Crystal growth and characterisation of mixed niobates for non-linear optical applications

By employing ferroelectric phenomenological theory, it is revealed that the birefringence of a ferroelectric crystal consists of two parts: one relating to a ferroelectric free of any electrical displacement and the other depending on the spontaneous polarisation. The theoretical outcomes provide a brief explanation of the experimental results in modifying KTP and suitable guidelines for this work. High-quality potassium lithium niobate crystals have been grown reproducibly from K2O and Li2O-rich solutions by top-seeded solution pulling (TSSP) method. K2O and Li2O play a similar role in that they decrease the crystal growth temperature so as to allow more Li- ions to incorporate into the crystal. Crystals having higher Li+ concentration possess higher Curie temperatures, larger spontaneous polarisations and larger distortions in the structure, giving rise to larger birefringences that make KLN crystals to have suitable birefringence for room temperature non-critical phase matching (NCPM) over the spectrum of available diode lasers. KLN crystals have also been grown from TiO2-doped or V2O5-doped solutions, but they have smaller birefringence than the crystals grown from undoped solutions. In addition, the domain structure was observed and poling conditions have been developed for KLN crystals. The non-linear optical properties of various KLN compositions have been investigated at λ=1064nm and λ=985nm. Temperature tuned NCPM has been realised by using both wavelengths. The measured phase matching temperatures increase with increasing spontaneous polarisation. KLN also has large non-linear optical coefficient (d31=2.14 d31liNbO3), a reasonably high damage threshold (1.75 times that of LiNbO3), wide temperature acceptance (∼5 deg C) and angular acceptance (∼8 deg). Potassium sodium niobate (KxNa1-xNbO3, KNN) crystals have been grown and they are confirmed to be ferroelectric. However, they are unstable and break up into small pieces after a short period of time. (author)