Broadband second harmonic generation from a cladding waveguide fabricated by ultrafast laser ablation in KTN crystal

In this paper, we report a cladding waveguide with a broadband second harmonic generation (SHG) based on second-order nonlinearity in relaxation ferroelectrics. The cladding waveguide was fabricated in a KTa_1-xNb_xO₃ (KTN) crystal by the femtosecond laser direct writing (FLDW) technology. The end-face coupling system has been used to measure the near-field intensity distribution, the spectra of SHG have been detected by the spectrometers and the local paraelectric-ferroelectric phase transition has been verified by Raman spectra. The experimental results indicate that the cladding waveguide presents excellent performance with polarization independence 515-nm SHG output under a 1030 nm femtosecond input laser, and 200-nm bandwidth SHG output under a picosecond supercontinuum source. This work demonstrates the application potential of FLDW technology for fabricating miniaturized nonlinear photonic components of integrated devices on an optical crystalline platform.