The starting materials were analytical grade reagents Na₂CO₃, CaCO₃, SiO₂, and with the composition (in mol%) of 1Na₂O:2CaO:3SiO₂. #0-5 represent the samples added with 0, 1.0, 2.0, 3.0, 4.0, 5.0 wt% Nd₂O₃. Powder mixtures of reagent grade chemicals were mixed by ball-milling for 36 h and thereafter melted in a platinum crucible in an electrically heated furnace at 1600 ° C for 3 h. The melt was poured onto a stainless steel plate, then crushed and re-melted at least three times to ensure homogeneity. After the last melting, the mixture was cast in a preheated metallic mold at 300 ° C, then transferred to an annealing furnace and held at 600 ° C for 1 h.

X-ray diffraction (XRD) investigations were carried out using a D-max-RB diffractometer with CuKα radiation in the 2θ range from 5° to 80° at 0.02° steps.

Optical absorption spectra of the samples were measured on UV-3100 spectrophotometer. The luminescent spectra were obtained on Spectrapro-500i spectrophotometer with excitation of LYPEX-SG-WLX-FX model 808 nm fiber coupled semiconductor laser. All measurements were taken at room temperature.

The crystallized samples were tested by powder XRD pattern, and the main crystal phase obtained was Na₂Ca₂Si₃O₉ only, and no other crystalline was detected. From the XRD peaks width and Scherrer equation, the degree of crystallites after nucleation and crystallization was estimated, which is shown in Table 1.

Table 1

Table 1

Table 1 Crystallization degree of the samples doping with different Nd2O3 contents and after different heat-treatments Sample Heat-treatment Crystallization degree #0 — 0 #1 680 ° C/2 h + 700 ° C/2 h ~35% #1-1 680 ° C/2 h + 720 ° C/2 h ~55% #1-2 680 ° C/2 h + 750 ° C/4 h ~85% #1-3 680 ° C/2 h + 750 ° C/6 h ~90% #2 680 ° C/2 h + 750 ° C/3 h ~80% #3 680 ° C/2 h + 750 ° C/3 h ~80% #4 680 ° C/2 h + 750 ° C/3 h ~80% #5 680 ° C/2 h + 750 ° C/3 h ~80%

Table 1 Crystallization degree of the samples doping with different Nd2O3 contents and after different heat-treatments

High transparency is the basic requirement of laser material, and absorption or transmittance spectrum is the general judgment of transparency^[16]. The transmittance spectra of the samples #0, #1, and #1-3 are shown in Fig. 1.

Fig 1.

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	Fig 1. Transmittance spectra of the samples #0, #1 and #1-3.

Traditionally, the deeper crystallization and larger grain size will reduce the transmittance of the glass-ceramics. But in this study, it can be seen from Fig. 1 that even for the highly crystallized sample #1-3 (crystallinity is about 90%), the transmittance is still above 80% (Table 2). The high transparency of such highly crystallized glass-ceramics mainly results from simultaneous variations of the glass matrix and crystal compositions during crystallization, which considerably decreases the difference between the respective refractive indexes^{[13, 15]}. High transparency makes the consumed energy extremely low, which is beneficial to the effective pumping of solar energy, also greatly reducing thermal load, guaranteeing the stability of the laser^{[9, 17]}.

Table 2

Table 2

Table 2 Transmittance, 808 nm absorption peak width and absorption cross section No Transmittance (%) FWHM of absorption peak (nm) Absorption cross section, σ abs (10-20 cm2) #0 ~93 0 0 #1 ~90 21.35 5.46 #1-1 ~87 21.54 5.76 #1-2 ~85 22.52 6.01 #1-3 ~83 22.04 5.73 #2 ~80 25.21 4.75 #3 ~86 25.59 4.75 #4 ~91 26.04 4.06 #5 ~70 29.40 3.82

Table 2 Transmittance, 808 nm absorption peak width and absorption cross section

According to the above spectra data, absorption cross sections of curve peak for each sample at 808 nm were calculated by the general formula (1)^[18]:

where I_o and I are the intensities of incident light and transmission light, respectively; N is the volume concentration of doping elements; and l is the thickness of the sample (2 mm).

