Investigation on the fabrication of femtosecond laser induced waveguides and gratings inside quartz crystals

When femtosecond laser pulses are tightly focused on transparent material, high pressure caused by temperature increase in a confined volume of material will result in microexplosion. Thus the materials in such area will be densified, which causes the variation in refraction index. Continuous explosive lines that are just under the surface of quartz crystal wafers are micromachined by femtosecond lasers. These lines can serve as optical waveguides. Surface morphologies and nanomechanical properties in the area above the explosive lines were measured. The surface irradiated by laser beam is much rougher than that of the unirradiated zone, and there is a clear boundary between laser-induced rough and smooth areas. The size of the rough areas is much smaller than the laser spots that are irradiated on the surface. Nanomechanical changes occur in the area that has no observable damages in microscope and AFM. The position dependent variations of mechanical properties around the surface area that is just above the irradiated line were described in this paper. The values of elastic ratio changes significantly in different positions, which reveals the position dependent variations of femtosecond laser induced stresses and structural changes. Shocks generated during microexplosive process response for such changes. The changes can cause the variation in the degree of densification in different positions, which is corresponding to the modification of refractive index. By focusing the femtosecond laser pulses into the bulk of quartz crystal, internal waveguides and grating structures were fabricated.