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A 0.3 at% Nd doped La0.11Y0.89VO4 mixed crystal was grown by the Czochralski method. The thermal properties including the thermal expansion coefficient, specific heat, thermal diffusion coefficient, and the thermal conductivity were systematically studied. Diode-pumped continuous-wave (CW) laser performance at 1.06 μm with a- and c-cut crystal were demonstrated, and the maximum output power is about 8.76 W, which is obtained at the incident power of 19.15 W. All the results show that Nd:La0.11Y0.89VO4 is an excellent crystal for application as high-power lasers. What’s more, different with the a-cut crystal, the laser spectrum of c-cut one is found to be dual-wavelength.
©2012 Optical Society of America
1. Introduction
Nowadays, based on the development of laser-diode (LD), neodymium (Nd) doped vanadate crystals are identified to be excellent laser materials and play more and more important roles in the lasers with low and even moderate powers. Compared with commercial Nd:YAG materials, vanadate crystals possess larger emission cross-sections and shorter fluorescence lifetimes [1,2], which determine the much lower laser threshold and smaller energy storage capacity. However, in the Q-switched laser operations, the smaller energy storage capacity limits the output energy, and in the mode-locking lasers, the relative narrow spectra of single Nd doped vanadate crystals make it difficult to get short pulse such as sub-picosecond pulse. Inhomogeneous broadening of the spectra is a practicable technology for reducing the emission cross-section and increasing the fluorescence lifetime, meanwhile, conserving the advantages of the vanadate crystals. Comparing with the single Nd-doped vanadate crystals, the enhancement of the pulse energy in the passive Q-switching and much shorter pulse width achieved in the passive mode-locking with Nd doped vanadate mixed crystals had been reported [3,4]. The difference of ions radii in the host materials determines the variant level of the crystal fields, the degree of inhomogeneous broadening of the spectra, and the energy storage capacity. In rare earth ions, La has the largest ion radius. LaVO4 also possesses different structure with the other familiar vanadate crystals, such as YVO4, GdVO4, LuVO4 and ScVO4. Therefore, it can be proposed that, when mixing Nd:LaVO4 with other Nd doped vanadate crystals, the mixed crystals possess larger energy storage capacity than the previous reported Nd:LuxY1-xVO4, Nd:YxGd1-xVO4 and Nd:LuxGd1-xVO4. In the previous, Nd:LaxGd1-xVO4 and Nd:LaxY1-xVO4 were grown and reported [5–8]. However, the detailed characterization of this mixed crystal ions has not been reported, including the thermal properties and laser performance, which determine the future applications.
Based on the previous reported, when the ratio of the mixed ions in the host materials is almost one such as Nd:Lu0.5Gd0.5VO4, the absorption and fluorescence spectra have largest inhomogeneous broadened spectra and worst thermal properties [3]. While the larger or smaller ratio such as Nd:Lu0.8Gd0.2VO4 or Nd:Lu0.14Gd0.86VO4 determines the larger thermal conductivity [9]. Considering the large difference in the radii of La and Y ions, it may be believed that the replacement of Y with La ions in Nd:YVO4 would generate large inhomogeneous broaden spectra. Combining the relative higher thermal conductivity, the crystal with larger (or smaller) ratio should more promising applications in the pulsed lasers with moderate and even high power. In this paper, we reported the characterization of Nd:La0.11Y0.89VO4 with the ratio of Y and La ions of 9. The components and thermal properties of the crystal were systematic studied. The continue wave (CW) laser output of the Nd:La0.11Y0.89VO4 at 1.06 μm has been realized using a LD as a pump source.
2. Experiment details
Polycrystalline materials NdVO4, YVO4, and LaVO4 with purity of 99.99% were used to grow the crystal, and they were weighted according to the following equation
The crystal was grown by the Czochralski method under a nitrogen atmosphere with 2% oxygen (v/v) in an iridium crucible. A rectangular a-axis YVO4 single crystal was used as the seed.The average linear thermal expansion coefficients were measured using a thermal mechanical analyzer (Perkin Elmer model Diamond TMA). The sample used for the measurement was cut into a rectangular piece with dimensions of 3 × 4 × 5 mm3 (a × b × c). The density of the mixed crystal was measured by the buoyancy method at room temperature (300 K). The density can be calculated by the following equation:
where m is the sample weight in air, m’ is the sample weight immersed in distilled water, and ρwater is the density of distilled water at room temperature. The samples used in the thermal diffusion coefficient measurement were cut with the dimensions of 6 × 6 × 2 mm3 (a × b × c) and 6 × 6 × 2 mm3 (a × c × b), respectively. The thermal diffusion coefficients (λij) of the mixed crystal were measured on a laser flash apparatus (LFA447) in the temperature range of 293 to 873 K. The specific heat was measured using a differential scanning calorimeter (diamond DSC) with a small sample of the Nd:La0.11Y0.89VO4 crystal (55 mg).The continuous-wave (CW) laser performance of the mixed crystal Nd:La0.11Y0.89VO4 was based a plano-concave resonator with end pumped by a LD. The schematic diagram of the setup is shown in the Fig. 1 . The pump source employed in the experiment was a fiber-coupled LD with a central wavelength around 808 nm. Through the focusing optics (N.A = 0.22), the output of the source was incident onto the laser material. M1 has a radius of curvature of 200 mm. It was anti-reflection (AR) coated at 808 nm on the pump side, and high-reflection coated at 1.06 μm and high-transmission (HT) coated at 808 nm on the other side. M2 was a flat output coupling (OC) mirror with different transmissions of OC = 10% or 15% at 1.06 μm. In order to remove the heat generated in the laser crystal in the experiment, it was wrapped with indium foil and mounted on a water-cooled copper block. The cooling water was maintained at about 15 °C throughout the experiments. The output power was measured with a power meter (EPM 2000. Melectron Inc).