By the data from Fig. 2 and Fig. 3 and the results of calculation, the transmittance, full width at half maximum (FWHM) of 808 nm absorption peak, and absorption cross section of each sample were obtained and listed in Table 2. From Table 2, it can be seen that the crystallization behavior of the glass-ceramics greatly influences its FWHM of absorption peak at 808 nm. That is, during the crystallization process, the FWHM enhanced from 21.35 nm to 22.52 nm with the crystallinity increase of from 35% to 85%, but when the crystallinity increased more from 85% to 90%, the FWHM does not increase anymore; it decreases from 22.52 nm to 22.04 nm. There are two factors leading to this result, the first is that, before the process of crystallization, Nd³⁺ ions stay in the structure of glass, their emission spectra is Gauss type for wide glass structure; with the deepening of crystallization degree, the fluorescence emission spectra gradually changed to Lorentz model for crystal structure, which resulted in the narrowing of FWHM in early crystallization process; the second factor is that, when the fluorescence emission spectra changed, the whole fluorescence emission strength of the Nd³⁺ ions is changed accordingly. To some extent, this leads to the expansion of the emission peak and increases the measuring results of FWHM. Just these two factors make FWHM increase firstly and then decrease with deeper crystallization. Meanwhile, for the second factor, the increase of neodymium oxide content obviously leads to the increase of FWHM height width. Direct absorption cross section σ _abs is proportional to the FWHM of absorption peak, so with the increase of crystallinity, it also appears the phenomenon increased first and then decreased. In addition, the FWHM of absorption peaks of this system materials is more than 20 nm, way higher than that of commercial YAG laser materials (about 9 nm), and the absorption cross section is also higher than that of YAG laser materials (about 2.3 ×  10^-20cm²). This is much beneficial to the conversion efficiency of solar energy^[19].

Fig 2.

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	Fig 2. Absorption spectra of the samples with different Nd3+ doping contents.

Fig 3.

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	Fig 3. Absorption spectra (a) and the enlarged part (b) of samples with different crystallinities.

Under the excitation of 800-808 nm semiconductor solid state laser, the fluorescence emission spectra of ⁴F_3/2→ ⁴I_9/2 (hereinafter referred as the first fluorescent band, about 900 nm), ⁴F_3/2→ ⁴I_11/2 (hereinafter referred as the second fluorescence spectrum, about 1062 nm) and ⁴F_3/2→ ⁴I_13/2 (hereinafter referred as the third fluorescence spectrum, about 1332 nm) are obtained. The fluorescence emission spectra of the samples are shown in Fig. 4.

Fig 4.

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	Fig 4. Fluorescence emission spectra (a) and the enlarged part (b) of the samples with different neodymium oxide doping contents.

From the data of Fig. 4, the curve of I900 (the other two are the same) fluorescence peak intensity changing with the neodymium oxide content (Fig. 5) was obtained. It shows that with the increase of neodymium oxide content, the fluorescence intensity increased, but when the Nd³⁺ doping content is more than 3.0%(sample #3), it decreased with the increase of neodymium oxide content. This phenomenon shows that only a moderate amount of Nd₂O₃ doping can realize the best fluorescent output. The possible reason is that when the doping content of Nd₂O₃ is more than 3.0%, the concentration of Nd³⁺ iron makes the energy transfer between each other possible and happen frequently, which leads to the outputting energy being absorbed or transferred secondly, and results in the decline of fluorescence intensity. So an excess of Nd₂O₃ doping, more than 3.0%, is not conducive to yield effective laser output.

Fig 5.

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	Fig 5. I1062 fluorescence peak intensity curve along with the change of neodymium oxide content.

The effects of crystallinity on the fluorescence intensity is same as the Nd³⁺ doping content, increasing first and then decreasing with deeper crystallization. It is concluded that it is expected to obtain the optimal fluorescence output only with the optimal doping neodymium oxide content and optimal crystallinity.