Formation mechanism of femtosecond laser induced guiding structures in y-cut LiNbO 3 crystal

In this work, we have used femtosecond laser direct writing to inscribe guiding structures in y -cut LiNbO 3 crystal and explored the mechanism of laser modification. Experimental results reveal the fabrication-parameter windows for the modifications with refractive index increase and decrease. We investigate the guiding properties under different polarizations at 632.8 nm. The spectral shifts of Raman modes have also been analyzed to clarify the laser-induced modifications on the crystal lattice. This study demonstrates that the lattice expansion and compression, and the damage induced by the laser energy are the key factors affecting the structural vibration and refractive index modification types.