3. Results and discussions
3.1 Crystal growth and crystal structure
The as-grown crystal has an excellent quality and is shown in Fig. 2(a) , it is seen that the dimensions of the crystal are about 22 × 28 × 15 mm3 and there are no cracks in evidence (no scatter pellets can be observed under 5 mW He-Ne laser). Before cutting for the measurements, the as-grown Nd:LaxY1-xVO4 mixed crystal was annealed following the same procedure as employed previously [10]. Nd concentrations in the crystal was measured to be 0.3 at%, and the components of the mixed crystal was determined to be Nd:La0.11Y0.89VO4 by the X-ray fluorescence method.
Fig. 2 (a) As-grown crystal Nd:LaYVO4 boule; (b) XRPD patterns of the Nd:La0.11Y0.89VO4 and standard YVO4data.
The structure of the as-grown crystal was studied by x-ray powder diffraction (XRPD), and the results are shown in Fig. 2(b). All the x-ray peaks of this crystal are sharp, indicating that it is of high quality. The as-grown crystal process the ZiSiO4 structure with I41/amd space group. Figure 2(b) also shows that all the peaks can be indexed in accordance with the standard JCPDS Card File 72-274 for YVO4. Based on the XPPD data, the unit-cell parameters of Nd:La0.11Y0.89VO4 were calculated to be: a = b = 7.136 Å, c = 6.301 Å, α = β = γ = 90°. Because the radius of La (1.061 Å) ions is bigger than that of Y (0.9 Å) ion, using La ion replace Y ion results that the unit parameters is bigger than that of Nd:YVO4 (JCPDS data of YVO4: a = b = 7.123 Å, c = 6.291 Å).
3.2 Thermal expansion
The thermal expansion coefficients along a- and c- axis of the crystal were shown in the upper right of Fig. 3 . The thermal expansion curves are almost linearly increasing within the measuring temperature range over the temperature range from 303 to 739 K, and the average linear expansion coefficients of the a- and c-axis were calculated to be 1.38 × 10−6 /K and 9.56 × 10−6 /K, respectively.
3.3 Density
The ρexp of Nd:La0.11Y0.89VO4 is calculated to be 4.276 g/cm3 at room temperature, which is larger than that of YVO4 because of the much larger atomic weight of La ions. For vanadate materials, the volume will expand when the temperature increases, so the density of the mixed crystal will decrease with the temperature increasing. With the measured thermal expansion coefficients and ρexp, the density of Nd:La0.11Y0.89VO4 at different temperatures can be calculated using the following the equation
where ρ0 is the density of the crystal at room temperature, a and c is the cell parameters of unit cell at the same temperature, ∆a and ∆c are the variation of the cell parameters with the temperature changing ∆T. ∆a/a and ∆c/c can get from the experiment date of the thermal expansion coefficient. The calculated density curve with the increase of temperature is shown in Fig. 3, which shows the density decreases from 4.276 g/cm3 to 4.256 g/cm3 slowly as the temperature rising from 303 K to 737 K.3.4 Specific heat
The specific heat of Nd:La0.11Y0.89VO4 dependence on temperature is shown in upper right of Fig. 4(a) . It can be seen that the specific heat of the crystal increases from 0.542 to 0.608 Jg−1k−1 slightly with temperature increasing. The specific heat of Nd:La0.11Y0.89VO4 is 27.15 cal/(mol K) at 298 K, which is larger than that of Nd:LaVO4 [12] crystal (24.6 cal/(mol K) at 298 K) and Nd:YVO4 [15] crystal (24.6 cal/(mol K) at 298 K). The comparisons with the other vanadate crystals are list in Table 1 .
Fig. 4 (a) thermal diffusion coefficient versus temperature (b) thermal conductivity versus temperature.
Table 1. Comparison of Main thermal Properties of several Nd-Doped vanadates
3.5 Thermal diffusion coefficient and thermal conductivity
Figure 4(a) shows the thermal diffusion coefficients of the crystal over the temperature range from 293 to 873 K. it is seen that the thermal diffusion coefficient along a- and c- axis decreases with the increase of temperature. At room temperature the thermal diffusion coefficients λ11 and λ33 are 2.198 mm2s−1 and 2.502 mm2s−1, respectively, which is smaller than that of Nd:YVO4 [13] (1 at% Nd doped, λ11 = 3.6 mm2s−1 λ33 = 4.7 mm2s−1).
