Crystal Growth and Characterization of a New Quaternary

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Crystal Growth and Characterization of a New Quaternary Chalcogenide Nonlinear Crystal for the Mid-Infrared: PbGa2GeSe6 Valeriy V. Badikov,† Dmitrii V. Badikov,† Li Wang,‡ Galina S. Shevyrdyaeva,† Vladimir L. Panyutin,‡ Anna A. Fintisova,† Svetlana G. Sheina,† and Valentin Petrov*,‡ †

High Technologies Laboratory, Kuban State University, Stavropolskaya Str. 149, 350040 Krasnodar, Russia Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, Max-Born-Strasse 2a, 12489 Berlin, Germany

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‡

ABSTRACT: Non-centrosymmetric crystals of PbGa2GeSe6 are grown in large sizes with good optical quality for the first time. Using them, the linear (transmission, dispersion, and birefringence) and nonlinear (second order susceptibility) properties are characterized.

T

birefringence, and the components of the second order nonlinearity tensor. These properties have been analyzed on the basis of Kurtz−Perry powder tests or theoretical models (electronic structure calculation), both of which provide only a rough estimate. We have been systematically trying to grow large and high optical quality crystals in order to characterize these essential properties and open the way for real applications. The development of high optical quality BaGa4S7 (BGS) and BaGa4Se7 (BGSe) crystals as alternatives to AGS and AGSe was the first such example.3−5 BGS is orthorhombic (space group Pmn21, point group mm2), while BGSe is monoclinic (space group Pc, point group m), and thus both of them are biaxial. The nonlinear effect is due to the distorted GaS(e)4 tetrahedra, just as in the chalcopyrites AGS(e), with slightly stronger distortion in the selenide compounds. In contrast to the chalcopyrites, these two new crystals are, however, chemically stable and show much larger bandgaps and higher optical damage thresholds at similar transmission windows in the mid-IR.5 Subsequently, a new class of quaternary Ba chalcogenides BaGa2AB6 (A = Si, Ge; B = S, Se) has been identified,6,7 and the growth of the two crystals BaGa2GeS6 (BGGS) and BaGa2GeSe6 (BGGSe) was developed.8 Adding Ge was inspired by older work on the quaternary AgGaS(e)2−GeS(e)2 systems9 where this was found to increase the bandgap (damage threshold) and reduce the melting temperature compared to the classical chalcopyrites AGS(e). In addition, quaternary compounds offer more flexibility in engineering their properties. These two quaternary Ba-compounds belong to the trigonal space group R3, point group 3, and as uniaxial

he great progress in the development of all-solid state (diode-pumped) laser sources emitting between 1 and 2 μm in the near-IR greatly stimulated the search for new highly efficient nonoxide nonlinear crystals for their frequency downconversion to the mid-IR part of the spectrum in all possible time formats.1 The widely spread and commercially available nonlinear crystals that can be applied are in fact only three: the I−III−VI2 chalcopyrites AgGaS2 (AGS) and AgGaSe2 (AGSe) which represent the benchmarks for pumping near 1 μm (Ndor Yb-laser systems) and near 1.5 μm (Er-laser systems), and the II−IV−V2 chalcopyrite ZnGeP2 (ZGP), which is the benchmark for pumping near 2 μm (Tm- and Ho-laser systems). In addition, since the other two compounds exhibit multiphonon absorption at this wavelength, AGSe is the primary choice for second harmonic generation (SHG) of 10.6 μm CO2 lasers. Several problems restrict, however, the practical application of the AGS(e) compounds including the chemical instability of the polished surface in air and the low optical damage thresholds, in particular, for down-conversion. ZGP is free of these limitations and exhibits high thermal conductivity but as a phosphide, multiphonon absorption sets in from as early as ∼8.5 μm, which restricts its application at longer wavelengths. For all the three chalcopyrites, annealing at high temperature for long periods is required to obtain optically uniform samples. In the past decade, an impressive amount of nonchalcopyrite structure, non-oxide crystals were explored for their nonlinear optical properties, and such new structures were discovered by a few research groups in China.2 The focus has been on alkali or alkaline-earth metal compounds as they tend to exhibit larger bandgaps, to ensure high optical damage threshold and avoid detrimental two-photon absorption (TPA) effects when pumping with pulsed laser sources. However, in most cases, if crystals were grown at all, their size has been insufficient to evaluate the most important properties for real applications in nonlinear optics including the dispersion, the © XXXX American Chemical Society

Received: January 24, 2019 Revised: April 25, 2019 Published: July 2, 2019 A

DOI: 10.1021/acs.cgd.9b00118 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

