Flux Growth and Properties of Volatile Bromine-Containing UV Nonlinear Optical Crystal K₃B₆O₁₀Br

Abstract

A UV Nonlinear optical (NLO) crystal is one of the key devices in all-solid-state laser technology, and borate halides show outstanding potential due to their abundant structural diversity and short UV cut-off edges. In this article, the sizable UV NLO crystal of K₃B₆O₁₀Br (KBOB) has been grown with lead-containing and lead-free fluxes systems using the high-temperature top-seeded solution growth (TSSG) method. Energy Dispersive X-ray Spectroscopy (EDS) and transmittance spectra illustrate the influence of Pb²⁺ ions on the transmittance properties and laser-induced damage threshold (LDT). The thermal property, namely, thermal expansion, thermal conductivity, and thermal diffusivity curves, were characterized. Moreover, a small variation of thermal refractive indexes was analyzed to illustrate the advantage of KBOB in the application for temperature-fluctuated specific regions.

1. Introduction

Due to the good monochromaticity, strong directivity, and high brightness, lasers have been used extensively in the fields of industry, communication, and information technology, through applications such as laser welding, laser etching, and free special optical. Benefiting from the frequency conversion by NLO crystals, a UV laser can be expediently and efficiently obtained [1,2,3,4]. Nowadays, some well-known borates, such as β-BaB₂O₄ (β-BBO) [5], (LiB₃O₅) (LBO) [6], CsLiB₆O₁₀ (CLBO) [7] have been commercially applied. While these classic crystals cannot meet practical needs, the investigation of NLO crystals with a UV absorption deeper than 400 nm, a large second harmonic generation (SHG) response, and a moderate birefringence is of urgent demand.

Notably, introducing halide elements into borate can regulate the structure configuration and increase the structure diversity, and this strategy leads to the discovery of plenty of excellent UV NLO materials [8,9,10,11,12]. In 2006, KBOB was firstly synthesized by Belokoneva et al. via the hydrothermal method [13], which crystalized in the R3m space group. Then, centimeter-sized KBOB crystals were grown by our group via the high-temperature TSSG method using lead-containing fluxes to reduce the crystallizing temperature and hinder the volatilization of bromine [14,15,16]. The property characterizations of the crystal indicates that KBOB crystals have a short UV absorption edge (about 182 nm), large SHG response (d₂₂ = 0.83 pm/V), moderate birefringence (0.0506@355 nm), and short type I phase-matching SHG wavelength (290 nm), which was regarded as a potential UV NLO crystal. Due to the thriving performance, R.K. Li et al. [17,18,19] continued the growth of KBOB and estimated the optical properties. The autonomous studies by these groups verified the promising application of KBOB crystals for laser frequency conversion in the UV region, such as 355 nm laser outputting by frequency tripling of Nd: YAG. However, in the previous sizable growth of KBOB, the lead-containing fluxes were adopted to decrease the viscosity and crystallization temperature of the solution. At the same time, the introduction of Pb²⁺ cations causes UV absorption below 280 nm. Although decreasing the content of the PbO in the flux can reduce absorption, the issue remains unsolved [17]. Hence, we propose the exploration of lead-free fluxes to completely remove the absorption and propose the dominant application fields of KBOB crystal.

In this article, we present research regarding growing the KBOB crystal to analyze the difference of optical absorption between lead-containing and lead-free fluxes systems. Additionally, the thermal expansion, thermal conductivity, thermal diffusivity, and thermal refractive index of KBOB were compared with the benchmark UV NLO crystal LBO. The results indicate that KBOB is preponderant in the circumstance with temperature fluctuation due to the weak temperature dependence of physical properties of KBOB.

2. Materials and Methods

KBOB has been grown by the top-seeded solution growth (TSSG) method as KBOB melts incongruently. All of the regents are analytically pure and were used as received. The method to grow KBOB with lead-containing fluxes (KBOB-L) can be found in a previously published article. For the lead-free fluxes (KBOB-LF), H₃BO₃, KBr, K₂CO_3, and KF were mixed in the proper molar ratio and pre-calcined, then the mixture was transferred to the center of a vertical, programmable temperature furnace with a platinum plate to restrain the volatilization and heated to at 740 °C for 24 h. Then, the temperature was rapidly cooled to 720 °C, and a seed crystal was dipped into the solution. After this operation, the rotation speed of the seed crystal was set to 20 rpm, and the cooling rate was set as 0.1 °C per day. Finally, a crystal with the size of 25 mm × 10 mm × 30 mm was obtained, and the experiment was completed by removing the crystal from the solution.

The powder X-ray diffraction (PXRD) was recorded by Bruker’s D2 PHASER XRD diffractometer (Germany) at room temperature. The sample was ground and placed in the instrument. Comparing the results with the powder diffraction file (PDF) cards in Jade helped to identify the composition of the powder.

