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Stimulated Raman scattering (SRS) is an important technique for broadening the laser spectra. The Raman crystals with large dimensions and high thermal conductivity are favorable by SRS. Zinc tungstate (ZnWO4) belonging to a monoclinic system has been grown by the Czochralski method with the dimensions of Φ 35 mm × 70 mm. The thermal properties were evaluated and analyzed in several aspects, including the density, specific heat, thermal expansion coefficient, and thermal conductivity. Thermal conductivity was calculated to be 5.412 Wm−1K−1 along the c-axis, which is about two times larger than the famous BaWO4 and SrWO4. Meanwhile, the optical properties including transmission and Raman spectra and Raman shift were measured and the stimulated Raman scattering performance of ZnWO4 crystal was demonstrated. The results show that the thermal conductivity decreases with increasing temperature. Furthermore, the thermal focal lengths were theoretically estimated. Along the a direction, the light with a wavelength of 532 nm can be shifted to the light at 558.95 nm, which is in good accordance with the spontaneous Raman spectrum. All the analyses and results above show that ZnWO4 is a promising solid-state Raman laser crystal.
© 2017 Optical Society of America
1. Introduction
Stimulated Raman scattering (SRS) is a third-order, nonlinear optical progress that offers a way to shift frequency of lasers to a new spectra regions from ultraviolet to near infrared by changing pump lasers and Raman crystals, in theory [1–4]. The application of SRS could generate eye-safe lasers which can only be produced by few solid-state sources at present. Because of this excellent property, solid-state Raman laser plays an important role in many aspects such as laser guide star, optical remote sensing, medical treatment, liar techniques and coastal bathymetry, etc [5-6]. As an inelastic scattering process, there is a mass of heat during the SRS process. Thus, Raman-active crystals require large dimensions and great thermal properties, specifically, high thermal conductivity, low thermo-optical coefficient and thermal expansion. Up to now, KRE (WO4)2 (RE represents for the rare earths), CaWO4 [1], SrWO4 [6], BaWO4 [7], YVO4 [8] have already been reported and even been used as promising Raman-active materials.
Tungstate crystals have a relatively low melting point and the crystals with large dimensions can be obtained by the Czochralski method [9-10]. Metal tungstate with a formula of AWO4 could have scheelite or wolframite structure depending on the ionic radius of the A2+ cation. When the bivalent cations have a relatively large radius, such as Ba, Sr, Pb (ionic radius ≥ 0.99 Å), tungstates tend to have scheelite structure where tungsten shows a tetrahedral coordination. On the contrary, when the bivalent cations have a smaller radius, such as Zn, Mn, Fe (ionic radius≤ 0.77 Å), tungstates would have wolframite structure and tungsten tends to have an overall 6-fold coordination, which is easy to have large thermal conductivity [11–14, 31]. Zinc tungstate (ZnWO4) which belongs to wolframite structure has the monoclinic structure with C2/h point-group symmetry and P2/c space group. Its lattice parameters are a = 4.6926 Å, b = 5.7213 Å, c = 4.9281 Å and β = 90.6321° [15]. Studies concerning the crystal growth and the optical properties including the Raman spectra of ZnWO4 crystal have been reported before [9-10], indicating that it is a promising Raman-active material. However, the completely basic studies of the thermal expansion, specific heat and the thermal conductivity, which is very important for Raman-active material, have not been studied in detail up to now. It is well known that thermal properties mentioned above play a vital role on the application in all-solid-state Raman lasers, specifically the specific heat and thermal conductivity. According to Einstein Debye model, thermal conductivity is connected with the mean phone free path, which is decided by the unit cell dimensions [16]. Compared with BaWO4 (b = 5.6041 Å) and SrWO4 (b = 5.4168 Å), ZnWO4 with wolframite structure would have a higher thermal conductivity along b direction predicting that it should be a promising Raman-active material. The specific heat is a vital factor that affects the damage threshold of the material [17]. If a Raman-active crystal has a high thermal conductivity and specific heat at the same time, the heat can be transferred to the environment as soon as possible, accordingly decreasing the thermal loading effect. This provides the possibility for pumping the crystal at high repetition rate (~10 KHz) [18]. Therefore, the thermal properties of ZnWO4 crystal along each axis over a temperature range could guide the Raman laser designs.
