A new laser crystal Nd:Gd0.69Y0.3TaO4 (Nd:GYTO) with high quality was grown by Czochralski method. The physical properties, including temperature dependent density, specific heat, thermal expansion coefficient, and thermal conductivity, were systematically characterized. The maximum absorption cross section at 809 nm and the stimulated emission cross section at 1066.6 nm are 6.886 × 10−20 cm2 and 22 × 10−20 cm2, respectively. The fluorescence lifetime is measured to be 182.4 μs. Up to 2.37 W of continuous wave (CW) laser operating at 1066.5 nm has been successfully realized, corresponding to an optical conversion efficiency of 36.5% and a slope efficiency of 38%. Compared with Nd:GdTaO4, Nd:GYTO shows an enhancement of the laser performance. These results demonstrate that Nd:Gd0.69Y0.3TaO4 is a novel laser crystal with low symmetry and has great potential as low to moderate power lasers.
© 2015 Optical Society of America
Recently, Nd3+ doped GdTaO4 (GTO) single crystal has been reported to be an efficient laser material for diode pumped solid state lasers (DPSSLs) . Continuous-wave (CW) laser output at 1066 nm is realized with an optical conversion efficiency of 34.6% and a slope efficiency of 36%. Compared with Nd:YAG crystal , Nd:GTO possesses larger emission cross section and shorter fluorescence lifetime, resulting in the lower laser threshold and lower energy storage capacity. Compared with Nd:YVO4 , Nd:GTO has longer fluorescence lifetime but its absorption cross sections is much smaller. The shorter lifetime and smaller absorption cross section limit Nd:GTO in pulse laser applications, especially in ultrashort pulse laser.
It is widely accepted that inhomogeneous broadening of spectra caused by mixed crystals is a practical way to reduce the emission cross section and to increase the fluorescence lifetime . For example, the mixed crystals Nd:GdxY1-xVO4 and Nd:GdxLu1-xVO4 have shown excellent CW and Q-switched laser performances, hence have attracted great attention in recent years [5, 6]. We anticipate that this strategy may improve the laser performace of Nd:GTO. Y3+ seems to be a good choice considering that the ionic radius of Y3+ is similar to that of Gd3+. Moreover, YTaO4 and GdTaO4 have the same crystal structure. Mixing GdTaO4 crystal with YTaO4 will not bring structural change or damage. Herein, we present the successful growth of Nd:Gd/YTaO4 (Nd:GYTO) crystal by Czochralski (Cz) method for the first time. The physical properties and spectral properties are investigated systematically. An efficient laser diode end-pumped CW laser operated at 1066 nm is demonstrated.
2. Crystal growth and analysis
2.1 Crystal structure and quality
Nd3+ doped Gd/YTaO4 single crystal was grown by Cz method in an iridium crucible under N2 atmosphere. The raw materials of Nd2O3 (6N), Gd2O3 (5N), Y2O3 (4N), and Ta2O5 (4N) were weighed according to the following equationFig. 1(a). No scattering point is observed under irradiation of a 5 mW 532 laser, namely, the crystal is of high optical quality.
The X-ray diffraction (XRD) patterns of the as-grown crystal were recorded with a Philips X’pert PRO diffractometer using Cu Kα radiation in the 2θ range 10° - 90° at a step rate of 0.02° min−1, as shown in Fig. 1(b). The peaks in the XRD pattern of Nd:GYTO can be well indexed with those in the JCPDS file 24-441, which indicates that the as-grown crystal has the same structure with M-GdTaO4, space group I2/a. The structural parameters of Nd:GYTO are fitted to be a = 5.394 Å, b = 11.051 Å, c = 5.083 Å, β = 95.561° by the Rietveld refinement. Compared with that of Nd:GTO (a = 5.4 Å, b = 11.059 Å, c = 5.083 Å, β = 95.61°) , the unit-cell of Nd:GYTO is smaller because the ionic radius of Y3+ (0.9 Å) is smaller than that of Gd3+ (0.938 Å). The rocking curves were collected by a high resolution X’pert Pro MPD diffractometer with a hybrid Kα1 monochromator. Wafers with dimensions of Φ12.6 mm × 2 mm were polished on both sides and used to measure the rocking curves. The Φ12.6 mm facet of each wafer was (100), (010), (001) planes, respectively. The photograph of wafer samples and the rocking curves are shown in Figs. 1(c) and 1(d), respectively. The full width at half maximum (FWHM) of (100), (010), and (001) is 0.028°, 0.037°, and 0.026°, respectively, which indicates a high crystalline quality.
