Lap Kin Cheng

Crystal growth and characterization of KTiOPO4 isomorphs from the self-fluxes

Abstract We report the crystal growth of KTiOPO 4 (KTP) and its isomorphs from the self-fluxes. Common features in the crystal growth properties of MTiOXO 4 within the M 2 O-TiO 2 -X 2 O 5 ternary, where M={K, Rb or Cs} and X={P or As (for M=Cs only)}, are noted. The solubility of these crystals in the M 5 X 3 O 10 and M 6 X 4 O 13 fluxes are presented. A previously unreported (00-1) growth face was observed in KTiOAsO 4 (KTA) and the morphological stability of this face in KTP is also discussed. Optimization of the crystal growth parameters allows us to grow inclusion-free crystals of up to 18×35×35 mm 3 from 250 ml charges. We discuss the domain characteristics of these crystals and present the optical, electro-optical and dielectric properties of these materials.

Properties of doped and undoped crystals of single domain KTiOAsO4

Large single crystals of KTiOAsO4 (KTA) measuring up to 35×31×58 mm3 (along abc) are grown from the pure potassium arsenate self-fluxes and from the tungstate flux using the seeded high-temperature solution growth technique. Small amounts of Fe2O3, Sc2O3, or In2O3 (∼0.2–0.4 wt %) are added to the melt to promote single-domain formation in the flux-grown crystals. We report here the linear optical and nonlinear optical properties of these crystals. We observed an unusually strong optical birefringence increase with Fe2O3 dopant concentration [δ(nz−nx)∼0.017/wt % Fe]. This increase leads to a blue shift in the second-harmonic generation cutoff wavelength of as much as 37 nm for a ∼0.47 wt % Fe-dopant level in KTA.

Crystal growth of KTiOPO4 isomorphs from tungstate and molybdate fluxes

Abstract Large inclusion-free crystal of KTiOPO 4 , RbTiOPO 4 , KTiOAsO 4 and RbTiOAsO 4 have been grown from the tungstate and molybdate fluxes. In this paper, we report the typical growth parameters, the solubilities, and the heats of solution (δ H sol ) for these crystals, and briefly comment on their optical quality. In addition to being nonvolatile and of low viscosity, the tungstate and the molybdate fluxes also have significantly faster dissolution kinetics than the pure phosphate or arsenate self-flux. This makes them especially versatile for the exploratory crystal growth of the KTP isomorphs and their solid solutions. Using these fluxes, single crystal boules of up to 2.5×3×5 cm 3 have been grown from a 250 ml melt. We have also observed a modification in the crystal growth habit, which in the case of KTiOAsO 4 , results in the formation of stable, singular {111} faces.

Periodic poling of RbTiOPO4 for quasi-phase matched blue light generation

RbTiOPO4 has been periodically poled for first-order quasi-phase matching. The poling was monitored using a technique based on the electro-optic effect, which enabled fabrication of high-quality domain gratings for efficient frequency doubling to the blue.

Development of the nonlinear optical crystal CsTiOAsO4 I. Structural stability

Abstract We have systematically investigated the structural stability of the KTiOPO4 family of crystals, specifically of CsTiOAsO4. High temperature differential thermal analysis (DTA) reveals for the first time that CsTiOAsO4 thermally decomposes into a non-stiochiometric, Cs-deficient cubic compound at ∼960°C. This decomposition is actually more general, and is observed in two other KTiOPO4 isomorphs, namely RbTiOPO4 and TITiOPO4. A “threatened-structure model” is proposed to explain the decomposition and evidence supporting the validity of this model are presented. The model further explains the observed anomaly in CsTiOAsO4s crystal growth properties using various fluxes. CsTiOAsO4s relatively low decomposition temperature creates problems for bulk crystal growth and methods for reducing these growth problems are proposed.

Growth of epitaxial thin films in the KTiOPO4 family of crystals

Abstract We report the growth of thin (4–20 μm) epitaxial films of KTiOAs x P 1− x O 4 on KTiOPO 4 substrates. The films were grown at ∼ 850°C by liquid phase epitaxy using the tungstate and the pure arsenate-phosphate self-fluxes. Typical growth rates are ∼ 50–100 A/s, although rates as slow as 10 A/s have been observed. The orientations of these films are {011}, {100}, {110} and {201}, and no significant variation in their growth properties was observed. Unlike epitaxial films obtained by the ion-exchange process, these films possess a step-like refractive index profile which makes them especially attractive for micro-optical devices for frequency conversion applications. The large refractive index change, δn = n film − n substrate ∼ 0.01–0.02, and thegood optical quality of these films allows us to readily demonstrate optical waveguiding. We also report on results of other related material systems, and address the problem of lattice matching in the growth of these films.

