Incongruent Melting LaxYySc4-x-y(BO3)4 - LYSB Nonlinear Optical

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Functional Inorganic Materials and Devices x

y

4-x-y

3

4

Incongruent Melting LaYSc (BO) - LYSB Nonlinear Optical Crystal Grown by the Czochralski Method Lucian Gheorghe, Madalin Greculeasa, Alin Broasca, Flavius Voicu, George Stanciu, Konstantin N. Belikov, Ekaterina Yuriivna Bryleva, and Olga Gaiduk ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 22, 2019

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ACS Applied Materials & Interfaces

Incongruent Melting LaxYySc4-x-y(BO3)4 - LYSB Nonlinear Optical Crystal Grown by the Czochralski Method Lucian Gheorghe*,†, Madalin Greculeasa†‡, Alin Broasca†, Flavius Voicu†, George Stanciu†, Konstantin N. Belikov§, Ekaterina Yu. Bryleva§, and Olga Gaiduk§ † National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-36, 077125 Magurele, Romania §

Institute for Single Crystals of NAS of Ukraine, 60 Nauky Ave., Kharkov 61001, Ukraine

‡ Doctoral School of Physics, University of Bucharest, Romania

KEYWORDS: borate crystals, crystal growth, Czochralski method, nonlinear optical materials, SHG, SFG ABSTRACT: Nonlinear optical (NLO) crystals with incongruent melting of LaxYyScz(BO3)4 (x + y + z = 4) (LYSB) - type were grown for the first time, to the best of our knowledge, by the Czochralski method. A special thermal assembly was used and the melt composition, growth direction, and the pulling and rotation rates have been optimized. Good optical quality LYSB crystal with a diameter of about 13 mm and a length of 25 mm has been grown from the La0.765Y0.485Sc2.75(BO3)4 starting melt composition, along the c-axis direction, using a slow rotation rate of 8 - 10 rpm and a high pulling rate of 2 mm/h. The grown crystal has an acentric huntite-type structure (space group R32, Z = 3) with cell dimensions a = 9.8098(4) Å and c = 7.9802(3) Å, and its chemical composition was determined to be La0.78Y0.32Sc2.90(BO3)4. The optical transmission and the refractive indices were determined, and the second harmonic generation (SHG) and sum frequency generation (SFG, ω + 2ω) properties were reported. The laser damage threshold was also determined to be ~ 2.0 GW/cm2 at 1064 nm (6 ns pulses). The main nonlinear properties of Czochralski-grown LYSB crystal were found to be similar to those of flux-grown LYSB, and comparable to YAl3(BO3)4 (YAB) nonlinear properties. The big advantage of Czochralski-grown LYSB crystals is that they can be grown with large size and high quality, making them promising candidates for various NLO applications, including frequency conversion of high-average power radiation sources.

I. Introduction Acentric borate crystals with trigonal BO3 groups parallelly aligned have attracted great interest for the researchers due to their potential for nonlinear optical (NLO) applications. In general, they are characterized by a wide UV transparency range, high NLO coefficients, and moderate birefringence.1 The NLO crystals of binary borates LnM3(BO3)4 (Ln = La, Y and M = Al, Ga, Sc) have as a structural prototype the mineral huntite (CaMg3(CO3)4, trigonal structure, space group R32),2 presenting an advantageous disposition of BO3 anionic groups in their structure.3,4 YAl3(BO3)4 (YAB) is the most representative crystal of LnM3(BO3)4 borates family due to its high nonlinear efficiency and laser damage threshold.3,5-7 The research done so far on YAB crystal has focused mainly on selffrequency doubling (SFD) properties of Nd3+ or Yb3+ doped crystals, and very good results were obtained for this kind of application.8-10 However, the main disadvantage of YAB crystal is that it can be grown only by the flux method, which imposes limitations in terms of size and optical quality of the grown crystals. Another impediment of YAB crystals is the presence of some absorption bands determined by the impurification with different flux components (Fe, Mo …) which limit the NLO applications in the UV wavelength range.4,11,12 To avoid the crystal growth issues, the research has been oriented to Sc derivatives binary borates of LnSc3(BO3)4 - type (Ln =