2.2 Effective segregation coefficient
The effective segregation coefficients keff of the Nd3+, Y3+, Gd3+, and Ta5+ ions in the Nd:GYTO crystal were calculated according to the equation keff = Cs / Cl, where Cs and Cl are the respective concentration of ions in the crystal and the melt. X-ray fluorescence (XRF) analysis was utilized to measure the elemental concentrations of Nd3+, Gd3+, Y3+, and Ta5+ ions in the as-grown crystal. The measured specimens were cut from the shoulder part of the as-grown crystal and ground into powder. The initial composition of raw materials for growing the crystal was taken as the initial crystal growth melt composition (Cl). Results of XRF are summarized in Table 1. The effective segregation coefficient of Nd3+ ions is 0.63, which is consistent with that in GdTaO4 crystal , indicating that Nd3+ ions have been successfully doped into GYTO crystal as the active ions. It is also confirmed that Gd3+ ions can be easily substituted by Y3+ ions by the large effective segregation coefficient of Y3+ ions (1.137). This may be explained by the quite close radii between them. In addition, the redundant Y3+ ions can compensate the deficiency of Nd3+ ions in the Nd:GYTO crystal structure, and thus avoid the lattice distortion during the growth process .
3. Physical properties
3.1 Thermal expansion coefficient
Thermal expansion coefficients were obtained from the temperature-dependent XRD data collected by a Brucker D8 ADVANCE diffractometer system coupled with an Anton Paar HTK 1200N temperature control unit. Nd:GYTO crystal was ground into powders and loaded in alumina holder disks. The continuous scans covered the 2θ range of 10° - 90° with a step of 0.0105° and a counting step time of 2 s. The XRD patterns were collected at 313, 373, 453, 533, 613, 693, 773, and 853 K, respectively. Within the temperature range, the crystal symmetry remains unchanged according to the XRD data. The lattice parameters of Nd:GYTO were obtained by the Rietveld refinement of XRD patterns using the data of the JCPDS file 24-441 as the initial values.
Thermal expansion coefficient is defined as α = ΔL/L0 × ΔT, where L0 is the lattice length at room temperature and ΔL is the lattice expansion upon the temperature change ΔT . It is a second-rank tensor αij which can be represented with a second-order matrix. In monoclinic symmetry, the tensor is characterized in the orthogonal crystallo-physical frame (a*, b, c) by four nonvanishing components, α11, α22, α33, α13. The procedure to determine the four components was described in detail in , Sun et al. Herein, we provide the results for brevity. Thermal expansion coefficient can be obtained from dL/ΔL vs ΔT plots along a, b, c, a* and c* (presented in Fig. 2) as α11 = αa* = 3.79 × 10−6 K−1, α22 = αb = 10.82 × 10−6 K−1, α33 = αc = 10.41 × 10−6 K−1, αc* = 10.62 × 10−6 K−1, αa = 3.59 × 10−6 K−1. Therefore, α13 is calculated to be 1.36 × 10−6 K−1. The second-order matrix of thermal expansion coefficients in the crystallo-physical coordinate can be written as:
With the aid of the Mohr’s circle, the value of the tensors in the principal coordinates (X’, Y’, Z’) can be determined. It should be noted that in monoclinic system the principal Y’ axis is parallel to the b axis, and crystallographic axes (a and c), crystallo-physical axes (a* and c), and thermal expansion principal axes (X’ and Z’) are in the same plane but rotated. The angle f is measured counterclockwise from the principal Z’-axis towards the crystallographic c-axis. According to the matrix diagonalization procedures, it is given byEqs. (5) and (6) in , Sun et al, the second-order matrix of thermal expansion coefficients in the principal coordinate can be expressed as:
Density is one of the most basic physical properties. The density of Nd:GYTO crystal was measured by the Archimedian buoyancy method and calculated with the following equation11]. According to the thermal expansion measurement, the volume of crystal expands with the increase of temperature. As a result, the density will decrease when the temperature increases. The density of Nd:GYTO at different temperature can be calculated with Eq. (6):Fig. 3(a).
