Nd³⁺ ions have received considerable attention for laser applications owing to their unique energy-level structures. Calcium niobium gallium silicate (CNGS) crystals, members of the gallium lanthanum silicate system, exhibit superior mechanical and thermal properties. In this study, Nd³⁺-doped CNGS crystals with the diameter of 30 mm and isometric section length of 45 mm are grown using the pull-down method, and their optical properties, including refractive index, absorption spectra, and emission spectra, are characterized. The laser output of 1065 nm is obtained along the b-direction using 880 nm pumping, and the laser output power is 1.88 W with the conversion efficiency of 28.1% at the pumping power of 6.69 W.

Pure CNGS and neodymium-doped CNGS crystals with the diameter of 30 mm and isometric part length of 45 mm are prepared via the Czochralski method, using 99.99% high-purity (mass fraction) CaCO₃, Nb₂O₅, Ga₂O₃, and SiO₂ raw materials, as shown in Fig. 1. Single-crystal X-ray diffraction (XRD) is performed using a diffractometer. The corresponding data are collected using a diffractometer, processed with the SHELEX software, and then all the atoms in the structure are refined using full matrix least square method. The grown CNGS and Nd∶CNGS crystal samples are processed into prisms along the c-direction and adapt for refractive-index measurements using a spectrometer at room temperature. The absorption spectra of crystals with size of 6 mm×6 mm×2 mm are recorded in the wavelength range of 400?950 nm along the a and b directions by an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometer. The excitation spectra, fluorescence lifetimes, and photoluminescence spectra are determined using a fluorescence spectrum analyzer. Laser experiments are performed using a lens combination with numerical aperture (NA) of 0.22, laser spot radius of 200 μm, and focal length of 7.5 cm. As shown in Fig. 2, the input mirror is concave with curvature radius R=50 mm, the output mirror is calm, and the transmission rates (T) are 1%, 8%, 10%, and 15%. The crystal processing size is 3 mm×3 mm×13 mm, the resonant cavity length is 2 cm, and the cooling system temperature remains constant at 12 ℃.

Figure 3 presents the single-crystal XRD data. Because of the similar radii of Nd³⁺ ions and Ca³⁺ ions, the analysis of the diffraction results reveals no difference in the structural test results between the pure-phase CNGS crystals and Nd∶CNGS crystals. All hexahedra and tetrahedra are distorted to some extent, causing an increase in the disorder of the crystal structure, which in turn increases the absorption and emission cross-sections of the crystal. The refractive indices of the Nd∶CNGS crystals at 1064 nm are calculated from the fitted Sellmeier equation as n_o=1.7721 and n_e=1.8534, where n_o represents the refractive index of unusual light and n_e represents the refractive index of non-unusual light. Comparing the absorption spectra in the two directions, the absorption is stronger along the c-axis at 586, 741, and 808 nm, and therefore the c-direction can be considered as the most effective laser pumping direction. The full width at half maximum (FWHM) of the fluorescence emission spectra of the Nd∶CNGS crystals is approximately 22.58 nm, suggesting that the Nd∶CNGS crystals can be incorporated to generate ultrafast pulses. Figure 6 shows that the fluorescent lifetimes of the Nd∶CNGS crystals are 0.223 ms (b-direction) and 0.219 ms (c-direction). The excited emission cross section of the Nd∶CNGS crystal is calculated using the Fuchtbauer-Ladenburge (F-L) equation, as 9.89×10^-20 cm². Laser experiments are conducted using the Nd∶CNGS crystals in various directions. When the output mirror with 8% transmittance is employed, the highest output power is 1.88 W for the pump power of 6.69 W, slope efficiency of 28.1%, and output wavelength of 1065 nm with the corresponding full width at half-maximum of 1.5 nm. Additionally, laser experiments are conducted on the c-cut Nd∶CNGS crystal using output mirrors with transmittance values of 8%, 10%, and 15%, and the results are provided in Figs. 8(c) and (d). The laser performance of the Nd∶CNGS crystal along the b-direction is significantly better than that along the c-direction. For the b-directional crystal, the laser threshold is 0.28 W at T=1%, 0.35 W at T=8%, 0.45 W at T=10%, and 0.5 W at T=15%, while for the c-cut crystal, the laser threshold is 0.45 W at T=8%, 0.6 W at T=10%, and approximately 0.69 W at T=15%. The laser threshold increases with an increase in output transmittance T, mainly because the intracavity loss increases with an increase in T, which leads to an increase in the laser threshold.

Nd∶CNGS crystals are successfully grown via the Czochralski method, and their optical properties are examined in the b- and c-directions with the emission cross section of 9.89×10^-20 cm² at 1064 nm. The continuous laser performance of the Nd∶CNGS crystals is evaluated using a pump light source at 880 nm, and the laser output power and conversion efficiency along the b-direction of the crystal are better than those in the c-direction, with the output power of 1.88 W at 1065 nm and slope efficiency of 28.1%. Moreover, the output power and conversion efficiency of the crystal along the b-direction are better than those in the c-direction, with the output optical power of 1.88 W at 1065 nm and slope efficiency of 28.1% at the output mirror with 8% transmittance. The results obtained demonstrate that the Nd∶CNGS crystal is a promising laser gain medium; combined with its emission cross-section and spectral full width at half-maximum, it is well suited for Q-modulation and ultrafast laser applications.