We report on recent technological improvements concerning nonlinear patterning of lithium niobate and lithium tantalate in the micrometer and submicrometer scales using surface periodic poling for ferroelectric domain inversion. The fabricated samples were employed for frequency doubling via quasiphase-matching both in bulk and guided wave geometries, including forward and backward configurations and wavelength conversion in bands C and L. We also investigated short-period quasiperiodic samples with randomly distributed mark-to-space ratios. 1. Introduction Optical frequency conversion in parametric media is one of the most important applications in nonlinear optics, as it allows generating coherent light in regions of the spectrum where laser emission is not available. Moreover, through a careful engineering of the phase mismatch and excitation(s), several signal processing schemes can benefit of parametric nonlinearities via cascading [1–7]. Since the first experiments on second-harmonic generation (SHG) [8–10], a great effort has been devoted towards developing configurations with the highest conversion efficiencies [11, 12]. Among them, quasi-phase matching (QPM) is one of the most useful and versatile techniques for efficient frequency conversion, allowing continuous growth of the generated wave along the propagation length despite the local phase velocity mismatch (hence, lack of momentum conservation) between the interacting waves [13, 14]. This can be achieved thanks to a correction of their relative phases by means of a structural periodicity built in the nonlinear medium; more precisely, a periodic sign change of the quadratic coefficient as it can be achieved in noncentrosymmetric crystals, for example, lithium niobate (LN) and lithium tantalate (LT), via periodic poling (PP). Several techniques have been developed for PP, including domain inversion during crystal growth [15], spontaneous Curie temperature poling using heat [16], chemical reactions or substitution of impurities [17, 18], electron beam [19–22], and electric field assistance [23–25]. In recent years, surface periodic poling (SPP) was introduced as a PP approach for the fine tuning of the mark-to-space ratios in short-period QPM, aiming at the realization of smaller and smaller domain lengths for sum-frequency generation at shorter wavelengths, as well as backward or counterpropagating wave mixing [26–36]. Surface periodic poling is based on the overpoling of a ferroelectric substrate and provides periodic domain inversion with shallow domain depths as compared to the thickness
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