3.3 Specific heat, thermal diffusion coefficient and thermal conductivity
Specific heat (Cp), thermal diffusion coefficient (λ), and thermal conductivity (k) were recorded on NETZSCH LFA457 equipment using the laser pulse method. Wafers were coated with graphite on the faces and measured in the temperature range of 298 - 973 K. Figure 3(b) shows the Cp variation of Nd:GYTO crystal versus temperature. The specific heat increases slightly from 28.98 to 35.08 cal/(mol K) with the increase of temperature. The value is 28.98 cal/(mol K) at 298 K, which is smaller than that of Nd:YAG  crystal (83.6 cal mol−1 K−1) but larger than that of Nd:YVO4  crystal (24.6 cal mol−1 K−1).
As with thermal expansion coefficient, both thermal diffusion coefficient and thermal conductivity are second-rank tensors expressed with four independent non-vanishing components in monoclinic crystal system. However, for the limit of samples, only the thermal diffusion coefficients and thermal conductivity along the crystallographic axes are measured to reveal the thermal anisotropy. The thermal diffusion coefficients of Nd:GYTO crystal in the temperature range from 298 K to 973 K are shown in Fig. 3(c). The thermal diffusion coefficients decrease with the increase of temperature and the thermal diffusion coefficient along c axis is larger than those along a and b axes. The thermal diffusion coefficients of Nd:GYTO crystal along a, b, and c axes at 298 K are 1.606 mm2s−1, 1.346 mm2s−1, and 1.962 mm2s−1, respectively.
The thermal conductivity can be calculated by the equation: k = λρCp with the temperature dependent density, specific heat and thermal diffusion coefficients. The thermal conductivity of Nd:GYTO crystal, which is presented in Fig. 3(d), decreases with the temperature increasing. The values of ka, kb, and kc at 298 K are 4.366 W/mK, 3.497 W/mK, and 5.171 W/mK, respectively. The thermal conductivity of Nd:GYTO crystal is much larger than those of the potassium double tungstates (in the range of 2.36 to 4.4 W/mK)  and the borate crystals (in the range of 2.6 to 3.01 W/mK) .
4. Spectral properties
The absorption spectra of Nd:GYTO crystals along a, b, and c axes, as shown in Fig. 4, were recorded by a Perkin-Elmer Lambda-950 UV/VIS/NIR spectrophotometer with a resolution of 0.2 nm at room temperature. There are nine bands in the spectra corresponding to the transition of Nd3+ ion from the ground state 4I9/2 to different excited states. The final states are assigned and denoted in Fig. 4. Nd:GYTO crystal exhibits strong absorption at 809 nm and can be well matched with diode laser. Due to the anisotropy of monoclinic crystal, the absorption coefficients of Nd:GYTO along a, b, and c axes are different, which are 6.7992 cm−1, 5.3235 cm−1, and 11.4991 cm−1, with FWHMs of 8 nm, 13 nm, and 6 nm, respectively. Whereas, the FWHM of Nd:GTO crystal along a direction is 6 nm. Such inhomogeneous broadening behavior is the result of the structural disorder in mixed crystal. The absorption cross section can be determined by , where α is the absorption coefficient and N is the concentration of active ions in crystal. Based on the XRF analysis, the concentration of Nd3+ ions in Nd:GYTO is 1.67 × 1020 cm−3. Accordingly, the maximum absorption cross section (σc) of Nd:GYTO is 6.886 × 10−20 cm2. Compared with Nd:GTO crystal , Nd:GYTO shows larger absorption cross section at 809 nm and thus has better absorption of pumping energy.
The fluorescence spectrum and fluorescence decay curve were measured by an Edinburgh FLSP 920 spectrometer excited by a 450 W Xe lamp and an Opolette 355 I OPO laser, respectively. Figure 5(a) shows the fluorescence spectrum of Nd:GYTO single crystal. The strongest emission peak is located at 1066.6 nm, which corresponds to the 4F3/2→4I11/2 transition of Nd3+ ions. Figure 5(b) shows the comparison of the fluorescence at 1066 nm between Nd:GTO (red) and Nd:GYTO (black) at 8 K. The FWHM of Nd:GYTO (2.18 nm) is wider than that of Nd:GTO (1.74 nm). The structure disorder shall be responsible for the broadening of the fluorescence spectrum. It is obvious at room temperature that the FWHM of Nd:GYTO (3 nm) is much wider than that of Nd:GTO (1 nm), as shown in Fig. 5(d). The fluorescence decay curve of 4F3/2→4I11/2 transition at 300 K is presented in Fig. 5(c). With single exponential function, the fluorescence lifetime of 4F3/2→4I11/2 transition is fitted to be 182.4 μs. The stimulated emission cross section (σem) can be calculated from the fluorescence spectrum by13], Liu et al. The stimulated emission cross section of Nd:GYTO single crystal is 22 × 10−20 cm2, which is smaller than that of Nd:GTO crystal. The smaller σem and longer fluorescence lifetime will result in a higher energy storage capacity, which is beneficial for its application in Q-switched laser.