lanthanide),13-15 which presents polymorphism, crystallizing in different structural phases as a function of the ionic radii ratio of Ln and Sc (rLn/rSc). For example, the LaSc3(BO3)4 - LSB crystal has a monoclinic structure (space group C2/c)15 and therefore the second-order nonlinear susceptibility �(2) is null, while the YSc3(BO3)4 - YSB crystal is isostructural with the mineral huntite (trigonal structure, space group R32) allowing secondorder nonlinear effects.15,16 Nevertheless, by doping with small ionic radii lanthanides, the structure of LSB can be modified into trigonal structure (space group R32) as in the case of fluxgrown crystals La1-xNdxSc3(BO3)4,17 or LaxYyScz(BO3)4, LaxLuyScz(BO3)4, BixLayScz(BO3)4, and LaxGdyScz(BO3)4 with x + y + z = 4.18-24 Even if LSB crystal has an incongruent melting, it has been grown by the Czochralski method for the first time by Ivonina et al.13 in 1991. Later, Durmanov et al.15 have grown LSB crystals doped with diverse rare-earth ions by the Czochralski method, having different structures in the function of the ionic radii ratio of the doping ions (in the lanthanide sites) and the scandium ion. They also concluded that an excess of LaBO3 in the starting melt composition is needed to grow high-quality crystals. Recently, we have succeeded to grow for the first time NLO LaxGdyScz(BO3)4 (x + y + z = 4) - LGSB crystals with incongruent melting by the Czochralski method.25 Based on all these results, the Czochralski growth of LaxYyScz(BO3)4 (x + y + z = 4) - LYSB crystals was successfully approached in this

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work. The novelty of this work consists of the development of the Czochralski growing procedure of incongruent melting LYSB crystals, which offers the possibility to obtain large-size and high-quality crystals, and also the characterization of the grown crystals. In this aim, NLO crystals with incongruent melting of LaxYySc4z(BO3)4 (x + y + z = 4) (LYSB) - type were grown for the first time, to the best of our knowledge, by the Czochralski method. Good optical quality LYSB crystal with a diameter of about 13 mm and a length of 25 mm has been grown along the c-axis direction, and the chemical composition along the growth direction was investigated. The laser damage threshold at 1064 nm (6 ns pulses) was determined. The optical transmission spectrum and the refractive indices were also determined, and type-I and II phase-matching curves for second harmonic generation (SHG) and sum frequency generation (SFG, ω + 2ω) have been calculated on the basis of Sellmeier equations.

II. Experimental section Material Synthesis. Taking into account that in the 6-fold coordination the ionic radius of Y3+ ion is closer to La3+ than Sc3+ ion (rLa=1.032 Å, rY=0.900 Å, and rSc=0.745 Å),20 we can approximate, in a first stage, that Y3+ ions mainly occupy the La3+ sites in the LYSB compounds. Thus, for the determination of the minimum yttrium content that induces the formation of the trigonal phase of LYSB as dominant phase, we have considered the LYSB general formula as being La1-xYxSc3(BO3)4 (0 < x < 1). For this purpose, La1-xYxSc3(BO3)4 compounds with compositional parameter x in the range of 0.0 - 0.6 have been synthesized by the solid-state reaction method. La2O3, Y2O3, and Sc2O3 of 5N purity and B2O3 of 99.98% purity were used as raw materials. To compensate B2O3 losses by evaporation during the sintering, an excess of 5 wt. % B2O3 was added to the stoichiometric quantities. La2O3, Y2O3, and Sc2O3 powders were preheated in air at 1000°C for 12h, and then all the raw materials were immediately weighed, according to their stoichiometric composition, and mixed. The obtained powders were then pressed into a mold, and heated in air at 1300°C for 24h. The same procedure was adopted to synthesize LaxYyScz(BO3)4 (x + y + z = 4) starting materials. X-ray Powder Diffraction. The X-ray powder diffraction (XRPD) patterns were collected at room temperature on a PANalytical Empyrean X-ray diffractometer (Cu Kα = 1.5406 Å) in a conventional Bragg-Bretano geometry. The measurements were recorded in the 2θ range between 10° and 60° using a scan step of 0.01° and a step-counting time of 120 s. The structure of the best quality grown crystal was refined by the Rietveld method using the X’Pert High Score Plus software. In this case, the crystal powder used for the X-ray spectrum acquisition was firstly grinded and sieved through a 200-mesh screen, and then the measurements were recorded in the 2θ range between 10° and 145° using a scan step of 0.01° and a step-counting time of 240 s. The X-ray rocking curve of the (003) reflection plane was also measured using the same X-ray diffractometer. The measurement was made on a crystal slice cut parallel to the (001) plane and polished at laser grade. Thermal properties analysis. Thermogravimetric (TG) analysis and differential thermal analysis (DTA) were performed on an SETARAM Setsys Evolution 18 thermal analysis system. Polycrystalline powders in an amount of 75 100 mg were placed in platinum crucibles and heated to 1600°C and then cooled to room temperature in synthetic air flux (79% N2 and 21% O2 of 5N purity). The heating and cooling rates were 10°C/min.

