Nonlinear waveguides by femtosecond laser writing of lithium triborate crystals

Nonlinear optical waveguides play a crucial role in the wavelength/frequency conversion of light. Femtosecond laser direct writing is an efficient and flexible technique to produce waveguides in dielectrics. We report on the nonlinear waveguides in LiB3O5 crystals by femtosecond laser writing. The double-line and depressed-cladding structures inscribed by femtosecond laser pulses show excellent guiding properties at 405 nm or 810 nm for both TE and TM polarized light. The investigations of the confocal micro-Raman spectroscopy have indicated that lattice structures in waveguide cores remain almost unchanged compared with those of the bulk. Based on type-I phase matching condition, both second harmonic generation of 810 nm and spontaneous parametric down conversion of 405 nm have been obtained in the laser-written lithium triborate waveguides, suggesting potential applications in integrated nonlinear optics and quantum photonics.