Comparisons are also made between Nd:GYTO and other laser crystals, which is listed in Table 2. Nd:GYTO has comparable thermal properties and longer fluorescence lifetime compared with Nd:YVO4. Besides, it is easy to grow high-quality and large-size Nd:GYTO crystal since no composition volatilization exists during the growth process. Therefore, it is believed that Nd:GYTO single crystal has great potential as low to moderate power lasers.
5. Laser performance
Given the anisotropy of thermal conductivity and absorption at 809 nm, Nd:GYTO crystal along c-axis is chosen to perform the laser characterization. The crystal was cut into the dimensions 2 mm × 2 mm × 6 mm, of which the 2 mm × 2 mm faces were perpendicular to c-axis and the 6 mm edge was along c-axis. The end faces were polished and coated with anti-reflection (AR) dielectric layers for 808 nm and 1066 nm. The pump source was a fiber-coupled diode laser with a core size of 200 μm and a numerical aperture of 0.22. The diode laser is centered at 809 nm with a bandwidth of 3 nm. The pump light was collimated and focused into the crystal by a focusing optics with a focus length of 6 mm. A plane-plane cavity was utilized as a resonator. The input mirror was coated with AR at 808 nm, high-reflection (HR) at 1066 nm on the pump face and high-transmission (HT) at 808 nm on the other face. The cavity length is 14 mm. The crystal was wrapped with indium foil and mounted in a water-cooled copper block which was maintained at 20 °C. The output light was collected by an OPHIR 30A-BB-18 power meter, and the laser spectrum was recorded on Edinburgh FLSP 920 spectrometer with a resolution of 0.1 nm.
We utilized two kinds of output coupler with the transmittance of 2.6% and 5.2% at 1066 nm. The output power of c-axis Nd:GYTO crystal versus incident pump power is plotted in Fig. 6. The threshold power (Pth) is 0.427 W and 0.438 W with the output coupler of 2.6% and 5.2% transmission, respectively. The output efficiency with the 5.2% output coupler is higher than that with the 2.6% output coupler. With the 5.2% output coupler, the maximum output power is 2.37 W under the pump power of 6.5 W, corresponding to an optical-optical conversion efficiency of 36.5% and a slope efficiency of 38%. The laser spectrum is presented in the inset of Fig. 6. Compared with Nd:GTO (the slope efficiency is 36%), Nd:GYTO shows an enhancement of laser performance.
A new laser crystal (0.63 at%) Nd:Gd0.65Y0.34TaO4 (Nd:GYTO) was grown by the Cz method. The effective segregation coefficient of Nd3+ is determined to be 0.63, which indicates that high Nd-doped GYTO crystal with high optical uniformity can be grown. The physical properties of Nd:GYTO crystal were systematically studied. The density is 8.34 g/cm3 and the specific heat is 28.98 cal/(mol K) at 298 K. Thermal expansion coefficients were obtained for the first time. The thermal conductivities of Nd:GYTO crystal along a, b, and c axes at 298 K are 4.366 W/mK, 3.497 W/mK, and 5.171 W/mK, respectively. The maximum absorption cross section of Nd:GYTO at 809 nm is 6.886 × 10−20 cm2 and the stimulated emission cross section at 1066.6 nm is 22 × 10−20 cm2. Compared with Nd:GTO, Nd:GYTO shows broadening in absorption and fluorescence spectra, smaller emission cross section, and longer fluorescence lifetime. With a diode laser as pump source, the efficient CW laser at 1066.5 nm is successfully realized. The maximum output power is 2.37 W with an optical-optical conversion efficiency of 36.5% and a slope efficiency of 38%. The results demonstrate that Nd:Gd0.69Y0.3TaO4 is a promising laser crystal which may find applications in low to moderate power lasers.
This work is supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 51172236, 51272254, 51102239, 61205173, and 61405206).
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