In this work, the single ZnWO4 crystal was grown with an a-cut seed crystal by Czochralski (CZ) method. CZ method could grow crystals with great optical properties in short time which is efficient. The anisotropic thermal properties were then measured including specific heat, thermal expansion, and thermal conductivity. The thermal properties of ZnWO4 were compared with some other Raman crystals. The optical damage threshold was determined and compared with BaWO4 and SrWO4 single crystal.
2. Experiments
Before the single crystal ZnWO4 was grown, polycrystalline raw material was synthesized through conventional solid-state reaction. The raw materials including ZnO (99.99%) and WO3 (99.99%) were put in a platinum crucible. The stoichiometry of ZnO and WO3 are determined by the formula of ZnWO4. In order to compensate for the mass loss during the solid-state reaction and crystal growth process, 1-2 wt% extra ZnO was added to the starting materials. In order to make the starting raw material homogeneous, it was well mixed in a rolling instrument for at least 24 h. After that, the mixed material was sintered in a platinum crucible for about 10 h at 900°C to get polycrystalline materials. Then the material was mixed again and pressed to thick pieces in a cylindrical shape. Whereafter, the thick pieces were sintered at about 1000°C according to the same procedure in the first step above.
The ZnWO4 crystal was grown by CZ method using a platinum crucible, which was 60 mm in diameter, 45 mm in height and the crucible contained about 500 g polycrystalline materials. The platinum crucible was heated with a 3 kHz intermediate frequency heater in a TDL-H50AC crystal-pulling apparatus. The temperature-control apparatus was an EUROTHERM 818 Controller/Programmer. The experiment was performed in the air atmosphere. At first, a single crystal was obtained from polycrystalline materials by using platinum wire. Then a rectangular a-direction crystal with dimensions 3 mm × 3 mm × 20 mm, cut from the as-grown crystal, was used as the seed which was tied on the seed crystal rod with Pt wire. In order to get more homogeneous melt, the melt was kept for about 2 h at the temperature which is 30-50°C higher than the melting point. The seed should be preheated about 30 min upon the interface of the melt before introduced to the melt at a suitable temperature. During the growth process, the rotation was at a rate of 8-15 r/min and pulling was at a rate of 0.6-1 mm/h. The crystal was pulled out of the melt when the growth was finished and was cooled to room temperature at a rate of 10-20 °C /h. The as-grown crystal shown in the Fig. 1(a) has an excellent quality:no scatter pellets can be seen under a 5 mw He-Ne laser and there is no cleavage, inclusions, obvious striations or other macroscopic defects.
Fig. 1 (a) The as-grown ZnWO4 Crystal;(b) X-ray powder diffraction patterns of as-grown crystal and standard data.
X-ray powder diffraction (XRD) was performed with an X-ray powder diffractometer using the Cu Kα1 radiation (λ = 1.5406 Å) at room temperature. The data was measured under the condition that is 10° < 2θ < 90° and 0.01°/step, 6 s/step. Then the data was collected by using Ni-filtered Cu-target tube. Each strong diffraction band was indexed and was used to analyze the orientation of the as-grown crystal.
The density of the crystal at room temperature could be measured by buoyancy method and calculated by the following equation:
where m stands the sample weight in air, m' is the sample weight in distilled water, and ρwater is the density of the distilled water at room temperature, respectively. At first, we measured the mass of the sample in air. Then, we tied the sample and measured the weight in distilled water. During the process, the sample can’t touch the beaker wall. In theory, the volume of the sample is approximate to the volume of drainage. According to that, we can calculate the density by the Eq. (1).Thermal expansion of the ZnWO4 crystal was measured in the temperature range from 298 K to 773 K by using a thermal dilatometer made by Perkin Elmer. Two rectangular pieces were prepared for thermal expansion test with dimension 4 mm × 5 mm × 6 mm, one was with faces perpendicular to the a*, b and c, the other was with faces perpendicular to the a, b and c*. The direction of a* lies in (010) plane and perpendicular to [001] direction and c* lies in (010) plane and perpendicular to [100] direction. During the thermal expansion measurement, thermal expansion ratio versus temperature along a, a*, b, c and c* were measured.
