Second harmonic generation into the near ultraviolet in Zr-diffusion-doped lithium tantalate ridge waveguides.

We report on the efficient second harmonic generation at a near-UV wavelength of 399.7 nm in periodically poled Zr-doped LiTaO₃ waveguides. High-temperature in-diffusion of ZrO₂ layers into congruent LiTaO₃ substrates with simultaneous improvement of crystal stoichiometry by vapor transport equilibration in a Li-rich atmosphere was performed for the fabrication of planar waveguides. This new method enables very high photorefractive damage thresholds and allows us to avoid the known disadvantages of, e.g., proton exchange or titanium in-diffusion waveguide fabrication methods, which lower resistance to photorefractive damage. Low-loss ridge waveguides were then fabricated using a diamond blade saw. At a maximum available coupled power of 540 mW of the near-infrared pump laser, 49 mW of second harmonic light was generated in a 22 mm long sample with a normalized conversion efficiency of 3.5%·W^-1·cm^-2, exceeding the previously reported powers for LiTaO₃ waveguides in this wavelength range. Furthermore, by heating the sample, a temperature tuning coefficient of 32 pm/°C was measured for the generated second harmonic wave.