With the development of the diode pumped all solid state lasers based on neodymium doped crystals, research on new laser host materials has gained much interest. Since the emission wavelength of AlGaAs diode laser (λ= 808 nm) is increased at a rate of 0.2–0.3 nm/°C with the operating temperature of the laser device, the temperature stability of the output wavelength of the diode laser needs to be crucially controlled. Therefore, it is necessary to explore new more efficient crystal materials whose absorption band should have a large full-width at half-maximum (FWHM) near the output wavelength of AlGaAs.

Host crystals with low symmetry present strong anisotropy of physical properties which maybe rich the spectroscopic properties of doped active ions. Double tetra-metaphosphates of potassium and gadolinium have a low symmetry (monoclinic system), including three structural types: type III (P2₁) and IV (P2₁/n) KGd(PO₃)₄, both of which have a PO₄ long chain geometry, and type A KGdP₄O₁₂, which has a [P₄O₁₂]⁴⁻ cycling geometry. Compared with the former two types, KGdP₄O₁₂ crystal is not easy to cleavage from the point of view of bonding and structure. So we studied the growth, thermal and spectral properties of Nd³⁺ doped KGdP₄O₁₂ crystal.

Nd³⁺:KGdP₄O₁₂ crystal has been grown successfully by the top seeded solution growth (TSSG) technique using Nd₂O₃, Gd₂O₃, K₂CO₃ and NH₄H₂PO₄ as starting materials. It crystallizes in space group C2/c with cell parameters a = 7.812(2) Å, b = 12.307(3) Å, c = 10.474(2) Å, β = 110.84(3)° and Z = 4. The IR and Raman spectra also testified that the phosphoric polyhedra of Nd:KGdP₄O₁₂ has a cycling symmetry. The chemical compositions of the as-grown crystal were analyzed, and the distribution coefficient of Nd³⁺ was calculated. The as-grown crystal was made up of the crystalline forms {001}, {010}, {110}, {021}, which were consistent with the simulated results of the morphology.

The TG-DSC analytical result shows that KGdP₄O₁₂ crystal has a good thermal stability and decomposes at 920°C. The specific heat of the Nd:KGdP₄O₁₂ crystal increases almost linearly from 0.485 J·g⁻¹·K⁻¹ to 0.799 J·g⁻¹·K⁻¹ with the temperature increase from -10°C to 510 °C, which means that the Nd:KGdP₄O₁₂ crystal can tolerate more thermal energy at high temperature. The thermal diffusion coefficient was also measured and then the thermal conductivity was calculated (1.66 W/m·K along the c* direction). This implies that Nd:KGdP₄O₁₂ crystal has a moderate thermal conductivity.

The spectroscopic properties of the Nd:KGdP₄O₁₂ crystal were studied in detail. The unpolarized absorption spectrum [Fig. 1(a)] of the 5at% Nd³⁺-doped KGdP₄O₁₂ crystal was measured at room temperature. The absorption band around 798 nm has an FWHM of 14.7 nm. The Judd-Ofelt theory was applied to calculate the optical parameters. The excitation fluorescence spectrum [Fig. 1(b)] shows that Nd:KGdP₄O₁₂ crystal can be effectively pumped by AlGaAs in a very broad range (FWHM = 39 nm), which implies that stringent control of the diode laser temperature would be unnecessary in laser action. The emission spectrum at room temperature excited at 808nm wavelength was shown in Fig. 1(c). The FWHM of the emission peak and the emission cross section for the transition ⁴F_3/2 → ⁴I_11/2 were 14.5 nm and 6.25×10⁻²⁰cm², respectively. The fluorescence lifetime of the ⁴F_3/2 energy level of Nd³⁺ was determined to be 300μs. The higher value should be contributed to the structural characteristic of isolated GdO₈ polyhedra and would be beneficial to high energy storage in laser operation. In addition, the higher doping concentration of Nd³⁺ and the bigger fluorescence lifetime will also be useful in increasing the quality factor of laser materials. The fluorescent quantum efficiency was about 96%. All the results indicate that Nd:KGdP₄O₁₂ crystal can be considered as a very promising material for solid-state laser application.

Fig. 1. The absorption spectrum (a), the excitation (b) and emission (c) fluorescence spectra of the 5at%Nd³⁺ doped KGdP₄O₁₂ crystal at room temperature.

References:

[1] Kaminskii A.A. Laser & Photon. Rev. 2007, 1(2): 93-177.

[2] Ettis H., Naïli H., Mhiri T. Cryst. Growth & Des., 2003, 3(4): 599-602.

[3] Rekik W., Naïli H., Mhiri T. Acta Crystallogr. Sect. C 2004, 60: i50-i52.

[4] Parreu I., Solé R., Gavaldà J., et al. Chem. Mater. 2005, 17(4): 822-828.

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