Crystal Growth. LaxYyScz(BO3)4 (x + y + z = 4) crystals with various x, y, and z compositional parameters have been grown by the Czochralski method using an induction-heated furnace. To obtain good quality LYSB crystals, the optimization of the starting melt composition and the growth parameters is required. A particular heating assembly was used to decrease the B2O3 evaporation and implicitly to avoid condensation of vapors on the growing crystal surface.25 The crystals were grown in a static N2 atmosphere using Ir crucibles with a height and diameter of 30 mm. By using an infrared pyrometer, the growth temperatures were determined to be in the range of 1490 - 1520°C. Good optical quality LYSB crystal has been grown along the c-axis direction using a slow rotation rate of 8 - 10 rpm and a high pulling rate of 2 mm/h. More details related to the growth conditions can be found in our previous work.25 Elemental Analysis. The elemental composition of the grown crystals was determined by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) method using a Trace Scan Advantage spectrometer (Thermo Jarrell Ash, USA). The compositional uniformity along the growth direction was determined by measuring the elemental composition of samples from different zones of the grown crystal. Optical Characterization. A pulsed Nd: YAG laser Q-smart 850 (1064 nm, 10Hz, pulse duration 6 ns) has been used for SHG preliminary tests on the synthesized compounds and the grown crystals. By using the same Nd: YAG laser, the laser damage threshold was also determined. The laser beam was focused with a diameter of 84 µm on the surface of laser grade polished (001) crystal sample, and the laser power was slowly increased until the damage was observed on the sample surface. The room temperature optical transmission spectrum of a (0 0 1)-cut LYSB crystal sample with a thickness of 1.75 mm was measured using a Varian Cary 5000 UV-Vis-NIR (200 2000 nm) spectrophotometer. The refractive indices were determined by the minimum deviation method using a prism with the apex angle of 60°28' and the prism axis parallel to the c-axis of the as-grown crystal. Different gas-discharge lamps (Cd, Na, Hg, Zn) in the visible range and three laser diodes emitting at 808, 877 (Coherent, USA), and 974 nm (Limo, Germany) were used for the measurement of the refractive indices depending on the wavelength.

III. Results and Discussion Figure 1 shows the XRPD patterns of polycrystalline La1compounds synthesized with various xYxSc3(BO3)4 compositional parameter x. We can observe that for a Y content larger than 40 at. % the structural change from monoclinic to trigonal phase (C2/c → R32 space group) is almost complete, as evidenced by the disappearance of several diffraction peaks associated to monoclinic phase (highlighted with stars in Figure 1). According to the Rietveld analysis, for a Y doping concentration larger than 40 at. % (x > 0.4) one dominant trigonal phase (R32) over 93 wt. % is identified, and two residual phases of Sc2O3 and monoclinic phase (C2/c), which are always below 5 wt. % and 2 wt. %, respectively, can be also observed. These results are in good agreement with those published by Ye et al.18 The property of La1-xYxSc3(BO3)4 compounds (with the compositional parameter x > 0.4) to generate 532 nm radiation by SHG process of 1064 nm laser radiation was successfully tested. Because LaxYyScz(BO3)4 (x + y + z = 4) are peritectic compounds, an excess of LaBO3-YBO3 (or Sc(BO3) deficiency, z < 3) in the starting melt composition is needed to directly crystallize the


