The influence of illumination on ferroelectric domain engineering in lithium niobate and lithium tantalate is investigated. The conventional method of domain inversion is electric field poling, which suffers from several limitations such as a requirement for photolithography and high-voltage equipment, the formation of inhomogeneous electric fields, and a minimum domain size of micrometres. Through the use of directed laser light, either in the presence or absence of an externally applied electric field, these limitations can be overcome and new fabrication capabilities are revealed. Light-assisted poling is the simultaneous application of an external electric field and laser illumination. Using wavelengths ranging from near-UV to near-IR, the electric field required for domain nucleation was reduced for increasing intensities. This effect was most prominent in crystals highly doped with MgO, achieving a reduction of 90% and 98% for cw and fs-pulsed light, respectively. Arbitrary domain patterns were directly written by the scanning of a focused beam. Periodically poled gratings were formed using periodic intensity patterns via a phase mask, forming domain engineered crystals suitable for quasi-phase-matched nonlinear frequency conversion.
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