The specific heat of the ZnWO4 crystal was measured in the temperature range from 293 K to 573 K by differential scanning calorimetry with a simultaneous thermal analyzer (Perkin Elmer Diamond: DSC). The sample with dimensions 4 mm × 4 mm × 1 mm was prepared for measurement with the weight of 130 mg. The experiment was performed in a N2 atmosphere. At the start, an empty aluminium crucible was heated from 293 K to 573 K with a constant rate of 10 K∙min−1 for measuring the baseline. An Al2O3 sample was used as a standard specimen. After measurement of Al2O3, the ZnWO4 sample was put in the crucible processed with the heating procedure as shown above. The results were gotten by the associated software supplied by Perkin Elmer Co.
The thermal diffusion coefficients of the ZnWO4 crystal were measured in the temperature range from 303 K to 573 K by the laser flash method using a laser flash apparatus (Netzsch Lfa457). Two samples were processed into rectangular with dimensions of 6 mm × 6 mm × 2 mm, one possessed faces perpendicular to the a*, b and c, the other had faces perpendicular to the a, b and c*. The directions of a* and c* were in accordance with the sample of thermal expansion measurement above. In order to ensure complete and uniform absorption of the laser pulse, all the samples were sprayed with a thin layer of colloidal graphite on both sides.
Absorption spectra along the b axis of the as-grown crystal was measured at the room temperature by the laser flash method using a probe to receive the signal (Jasco V-570). The crystal was processed into dimension of 3 mm × 3 mm × 3 mm along a, b, c respectively. Then, the sample was optically polished along b-axis to get the flat face. The wavelength is in the range from 190 nm to 3000 nm at the room temperature. All the data received could be analyzed by the associated software.
Infrared transmission spectra of the as-grown ZnWO4 crystal was measured at the room temperature in wave numbers from 600 cm−1 to 4000 cm−1 by using a laser flash. The dimensions of the sample are the same with those of the sample of absorption spectrum mentioned above. The sample was placed on a diamond pedestal to ensure the sample could get homogeneous transmission. The data was collected by a probe, then a curve can be made by the associated software.
The Raman spectrum of the ZnWO4 crystal was tested with the same sample at the room temperature. The data was collected through the detector by using the laser flash apparatus. The sample was put on a diamond in order to decrease the effect from the platform. Then a curve could be get from the associated software.
An a-cut sample from the as-grown crystal was used to analyze the stimulated Raman scattering phenomenon. The dimensions of the sample were 51 mm × 3 mm × 3 mm along a, b, c, respectively. The end face was polished and uncoated. The sample was placed in a laser cavity, which was shown in the Fig. 8. The laser resonant cavity was made up with a pump source, a second harmonic generation crystal KTP, a plane reflector (M1) and a focus mirror (M2). A mode-locked Q-switched Nd:YAG laser (PY61C-10 type, Continuum) system was used to produce single pulse radiation, which was at the wavelength of 1064 nm with 40 ps duration. The repetition rate is 10 Hz. As a second harmonic generation (SHG) crystal, KTP was used to generate green light at the wavelength of 532 nm. The front mirror (M1) was coated for high reflectance at 1020-1200 nm (R > 99.9%) and high transmission at 500-580 nm, which was over 99.5%. A focus mirror (M2) with a radius of curvature of 40 mm and a high transmittance at 540-600 nm was used as the output coupling. The sample was placed in a platform, which was in the same height with the output laser. At the last, a showing card was used to receive the signal.
By doubling in SHG progress, the second harmonic was get and pulses duration of TEM00 mode was 30 ps. The power was a constant, which was 10 Hz repetition. Through the focus optics, the second harmonic output was incident through a polished face of the Raman crystal (ZnWO4 crystal) and the Raman laser was reflected onto the showing card. The Stokes lines spectrum was measured with a spectrum analyzer (USB200-VIS-NIR).
3. Results and discussion
3.1. X-ray diffraction
The result of the as-grown ZnWO4 crystal is shown in the Fig. 1(b). Compared with the standard JCPDS Card, it can be seen that the data is in good accordance with standard value. The background was fitted using a Chebyshev Polynomial and the peak shape was employed to be a modified Thompson-Cox-Hasting pseudo-Voigt (TCHZ_PV) function. The accurate unit cell parameters were calculated from the XPRD data, and they are a = 4.6920 Å, b = 5.7186 Å, c = 4.9284 Å, and β = 90.642°. The result is similar to the standard cell parameter of ZnWO4 crystal which are a = 4.6926 Å, b = 5.7213 Å, c = 4.9281 Å and β = 90.6321°. However the intensity of face (010) is larger than the standard data and it is similar to the data of Co2+: ZnWO4 reported in a literature by Fugui Yang [15].
