M. Tseitlin

On the optical quality of KTP crystals for nonlinear optical and electro-optic applications

Correlation between conditions of the top-seeded solution growth (TSSG) of potassium titanyl phosphate (KTP) crystals from self-fluxes and their optical uniformity has been studied with respect to the crystal morphology, variation of stoichiometry and formation of point defects. It has been shown that growth with pulling on X-oriented seeds yields single sector crystals exhibiting the ideal transverse optical uniformity necessary for the design of eye-safe optical parametric oscillations (OPO) and electro-optic elements based on the KTP crystals. Variation of stoichiometry has been studied using powders synthesized by solid state reaction and measuring the Curie temperatures of samples synthesized from batches with different starting compositions. Growth from self-fluxes at lower temperatures is found to reduce the concentration of potassium vacancies and their gradients and thus to suppress the detrimental gray-tracking phenomenon during frequency doubling of the 1064 nm Nd:YAG laser radiation into the green.

Growth-dependent properties of KTP crystals and PPKTP structures

Growth of optically uniform and single ferroelectric domain KTP crystals is of prime importance for frequency-conversion applications. In the course of KTP growth from pure self-fluxes the flux becomes enriched in potassium causing a gradual increase of potassium content in the crystal as well. We have shown that such an effect can be well characterized by a corresponding increase in the Curie temperature of the crystal. Establishment of the potassium concentration gradients is followed by charge separation and production of a built-in electric field, which can be enhanced or diminished also by the incorporation of charge-compensating residual impurities. The magnitude of the built-in electric field is directly proportional to the projection of the potassium concentration gradient on the crystals Z-axis, and it defines the domain direction in immersion seeded or different configurations of the top-seeded growth of KTP crystals. Detailed investigation of the domain formation mechanisms has allowed us to suggest a number of ways of growing single domain crystals, such as top-seeded growth with pulling in the Z-direction. Pulling in the X-direction is shown to yield predominantly bi-domain crystals. The formation of bi-domains and complex domain structures along the growth sector boundaries is explained in terms of edge-like and apex-like growth perturbations, respectively, which are due to temperature fluctuations at the growth interface. The knowledge of parameters influencing the domain formation mechanisms has allowed us to develop a technique for obtaining as-grown periodic domain structures necessary for large aperture (high-power) frequency conversion applications.

Pyroelectric properties of high-resistant KTiOPO4 crystals in the temperature range 4.2–300 K

The results of measuring the pyroelectric coefficient γsσ of nominally perfect KTiOPO4 (KTP) crystals grown from solution in a melt with a potassium to phosphorus ratio of ∼2 are presented. The γsσ (T) dependence is monotonic in the range from 4.2 to 250 K. Deviations from a linear dependence are observed beginning from 250 K, which is considered to be due to interstitial-potassium transport in the KTP crystal field. The spontaneous polarization of unclamped KTP samples is estimated from the results of the measurements. In terms of the crystal-physics approach, it is shown that the main contribution to a polar state of KTP is made by the dipole moments of two nonequivalent mesoscopic tetrahedra forming two sublattices that are polarized in opposite directions and bound by Ti(1) ions.

Structure of KTiOAsO4 single crystals at 293 and 30 K

The unit cell parameters of KTiOPO4 and KTiOAsO4 single crystals are measured in the temperature range from room temperature to 20 K. It is found that the unit cell volume of the single crystals changes smoothly. With a decrease in temperature, the c parameter remains almost unchanged. In a certain temperature range, the linear temperature dependence of the a and b parameters is violated. An X-ray diffraction study of KTiOAsO4 single crystals is performed at T = 293 and 30 K. With a decrease in temperature, the electron density in the channels of the structure undergoes a redistribution, suggesting that at room temperature the state of the potassium ions is characterized by the dynamic and static disordering. The nonuniformity of the distribution of the electron density at the junctions of TiO6 octahedra and AsO4 tetrahedra is significantly enhanced in relation to that at the corresponding junctions in the KTiOPO4 structure. It has been experimentally established that the geometry of the tetrahedral anions makes a decisive contribution to the nonlinearity of KTiOAsO4 single crystals.

Structure of KTiOPO4 single crystals grown by the top-seeded solution and spontaneous flux crystallization methods

This paper reports on the results of precision X-ray structural investigations of KTiOPO4 single crystals grown by one method (crystallization from a solution in the melt) in two variants (the spontaneous formation of crystallization centers or top-seeded solution growth during slow cooling of saturated solution melts). It is shown that spontaneous flux crystallization leads to the formation of a larger number of defects. Potassium atoms are found to be disordered. The splitting of the K1 and K2 potassium positions is equal to 0.347(4) and 0.279(3) Å, respectively, for the crystals grown by the top-seeded solution method and 0.308(5) and 0.321(4) Å, respectively, for the crystals grown through the spontaneous flux crystallization.

Structural Conditionality of the Ionic Conductivity of MTiORO 4 (M = K, Rb; R = P, As) Single Crystals

The ionic conductivity σ||c along the crystallographic axis c and the structural imperfection of the lattices of KTiOPO4, RbTiOPO4, and RbTiOAsO4 single crystals with low defect concentration, grown by the high temperature solution growth technique, have been investigated by impedance spectroscopy and X-ray diffraction analysis. Isomorphic substitutions of Rb+ ions for conduction K+ cations in MTiOPO4 crystals decreases the σ||c value, whereas the substitution of As5+ ions for framework P5+ cations in RbTiORO4 crystals increases the σ||c value. The σ||c values at 573 K are found to be 1.0 × 10–5, 5.7 × 10–6, 2.0 × 10–6, and 3.3 × 10–5 S/cm for the KTiOPO4, RbTiOPO4 {100}, RbTiOPO4 {201}, and RbTiOAsO4 crystals, respectively (the growth zone of the crystalline boule from which the samples were cut is indicated in braces).

Pyroelectric Properties of Potassium and Rubidium Titanyl-Arsenate Single Crystals in the Temperature Range of 4.2-300 K

The temperature dependences of the pyroelectric coefficients of KTiOAsO4 and RbTiOAsO4 single crystals grown by flux crystallization have been investigated in the temperature range of 4.2–300 K. With an increase in temperature, superionic conductivity first arises in KTiOAsO4 (at T > 200 K) and then (at T > 270 K) in RbTiOAsO4. This conductivity is much higher in the samples polarized at T = 4.2 K. An exponential change in the crystal resistivity along the polar direction is simultaneously observed. The results of measurements in the range of 4.2–200 K indicate larger values of pyroelectric coefficients when compared with potassium and rubidium titanyl-phosphate crystals. A correlation between the pyroelectric coefficients and a change in the lattice constants at isomorphic substitutions of K atoms for Rb and P atoms for As has been revealed within the symmetry approach.

High conversion efficiency of an eye-safe KTP monolithic OPO

We have compared the performance of pulsed eye-safe (1.576 μ) OPO resonators based on flux-grown KTP crystals in a plano-plano configuration with the monolithic and conventional external mirrors design. The former yields a higher frequency conversion efficiency of 40% and a slightly lower conversion threshold.