The energy dispersive X-ray spectrum of the crystals was recorded by OXFORD INSTRUMENTS X-MaxN’s (Britain) microscopic analysis instrument consisting of SEM (Scanning Electron Microscope) and EDS. A piece of crystal was taken from the bulk crystal and was placed in the equipment to be tested.

The transmittance spectrum was recorded by Shimadzu SolidSpec-3700 DUV UV-VIS-IR spectrometer (Japan) at room temperature, and the measurement range extends from 180 nm to 1600 nm. Polished crystal plates of KBOB-L and KBOB-LF were used in the characterization.

The thermal expansion coefficients were measured with a thermal-mechanical analyzer (Netzsch DIL402PC, Germany). Two crystal plates, oriented in the direction of [100] and [001], respectively, were used to study the thermal expansion coefficients of KBOB along the different crystallographic axis. The measuring directions of the crystal plates were polished to 4.359 mm and 5.017 mm, respectively. The thermal conductivity and thermal diffusion coefficient were tested by laser thermal conductivity analyzer (Netzsch LFA 457, Netzsch, Selb, Germany).

The thermal refractive index of KBOB was measured using the prism coupling technique (Metricon Model 2010, Metricon, Pennington, NJ, USA), and a [001] crystal plate was used for the test at temperatures of 25, 40, 60, 80, and 100 °C, respectively.

The LDT was measured on a pulsed Q-switched Nd: YAG laser (Continuum, 1064 nm, 5 ns, 10 Hz) using KBOB-L and KBOB-LF crystal plates with 1 mm thickness. An optical concave lens was used to obtain the appropriate laser beam diameter (0.5 mm).

The rocking curves were recorded by a high-resolution X-Ray Diffractometer (D2 phaser, Germany), using polished KBOB-L and KBOB-LF crystal plates with the size of 5 mm × 5 mm × 1 mm.

3. Results and Discussion

3.1. The Crystal Growth of KBOB with Lead-Containing and Lead-Free Fluxes

The following reagents were purchased and were not further purified: H₃BO₃ (Aladdin, Shanghai, China, 99.99%), KBr (Aladdin, Shanghai, China, 99.99%), KF (Tianjin Hengxing, Tianjin, China, ≥99.0%), PbO (Aladdin, Shanghai, China, 99.9%), K₂CO₃ (Tianjin Hengxing, Tianjin, China, ≥99.0%)_, KF·2H₂O (Tianjin Hengxing, Tianjin, China, ≥99.0%).

For the KBOB-L (the ratio of the raw materials was H₃BO₃:KBr:KF:PbO = 6:1:2:1), the orientation of the seed crystal was selected as described in [101], and a high-quality, transparent crystal (30 mm × 30 mm × 10 mm) with the developing faces of [101], [211], and [110] was obtained (Figure 1a). The volatile was collected and characterized, and the result indicates that the volatile mixture consists of PbBr₂, PbBrF, and KPb₂Br₅. At the same time, the prior fluxes used were, the self-fluxes for the TSSG method to avoid introducing the impurities. Our group explored the fluxes in the system of [H₃BO₃-KBr-KF], [H₃BO₃-KBr-K₂CO₃-KF], [H₃BO₃-KBr-K₂CO₃-KF·2H₂O], and [B₂O₃-KBr-K₂CO₃-KF], respectively. Speculated from the experiment results, [H₃BO₃-KBr-K₂CO₃-KF] was the best growing system, as the main phase of the crystallization product was KBOB. Finally, the ratio of H₃BO₃:KBr:K₂CO₃:KF was selected as 6:0.6:0.5:1, Additionally, a block KBOB crystal with the size of 25 mm × 10 mm × 30 mm was obtained (Figure 1b). Apparently, the inclusion was found in the crystal, which is due to the heavy volatilization, (Mainly potassium bromide) leading to deviated components at the high growth temperature. The transmittance spectra indicate that the KBOB-LF performs better than KBOB-L in the UV region (Figure 1c) due to the absence of lead compounds. Specifically, the removal of lead can result in a 70 nm blueshift, which is effective for application in shortwave regions, for example, 266 nm laser frequency doubling via sum-frequency. Hence, the optimized growth technique, such as the closed high-temperature solution method, needs to be developed to prevent volatilization in future studies. We recorded the PXRD of KBOB-L and KBOB-LF, and both of were in accordance with those that were calculated (Figure 2a). The Raman bands (Figure 2b) around 1011–1159 cm⁻¹ can be attributed to the BO stretching vibrations of both the trigonal and tetrahedral boron, and the bands located from 600 cm⁻¹ to 800 cm⁻¹ are attributed to the bending modes of trigonal and tetrahedral boron, which means both KBOB-L and KBOB-LF consisted of the ideal KBOB structure.