3.2. Density
The density of the as-grown ZnWO4 crystal was measured by buoyancy method at room temperature. The experiment data and results are shown in the Table 1.
Table 1. Experimental density of as-grown ZnWO4 at room temperature
In theory, the density of ZnWO4 crystal at the room temperature can be calculated by the following equation:
where M is the molar mass of the crystal, Z is the number of molecules of per unit cell, V is the volume of per unit cell and N0 is the Avogadro constant. According to the equation, the theoretical density is 7.862 g·cm−3, which is in a good accordance with the experiment data.When the temperature changes from T0 to T, the dimension of the sample is also changed. The volume of the sample can be calculated by the following equation [19]:
where Δa*/a*0, Δb/b0 and Δc/c0 are the thermal expansion ratios when temperature increases from T0 to T. With the thermal expansion, the density decreases when the temperature increases. As a result, the density of the as-grown crystal is:where ρ0 is the density at T0. The density is 7.892 g·cm−3 at the room temperature which is a little different from the density at 300 K. We assure the density at 300 K is also 7.892 g·cm−3 so that ρ0 is 7.892 g·cm−3. The relationship between density and temperature is shown in the Fig. 2.3.3. Thermal properties
3.3.1. Specific heat
The relationship between the specific heat and the temperature of the as-grown ZnWO4 crystal is shown in the Fig. 3. It can be seen that the specific heat of the crystal increases from 0.370 J·g−1K−1 to 0.542 J·g−1K−1 smoothly when the temperature increases from 293 K to 573 K, which is equivalent to 115.81 J·K−1mol−1 to 169.65 J·K−1mol−1, and it varies approximately linearly with temperature. It is not similar to BaWO4 [19] and SrWO4 [20] which obey the Dulong-Petit law. But the specific heat of ZnWO4 is higher than that of SrWO4 (100.64 J·K−1mol−1) and is similar to that of BaWO4 (115.56 J·K−1mol−1) at room temperature. A large specific heat is in favour of a high damage threshold of the crystal [17]. Thus ZnWO4 should be a bit better than SrWO4 in the use of solid-state-Raman lasers.
3.3.2. Thermal expansion
Measurements of thermal expansion broaden our comprehensions of the crystal properties such as electronic interactions, lattice dynamics, thermal defeats and phase transition [21]. Thermal expansion coefficient of a crystal is a second-rank tensor and it can be described by the representation quadric [22].
Thermal expansion was measured along a, b, c, a*, c* in the range from 298 K to 773 K and the dimensions of the sample are 4 mm × 5 mm × 6 mm. The linear thermal expansion coefficient is defined as:
where L is the sample’s initial length at room temperature, ∆L is the change in length when temperature changes in an amount of ∆T. According to the experiment data, the thermal expansion coefficients could be calculated to be αa = 9.6369 × 10−6 K−1,αb = 8.6338 × 10−6 K−1,αc = 6.4471 × 10−6 K−1, αa* = 9.6298 × 10−6 K−1 and αc* = 6.4139 × 10−6 K−1 .Thermal expansion coefficient is connected with the symmetry of the crystal. And the relationship between the thermal expansion coefficient tensor axes in the conventional orientation is
and α13 is the thermal expansion of (101) direction. Least-squares method reported by Krishman was used to find all the other components of the tensor X [23]. It can be calculated by the following equationwhere α is 0.642°.In the conventional crystallographic axis coordinates, thermal expansion coefficients tensors were calculated to be
By the process of principle axis, thermal expansion coefficient tensor in principle axis X, Y, Z coordinates were calculated to be
The angle γ relates one of those principle coefficients with a crystallographic axis in the (010) plane and it is measured counter-clock from the c crystallographic direction. The angle could be calculated by the following equation:
So, the thermal expansion coefficients are 6.4341 × 10−6 K−1, 8.6338 × 10−6 K−1 and 9.6168 × 10−6 K−1 along X-, Y- and Z-axis and γ is 33.291°. The error bar of the apparatus is about 3% and the accuracy of the measurement is 97%.
The linear thermal expansion coefficient along a direction is maximum among these along a, b, c directions and the linear thermal expansion coefficients along b and c are close; thus the growth of crystal along a direction would not be easily crack, and it was consistent with the fact of crystal growth experiment. ZnWO4 crystal has a large and anisotropic thermal expansion, which suggests the as-grown crystal should be cooled to room temperature at low rate otherwise the crystal might crack.