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ACS Applied Materials & Interfaces trigonal phase (space group R32) of these compounds.15 Based on the results obtained on La1-xYxSc3(BO3)4 sintered compounds, and taking into account that Y3+ ions partially occupy the Sc3+ sites, several nonstoichiometric compositions of LYSB-type, such as La0.8125Y0.4375Sc2.75(BO3)4, La0.8625Y0.4375Sc2.70(BO3)4, La0.7875Y0.4625Sc2.75(BO3)4, La0.765Y0.485Sc2.75(BO3)4, and La0.754Y0.546Sc2.70(BO3)4, were selected as starting compositions for crystal growth by the Czochralski method. TG-DTA analyses were performed on all selected compositions and the obtained results demonstrate their incongruent melting. For example, the TDA curves obtained on the La0.765Y0.485Sc2.75(BO3)4 composition are presented in Figure 2. Two endothermic peaks situated at 1473°C and 1492°C, attributed to the melting of the composition, with a mass loss of about 1.4 % are observed during the heating. Two exothermic peaks at 1466°C and 1357°C can be also observed during the solidification process, and almost the same amount of mass is lost in the high temperature zone (over 1200°C). This thermal behavior indicates the incongruent melting character of La0.765Y0.485Sc2.75(BO3)4 compound.

Figure 1. XRPD patterns at room temperature of La1synthesized compounds. The vertical green, red, and blue sticks are associated with the peaks of C2/c monoclinic phase (PDF card 01-087-1665), R32 trigonal phase (PDF card 04-018-1225), and Sc2O3 cubic phase (PDF card 04001-2439), respectively. xYxSc3(BO3)4

The originality of the thermal assembly lies in the adding of two drilled rings, the first of Pt and the second one of ceramic Al2O3, to decrease the thermal gradients in the zone of the solid-liquid interface. The distances between the top of the crucible and the Pt ring and respectively the Al2O3 ring, are 2 mm and 20 mm, respectively. These distances were found to be optimal for our crucibles with a diameter of 30 mm. By making use of this thermal assembly, several LYSB crystal with various starting melt compositions were grown. Another important factor that influences the quality of the grown crystals, and also the growth morphology, is the crystal growth direction. Good quality LYSB crystals with regular hexagonal shape have been grown by using undoped LGSB seeds oriented in the ⟨0 0 1⟩ direction (parallel to the crystallophysic Z-axis or crystallographic c-axis), similar to LGSB crystal.25 Four of the LYSB grown crystals are presented in Figure 4. Good results with regard to the crystal transparency have been obtained for the crystal grown from the La0.7875Y0.4625Sc2.75(BO3)4 starting melt composition (Figure 4c) with a pulling rate of 1.8 mm/h at a rotation rate of 10 rpm. Nevertheless, the preliminary tests regarding the SHG of 1064 nm laser radiation have resulted in obtaining of a diffuse green (532 nm) radiation, revealing the presence of some defects like twins acting as a grating, similar to flux-grown LYSB and YAB crystals.27 By using La0.765Y0.485Sc2.75(BO3)4 starting melt composition, intermediate to the compositions from which were grown the crystals presented in Figures 4c and 4d, the best quality LYSB crystal in terms of transparency, defects, and preliminary tests related to the SHG of 1064 nm laser radiation (the green light is observed only in the phase-matching direction), was grown using a slow rotation rate of 8 - 10 rpm and a high pulling rate of 2 mm/h (Figure 5). All subsequent studies in this work were done on this crystal. The crystalline quality of the grown crystal was evaluated by Xray rocking curve. The peak corresponding to the (003) reflection plane is of good shape and has a full-width at halfmaximum (FWHM) of 0.013° (Figure 6), which indicates that the crystal is of good crystalline quality. The laser damage threshold was also measured and a value of ∼2 GW/cm2 was determined, which is about four times larger than that of the YAB crystal (i.e., 0.4 - 0.6 GW/cm2).7

Figure 3. Heating assembly designed and used for the Czochralski growth of LYSB crystals.

Figure 2. DTA curves obtained on La0.765Y0.485Sc2.75(BO3)4 starting composition. For the growth of LYSB crystals by the Czochralski method, a compromise between the thermal gradients and evaporations must be found to avoid the constitutional supercooling phenomenon.15 In this order, based on our experience on LGSBtype crystals,25 a special thermal assembly (Figure 3) was used.



