Fig. 1. Schematic of the sample fabrication and pyroelectric poling principle. (a) Schematic of the FLW process for fabricating single gratings and grating arrays. (b) Thermal treatment with a polarized microscopy system. (c) Principle of the pyroelectric poling process, including being stable at a low temperature (Stage 1), heating up to a high temperature (Stage 2), maintaining at the high temperature (Stage 3), and cooling down in the air and in vacuum, respectively (Stage 4). The spontaneous polarization, depolarizing field, screening field, and pyroelectric field are denoted by the dashed black arrows, blue arrows, red arrows, and purple arrows, respectively.

Fig. 2. Domain inversion of single gratings. (a) The dynamic changes of domain inversion during the cooling process in vacuum (cooling rate 30°C/min). (b) Comparison of domain inversion at different initial temperatures in vacuum cooling (cooling rate 30°C/min.). (c) Comparison of domain inversion at different cooling rates in vacuum (the same high temperature of 200°C). (d) The inverted domain structures with periods of 10, 8, 6, and 4 µm along with the higher-magnification insets. The top and bottom are the inverted domain structures after cooling in vacuum and in air, respectively.

Fig. 3. Diffraction of single gratings with inverted domain structures and FLW. (a), (b) Schematic diagrams for performing nonlinear Raman–Nath diffraction (RND) in gratings with domain inversion and the corresponding diffracted SH patterns with different periods. (c), (d) Schematic diagrams for performing linear RND in gratings induced by FLW and the corresponding diffracted patterns with different periods.

Fig. 4. (a) Dark-field images of periodic grating arrays domain in the y–z plane and x–y plane. (b) Schematic diagram of the QPM SHG in the grating arrays with inverted domain structures and corresponding diffracted SH spots. In the experiments, the propagation of the fundamental beam was along the x direction. (c) SH power versus wavelength using reciprocal vectors with l = 1 and l = 2, respectively. (d) Corresponding dependence of SH powers on input pump powers at the QPM wavelengths of 910 and 894 nm, respectively.