Fig. 4 Thermal expansion ratio and thermal expansion coefficient along a-, a*-, b-, c- and c*-axis of the ZnWO4 crystal.
3.3.3. Thermal diffusion coefficient and thermal conductivity
The thermal conductivity plays an important role in laser cavity design, which is also a symmetric second-rank tensor [11,30]. Just as the thermal expansion coefficient, thermal conductivity also need to calculate through the principle process.
Thermal diffusion coefficients were measured along a, b, c, a* and c* in the range from 303 K to 573 K at 50 K intervals between measured temperature points. It can be seen from Fig. 4 that thermal diffusion coefficients along each measured directions are anisotropic and decrease with the temperature increasing. At 303 K, thermal diffusion coefficients along a, b, c, a* and c* are 1.231 mm2/s, 1.027 mm2/s, 1.750 mm2/s, 1.278 mm2/s and 1.220 mm2/s, respectively.
Thermal conductivity of the as-grown ZnWO4 crystal can be calculated by the following equation:
where the ρ = 7.892 g/cm3 is the density of the crystal, cp stands for specific heat, λ denotes the thermal diffusion coefficient, respectively.The obtained thermal conductivity is shown in the Fig. 5. When the temperature was 303 K, the thermal conductivity was 3.807 Wm−1K−1 along a-axis, 3.176 Wm−1K−1 along b-axis and 5.412 Wm−1K−1 along c-axis. Through the transformation of principle axes, the thermal conductivity along X direction is 3.06 Wm−1K−1, along Y direction is 3.17 Wm−1K−1 and along Z direction is 4.52 Wm−1K−1. The error bar of the apparatus is about 3% and the accuracy of the measurement is 97%. It indicates that the thermal conductivity anisotropy of ZnWO4 crystal is relatively large at room temperature. Compared with SrWO4 and BaWO4, ZnWO4 has a relatively high thermal conductivity at the same temperature; thus the ZnWO4 crystal is more suitable for high power Raman laser applications.
Fig. 5 Thermal diffusion coefficient and thermal conductivity along a-, a*-, b-, c- and c*-axis of the ZnWO4 crystal.
3.3.4 Comparisons of thermal focal lengths between ZnWO4 and other Raman crystals
Thermal focal length is another significant parameter in designing laser system [20]. The thermally induced effects severely degrade the optical quality of the laser beam and eventually limit the laser output power. The fraction of the pump power that converting to heat strongly depends on the choice of laser material and the corresponding pump source [24-25]. The thermal focal length f can be calculated by the following equation [26]:
where κ is the thermal conductivity, A is the rod cross-sectional area, Pa is the total heat dissipated in the rod, dn/dt is the refraction index, Cr,Φ is the photoelastic coefficient, α is the thermal-expansion coefficient, r0 is the radius of the crystal rod, and L is the length of the rod. Using the method presented in [20] and Eq. (10),) above, the thermal focal length f of ZnWO4 is 33.32 (r0/L = 1/5). Compared with the data of other crystals presented in [20], listed in Table 2, we can see that ZnWO4 presents a weak thermal lensing effect, which is better than the other three tungstate crystals and is slightly stronger than vanadate crystals. For r0 /L = 1/5, take 1.095 × 105A/Pa as 1.Table 2. Relative thermal focal lengths of some Raman crystals
Thermal properties of the gain mediums are vital for laser system design. The thermal properties of ZnWO4 and some other familiar Raman laser crystals are shown in Table 3. It is easy to see that thermal expansion coefficient of the ZnWO4 is relatively small compared with that of the other Raman crystals. But the ZnWO4 crystal possesses a higher thermal conductivity compared with BaWO4 and SrWO4 crystals. Compared with YVO4 and GdVO4, ZnWO4 possesses a proper melting point and is easy to grow the crystal in large size with good quality. All these properties mean that it has more potential applications in high power systems [8, 27-28].
Table 3. Some thermal property parameters of several familiar Raman crystals at room temperature
3.4. Optical properties
3.4.1. Transmission spectrum
A solid state Raman crystal should possess a suitable absorption range. The transmission spectra of the ZnWO4 crystal are shown in Fig. 6. It is shown that this crystal has a high transmittance up to 80% in the range from 432 to 6180 nm which covers the visible, near infrared and mid-infrared range. The wide transmission range is beneficial for the Raman-laser frequency transition. Compared with BaWO4 (400 nm – 3000 nm) [20] and SrWO4 (300 nm – 2700 nm) [29], the penetration bandwidth is a little broader.