Figure 4. LYSB crystals grown from the starting melt compositions (a) La0.8125Y0.4375Sc2.75(BO3)4, (b) La0.8625Y0.4375Sc2.70(BO3)4, (c) La0.7875Y0.4625Sc2.75(BO3)4 (c), and (d) La0.754Y0.546Sc2.70(BO3)4. Figure 7 shows room temperature XRPD pattern of c-axis grown LYSB crystal along with the PDF card 04-018-1225 of the trigonal phase of flux-grown LGSB (space group R32, Z = 3, a = 9.7933(7) Å, c = 7.9540(12) Å).21 The analysis of the X-ray spectrum revealed the presence of single trigonal phase (space group R32). The unit cell parameters were determined to be a = 9.8098(4) Å and c = 7.9802(3) Å. The unit cell volume was calculated to be V = 665.07 Å3, which is slightly higher than that of flux-grown LYSB crystal (657.4 Å3), and very close to the unit cell volume of the Y0.32La0.76Sc2.92(BO3)4 synthesized compound (666.6 Å3) investigated by Ye et al.18

Figure 5. LYSB crystal grown along the c-axis from the starting melt composition La0.765Y0.485Sc2.75(BO3)4.

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Figure 7. X-ray powder diffraction pattern of LYSB crystal at room temperature. Red vertical sticks are associated with the peaks of R32 trigonal phase (PDF card 04-018-1225). The elemental composition of the LYSB grown crystal was measured by ICP-AES method, and the compositional uniformity along the growth direction was determined by measuring the elemental composition of samples from different zones (shoulder, body, and tail) of the grown crystal. The obtained results are presented in Table 1. The stoichiometry was calculated considering 12 O atoms and 4 B atoms in each formula unit, accordingly to the composition of huntite-type compounds. As it can be observed, the grown crystal has a homogenous composition along its length, and from the values of the La/Y ratio, it can be concluded that the segregation coefficient of Y in the LSB compound is lower than unity. The chemical composition was determined to be La0.78Y0.32Sc2.90(BO3)4 with a measurement error of ± 0.2%. The obtained results demonstrate that Y3+ ions occupy mainly the sites of La3+ ions, and to a lesser extent, the Sc3+ sites (the Sc stoichiometry is close to 3). The Rietveld refinement of the XRPD data obtained on LYSB crystal at room temperature was done to investigate the atomic positions and site occupancies. The occupancy factors for the La and Sc sites were fixed to the values determined by ICP elemental analysis. Figure 8 shows the experimental, calculated, and difference profiles after Rietveld refinement together with the reliability factors for the refined diffraction pattern. The difference in peak intensity for the same crystallographic index between the experimental and calculated patterns is believed to be caused by the preferred orientation of the powder sample. The atomic coordinates and site occupancy fraction are listed in Table 2.

Table 1. The stoichiometry of the LYSB crystal based on ICP-AES elemental analysis.

Figure 6. Rooking curve of the (003) reflection plane acquired on LYSB crystal.

Composition of the LYSB crystal

ratio of La/Y

Starting melt

La0.765Y0.485Sc2.75(BO3)4

1.57

Shoulder

La0.780Y0.321Sc2.899(BO3)4

2.42

Body

La0.783Y0.316Sc2.901(BO3)4

2.47

Tail

La0.779Y0.324Sc2.897(BO3)4

2.40


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O B1,B2 Sc/Y2 La/Y1

Figure 9. Crystal structure of the LYSB crystal.

Figure 8. Rietveld refinement of the XRPD pattern of the LYSB crystal at room temperature.

Table 2. Atomic positions and site occupancy fraction for the LYSB crystal.

Atom

x

y

z

Wyckoff position

Occupancy

La

0

0

0

3a

0.78

Y1(La)

0

0

0

3a

0.22

Sc

0.1235

0.6666

0.6666

9d

0.9667

Y2(Sc)

0.1235

0.6666

0.6666

9d

0.0333

B1

0.2317

0.6666

0.1666

9e

1

B2

0

0

0.5

3b

1

O1

0

0.4177

0.5

9e

1

O2

0.0263

0.1894

0.2009

18e

1

O3

0

0.1387

0.5

9e

1

The LYSB crystal has a classical huntite-type structure with BO3 groups arranged in almost planar layers as illustrated in Figure 9. The La3+, Y3+, and Sc3+ cations occupy 6-coordinate sites between these layers. La3+ and Y13+(La) cations are situated in the center of isolated (one from the other) distorted trigonal prisms, while Sc3+ and Y23+(Sc) cations are located in distorted octahedrons with shared edges. These two types of polyhedra share only one O2- anion.