PGGSe was selected on the basis of the physicochemical properties of the elemental and binary compounds, preliminary results of the thermal analysis, and data from the literature. The binary compounds were melted, and the melt was completely homogenized for 3 h before starting with the cooling. The synthesis of PGGSe took place in a graphitized quartz ampule filled with PbSe, Ga2Se3, and GeSe2 in a 1:1:1 molar ratio. The ampule was sealed off under vacuum (10−6 mbar) conditions by means of a gas burner and inserted into a horizontal oven. After heating it to 770 °C within 6 h, the melt was maintained at this temperature for additional 24 h, mixing it until complete homogenization. Finally, the heated ampule was transferred into the oven for vertical Bridgman−Stockbarger crystal growth. The melting temperature of PGGSe amounts to 720 °C, lower compared to BGGSe (877 °C).8 The growth was carried out at a speed of 6 mm/day in sizes of 12.5 to 24 mm (diameter) and 50 to 100 mm (length). PGGSe exhibits congruent melting character. No post growth treatment (annealing) was necessary for this compound. Figure 1 shows a photograph of an as-grown

crystals are simpler, presenting another alternative to AGS and AGSe, again with larger bandgaps and damage thresholds. Their structure is built up from corner-sharing (Ga/Ge)Se4 tetrahedra with 12-coordinated Ba2+ cations residing in the cavities. The large SHG response observed was attributed to the polar arrangement with one (Ga/Ge)−S(e)2 bond aligned roughly along the c direction for each (Ga/Ge)S(e) 4 tetrahedron.6 Another practical advantage of the BGGS(e) crystals is that no postgrowth treatment is necessary. Few additional isostructural crystals have also been reported, including BaGa2SiS(e)66 and BaGa2SnS(e)6,8,10 but only tiny samples were obtained by spontaneous nucleation, and the growth technology was not further developed, e.g., because of the instability in air established in ref 8 for the Si-containing compounds. More recently, substituting Ba2+ by the nonlinear-active ions Sn2+ and Pb2+, second-order Jahn−Teller (SOJT) distorted cations, analogues of the above compounds, have been synthesized in the hope to increase the second-order nonlinear response.11−15 The four ternary compounds with general formula IV−III4−VI7, i.e., AGa4S(e)7 (A = Sn, Pb), exhibit the same monoclinic symmetry as BGSe, while the four quaternary compounds with general formula IV−III2−IV−VI6, i.e., AGa2GeS(e)6 (A = Sn, Pb), belong to the orthorhombic Fdd2 space group, point group mm2. In the latter, two kinds of microscopic nonlinear-active units, namely, tetrahedral (Ga/ Ge)−S(e)2 units and polyhedral units centered by cations with a lone electron pair (Sn2+ or Pb2+) are combined. A chemically related compound, PbGa2SiSe6, was also reported in ref 15, but it has a different symmetry (space group Cc, point group m) due to the different ionic size of Si and Ge, and its growth was unsuccessful. The present work is devoted to the optical characterization of the orthorhombic PbGa2GeSe6 (PGGSe), first reported in ref 15, which contains three non-centrosymmetric chromophores, [PbSe4], [GaSe4], and [Ga/GeSe4], with covalent interactions between the X and Se atoms (X = Pb, Ga, Ga/ Ge). Unfortunately, only tiny crystals of PGGSe (∼1 mm) could be grown in ref 15 (see Supporting Information). Thus, the bandgap of PGGSe was determined from diffusereflectance vis/near-IR spectra, while the mid-IR transmission was measured with samples diluted with dry KBr and pressed into transparent sheets. The SHG response of PGGSe powder has been studied with a 2.05 μm Q-switched Ho-laser and AGS as a reference;15 however, the experimental results (12 to 5 times higher signal compared to AGS, depending on the powder size) sharply contradict the theoretical evaluations of the three nonlinear tensor components (of the order of 200 pm/V) according to which the SHG signal shall be about 100 times higher compared to AGS. Here we present measurements of the vis/near-IR/mid-IR transmission of PGGSe performed on thin plates, measurements of the dispersion and birefringence performed using prisms, and first SHG results from which the nonlinear coefficients are evaluated relative to AGS. In addition, we present the first Sellmeier equations constructed for this new nonlinear crystal and some laser damage threshold results. Pure chemical elements were used for the synthesis of PGGSe: Pb (99.999%), Ga (99.999%), Ge (99.999%), and Se (99.999%). First, the binary compounds PbSe, Ga2Se3, and GeSe2 were synthesized from these elements in evacuated (10−6 mbar) quartz ampules at a high temperature using a horizontal furnace. The optimum regime for the synthesis of