3.4.2. Raman spectrum
SRS is widely reported because it’s easy to get “eye-safe” laser with high optical stability by changing the frequency of the pump laser. Raman shift is a key factor in the progress of frequency conversion. In order to identify the Raman shift of as-grown crystal, Raman spectrum was measured at the room temperature with the normal state. ZnWO4 is of the monoclinic structure with C2/h point-group symmetry and P2/c space group with two formula groups per primitive cell. The result of group theory calculation indicates that each primitive cell contains 12 free atoms and they are divided into 36 vibrations including 8Ag + 10Bg + 8Au + 10Bu. Each Raman vibration modes has been completely reported before in [9]. The typical normal state Raman spectrum of ZnWO4 crystal that we measured is shown in Fig. 7, which is in good accordance with the theory. It can be seen that the Raman shift is at 906.4 cm−1 which is belonging to Ag mode.
Many crystals are made up with tightly bond atoms in a molecular group which is loosely bound to other lattice. Therefore, it might be well matched with label vibration whereas internal or external. But for ZnWO4, the two kinds of tungstate groups are so close that is difficult to distinguish and the Davydov splitting is so large. According to the XRD structure data, the octahedral (WO6) can be seen as a starting point instead of the tetrahedral (WO4). The Raman shift frequency 906.4 cm−1 is corresponding to W-O in the WO6 group.
3.5. SRS performance of the ZnWO4 crystal
To the best of our knowledge, a SRS performance has been demonstrated for the first time by the experimental setup shown in Fig. 8, and the data of the laser experiment is also presented in the Fig. 8. From the data shown in Fig. 8, we can see that 532 nm is shifted to 558.95 nm. From this phenomenon, we could calculate the Raman shift to be 906.4 cm−1, which is in good accordance with our measurement. With the Raman shift is 906.4 cm−1, the transmission range could be broadened to be 449.6 nm – 14050.4 nm, which is a promising range. When the pump pulse energy reached 0.28 mJ, the SRS pulse energy was 0.07 mJ and the conversion efficiency is 25%. We believe that the laser performance could be improved with optimizing the design of the laser cavity, including considering the design of inner cavity between the input mirror and output mirror, optimizing the transmission of the output couplers and suitable coating of the mirror in order to reduce reflection during the inner cavity. The threshold of the SRS could be decreased and the quality of the output could be improved.
Fig. 8 (a) The schematic diagram of the SRS performance experiment; (b) Relationship between output light and input light 532nm.
Optical properties play an important role in laser experiment. Compared with other tungstate crystals, such as BaWO4 and SrWO4, ZnWO4 has a wider range of transmission and a higher transmittance, which is of importance for Raman process. In summary, ZnWO4 crystal has the potential to be a promising Raman material.
4. Conclusions
A large size ZnWO4 crystal with good quality was successfully grown by the Czochralski method and the dimensions are up to Φ 35 mm × 70 mm. From the X-ray Powder Diffraction (XRD) measurement, the crystal was determined to be the monoclinic structure and P2/c space group. Furthermore, thermal properties of the crystal were measured, such as thermal expansion coefficient, specific heat, thermal diffusion coefficient and thermal conductivity. The principle coefficients of thermal conductivity are calculated to be 3.06 Wm−1K−1 along X direction, 3.17 Wm−1K−1 along Y direction and 4.52 Wm−1K−1 along Z direction. Transmittance and Raman spectra were measured at room temperature. Optical properties were also measured of the crystal, such as absorption and infrared transmission spectra. An a direction crystal was used to analyze the Raman phenomenon and the laser at 532 nm can be shifted to 558.95 nm. So the Raman shift can be calculated to be 906.4 cm−1. They indicate that ZnWO4 crystal has excellent optical properties. All the results that we tested show that ZnWO4 is a good nonlinear and Raman material.
Funding
National Natural Science Foundation of China (51422205 and 51272131); Natural Science Foundation for Distinguished Young Scholars of Shandong Province (JQ201415) and Taishan Scholar Foundation of Shandong Province, China.
Acknowledgments
The authors wish to thank Professor Qingming Lu for the help of crystal processing and Professor Shangqian Sun and Xiufeng Cheng for the help of thermal and optical properties measurement.
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