Figure 10 shows the optical transmission spectrum in the wavelength range of 200 - 2000 nm measured on a (0 0 1)-cut LYSB crystal sample with a thickness of 1.75 mm. As it can be observed, the LYSB crystal is characterized by a large transparency window, having a cut-off wavelength slightly lower than 200 nm (below to the lower limit of our spectrophotometer), making it a promising candidate for different NLO applications in the UV spectral range. The UV cutoff wavelength is similar to the values reported previously for flux-grown LYSB and LGSB crystals.18,21 Compared to our previous results on Czochralski-grown LGSB crystal25, LYSB has a lower UV cut-off wavelength (i.e., 230 nm) making it more suitable for UV applications.

Figure 10. Optical transmission spectrum of the LYSB crystal (thickness of 1.75 mm). The inset shows the transmission in the UV-Vis wavelength range. The refractive indices were determined by the minimum deviation method using a prism with the incident and emergent faces polished at laser grade (Figure 11).

Figure 11. Oriented LYSB prism cut from the grown crystal. The measured values of the refractive indices of the LYSB crystal are reported in Table 3, where no and ne are the refractive indices for ordinary and extraordinary polarizations, respectively, and λ is the wavelength expressed in micrometers. According to the obtained results, the LYSB crystal is a negative uniaxial crystal (no > ne).



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Table 3 Refractive indices of the LYSB grown crystal.

Waveleng th (µm)

no

ne

measurem ent

calculati on

measurem ent

calculati on

0.40466

1.8740

1.8739

1.7847

1.7846

0.43584

1.8650

1.8649

1.7773

1.7772

0.46782

1.8578

1.8578

1.7714

1.7713

0.46801

1.8578

1.8577

1.7714

1.7713

0.47222

1.8570

1.8569

1.7707

1.7706

0.47999

1.8555

1.8554

1.7695

1.7694

0.48105

1.8553

1.8552

1.7694

1.7692

0.50858

1.8508

1.8507

1.7656

1.7655

0.54607

1.8457

1.8456

1.7614

1.7613

0.57906

1.8420

1.8420

1.7584

1.7584

0.5893

1.8411

1.8410

1.7576

1.7575

0.63623

1.8371

1.8370

1.7544

1.7543

0.64385

1.8365

1.8365

1.7539

1.7539

0.8082

1.8278

1.8277

1.7472

1.7471

0.8773

1.8253

1.8253

1.7455

1.7454

0.9745

1.8226

1.8226

1.7437

1.7436

The measurements have been made with an accuracy of 0.0001 and the calculations were based on Sellmeier equations.

Figure 12. Dispersion curves of the refractive indices of the Czochralski-grown LYSB crystal.

Figure 13. The phase-matching curves for type-I and II SHG in the Czochralski-grown LYSB crystal determined on the basis of Sellmeier equations.

The experimentally determined values were fitted with Sellmeier equations in eqs. 1 and 2, and the refractive indices dispersion curves are shown in Figure 12. 0.03004

(1)

0.02536

(2)

𝑛2𝑜(𝜆) = 3.30146 + 𝜆2 ― 0.02237 ―0.0127𝜆2 𝑛2𝑒 (𝜆) = 3.01189 + 𝜆2 ― 0.01722 +0.0014𝜆2

Czochralski-grown LYSB crystal has a wider birefringence in comparison with the YAB crystal (Δn = 0.078 at 1064 nm compared to 0.070 for YAB),27 but the cut-off wavelength for type-I SHG in the LYSB (Figure 13) is higher for than that of YAB crystal (579 nm compared to 490 nm for YAB).27 Consequently, laser emission at 266 nm (the fourth harmonic of 1064 nm) cannot be reached by SHG of the 532 nm radiation in the LYSB crystal. However, radiation at 355 nm can be achieved in the LYSB crystal by type-II SFG (eoe) of 1064 and 532 nm at the phase-matching angle θ = 65.2°, as it can be observed from Figure 14. The phase-matching angle for type-I SHG of 1064 nm fundamental wavelength was found to be θ = 33.4° for Czochralski-grown LYSB crystal, which is in very good agreement to the value of θ = 33.5° obtained for flux-grown LYSB crystal.19

Figure 14. The phase-matching curves for type-I and II SFG (ω + 2ω) in the Czochralski-grown LYSB crystal determined on the basis of Sellmeier equations. Several NLO parameters, such as walk-off angle (ρ), angular acceptance (Δθ×L), and spectral acceptance (Δλ×L), can be derived from the refractive indices measurements according to the following relationships:28-30 1

{

𝑡𝑎𝑛 𝜌 = 2[𝑛𝑜(𝜔)]2


1 [𝑛𝑒(2𝜔)]2

―

1

}sin 2θ

[𝑛𝑜(2𝜔)]2

(3)