Figure 1. As-grown boule of PGGSe with partially polished surfaces.

boule of PGGSe containing an optically uniform single crystalline part of ϕ 12.5 × 20 mm2. The identification of the PGGSe compound was based on comparison of the recorded X-ray diffractograms (in particular, the interplanar spacings and relative intensities) using powder samples, with those from the literature,15 with the same choice for the crystallographic abc frame. The PGGSe samples were oriented by conoscopy and with a X-ray diffractometer in terms of azimuthal angle φ and polar angle θ defined in the orthogonal dielectric frame xyz. Three thin PGGSe plates with a-cut (0.11 mm), b-cut (0.114 mm), and c-cut (0.07 mm) were prepared for polarized transmission measurements. Such were performed using a standard calcite and a KRS-5 grid polarizer with a resolution ranging from 1 nm in the visible to 4 cm−1 in the mid-IR. However, the polarization dependence was found to be negligible within the typical accuracy in such measurements (∼1%). This concerns both the plateau region of clear transparency and the cutoff ranges except differences due to Fresnel reflections. Therefore, unpolarized spectra are presented in Figure 2. The 0-level transmission of the visible cutoff edge is at 0.55 μm, Figure 2b, corresponding to a bandgap of 2.25 eV, smaller compared to BGGSe,8 but considerably larger than estimations based on diffuse reflectance measurements (1.96 eV) in ref 15. Thus, PGGSe will suffer from TPA when pumped near 1 μm by pulsed Ndor Yb-laser sources. B

DOI: 10.1021/acs.cgd.9b00118 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

Figure 3. Measured (symbols) and calculated (curves) refractive indices of PGGSe.

overestimation of the nonlinear coefficients calculated in ref 15. The Sellmeier equations used to fit the measured refractive indices were of the form n2 = A + B/(λ2 − C) − Dλ2, i.e.. with one pole in the visible (√C) and an IR quadratic correction term (D). The fit parameters are summarized in Table 1. Figure 3 shows the calculated refractive indices. There are three independent nonzero tensor components for point group mm2: d24, d31, d32 (= d24 under Kleinman symmetry), and d33. The effective nonlinearity in the principal planes with c as the 2-fold symmetry axis is given by

Figure 2. Unpolarized transmission of a 70 μm thick c-cut plate of PGGSe in the entire spectral range (a) and near the bandgap (b). The plots correspond to a single pass through the sample including Fresnel reflections at both surfaces but after correction for multiple reflections.

x ‐y plane, ooe: deff = d31 sin φ

type‐I

y‐z plane, eeo: deff = d31 sin 2 θ + d32 cos2 θ

The transmission measurement error starts increasing above the usual value of 1% from 22 μm, and the error limit at 25 μm is considered to be an overestimation by up to 5% (absolute transmission value). This was the upper limit of the available FTIR spectrometer. Thus, it can be concluded that thin samples are applicable for ultrashort pulse interactions up to this limit. High transmission of ultrathin samples at such long wavelengths seems, however, not to be a peculiar feature of PGGSe, since we observed a similar behavior of BGGSe. The multiphonon absorption in PGGSe sets in from about 16 μm. However, a more accurate evaluation of the practical transparency range of PGGSe will require measurements performed also with thick samples. The present measurements did not reveal any absorption features within the clear transmission window of PGGSe. Thus, Figure 2a indicates more or less the ideal transmission window that could be achieved with thick crystals of perfect optical quality. The refractive indices of PGGSe were measured by the minimum deviation (auto collimation) technique. Two triangular semiprisms were fabricated for this purpose: one for propagation along the a-axis (apex angle: 13°52′49″) and one for propagation along the c-axis (apex angle: 14°01′08″). The three refractive indices were measured between 0.65 and 10 μm, see Figure 3, with an accuracy of 5 × 10−4 in the 0.65− 1.2 μm and 1−2 × 10−3 in the 1.3−10 μm range. PGGSe is an optically negative biaxial crystal with nx < ny < nz and Vz = 64° at 5 μm. The correspondence between the principal optical and the crystallographic axes is xyz = cba. The birefringence of PGGSe is very large, ∼0.25 at 5 μm. The measured refractive indices are lower than the theoretical predictions in ref 15. In fact the theoretical calculations predict twice lower birefringence and equidistant refractive indices, while the actual birefringence indicates quasi-uniaxial behavior. Since the second-order nonlinearity to a great extent depends on the linear susceptibility, this is already an indication of an

(1a)

type‐I (1b)

x ‐z plane, ooe: deff = d32 cos θ

type‐I (θ < Vz)

(1c)

x ‐z plane, oeo: deff = d 24 cos θ

type‐II (θ > Vz)

