Open Access
Add to BibSonomyAbstract
The linear and nonlinear optical properties of a new quaternary tungstate tellurite crystal Na2TeW2O9 (NTW) are reported. It transmits well from 0.45 to 5 μm. The refractive indices have been measured and the crystal exhibits flexible phase-matching properties. The complete set of second-order nonlinear optical coefficients were obtained using Maker Fringe techniques, with the largest d13 = 9.3 pm/V. The crystal supports noncritical phase-matching (θ = 90°, φ = ± 48.6°), at the incident wavelength of 1064 nm, the effective nonlinear coefficient (deff) is 6.9 pm/V. The crystal structure and calculations of the dipole moment are discussed to explain the second-harmonic generation of NTW compared with Cs2TeMo3O12. Considering its good linear and nonlinear optical properties, NTW is a good candidate for nonlinear optical applications, especially for 3-5 μm region.
© 2015 Optical Society of America
1. Introduction
Mid-infrared (3-5 μm) laser sources have attracted great attentions because of their military and civilian applications such as infrared spectroscopy, medical diagnosis and therapy, infrared countermeasures. Optical frequency conversion is an important method for producing infrared lasers. Nonlinear optical crystals play the most important role in frequency conversion progress. Phosphides and sulfides such as ZnGeP2, CdSiP2, LiInS2, and AgGaS2, are considered to be excellent infrared optical crystals, but there are still serious drawbacks which limit their application [1–4]. Therefore, continue interest has been paid in developing new crystals with improved linear and nonlinear optical properties, including higher nonlinear coefficients, wider transmission ranges, more flexible phase-matching properties, enhanced optical damage tolerance, and chemical, thermal, and mechanical stability [5,6]. In the crystal growth processes of phosphides and sulfides, there are great vapor pressure of sulphur and selenium. Therefore, it is harder to obtain high quality phosphides and sulfides crystals than that of oxide crystals. Compounds with d0 transition metal cations and cations with nonbonded electron pairs have been synthesized to take advantage of the second order Jahn-Teller (SOJT) effect, and these have attracted attention due to their relatively large powder second-harmonic generation (SHG) efficiencies. In the past few years, crystals of this kind such as β-BaTeMo2O9(BTM), Cs2TeMo3O12(CTM), Na2TeW2O9(NTW), and Na2Te3Mo3O16 have been grown [7–10], but only the second order optical properties of BTM and CTM crystals have been studied [11]. NTW is also one of these interesting compounds. Its Kurtz power SHG efficiency is about 500 × α-SiO2 [12], while that of BTM and CTM are 600 and 400 × α-SiO2, respectively. Hence, we consider that NTW should possess good nonlinear properties. So far, centimeter size NTW crystals have been grown by the top-seeded solution growth method [8]. However, the linear and second order optical properties, including transmission range, refractive indices, nonlinear optical coefficients and phase-matching etc., of this potential mid-infrared nonlinear crystal have not been reported.
In this letter, we report transmission spectra, the refractive indices and the relationship between the crystallographic axes and refractive index axes. The nonlinear optical coefficients have been studied by Maker fringe technology. The phase-matching and effective SHG coefficients at 1064 nm are also discussed in detail. Moreover, the crystal structures and dipole moments have been studied and calculated to explain the SHG activities of NTW and CTM. It is worth noting that the calculations are consistent with the experimental results.
2. Linear optical properties
All the optical properties should be studied in the orthorhombic coordinate system which is ordered as nx<ny<nz rather than a crystal physical coordinate system. Since NTW is a biaxial crystal with point group m, only one of the orthorhombic axes is parallel to crystallographic axis b, while the other two orthorhombic axes are located in the ac-plane, but they are not parallel to the a- or c-axes. A polarizing microscope and a polished NTW sample perpendicular to b-axis were used to determine the directions of orthorhombic axes. The results show that one of the orthorhombic axes is located in β-range, and the angle between a-axis and a orthorhombic axis is 42°, while the other one is perpendicular to it in ac-plane (see inset of Fig. 1). A 1 mm thick (010) sample of NTW was cut and double-face polished for transmission spectra measurements at room temperature. As shown in Fig. 2, NTW exhibits good transparency from 0.45 to 5 μm. The ultraviolet and infrared absorption edges are 0.36 and 5.5μm, respectively. It is possible that mid-infrared lasing can be generated with 1064 nm pumping.
Fig. 1 Dispersion of the refractive indices and calculated curves from the Sellmeir coefficients for the NTW crystal.
