation of defects which are responsible for the observation of emission in polymers is explained in the context of formation of myriad optical centers. The change in emission peak with excitation wavelength is observed in polymers...