Simple method for determining the crystalline axes of nonlinear uniaxial crystal by using the second harmonic transmission or reflection

For the periodic poling of Lithium Niobate, domain walls typically want to align with the x-planes and therefore the determination of X and Y axes is very important. However, this is not so trivial, normally requiring the x-ray diffraction measurements.We propose a simple method for determining these axes of the nonlinear uniaxial crystals such as Lithium Niobate by suing the second-harmonic generation. For several uniaxial crystals (with 3 m, 32, 62 m and 42 m point groups), the intensity change of the second harmonic output is calculated as the crystal is rotated azimuthally with respect to Z-axis. The appropriateness of our method is confirmed through the second harmonic transmission experiment on Lithium Niobate (3 m point group) and the second harmonic reflection experiment on quartz (32 point group) crystals.