3.2. The EDS Analysis of KBOB Crystals

In order to identify whether the fluxes, especially for the non-self fluxes, intrude into the crystal lattice as doped ions, EDS tests were performed to investigate the quantity and distribution of the elements (Figure 3). The abundance of the elements is uniformly distributed, which means the crystals are of high quality. When PbO flux was used, the lead ion was found to be distributed homogeneously in the transparent zone of KBOB-L, which means that the impurity ions were relatively evenly distributed in the crystal lattices. Furthermore, the distribution of lead and fluorine ions was localized in the inclusion zone (Figure 4). Although the concentrations of lead ions are very low, they significantly influence the optical performance, such as the transmittance in the UV region. For KBOB-LF, the main elements are K, B, O, and Br (Figure 5), and no other impurity elements were found. This means that the objective of removing the absorption in the UV region caused by the Pb element was achieved.

3.3. Rocking Curves and Laser Damage Threshold

Both of the rocking curves of KBOB-LF and KBOB-L have ideal symmetric shapes, and the full width at half maximum (FWHM) is relatively small, at 30.706″ and 28.044″, respectively, which means both of these two crystals are of high quality (Figure 6).

Additionally, the LDTs of KBOB-LF and KBOB-L are 2.5 and 1.4 GW·cm⁻², respectively, which means that the lead impurity can cause absorption and significantly affect the laser’s damage resistance.

3.4. The Thermal Properties

The thermal expansion coefficients (α) of KBOB in different directions are α_[100] = 8.84 × 10⁻⁶/K, and α_[001] = 13.86 × 10⁻⁶/K, respectively (Figure 7). The ratio of the thermal expansion coefficients between the c direction and a direction is 1.6, which is relatively smaller than LBO (α_[100] = 66.4 × 10⁻⁶/K, α_[001] = −52.8 × 10⁻⁶/K, and α_[110] = 27.3 × 10⁻⁶/K, respectively [20]). The thermal conductivity and thermal diffusivity were also considered for the applied perspective of the crystals, and these properties are shown in Figure 7b. Both of them decrease with increasing temperature, and the values of different crystallographic axis were kept approximate, which is beneficial for the uniform distribution of heat in different directions. The small thermal anisotropy of KBOB is beneficial for avoiding cracks in the process of crystal growth and application in laser devices.

3.5. Thermal Refractive Index

As the heating effect of the laser system, the thermal refractive index is very important for the application. As shown in

Table 1. Thermal refractive index at different temperatures.

λ (μm)/T (°C)		25	40	60	80	100
0.405	no	1.60344	1.60301	1.60259	1.60220	1.60179
	ne	1.55386	1.55394	1.55357	1.55317	1.55303
	no − ne	0.04958	0.04907	0.04902	0.04903	0.04876
0.514	no	1.59120	1.59098	1.59071	1.59030	1.59014
	ne	1.54350	1.54340	1.54299	1.54282	1.54260
	no − ne	0.04770	0.04758	0.04772	0.04748	0.04754
0.636	no	1.58358	1.58340	1.58320	1.58280	1.58251
	ne	1.53652	1.53630	1.53624	1.53602	1.53599
	no − ne	0.04706	0.04710	0.04696	0.04678	0.04652
0.965	no	1.57488	1.57456	1.57444	1.57412	1.57390
	ne	1.52895	1.52934	1.52886	1.52868	1.52856
	no − ne	0.04593	0.04522	0.04558	0.04544	0.04534
1.547	no	1.56611	1.56542	1.56531	1.56517	1.56490
	ne	1.52193	1.52182	1.52179	1.52170	1.52135
	no − ne	0.04418	0.04360	0.04352	0.04347	0.04355

, the refractive index waned slightly with the increased temperature, as did the birefringence. Then, the Sellmeier equation was fitted to map the curves of the type I phase matching angle (frequency-doubling of Nd: YAG laser) versus temperature (Figure 8). It is clear that the phase-matching angles of KBOB changed slightly (about 0.5°) when the temperature ranged from 25 °C to 100 °C. This suggests that the NLO property of KBOB is less affected by temperature, and it is not necessary to use the temperature control system during the frequency doubling applications, which is of benefit to the miniaturization of the equipment system and for applications in temperature-fluctuated circumstances.

4. Conclusions

In conclusion, the UV NLO crystal, KBOB, has been grown by the TSSG method with lead-containing and lead-free fluxes, respectively. The lead impurities in the crystal lattice cause UV absorption below 280 nm, and the problem was solved by exploring lead-free fluxes. The UV absorption edge of KBOB-LF has a blueshift of 70 nm above KBOB-L. The small differences in the thermal expansion, thermal conductivity, and thermal diffusivity between different crystallographic axes endow KBOB with excellent mechanical properties for wafer fabrication and application. Moreover, the KBOB crystals’ refractive indexes show weak temperature dependence, which indicates that KBOB crystals can avoid phase mismatching with temperature severely changing. Hence, KBOB is a potential material for the application in UV nm laser frequency doubling. Notably, upgraded equipment needs to be developed to restrain volatilization during the growth of KBOB-LF.