The refractive indices (nλ) of NTW, as a function of wavelength λ, were measured using the vertical-incidence method. The accuracy of the measurements is estimated to be 5 × 10−5, which permits a sufficiently accurate prediction of the phase-matching directions owing to the large birefringence. In our experiments, two orthorhombic refractive indices nx and ny were obtained when monochromatic sources were vertically incident on the (010) plane. The refractive indices nz and na were obtained when the monochromatic source was vertically incident on the (001) plane. The results show that the refractive index nz is parallel to crystallographic b-axis. The dispersion parameters of the refractive index ni were fitted by the least-squares method to the Sellmeir equations as follows:
where λ is the wavelength in micrometers. As shown in Fig. 1, the curves obtained from the equations and the experimental data are in good agreement. It is obvious that the difference between the values of nz and ny is larger than that between values of ny and nx, which indicates that NTW is an optically positive biaxial crystal.3. Second-order nonlinear optical properties
NTW belongs to the most complex point group except for the triclinic system. It has six independent second order nonlinear optical (NLO) coefficients, d11, d12, d13, d16, d22, and d34, under the Kleinman symmetry. They can be determined with three NTW samples using the Maker fringes (MFs). According to the Ref. (13), there are three kinds of MFs: the first is related to one NLO coefficient and the envelope function has a maximum value at incidence angle θ = 0°; the second is related to one NLO coefficient, but the envelope function is zero at θ = 0°; the third is related to two NLO coefficients and its envelope function only depends on one of coefficients. Measurement of d36 in KH2PO4 (KDP) was used as the standard. In our experiments, a Q-switched Nd:Yttrium lithium fluoride laser (Sunlight 200 SGR-10) at 1053 nm with a repetition frequency of 1 Hz was used as the fundamental light source. The second harmonic signal from the sample was selectively detected by a photomultiplier tube (PMTH-S1V1-CR131), averaged by a fast-gated integrator and a boxcar averager (Stanford Research Systems), and then recorded using data acquisition software.
The experimental power of SHG (I2) can be expressed using Eq. (4). With the relative measurement of d36 in KDP, the dij values can be obtained from the ratio of the central envelope values measured on NTW and KDP samples as in Eq. (5).
where f(θ) is a function of the incident angle θ, T is the transmittance, and d36(KDP) = 0.39 pm/V is the standard.The sample orientations, the polarizations of the fundamental and of the harmonic waves for the determination are listed in Fig. 3. An experimental MF platform was produced according to the procedure in Ref. [14]. All the experiments were performed at room temperature. For example, for the determination of d13, the experimental results, the calculated fringe patterns, and the calculated envelope curve (the dashed line) are presented in Fig. 4. It can be seen that the theoretical envelope fits the experimental data very well. When the largest SHG power and the values of f(0) for each sample were obtained, the coefficients of NTW can be calculated.
Fig. 4 Maker fringes measurement for the coefficient d13: a) experimental fringes b) theoretical fringes.
More attention should be paid to the effective NLO coefficients in crystal applications. As a biaxial crystal, the effective NLO coefficient (deff) of NTW should not be given using the simple equations for uniaxial crystals. Based on the Sellmeir equations for NTW, the phase-matching curves in XY-plane for SHG at 1064 nm were calculated. As shown in Fig. 5, NTW can achieve both type-I and type-II phase-matching SHG. It is worth noting that NTW supports noncritical phase-matching in the direction (θ = 90°, φ = ± 48.6°) for type-II phase-matching condition (o + e→o). The effective NLO coefficient in the noncritical phase-matching direction can be calculated as follows:
where the dijk are the NLO coefficients, and aie2, aje1 and ake2 are the coordinate rotation matrices corresponding to the incident angle θ, φ and refractive indices.Fig. 5 Phase matching curves of NTW at 1064 nm. a) Type-I phase matching curve b) Type-II phase-matching curve.
In the direction (θ = 90°, φ = ± 48.6°), the effective NLO coefficients are determined only by d13 and d23. The effective NLO coefficient was calculated to be 6.9 pm/V. Compared to other widely used NLO crystals as shown in Table 1, it can be seen that both the NLO and effective NLO coefficients are much larger than that of KTP, LBO and BBO. More interestingly, they are larger than that of CTM, which agrees with the values of the Kurtz power SHG efficiency very well.