(1d)

The calculated type-I and type-II SHG phase-matching curves for PGGSe in the x−z plane (the only plane interesting for the mid-IR spectral range) are shown in Figure 4a. We chose to present the calculated phase-matching angle as a function of wavelength. The nonlinear coefficients of PGGSe were measured by SHG at ∼2 μm because this enables simultaneous determination of d31 and d32, the two tensor components involved in birefringent phase-matching in the principal planes. One 0.441 mm-thick plate was cut for type-I SHG in the x−y plane at φ = 16.8°, and a second 0.446 mm-thick plate was cut for type-I SHG in the x−z plane at θ = 50.3°. A 0.446 mm-thick type-I AGS crystal was used as a reference, cut at θ = 58°, φ = 45° (deff = d36 sin θ, d36 = 13.9 pm/V at 2.53 μm8). The laser source was a Ho:YLF regenerative amplifier: pulse duration, ∼5 ps, bandwidth, ∼2 nm, wavelength, 2.051 μm, repetition rate, 1 kHz, and nearly Gaussian spatial mode, with only a small fraction (300 μJ) of the available energy used.16 The pulses were focused by a 1.5 m CaF2 lens to a Gaussian waist diameter in the crystal of 2w = 0.7 mm. The parameters of the laser radiation ensure that for the selected crystal thickness the effects of spatial walk-off, and angular and spectral acceptance can be neglected. The measurements were performed with a pyroelectric detector in the low conversion limit so that the spatial intensity distribution can also be ignored. Thus, the nonlinear coefficients were obtained from the relative SHG energy conversion efficiency by only taking into account the different refractive indices, the calculated Fresnel reflections (here the deviation from normal incidence could also be C

DOI: 10.1021/acs.cgd.9b00118 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

Table 1. Sellmeier Coefficients of PGGSe crystal

n

A

B [μm2]

C [μm2]

D [μm−2]

validity range [μm]

PGGSe

nx ny nz

6.02933 7.07629 7.37087

0.34641 0.48596 0.44463

0.07863 0.11060 0.17312

0.00055 0.00124 0.00165

0.65−10

plane where such processes are phase-matchable (x−z), see Figure 4b. However, the extremely broad spectral acceptance in this plane could be useful for parametric amplification of few-cycle pulses or femtosecond continua in the mid-IR with Ho-laser pumping.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Valentin Petrov: 0000-0001-7247-6145 Funding

This work was supported by the Deutsche Forschungsgemeinschaft (DFG), No. PE 607/14-1. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We thank Uwe Griebner for the opportunity to use the Ho:YLF regenerative amplifier. Figure 4. Calculated phase-matching curves in the x−z principal plane of PGGSe for SHG (a) and optical parametric amplification/ oscillation at a pump wavelength of 2.05 μm (b).

REFERENCES

(1) Petrov, V. Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using non-oxide nonlinear crystals. Prog. Quantum Electron. 2015, 42, 1−106. (2) Structure-Property Relationships in Non-Linear Optical Crystals. II The IR Region; Wu, X.-T.; Chen, L., Eds.; Springer-Verlag: Berlin, 2012. (3) Lin, X.; Zhang, G.; Ye, N. Growth and characteristics of BaGa4S7: A new crystal for mid-IR nonlinear optics. Cryst. Growth Des. 2009, 9, 1186−1189. (4) Yao, J.; Mei, D.; Bai, L.; Lin, Z.; Yin, W.; Fu, P.; Wu, Y. BaGa4Se7: a new congruent-melting IR nonlinear optical material. Inorg. Chem. 2010, 49, 9212−9216. (5) Badikov, V.; Badikov, D.; Shevyrdyaeva, G.; Tyazhev, A.; Marchev, G.; Panyutin, V.; Petrov, V.; Kwasniewski, A. Phasematching properties of BaGa4S7 and BaGa4Se7: Wide-bandgap nonlinear crystals for the mid-infrared. Phys. Status Solidi RRL 2011, 5, 31−33. (6) Yin, W.; Feng, K.; He, R.; Mei, D.; Lin, Z.; Yao, J.; Wu, Y. BaGa2MQ6 (M = Si, Ge; Q = S, Se): a new series of promising IR nonlinear optical materials. Dalton Trans 2012, 41, 5653−5661. (7) Lin, X.; Guo, Y.; Ye, N. BaGa2GeX6(X = S, Se): New mid-IR nonlinear optical crystals with large band gaps. J. Solid State Chem. 2012, 195, 172−177. (8) Badikov, V. V.; Badikov, D. V.; Laptev, V. B.; Mitin, K. V.; Shevyrdyaeva, G. S.; Shchebetova, N. I.; Petrov, V. Crystal growth and characterization of new quaternary chalcogenide nonlinear crystals for the mid-IR: BaGa2GeS6 and BaGa2GeSe6. Opt. Mater. Express 2016, 6, 2933−2938. (9) Badikov, V. V.; Tyulyupa, A. G.; Shevyrdyaeva, G. S.; Sheina, S. G. Solid solutions in the AgGaS2-GeS2 and AgGaSe2-GeSe2 systems. Inorg. Mater. 1991, 27, 177−180; transl. from Izv. Akad. Nauk SSR, Neorg. Mater. 1991, 27, 248−252. (10) Li, X.; Li, C.; Gong, P.; Lin, Z.; Yao, J.; Wu, Y. BaGa2SnSe6: A new phase-matchable IR nonlinear optical material with strong second harmonic generation response. J. Mater. Chem. C 2015, 3, 10998− 11004.

