KTiOPO4 (KTP) exhibits excellent nonlinear optical properties which have made it a widely used crystal in frequency conversion applications such as second-harmonic generation and optical parametric oscillation. When KTP is exposed to high power and high repetition frequency, the crystal will produce gray track effect. After gray track forms, the absorption will increase in visible and near-infrared light and thus, frequency doubling conversion efficiency will decline. So how to improve gray track resistance remains a challenge all the time.

There are always many potassium vacancies in flux-grown KTP crystals. Oxygen vacancies form to keep the lattice substitute for K⁺ ions to provide charge compensation for potassium vacancies. Thus, oxygen vacancies, which are responsible for creating the detrimental gray-tracking centers by reducing the Ti⁴⁺ ions to Ti³⁺, are needed no longer. In addition, PbO can reduce the viscosity of solution, increase growth velocity and decrease probability of sesondary nucleation events on the crucible wall.                          

This paper explores the reason of gray track formation. On the basis of the results, we succeed in improving the resistance of gray track using K₆-PbO as flux. The crystals grown in the solution containing PbO are difficult to produce gray track, while it tends to form gray track with K₆ as flux. We conducted X-ray absorption near edge structure(XANES) using total electron yield(TEY) method in the absorption of gray track formation in situ for the first time. The results of XANES and absorption spectrum of gray track region indicate that the formation of gray track is related to Ti³⁺ centers and the absorption of visible light will increase after gray track come into being. We also take a measurement of the optical and laser properties for comparing the difference between the crystal grown in the solution of K₆-PbO and the one grown in the solution of K₆. The results show that the crystal grown in the solution of K₆-PbO has lower absorption values at 532 nm and 1064 nm, lower absorption coefficients, higher resistance to gray track formation, and higher laser induced damage threshold(LIDT). PbO is a promising flux for crystal growth, and we expect that high quality and optically uniform gray-track-resistant KTP can be grown commercially.

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