Like thermal diffusion coefficient, the thermal conductivity [kij] is also a second-rank tensor and only have two independent principal components k11 = k22 and k33 along the principal axis for the tetragonal system. The thermal conductivity of the Nd: La0.11Y0.89VO4 mixed crystal can be calculated by the following equation
where λ, Cp and ρ denote the thermal diffusivity, specific heat and density of the Nd:La0.11Y0.89VO4 mixed crystal, respectively. The obtained thermal conductivity of the crystal along different directions is plotted in Fig. 4(b). The thermal conductivities of Nd:La0.11Y0.89VO4 mixed crystal are k11 = 5.13 Wm−1K−1 and k33 = 5.84 Wm−1K−1 at room temperature, and are larger than those of Nd:YVO4 [15]. According to the Eq. (4), although the thermal diffusion coefficient smaller due to the smaller average path of phonons generated by the disordered structure of the mixed crystal [14], the density and specific heat of the crystal both became larger than Nd:YVO4. Therefore, it is easy to understand why the thermal conductivity became larger than Nd:YVO4. The great thermal conductivity indicated that the crystal could be used in the middle and high power laser. The comparison with other common vanadate crystals was shown in Table. 1. It can be found that Nd:La0.11Y0.89VO4 has comparable thermal properties, including the specific heat and thermal conductivity, with Nd:YVO4 which has been widely used in the lasers with low and moderate power and low threshold [17–19]. Based on the generated larger inhomogeneous broaden spectra by introduction of La ions, it can be believed that the Nd:La0.11Y0.89VO4 should have promising application in the pulsed lasers with moderate power.3.6 Laser performances at 1.06 μm
Detailed laser performance of a-cut Nd:La0.11Y0.89VO4 is shown in Fig. 5(a) . It is found that the threshold power and the maximum output power are 0.353 W and 8.74 W, respectively, with a 15% transmission output coupler. The maximum output power is obtained under the pumped power of 19.15 W corresponding to optical conversion efficiency of 45.6%. The slope efficiency is 46.5%. From the plot, it can be seen that the laser output is not saturated thanks to the high thermal conductivity k33.
The laser output performance of the c-cut Nd:La0.11Y0.89VO4 is shown in Fig. 5(b). It is found that the threshold power and maximum output power are 1.093 W and 7.62 W with the 10% transmission output couple, respectively. The maximum output power is also obtained when the pumped power is 19.15 W, corresponding to optical conversion efficiency of 39.8%. The slope efficiency is 43.9%.
Using the Pin-hole method, the M2 value of the a-cut and c-cut CW laser beam at the incident pump power of about 12 W was measured to be 2.36 and 2.11, respectively. Compare the results of two a-cut and c-cut, we found that the c-cut crystal laser output is almost saturated when the pumped power reaches to 19.15 W. Just considering the thermal problems, for a-cut crystal, the thermal transfer depends on the thermal conductivity k11 and k33, while for c-cut crystal it mainly depends on k11.
With a spectra analyzer, the laser spectra are measured shown in Fig. 6 . For the a-cut crystal, the peak of laser wavelength is located at 1064 nm, while, the c-cut crystal laser possesses dual wavelength centered at 1064.8 nm and 1066.8 nm. With a polarizer, we found that both of the two modes of the dual-wavelength laser were unpolarized. In the inset of Fig. 6(b), the fluorescence spectrum of the c-cut crystal at the temperature of 78 K was obtained. It is found that there are two peaks located at about 1062.9 nm and 1065.1 nm, which can help us understand the reason of dual-wavelength output. Recently, multiple wavelengths lasing have been of great interest for many applications such as medical instrument, spectral analysis, and THz frequency generation, etc. So the dual-wavelength lasing property of the crystal will attract more people’s attentions.
Fig. 6 (a) Laser spectrum of a-cut Nd:La0.11Y0.89VO4; (b) Laser and fluorescence spectrum of the c-cut Nd:La0.11Y0.89VO4; Inset of Fig. 6(b) The fluorescence line of Nd:La0.11Y0.89VO4 at the temperature of 78 K.
4. Conclusion
The high-quality Nd doped La0.11Y0.89VO4 mixed crystal was grown by Czochralski method. The component measurement indicated that the mixed process the ZiSiO4 structure with space group I41/amd. The systematic thermal properties of the crystal were measured, the results show that the Nd:La0.11Y0.89VO4 crystal has relatively high thermal conductivity, and this crystal could be used as an moderate power laser materials. The CW laser operation with LD pumped was demonstrated, yield an output power of 8.17 W under the pumped power 19.15 W. Better results can be predicted by optimizing the cavity design and using a coated Nd:La0.11Y0.89VO4 crystal. All the results show that this crystal should be an excellent laser material.
Acknowledgment
This work was supported by National Natural Science Foundation of China (Grant Nos. 51025210, 51032004 and 51021062), the National Basic Research Program of China (Grant No. 2010CB630702), the National High Technology Research and Development Program (“863”Program) of China (No. 2009AA03Z436)
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