neglected), and the experimentally measured phase-matching angles. Using Miller’s rule, the reference value of d36 (AGS) at 2.05 μm is 14.1 pm/V. With this value, d31 (PGGSe) = 16.8 pm/V and d32 (PGGSe) = 3.6 pm/V. The relative sign remains unknown, but eq 1b is not relevant to the mid-IR spectral range. The results are considered to be highly reliable since type-I phase-matching in all cases simplified the comparison. Indeed, they were reproduced with ±5% in a few independent series of measurements. The measured SHG phase-matching angle (53.53°) exceeded the calculated one by +3.23° in the x−z plane (Figure 4). In the x−y plane, the measured value (17.45°) was only slightly (0.65°) larger than the calculated one. However, the values of the nonlinear coefficients obtained are lower by more than an order of magnitude compared to theoretical predictions, which cannot be attributed to the contribution of the diagonal d33 element in the powder tests.15 Optical damage was observed in some of the SHG efficiency measurements depending on the focusing conditions in the PGGSe samples. From this, we estimated an on-axis optical damage threshold of 250 MW/cm2 (1.25 mJ/cm2) at ∼2 μm (∼5 ps pulses at 1 kHz). In conclusion, high optical quality samples of the new noncentrosymmetric chalcogenide crystal PGGSe have been grown for the first time in large sizes, which enabled the assessment of its major properties relevant to nonlinear optical applications. The second-order nonlinear coefficients of PGGSe are much lower compared to theoretical predictions. The large birefringence and specific axis correspondence lead to rather low effective nonlinearity for down conversion from the 2-μm spectral range to the mid-IR in the only principal D

DOI: 10.1021/acs.cgd.9b00118 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

(11) Luo, Z.-Z.; Lin, C.-S.; Cui, H.-H.; Zhang, W.-L.; Zhang, H.; He, Z.-Z.; Cheng, W.-D. SHG materials SnGa4Q7 (Q = S, Se) appearing with large conversion efficiencies, high damage thresholds, and wide transparencies in the mid-infrared region. Chem. Mater. 2014, 26, 2743−2749. (12) Li, X.; Kang, L.; Li, C.; Lin, Z.; Yao, J.; Wu, Y. PbGa4S7: a widegap nonlinear optical material. J. Mater. Chem. C 2015, 3, 3060−3067. (13) Lin, Z.; Li, C.; Kang, L.; Lin, Z.; Yao, J.; Wu, Y. SnGa2GeS6: synthesis, structure, linear and nonlinear optical properties. Dalton Trans 2015, 44, 7404−7410. (14) Huang, Y.-Z.; Zhang, H.; Lin, C.-S.; Cheng, W.-D.; Guo, Z.; Chai, G.-L. PbGa2GeS6: an infrared nonlinear optical material synthesized by an intermediate-temperature self-fluxing method, Cryst. Growth Des. 2018, 18, 1162−1167. (15) Luo, Z.-Z.; Lin, C.-S.; Cui, H.-H.; Zhang, W.-L.; Zhang, H.; Chen, H.; He, Z.-Z.; Cheng, W.-D. PbGa2MSe6 (M = Si, Ge): Two exceptional infrared nonlinear optical crystals. Chem. Mater. 2015, 27, 914−922. (16) von Grafenstein, L.; Bock, M.; Ueberschaer, D.; Griebner, U.; Elsaesser, T. Ho:YLF chirped pulse amplification at kilohertz repetition rates − 4.3 ps pulses at 2 μm with GW peak power. Opt. Lett. 2016, 41, 4668−4671.

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DOI: 10.1021/acs.cgd.9b00118 Cryst. Growth Des. XXXX, XXX, XXX−XXX