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[

1+

𝛥𝜃 × 𝐿 = 0.443𝜆

𝛥𝜆 × 𝐿 = 0.443

tan (𝜃)|1 ―

𝑑𝑛𝑜

(

𝑑𝜆 (𝜆𝐹)

]

𝑛2 𝑜(2𝜔) tan 2(𝜃) 𝑛2 𝑒 (2𝜔)

( ) |𝑛 (2𝜔,𝜃)

―

𝑛𝑜(2𝜔) 2 𝑛𝑒(2𝜔)

𝑒

(4)

―1 1𝑑𝑛𝑒 2 𝑑λ (𝜆𝑆𝐻)

)

(5)

where no(ω), no(2ω), ne(2ω) are the ordinary (o) and extraordinary (e) refractive indices for fundamental and second harmonic frequencies, θ is the phase matching angle, L is the length of the crystal, and λF and λSH are the fundamental and second-harmonic wavelengths. Using the calculation method described by Ye et al.,19 the magnitude of the d11 coefficient for type-I SHG of the fundamental wavelength of 1064 nm was also estimated. The obtained results are compiled in Table 4 and compared to those of flux-grown LYSB and YAB crystals. As can be seen, the nonlinear properties of Czochralski-grown LYSB are similar to those of flux-grown LYSB and comparable to those of YAB crystal.

Table 4. Comparison of the NLO properties for type-I SHG at 1064 nm between flux-grown LYSB, Czochralskigrown LYSB, and flux-grown YAB crystals.

chemical composition was determined to be La0.78Y0.32Sc2.90(BO3)4. The optical transmission window and the refractive indices have been measured. The phasematching curves for type-I and II SHG and SFG (ω + 2ω) were determined on the basis of the Sellmeier equations. The main NLO properties of Czochralski-grown LYSB crystal were found to be similar to those of flux-grown LYSB, and comparable to YAB nonlinear properties. The big advantage of Czochralskigrown LYSB crystals is that they can be grown with large size and high quality, making them promising candidates for various NLO applications in the visible and UV wavelength ranges. Although laser tests were not performed in this work, based on the high value of the laser damage threshold, we believe that LYSB crystals could be also promising for frequency conversion of high-average power laser beams. Further experiments are currently in progress to determine the full potential of Czochralski-grown LYSB crystals, including the functioning as self-frequency doubling material.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID

Lucian Gheorghe: 0000-0001-7904-5540 Crystal

Fluxgrown LYSB19

Czochralskigrown LYSB [this work]

YAB

Phase matching angles (θ, φ) (deg.)

(33.5, 60)

(33.4, 60)

(30.8, 60)

Birefringence Δn

0.078

0.078

0.07031

2.5

2.41

2.2232

Walk-off (deg.)

angle

ρ

Angular acceptance Δθ × L (deg.·cm)

0.034

0.034

0.0432

Spectral acceptance Δλ×L (nm·cm)

-

0.60

-

Nonlinear coefficient d11 (pm/V)

1.35

1.35

1.6932

Conclusions. Nonlinear optical crystals with incongruent melting of LaxYyScz(BO3)4 (x + y + z = 4) (LYSB) - type were grown for the first time, to the best of our knowledge, by the Czochralski method. A special thermal assembly was used and the melt composition, growth direction, and the pulling and rotation rates have been optimized. Good quality LYSB crystal in terms of transparency, defects, and preliminary tests related to the SHG of 1064 nm laser radiation, was grown with a diameter of about 13 mm and a length of 25 mm from the La0.765Y0.485Sc2.75(BO3)4 starting melt composition, along the caxis direction, using a slow rotation rate of 8 - 10 rpm and a high pulling rate of 2 mm/h. The grown crystal has an acentric huntite-type structure (space group R32, Z = 3) with cell dimensions a = 9.8098(4) Å and c = 7.9802(3) Å, and its

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Romanian Ministry of Research and Innovation, CNCS-UEFISCDI, project number PNIII-P4-ID-PCE-2016-0853, under grant agreement no. 119/2017, and Romanian Ministry of Research and Innovation, Program NUCLEU-LAPLAS VI, under grant agreement no. 16N/2019.

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Incongruent Melting LaxYySc4-x-y(BO3)4 - LYSB Nonlinear Optical

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