Table 1. Second-order NLO coefficients and the largest effective nonlinear coefficient of NTW at room temperature
4. Discussion
It is well known that a noncentrosymmetric structure is a pre-requirement for SHG properties [15]. To study the structural and SHG relationships, the crystal structures and dipole moments of NTW and CTM have been studied to explain their SHG activity. As reported in previous work [12,16], the structure of NTW shows three-coordinated tellurium atoms link the two-dimensional tungsten oxide network, forming a three dimensional structure. There are eight crystallographically unique tungsten atoms and four crystallographically unique tellurium atoms, and all of them are located in a distorted environment. Each WO6 octahedron is distorted with three short and three long W-O bonds, whose length ranges from 1.711 to 2.252 Å. One of distorted octahedra has O-W-O angle of 71.4° and bond length ranging from 1.759 to 2.177 Å. The corrugated layers of W4O18−12 units show that all the WO6 octahedra are skewed with respect to each other, attributable to the distorted environment of each individual W atom. For the TeO4 tetrahedra, the Te-O bond lengths range from 1.852 to 1.944 Å. There are typical three-coordinated Te4+ that formed by distorted trigonal pyramidal interactions. It is obvious that both the distortions of the TeO4 tetrahedra and WO6 octahedra contribute to the noncentrosymmetric structure. The WO6 and MoO6 octahedra have similar distortions that were reported by Halasyamani P. S [17]. Following the method used to quantify the properties of asymmetric polyhedral [18,19], the distortion (Δd) and dipole moments have been calculated for NTW and CTM. The results are listed in Table 2. It can be seen that the magnitude of the distortion of WO6 is calculated to be in the range of 0.7552 to 1.2338, which is strong distortion. The magnitudes of the dipole moments show that three of the WO6 octahedra exhibit small dipole moments with values of 0.8739, 1.9243, and 2.2849 Debye. The other WO6 octahedra possess large dipole moments ranging from 8.2919 to 18.156 Debye, which are much larger than that of dipole moments of MoO6 octahedra in CTM crystals (5.7922 Debye). It is worth noting that the magnitudes of all of the TeO3 polyhedra are larger than that of TeO3 polyhedra in CTM. The magnitude of the net dipole moment in the NTW was calculated to be 628.96 × 10−4 esu*cm/A3, which is about twice that of CTM (302.14 × 10−4esu*cm/A3). The directions of dipole moments of each W-O6 and Te-O3 polyhedral and net dipole moment in NTW are shown in Fig. 6. Considering the larger net dipole moment in NTW than that in CTM, a larger NLO coefficient in NTW can be expected, which is in good agreement with the experimental results.
Table 2. Polyhedron distortion and net dipole moment in NTW and CTM
Fig. 6 Directions of dipole moments of each W-O6 and Te-O3 polyhedral and net dipole moment in NTW. The blue arrows indicate the directions of dipole moments for each W-O6 and Te-O3 polyhedron. The red arrow indicates the net dipole moment for each unit cell.
5. Conclusion
In summary, the linear and second order optical properties of NTW crystal have been studied in detail. All the independent NLO coefficients have been determined using the MF technique. The phase-matching curves indicate that NTW can achieve noncritical phase-matching with NLO coefficient of 6.9 pm/V. The crystal structure, distortion and net dipole moment were analyzed and calculated to explain the excellent NLO properties of NTW. Considering its wide transmission range and good NLO properties, NTW is expected to be an outstanding candidate crystal for nonlinear optical applications in mid-infrared range.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 61308088, 51021062, and 51272129), and the Program of Introducing Talents of Disciplines to Universities in China (111 program no. b06017).
