Short-wave ultraviolet lasers have attracted much attention due to their advantages of large photon energy, high spatial resolution, and excellent focusing ability, which are widely applied in the fields of laser processing, semiconductor inspection, medical diagnosis, data storage, and spectral analysis. Nonlinear frequency conversion is one route of realizing short-wave ultraviolet laser output. The generation of 266 nm through quadruple frequency converting from 1064 nm is comparatively mature at present, which benefits from the development of commercial β-BaB?O?(BBO) and CsLiB?O??(CLBO) crystals. However, the average output power of a 266 nm laser is relatively low at a high repetition rate, owing to the large walk-off angle and two-photon absorption of BBO. The serious deliquescence of CLBO crystals can be overcome by continuously heating at sealed condition, which limits the steady operation of a 266 nm laser. As a novel nonlinear optical crystal, RbLi(HC₃N₃O₃)·2H₂O (RLHCY) demonstrates the larger frequency doubling effect, the shorter phase-matching wavelength (calculated at 239 nm), and the higher laser induced damage threshold. More importantly, large-size crystals can be grown by solution method at low cost. This study is focused on crystal growth and optical properties of RbLi(HC₃N₃O₃)·2H₂O crystals.

The RLHCY crystal is grown by the solution method utilizing the raw materials of RbCl, LiOH·H₂O, and C₃H₃N₃O₃ powders with mass fraction of 98%?99%. The solubility is measured by the way of multiple small quantities at different temperatures. The piezoelectric coefficient is determined by quasi-static piezoelectric coefficient measuring instrument to distinguish the positive and negative polarity. The rocking curve of (100) wafer is measured by X-ray diffractometer to evaluate the crystalline quality. The transmittance spectrum at 200?800 nm is obtained using ultraviolet-visible-near-infrared spectrometer in the step of 0.5 nm with a polished sample of 1.0 mm thickness. The refractive indices of (100), (010), and (001) wafers are measured by prism-coupling method at 407, 514, 636, 965, and 1547 nm. Consequently, the refractive index curve is fitted and the phase matching curve is calculated. The type I frequency-doubling device in dimension of 4 mm×4 mm×5 mm is adopted for the 266 nm output experiment.

The solubility of RLHCY is increased with the temperature from 12.2 g (25 ℃) to 35.6 g (65 ℃) (Fig. 1). The optimized conditions are determined as the following: the growth temperature range of 50?25 ℃, the cooling speed of 0.5?1.0 ℃/d, and the rotation rate of 12 r/min. A transparent crystal is acquired in a large size of 50 mm×35 mm×20 mm (Fig. 2). Based on the plane orientating and the piezoelectric coefficient measurement, the growth rate is confirmed in the sequence of R_[010]>R_[001]>R_[100]>R_[00?1]. The full width at half-maximum (FWHM) of (100) X-ray rocking curve is 43.2″(Fig. 4), which indicates the excellent crystalline quality. The ultraviolet cut-off edge is around 238 nm and the transmittance at visible region is larger than 85% (Fig. 5). The calculated phase-matching curve demonstrates that the phase matched wavelength can be as low as 238 nm (Fig. 7). The 266 nm laser output is achieved under the 532 nm pumping (repetition frequency of 3 Hz, pulse width of 8 ns), using a type I device in a size of 4 mm×4 mm×5 mm. The conversion efficiency of 532 nm to 266 nm is about 2.8%.

This work focuses on the growth of RbLi(HC₃N₃O₃)·2H₂O by the solution method. A large crystal in a size of 50 mm×35 mm×20 mm is obtained under the optimal conditions of temperature range, cooling speed, and rotation rate. The FWHM value of X-ray rocking curve for (100) plane is 43.2″. The ultraviolet cut-off edge is measured to be 238 nm and the transmittance at visible region is higher than 85%. The laser output of 266 nm is realized using a type I frequency-doubling device. The conversion efficiency is about 2.8% from 532 nm to 266 nm. The output power and the conversion efficiency of 266 nm can be greatly improved by means of quality enhancement of both crystal and device.