Femtosecond mid-infrared optical parametric oscillator based onCsTiOAsO 4

A high-repetition-rate Ti:sapphire laser is used to synchronously pump a type-II angle-tuned CsTiOAsO(4) (CTA) optical parametric oscillator. When pumped at 809 nm, the optical parametric oscillator is tunable from 1.007 to 1.180 microm in the signal branch and from 2.590 to 4.120 microm in the idler branch. Powers as high as 235 mW are obtained in the signal branch. Pulse widths as short as 56 fs are generated at 1.115 microm. CTA is shown to have a unique combination of low walk-off and low dispersion that contributes to its high gain and conversion efficiency.

Development of the nonlinear optical crystal CsTiOAsO4 II. Crystal growth and characterization

Abstract We report here the crystal growth and characterization of the new nonlinear optical crystal CsTiOAsO 4 (CTA). We have studied the Cs 2 O-TiO 2 -As 2 O 5 ternary and identified the phase field within which orthorhombic CTA can be crystallized. Large inclusion-free single crystals measuring up to 30×20×25 mm 3 can be readily grown using the Cs 5 As 3 O 10 self-flux. As grown CTA crystals have the same crystal facets as KTP, although they tend to be multidomain. We present here two techniques for removing these domains: by trivalent oxide dopings and by ferroelectric poling. The refractive indices, nonlinear optical coefficients, electro-optic coefficients and dielectric properties of single domain CTA crystals are reported. Compared to the crystal KTiOPO 4 , the smaller birefringence of CTA makes it particularly suitable for frequency doubling fundamental wavelengths between 1.3 and 1.5 μm. Our results suggest that CTA should find practical applications in electrooptic and in frequency conversion applications, such as optical parametric oscillators and compact diode-pumped solid state lasers.

Curie temperatures and dielectric properties of doped and undoped KTiOPO/sub 4/ and isomorphs

The Curie temperatures and dielectric properties of KTiOPO/sub 4/ (KTP), RbTiOPO/sub 4/, KTiOAsO/sub 4/, RbTiOAsO/sub 4/, CsTiOAsO/sub 4/, Ba:KTP, and Ga:KTP were measured with small-signal relative dielectric permittivity ( kappa ) analysis, piezoelectric resonance analysis, and optical second harmonic generation. All the isomorphs and the doped KTP exhibit lower Curie temperatures than KTP, ranging from 637 degrees C for CsTiOAsO/sub 4/ to 955 degrees C for hydrothermally (HT) grown KTP. The Curie-Weiss law is obeyed in all samples. With the exception of CsTiOAsO/sub 4/, all doped and undoped crystals show large dielectric relaxation at frequencies below 100 KHz throughout the temperature range of 500 degrees C-800 degrees C. For the barium-doped KTP, it was found that the Curie temperature decreases with increasing Ba/sup ++/-doping concentration. Barium doping also significantly modifies dielectric and piezoelectric properties of KTP.<<ETX>>

KTIOPXAS1-XO4 OPTICAL WAVEGUIDES GROWN BY LIQUID PHASE EPITAXY

We report on a new type of optical waveguide based on the nonlinear optical material KTiOPO4 (KTP). Thin films of KTiOPxAs1−xO4 (5–50 μm) were grown on KTP substrates by liquid phase epitaxy using a tungstate flux. Energy‐dispersive x‐ray spectrometry reveals an abrupt increase in the arsenic concentration in the film, suggesting an abrupt, step‐like refractive index profile in these guides. Optical waveguiding at 0.633 μm was demonstrated in a 20‐μm‐thick KTiOP0.76As0.24O4 film grown on a {011} KTP substrate. The refractive index difference between the film and the substrate (Δn) was measured to be ∼0.012, which compares favorably with the value estimated from bulk crystal measurements of the end members. Variations in the design of these waveguides and several potential applications are discussed.