References and links
1. E. Lippert, G. Rustad, G. Arisholm, and K. Stenersen, “High power and efficient long wave IR ZnGeP2 parametric oscillator,” Opt. Express 16(18), 13878–13884 (2008). [CrossRef] [PubMed]
2. S. C. Kumar, M. Jelínek, M. Baudisch, K. T. Zawilski, P. G. Schunemann, V. Kubeček, J. Biegert, and M. Ebrahim-Zadeh, “Tunable, high-energy, mid-infrared, picosecond optical parametric generator based on CdSiP2,” Opt. Express 20(14), 15703–15709 (2012). [CrossRef] [PubMed]
3. G. M. H. Knippels, A. F. G. van der Meer, A. M. Macleod, A. Yelisseyev, L. Isaenko, S. Lobanov, I. Thènot, and J. J. Zondy, “Mid-infrared (2.75-6.0-µm) second-harmonic generation in LiInS2,” Opt. Lett. 26(9), 617–619 (2001). [CrossRef] [PubMed]
4. T. J. Wang, Z. H. Kang, H. Z. Zhang, Q. Y. He, Y. Qu, Z. S. Feng, Y. Jiang, J. Y. Gao, Y. M. Andreev, and G. V. Lanskii, “Wide-tunable, high-energy AgGaS2 optical parametric oscillator,” Opt. Express 14(26), 13001–13006 (2006). [CrossRef] [PubMed]
5. M. Ghotbi and M. Ebrahim-Zadeh, “Optical second harmonic generation properties of BiB3O6.,” Opt. Express 12(24), 6002–6019 (2004). [CrossRef] [PubMed]
6. I. Chung and M. G. Kanatzidis, “Metal chalcogenides: a rich source of nonlinear optical materials,” Chem. Mater. 26(1), 849–869 (2014). [CrossRef]
7. W. Zhang, X. Tao, C. Zhang, Z. Gao, Y. Zhang, W. Yu, X. Cheng, X. Liu, and M. Jiang, “Bulk Growth and Characterization of a Novel Nonlinear Optical Crystal BaTeMo2O9,” Cryst. Growth Des. 8(1), 304–307 (2008). [CrossRef]
8. W. Zhang, F. Li, S. H. Kim, and P. S. Halasyamani, “Top-seeded solution crystal growth and functional properties of a polar material-Na2TeW2O9,” Cryst. Growth Des. 10(9), 4091–4095 (2010). [CrossRef]
9. J. Zhang, X. Tao, Y. Sun, Z. Zhang, C. Zhang, Z. Gao, H. Xia, and S. Xia, “Top-Seeded Solution Growth, Morphology, and Properties of a Polar Crystal Cs2TeMo3O12,” Cryst. Growth Des. 11(5), 1863–1868 (2011). [CrossRef]
10. W. Zhang, J. Sun, X. Wang, G. Shen, and D. Shen, “Crystal growth and optical properties of a noncentrosymmetric molybdenum tellurite, Na2Te3Mo3O16,” CrystEngComm 14(10), 3490–3494 (2012). [CrossRef]
11. Q. Yu, Z. Gao, S. Zhang, W. Zhang, S. Wang, and X. Tao, “Second order nonlinear properties of monoclinic single crystal BaTeMo2O9,” J. Appl. Phys. 111(1), 013506 (2012). [CrossRef]
12. J. Goodey, J. Broussard, and P. S. Halasyamani, “Synthesis, structure, and characterization of a new second-harmonic-generating tellurite: Na2TeW2O9,” Chem. Mater. 14(7), 3174–3180 (2002). [CrossRef]
13. S. X. Dou, M. H. Jiang, Z. S. Shao, and X. T. Tao, “Maker fringes in biaxial crystals and the nonlinear optical coefficients of thiosemicarbazide cadmium chloride monohydrate,” Appl. Phys. Lett. 54(12), 1101–1103 (1989). [CrossRef]
14. X. Zhang, X. A. Wang, G. L. Wang, Y. C. Wu, Y. Zhu, and C. T. Chen, “Determination of the nonlinear optical coefficients of the LixCs(1 – x)B3O5 crystals,” J. Opt. Soc. Am. B 24(11), 2877–2882 (2007). [CrossRef]
15. C. Chen, Y. Wu, and R. Li, “The anionic group theory of the non-linear optical effect and its applications in the development of new high-quality NLO crystals in the borate series,” Int. Rev. Phys. Chem. 8(1), 65–91 (1989). [CrossRef]
16. W. Zhang, P. S. Halasyamani, Z. Gao, S. Wang, J. Wang, and X. Tao, “Anisotropic Thermal Properties of the Nonlinear Optical and Polar,” Cryst. Growth Des. 11(8), 3636–3641 (2011). [CrossRef]
17. K. M. Ok, P. S. Halasyamani, D. Casanova, M. Llunell, P. Alemany, and S. Alvarez, “Distortions in octahedrally coordinated d0 transition metal oxides: A continuous symmetry measures approach,” Chem. Mater. 18(14), 3176–3183 (2006). [CrossRef]
18. P. S. Halasyamani, “Asymmetric cation coordination in oxide materials: Influence of lone-Pair cations on the intra-octahedral distortion in d0 transition metals,” Chem. Mater. 16(19), 3586–3592 (2004). [CrossRef]
19. K. M. Ok and P. S. Halasyamani, “Mixed-metal tellurites: Synthesis, structure, and characterization of Na1.4Nb3Te4.9O18 and NaNb3Te4O16.,” Inorg. Chem. 44(11), 3919–3925 (2005). [CrossRef] [PubMed]
